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Artist's impression of a gaseous exoplanet closely orbiting its host star

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Detecting Exoplanets Closer to Stars with Moderate Spectral Resolution Integral-Field Spectroscopy
Authors: Shubh Agrawal et al.
First Author’s Institution: California Institute of Technology
Status: Published in AJ

Thus far, the vast majority of known exoplanets have been discovered indirectly, using techniques such as the transit or radial velocity methods, which allow us to infer the presence of planets based on their effects on their host stars. However, to fully characterize an exoplanet, we need to observe it directly. As you might guess, picking out the light coming from a planet, as opposed to the star it’s orbiting, is no small feat given how bright stars are compared to planets. Astronomers have come up with lots of tricks over the years to improve imaging techniques, from using coronagraphs to block out some of the star light to designing adaptive optics that correct for atmospheric effects and employing complex signal-processing algorithms. However, direct imaging is still typically restricted to observing planets that are massive, bright, and live quite far from their host stars. The relative brightness and physical separation from the star make these planets much easier to see than the smaller, closer planets whose signals are overpowered by starlight.

But today’s authors have a plan to directly observe planets orbiting closer to their host stars than ever before! Their idea hinges on using spectroscopy to better differentiate between planets and their host stars.

The new detection method involves a technique called integral field spectroscopy (IFS), in which a field of view is split into a grid, with a spectrum taken for each cell in the grid (Figure 1). The idea behind using IFS for finding planets depends on differentiating between the spectral features of planets and stars to identify which grid cells are sampling the planet’s light. For example, the planet might have features like water or carbon monoxide, whereas the star has a more complex spectrum with many features blended together.

Diagram describing integral-field spectroscopy

Figure 1: Diagram describing integral field spectroscopy, where an image is split into smaller cells, each with its own spectrum. [ESO; CC BY 4.0]

Currently, there’s a limit to how close a planet can be to its host star and still be observable due to speckle noise, which has to do with how the light from the host star is diffracted in the imaging process. Typically, one would try to eliminate the speckle noise while reducing the data, but today’s authors propose modeling the speckles along with the planet data. Figure 2 shows an example of a model planet spectrum (left) versus the components used to model starlight (right). By modeling all of the planet and star components together, the authors are able to avoid some of the systematic effects that typically cause speckle noise to hide planets that are too close to the host star. The authors then apply their model to all the spectra in an IFS grid to identify whether and where planets are hidden.

Plots showing the modeled planetary spectrum and five components of the starlight spectrum

Figure 2: The left panel shows a model spectrum for the planet, and the right panel shows a few of the many components that are used to model starlight. [Adapted from Agrawal et al. 2023]

To test the method, the authors used the OSIRIS instrument at Hawaii’s Keck Observatory to survey 20 target stars. They chose stars in the Taurus and Ophiuchus star-forming regions, which are most likely to have young planets. This is important because the young planets will be hotter and therefore brighter than their older counterparts, making them slightly easier to see. The authors also selected more massive stars, which have been found to be more likely to host gas giants.

Detection map for a test-case star

Figure 3: The resulting detection map for one of the test-case stars. The star is the larger bright area in the middle, and the M-dwarf companion is the small bright area marked by the red cross. [Agrawal et al. 2023]

It’s important to note that the test-case stars were much farther away from Earth than typical direct imaging targets are. Ideally, we want the planet to have as much angular separation from the star as possible; the farther away a system is, the smaller the angle between the planet and star becomes, and the harder it is to detect that planet. Despite the test-case stars being so far away, the authors found that the IFS technique is capable of recovering planets at least as well as typical methods! While no new planets were found for the particular stars in the test survey, the authors did identify an M-dwarf companion at a very small angular separation from one host star (Figure 3).

Based on the success of the IFS test, the authors conclude that IFS planet detection could be a really powerful way to find closer-in planets, especially given the IFS instruments on JWST and the capabilities of future Extremely Large Telescopes. Probing these close-in planets is especially important as radial velocity surveys have indicated that there should be quite a few Jupiter-mass planets within a few astronomical units of their host stars, but existing imaging techniques aren’t able to resolve those small separations. Finally, the authors show that their approach to modeling the planet and star light at the same time helps to retain more information about the planet’s atmosphere, and it could be a really promising method for measuring compositions and studying habitability in the coming years!

Original astrobite edited by Jack Lubin.

About the author, Isabella Trierweiler:

I’m a fifth-year grad student at UCLA. I’m interested in planet formation, and I study the compositions of exoplanets using polluted white dwarfs. In my free time, I like knitting, playing train games, and growing various fruit trees.

illustration of a super-Earth exoplanet with a watery atmosphere

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Prebiosignature Molecules Can Be Detected in Temperate Exoplanet Atmospheres with JWST
Authors: A. B. Claringbold et al.
First Author’s Institution: University of Cambridge
Status: Published in AJ

The search for life in the universe has always been a driving force for interest in and development of astronomy and the space sciences. Far from tales of little green men and aliens on Mars, today’s scientific investigations into extraterrestrial life usually involve trying to find the traces that life leaves behind in the light that we receive from the stars. This could be evidence of alien transmissions, structures, technology, or intelligence — “technosignatures” — or evidence of molecules or other indicators of the existence of life, regardless of its intelligence — “biosignatures.”

Given many arguments and discussions about the rarity of life in the cosmos, however, many consider it prudent to search not only for these signatures but also for prebiosignatures — molecules in planetary atmospheres that correspond not to the current existence of life, but to the conditions in which life (organic chemistry, in particular) arose on Earth. These include molecules created by volcanism, ultraviolet radiation, or even lightning. Given what we currently understand about how proteins and RNA came to be on our planet, searching for signs of these molecules may help us find the precursors for life elsewhere in the universe.

The authors of today’s article wish to test the sensitivity of JWST to detecting traces of these prebiosignature molecules in the atmospheres of various kinds of exoplanets. To do this, they use atmospheric models to simulate what the transmission spectra of different exoplanets would look like and test whether JWST’s instruments can recover the prebiosignatures from within the simulated data.

Molecular Mission

The authors focused their analysis on a selection of prebiosignature atmospheric molecules informed by a series of origin scenarios for life. The particular molecules chosen were hydrogen cyanide (HCN), sulfur dioxide (SO2), hydrogen sulfide (H2S), cyanoacetylene (HC3N), carbon monoxide (CO), methane (CH4), acetylene (C2H2), ammonia (NH3), nitric oxide (NO), and formaldehyde (CH2O).

In order to detect these molecules in the atmospheres of distant planets, scientists use a technique called transmission spectroscopy. Essentially, when a planet crosses in front of its star (or “transits”) from our point of view, some light from the host star passes through the planet’s atmosphere. Specific wavelengths of this light are absorbed by molecules in the atmosphere, leaving a telltale “fingerprint” of said molecules’ existence in the spectrum of light we observe. All the molecules chosen happen to have spectral signatures in the infrared, which JWST’s instruments can measure.

Wonderful Worlds

To carry out their investigation, the authors first simulated transmission spectra for a particular set of planets, using models of different types of atmospheres as a background. For the best possible chance of atmospheric detection and characterization, the authors elected to model planets whose atmospheres are rich in hydrogen and helium, have a low mean molecular weight, and are orbiting a smaller star.

Specifically, the authors modeled five different types of possible worlds (Table 1 and Figure 1): a “Hycean” world (an ocean planet with a hydrogen atmosphere), an “ultrareduced volcanic” world (active volcanism with hydrogen- and nitrogen-rich outgassing), a “post-impact” world (a planet recently impacted by another planetary body) at two different times after the collision, a super-Earth planet with a thin hydrogen envelope, and a model that simulates the early conditions on Earth, based on TRAPPIST-1e. This final model is not a light, hydrogen-rich atmosphere, but it’s an important one to study given the history of life’s evolution on our planet. All planets are assumed to be orbiting an M-dwarf star for consistency.

Table listing the ratios of molecules in each model

Table 1: Ratios of molecules in each atmospheric model. [Claringbold et al. 2023]

plots showing transit depth as a function of wavelength for five different types of planets

Figure 1: Simulated transmission spectra for each atmospheric model tested, with important spectral lines labeled. [Claringbold et al. 2023]

Having generated the model transmission spectra, the authors simulated realistic noise that JWST would observe in the data given the M-dwarf star and JWST’s various spectroscopic instruments. Afterward, they performed a series of Bayesian detection tests to attempt to retrieve individual molecule abundances from their data. The overall goal of this analysis is an order-of-magnitude estimate of how abundant these molecules would have to be in exoplanet atmospheres in order for JWST to detect them, assuming a “modest amount” of observation time (around five transits or less) dedicated to each exoplanet.

Rousing Results

The authors find that for the model Hycean world, all prebiosignatures are detectable with JWST’s instruments. The hydrogen-rich super-Earth also has very good detectability, despite having the atmosphere with the smallest scale height (the “higher” the atmosphere extends, the more light passes through its molecules, and thus the stronger the signal received on Earth). The ultrareduced volcanic world, while it has a large scale height like the Hycean world, generally has worse detection thresholds due to strongly absorbing CH4 and HCN in its atmosphere. The post-impact planets have the highest scale height, and thus are the best suited for detection, with low thresholds for the prebiosignature molecules. Finally, prebiosignatures in the early Earth model were very difficult to detect with a low number of transits — while some molecules became detectable within 5–10 transits, others require somewhere between 40 and 100, which might be prohibitively long.

Given the models and method of analysis used, the authors note that these results are optimistic at best and may not correspond to real observational thresholds. Features such as clouds and atmospheric haze can increase the detectability threshold for different spectral features by hundreds or thousands of times, possibly rendering them undetectable.

Furthermore, a realistic retrieval method (where there is uncertainty in the atmospheric composition or planetary properties of the exoplanet being observed) may affect the detected abundances of trace molecules. That being said, the authors’ attempts to simulate such an analysis show that the primary prebiosignatures are still well detected within an order of magnitude of the previous results.

The key conclusion of this article is that in the case of light atmospheres and optimal target planets/systems, the detection of prebiosignatures and exploration of the origin of life is well within the capabilities of JWST. As such, a wealth of data on planetary atmospheres is absolutely within the capabilities of the telescope, and detections of said molecules could very well make the news in the years to come.

Original astrobite edited by William Balmer.

About the author, Aldo Panfichi:

Hello! I’m currently finishing up my Master’s degree in Physics at the Pontificia Universidad Católica del Peru in Lima, Peru, writing a thesis project related to asteroids. I previously got my BSc in Astronomy and Astrophysics at the University of Chicago. In my free time, I like spending time with my friends (and my dogs!), going swimming in the summer, and cozying up inside in the winter, playing games or reading science fiction.

extreme-ultraviolet image of the Sun

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: A Type II Radio Burst Driven by a Blowout Jet on the Sun
Authors: Zhenyong Hou et al.
First Author’s Institution: Peking University
Status: Published in ApJ

The Background

One of the most exciting qualities of the Sun is its magnetic activity that can manifest in a variety of ways and across the entire electromagnetic spectrum. X-ray photons reveal flares produced through magnetic reconnection, microwaves arise from synchrotron and gyrosynchrotron emissions from electrons accelerated along the Sun’s magnetic field, and low-frequency radio waves provide information on the plasma properties in the upper atmosphere of the Sun, called the corona. Magnetic activity also involves particle motion; plasma ejected by this activity can take many forms, and, if the plasma is energetic enough, it can escape from the Sun entirely as a coronal mass ejection.

Because there are so many aspects to how magnetic activity manifests and evolves, it’s difficult to piece together a cohesive picture for how it all works. One phenomenon that has eluded explanation for a long time is the origin of Type II radio bursts. Type II bursts are an example of plasma emission — emission from the coherent oscillation of electrons in a plasma that then produces coherent emission. Like laser light, the coherent emission is bright only at specific frequencies, namely, the plasma frequency, which depends on the plasma density. As the accelerator of the burst moves outwards through the corona, the ambient plasma density decreases and so the plasma emission frequency drifts to lower frequencies, giving Type II bursts a very distinctive “sweeping” signature in time–frequency space, as shown in the bottom frame of Figure 1.

solar magnetic field, X-ray flux, and dynamic spectrum

Figure 1: The solar magnetic field as seen by the Helioseismic and Magnetic Imager (top left) and the jet as seen by the Atmospheric Imaging Assembly and the Solar Upper Transition Imager, (top middle and right, respectively). The middle plot is the light curve of the flare in the X-ray as seen by the Geostationary Operational Environmental Satellite, and the bottom plot is the dynamic spectrum (brightness as function of time and frequency) including the Type II burst as seen by the Chashan Solar Radiospectrograph. The Solar Upper Transition Imager images of the jet are from soon after the peak of the flare’s intensity, by which time the jet was in its ejection phase, traveling at almost 600 km/s. [Hou et al. 2023]

For a long time, folks believed that all Type II bursts are associated with coronal mass ejections. This is because coronal mass ejections are one of the few phenomena from the Sun that are capable of generating the necessary shock in the corona for producing plasma emission. Producing a coronal shock requires moving faster than the Alfvén speed — which helps define the sound speed in a magnetized plasma — which can be hundreds to thousands of kilometers per second (km/s) in the Sun’s corona. However, there have been observations in recent years that suggest there are other processes on the Sun that can produce Type II bursts. This article presents one such case, where the Type II burst may be associated with a jet instead of a coronal mass ejection.

The Events

Because different parts of the electromagnetic spectrum are sensitive to different components of magnetic activity, properly associating two events or phenomena with each other requires collecting simultaneous data across multiple instruments. Data from nine instruments were used in this study! A few of the major* contributors were:

  1. the Solar Upper Transition Imager (SUTRI), Atmospheric Imaging Assembly (AIA), and Extreme Ultraviolet Imager (EUVI), all of which take pictures of the Sun at various extreme-ultraviolet wavelengths and from different satellites;
  2. the Helioseismic and Magnetic Imager (HMI), aboard the same satellite as AIA, which is responsible for measuring the magnetic field of the Sun;
  3. the Geostationary Operational Environmental Satellite (GOES) for observing X-ray photons; and
  4. the Chashan Solar Radiospectrograph (CBSm), operating at about 100–500 megahertz (or wavelengths of about 1–3 meters!).

The story of this event starts with AIA and SUTRI detecting a solar jet — a narrow burst of plasma from the Sun’s atmosphere — during its initial phase. During this phase, the jet moves at about 370 km/s (about 1,000 times faster than the speed of sound on Earth)! Soon after, a flare is detected in the X-ray by GOES, as shown in the middle frame of Figure 1. Following this, the jet accelerates to 560 km/s and transitions to the ejection phase.

extreme-ultraviolet intensity of the jet and the wave

Figure 2: Extreme-ultraviolet intensity (color scale) as a function of time and distance from the flare’s base along the direction of the jet’s path, as observed by AIA at 211 Angstroms (Å). The jet is the bright feature outlined by the cyan lines. The extreme-ultraviolet wave is the bright crest outlined by the bright green line, preceded by a dark feature thought to be the trough of the wave. [Adapted from Hou et al. 2023]

Things get extra interesting after the ejection phase has begun. At this point, AIA now detects a wave-like structure propagating through the Sun’s corona in the same direction and at the same speed as the jet during its initial phase (see Figure 2). Not even a minute later, a Type II burst is detected by CBSm. By the time that the wave structure and the Type II burst are no longer detectable by their respective instruments, they are moving at the same speed as (or somewhat faster than) the jet.

The Big Picture

The authors claim that the similarities of the jet, flare, wave structure, and Type II burst in terms of occurrence time, location, and speed suggest that the three phenomena are related to one another. The data paint a picture of a flare causing the eruption of material in the form of a jet. The jet excites the surrounding material as it moves through the corona, producing the extreme-ultraviolet waves and the Type II burst. All of this happens without any evidence of a coronal mass ejection.

This is significant in several ways. It’s amazing 1) to have simultaneous data spanning so many observing methods and, by extension, 2) to be able to analyze the flare, the jet, and the coronal excitation (the extreme-ultraviolet wave and Type II burst) for a single event, and 3) to see a Type II burst without any evidence of a coronal mass ejection! This article represents an exciting step towards understanding how our Sun’s activity, and the emission from it, are produced and evolve.

* Unfortunately, there’s not enough space in a single article to meaningfully reference all nine instruments and their results (although all were important). For those interested in the other instruments that were used, they were the COR2 coronagraph on the STEREO satellite, LASCO on the Solar and Heliophysics Observatory satellite, and the H-alpha imaging system on the New Vacuum Solar Telescope.

Original astrobite edited by Lynnie Saade.

About the author, Ivey Davis:

I’m a third-year astrophysics grad student working on the radio and optical instrumentation and science for studying magnetic activity on stars. When I’m not crying over radio frequency interference, I’m usually baking, knitting, harassing my cat, or playing the banjo!

Illustration of a galaxy trailing stars in its wake

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Flyby Galaxy Encounters with Multiple Black Holes Produce Star-Forming Linear Wakes
Authors: Nianyi Chen et al.
First Author’s Institution: Carnegie Mellon University
Status: Published in ApJL

Earlier this year, a team of astronomers discovered a strange narrow twinkling trail of stars using the Hubble Space Telescope. This “linear feature” originates from the galaxy RCP 28 (Figure 1), stretching 200,000 light-years in length, and its origin is shrouded in mystery. Initially, this trail was thought to be caused by a “rogue” supermassive black hole that was ejected from its host galaxy. The rogue black hole would produce shock waves and light up the gas, leaving a long starry trail in its wake. The authors of today’s article, however, suggest an alternative scenario for the formation of this star-forming linear wake, backed by cosmological simulation studies.

Color image of the linear stellar wake

Figure 1: Color image of the linear stellar wake generated using the filters F606W and F814W on the Hubble Advanced Camera for Surveys. [Adapted from van Dokkum et al. 2023]

Supermassive black holes, with masses millions to billions times that of the Sun, are found in the centers of most galaxies, even our own Milky Way. When galaxies merge, they give rise to a binary supermassive black hole system at the center of the resulting merged galaxy. Now, picture a third galaxy, with its own supermassive black hole, interacting with this binary system. In this scenario, through complex three-body dynamics, the least-massive black hole is likely to be ejected, leading to the formation of a runaway black hole. If the observed linear trail of stars indeed stems from a runaway black hole, the implications are exciting! It opens up a new channel to look for massive black holes that have undergone complex interactions with multiple other black holes.

However, it is still not clear if a supermassive black hole (with a mass around 10 million solar masses) can produce such a high level of star formation in its wake. This requires theoretical studies, and the first step would be to investigate linear wake features in cosmological simulations and trace their origins. This is precisely the approach that the authors undertake in today’s article.

Looking for Linear Wakes in Cosmological Simulations

The authors use ASTRID, a cosmological simulation with a large galaxy population, to explore potential connections between linear stellar wakes and runaway black holes. The updated black hole dynamics of ASTRID yield a large number of wandering black holes, some of which may arise due to interactions involving multiple massive black holes.

The authors initiate searches to find galaxies that match the properties of the one observed in the linear wake feature. The feature was observed at a cosmological redshift of z = 0.964, and the galaxies in the search are at a redshift of z = 1.3 (the current lowest-redshift simulation in ASTRID) and z = 2. Additionally, the authors seek out three-black-hole systems in these galaxy searches, as a three-body interaction is necessary for the production of a runaway black hole. They find around 200 potential runaway black holes in the target galaxies at a redshift of z = 2.

To search for linear wakes, the authors look for linear star-forming features around the target galaxies. They visually inspect the simulation output to find the features with the largest signal-to-noise ratio, finding around 30 linear feature candidates at each of the redshifts.

Now the big question is, do they find any star-forming wakes that match up with the passage of a runaway black hole? The authors surprisingly find no discernible association between runaway black holes and linear stellar features! So, if not runaway black holes, then what mechanism is responsible for generating these linear wakes?

Could a Fly-By Galaxy Encounter Be the Answer?

The authors find that most of the star-forming wakes originate from a fly-by encounter of a massive galaxy with a newly merged young galaxy hosting a black hole binary. To illustrate this, they present two representative cases of such systems observed in the simulation (see Figure 2). In the first two rows, we can see the galaxy that does a fly-by encounter with a younger galaxy containing two black holes (all black holes are marked by crosses). They also produce mock images of the systems as would be seen by the Hubble Space Telescope (third row) and JWST (fifth row). The ages of the stars along the wake are plotted in the fourth row, showing that half of the stars are relatively old (around 1 billion years) and the other half are younger (with age less than 100 million years). The Hubble filters effectively capture the younger stars within the linear wake, while JWST can detect older stars through longer-wavelength bands.

Data and mock images of the linear stellar wake in filters relevant to Hubble and JWST

Figure 2: Row 1 and 2: The stars and gas associated with the linear stellar wake at redshifts of z~2 (left) and z~1 (right) systems. The linear feature observed is in the middle with the feature extending up to 160,000 light-years from the galaxy on the left of the image. Row 3: Mock image of the systems in Hubble F606W and F814W filters. Row 4: The color and age of the stellar population along the stellar wake. Row 5: Mock images of the systems in the longer-wavelength bands of JWST, revealing an older stellar population. [Chen et al. 2023]

In addition, the authors also trace the time evolution of the linear wake in System 1 (left-hand column of Figure 2) over a span of a billion years, both prior to and following the prominence of the linear feature. Figure 3 shows this time-evolution sequence of the two galaxies (seen face on) involved in producing the linear stellar feature, with their central black holes highlighted by crosses. During their fly-by encounter at a velocity of 580 km/s, Galaxy 1 remains undisturbed, while Galaxy 2 undergoes tidal disruption, leading to the development of elongated arms on both sides (see panels 2 and 3 in Figure 3). The linear features last for a considerable duration — around 200 million years — after which the two galaxies eventually undergo a head-on collision. Notably, the authors observe that the majority of linear features involve a galaxy with dual black holes along the trailing end. If these dual black holes are actively accreting matter, follow-up observations might detect X-ray emission originating from them.

Time evolution of the two galaxies and the associated black holes undergoing fly-by encounter leading to the formation of the stellar wake

Figure 3: Time evolution of the two galaxies (seen face on) and the associated black holes undergoing a fly-by encounter leading to the formation of the stellar wake. The central black hole in galaxy 1 is marked by the blue cross, and the two black holes in the tidally disrupted galaxy are in green and magenta. The green line indicates the path of black hole 2 relative to black hole 1, and the red line is for the orbit of black hole 3 relative to black hole 2. [Chen et al. 2023]

While the simulations indicate the stellar wake is primarily from fly-by galaxy encounters, the authors acknowledge that the simulation’s resolution limit could allow for the possibility of a runaway black hole being responsible for the origin. The resolution of ASTRID is unable to fully resolve an ejection of a black hole due to three-body interactions. Therefore, the potential origin of linear wakes through runaway black holes cannot be entirely ruled out. To definitively distinguish between these two scenarios, future studies employing simulations with higher resolution are needed.

Future observations with JWST could shed further light on the origins of the star-forming wake. Using JWST’s long-wavelength bands, we can examine older stellar populations along the linear feature. Should such older stellar populations be identified along the feature, they would provide support for the galaxy fly-by encounter formation channel. Conversely, the lack of older stars would lend credence to the runaway black hole formation channel for the linear stellar wake.

Original astrobite edited by Lili Alderson.

About the author, Pranav Satheesh:

I am a first-year graduate student in physics at the University of Florida. My research focuses on studying massive binary and triple black holes, their interaction, and the outcomes of their mergers. In my free time, I love drawing, watching movies, cooking, and playing board games with my friends.

artist's impression of the dwarf planet Eris

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Can the Gravitational Effect of Planet X be Detected in Current-era Tracking of the Known Major and Minor Planets?
Authors: Daniel C. H. Gomes et al.
First Author’s Institution: University of Pennsylvania
Status: Published in PSJ

Even though astronomy deals with profound questions about the universe that resonate with the public, it’s no secret that most open questions in astronomy require a hefty dose of context to explain. The cutting edge is necessarily convoluted: the vast majority of scientific progress is made when nibbling at the edges, and as a field we are lucky that our intermediate data products are pretty enough to appear on the front page news without an accompanying treatise on the true topics of interest like redshift, inflation, dark matter, and more.

In this context, the pure simplicity of the question at the center of the Planet Nine debate is almost charming: is there another giant planet lurking at the edges of our solar system, one that we just haven’t seen yet? Some say yes, pulling from observations of trans-Neptunian objects that seem to travel on aligned orbits, possibly shepherded by a stealthy attendant planet. Others say no, and they claim either that the noted alignment is actually a mirage, the manifestation of observational biases, or that it’s real but caused by something else. Who is correct remains unclear, since although astronomers haven’t found such a planet yet, doing so would require overcoming immense observational challenges.

Alternative Solutions

If it’s real, this prowling giant (which some refer to as Planet Nine, and others as Planet X) would be so far away, so faint, and so slow, that its discovery will likely require large telescopes, enormous cameras, and a decent amount of luck. Finding something more than 10 times more distant with Neptune is hard; the community therefore has a strong motivation to consider “alternative” ways to find or rule out this purported planet.

Enter today’s article, led by graduate student Daniel C. H. Gomes of the University of Pennsylvania. This exhaustive yet enjoyable 41-page juggernaut joins a growing literature focused on these “alternative” searches. Instead of trying to uncover Planet X’s hiding spot directly, the authors wonder if it might give itself away via its influence on the rest of the solar system.

Planet Ranging

Figure 1: The accuracy with which we could measure the mass of an object sitting at 400 au from the Sun as a function of its position on the sky. Since a “confident detection” generally requires measuring something with a precision five times smaller than the value in question, a 5-Earth-mass planet could be detectable everywhere smaller than σM = 1 here. Click to enlarge. [Adapted from Gomes et al. 2023]

Planet X, should it exist, would gently tug on every member of the solar system through gravitational interactions. In the first half of this article, the authors consider whether these nudges would knock any of the spacecraft we’ve dispatched throughout the solar system off course in a noticeable way. Using a statistical object known as a Fisher information matrix, they calculate the best possible precision with which we could hope to measure the mass of an unseen planet as a function of its location given historic spacecraft ranging data (Figure 1).

They find that by including all of the data from the Juno, Cassini, and various Mars missions, we could in principle conclusively detect a 5-Earth-mass planet sitting at 400 au anywhere within 99.2% of the sky. That’s a startlingly strong constraint, and on its face potentially bad news for the reality of Planet X since previous searches have turned up empty. However, its resurrection lies in the subtleties and assumptions. If the planet were instead farther out, at say 800 au, we would only be able to detect it if it sat in a very special pocket spanning just 4.8% of the sky. Alternatively, if it were less massive than the inferred super-Earth value, it would have to sit closer in than the nominal 400 au to be revealed by the ranging data.

Future Prospects

After demonstrating the usefulness of historical ranging data in the first half of the article, the authors barely pause before tackling a related but distinct question: will we be able to place even stronger constraints on the mass and position of any distant planets using future measurements? In particular, they consider the thousands of Jupiter trojans that will be observed by the upcoming Legacy Survey of Space and Time (LSST). Might careful tracking of these tiny objects reveal Planet X?

illustration of the offset between the photocenter and the barycenter of an object

Figure 2: An illustration of one of the complications with making extremely precise measurements of a trojan’s position. We see light coming from the photocenter, but the true center of the object, its barycenter, is offset from this by an amount that depends on the shape of the object. This distance is often comparable to the offset caused by Planet X. [Gomes et al. 2023]

Probably not. As revolutionary as LSST promises to be to solar system science (and many, many other subfields of astronomy as well), it’s not capable of measuring the position of these tiny sources accurately enough from the ground to make much of a difference to their earlier constraints. Planet X would shift each object on the order of only a few meters over the course of their orbits, a distance similar to the width of many objects themselves and comparable to the error accumulated through timing errors, photocenter–barycenter offsets (see Figure 2), and chromatic refraction through the atmosphere. In order to improve the constraints derived using spacecraft data, LSST would need to outperform its already high expectations by about an order of magnitude.

So, does Planet X exist? Still maybe, but it likely has fewer places to hide now. Between studies like this one that creatively consider the consequences of a distant planet and the direct searches for it, astronomers are pinching closed the parameter space in which it could live.

Original astrobite edited by Macy Huston.

About the author, Ben Cassese:

I am a second-year Astronomy PhD student at Columbia University working on simulated observations of exomoons. Prior to joining the Cool Worlds Lab I studied planetary science and history at Caltech, and before that I grew up in Rhode Island. In my free time I enjoy backpacking, spending too much effort on making coffee, and daydreaming about adopting a dog in my NYC apartment.

artist's impression of a rocky object in the outer solar system

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Impact Rates in the Outer Solar System
Authors: David Nesvorný et al.
First Author’s Institution: Southwest Research Institute
Status: Published in PSJ

Sometimes astronomy research articles detail glamorous, revolutionary results. Sometimes they produce gorgeous images using the latest and greatest telescopes. And sometimes they meticulously combine data and models to make incremental improvements to estimates of numbers last updated two decades ago.

Today’s article falls squarely into the latter category. Using the most up-to-date models for populations of comets and other small bodies in the outer solar system, the authors make an updated estimate at how often big-ish rocks collide with our four friendly gas giants and their moons.

It’s a fairly straightforward article, but it offers a great introduction to both the dynamics of the solar system and the dynamic between observations and simulations in making advances in astronomy.

Beyond that, the finer points of their conclusions have exciting implications for our understanding of some of the most intriguing bodies in the search for life beyond Earth. But before we can get into that, we need to talk about some of the different types of rocks one can find in space.

The Outer Solar System: A Guided Tour

Out beyond Jupiter, Saturn, Uranus, and Neptune, more than 30 times farther from the Sun than Earth, lies a field of smaller bodies (of which Pluto is, infamously, just one of many). These icy rocks make up the population of trans-Neptunian objects (Figure 1).

plot of the locations of various small solar system bodies

Figure 1: The locations of known comets and other bodies in the outer solar system, with scales in either dimension showing distance, in astronomical units (au), from the Sun. Jupiter-family comets are shown in blue, centaurs in magenta, and trans-Neptunian objects in yellow. Light grey points show all other types of small bodies beyond the asteroid belt, which are largely out of the plane of the planets’ orbits. Dark green rings show the approximate orbits of the gas giants (Jupiter, closest to the Sun, is almost entirely obscured by its family of comets), and the black ring shows 2.5 au from the Sun. Our observational abilities are best for objects closer to us, meaning observations of the population of objects out beyond the classical Kuiper belt are limited. Apparent “clumps” of objects are largely the result of staring at one patch of sky from the Earth to find things in that small, specific region. [Mark Dodici; data from Solar System Database (Downloaded August 1, 2023)]

Under the Nice model of the solar system’s early history, many of these objects were kicked out to their present locations by a “late instability,” which likely saw Neptune (and the rest of the gas giants) move outward from closer-in birthplaces to where they orbit today. Nowadays, these kicked-out bodies have orbits that come close to that of Neptune at their perihelia (their closest point to the Sun in every orbit). This population of objects, which tend to have highly eccentric and/or highly inclined orbits, make up the scattered disk of trans-Neptunian objects.

Within this outer reservoir of bodies, slight perturbations over long times can cause eccentricities to change and perihelia to shift. The objects with perihelia closest to Neptune will tend to be disrupted over time — often being flung inwards toward the rest of the solar system. This disruption is common enough that the planet-inhabited region of the solar system is the constant recipient of a slow (but steady!) stream of encroaching trans-Neptunian objects.

While their perihelia are between the orbits of Jupiter and Neptune, these objects become known as centaurs. But these freshly disrupted bodies are incredibly transient interlopers. Since their orbits cross that of at least one of the gas giants, they tend to undergo more major orbital changes through close encounters on (astronomically speaking) quick timescales.

These changes lead, in general, to one of three outcomes for our centaurs: reclassification (most likely drifting inward to become Jupiter-Family comets), collision (with the Sun, or, in the interests of this article, something else!), or ejection from the solar system altogether.

From Centaur to Collider

There’s one class of small body relevant to today’s article that we haven’t touched on yet: the ecliptic comets. 

This family technically includes much of the Jupiter family; specifically, it encompasses comets that orbit the Sun with low inclinations (i.e., with orbits mostly close to Earth’s orbital plane, known as the ecliptic). Most ecliptic comets were first low-inclination centaurs, whose orbits drifted inwards until they were more strongly associated with Jupiter.

It’s been a few paragraphs, but you might recall that today’s article is interested in the impact rates between smaller bodies and the gas giants and their moons. When two things have similar inclinations — imagine their orbits as dinner plates, stacked together — they have more chances to bump into each other than when they’re significantly misaligned — imagine one dinner plate standing on edge atop another.

Because of this, ecliptic comets (and the low-inclination centaurs they come from) are the most likely suspects for collisions in the outer solar system. Understanding the rate of these collisions, then, requires a good understanding of the populations of ecliptic comets and centaurs.

Today’s Article

Previous studies of impact rates (including their main point of comparison, dating back 20 years) have been hindered by uncertainties in our models of these populations. In the last few years, however, new observations by the Outer Solar System Origins Survey (OSSOS) have allowed for more accurate calibration of models, giving a better picture of the current state of play of ecliptics and centaurs, as well as the population of trans-Neptunian objects from which those colliders came. The time is ripe for reevaluation of these previous rate estimates; enter today’s article.

To calculate how often small bodies crash into planets, the authors simulate the orbits of a whole bunch of them, then calculate the percent that crash into a planet, then multiply that percent by the number of progenitors they expect there to have been in the solar system (using that OSSOS-updated understanding of trans-Neptunian objects!).

In the end, they estimate that Jupiter should be struck by bodies with diameters bigger than 1 kilometer every 230 years or so (consistent with previous work, which is always a good sign). They do note that there’s still uncertainty, as the calculation requires some extrapolation from the collision rates of bigger bodies based on a size distribution — if there’s a “steeper” distribution (more small bodies for every big one), the rate could be as high as 1 every 120 years.

When the authors consider that comets will be disrupted if they spend too much time too close to the Sun, though, the impact rate on Jupiter drops to 1 every 315 years. (Impact rates on all other bodies scale down, too!) From this, they draw an interesting conclusion related to icy moons — satellites that are some of the most interesting bodies in the search for life elsewhere in the solar system.

These icy moons, much like geologically active planets, “refresh” their surfaces over long timescales, wiping clean any craters that might have formed from impacts; the time it takes for this refresh to happen can give us details on the moons’ internal processes. If the moons are impacted less frequently, then it would take longer to accumulate the number of craters we see today. If it takes longer to accumulate craters, then the refresh rate must be slower than we expected — their surfaces must be older to accumulate the same number of craters.

Jupiter's moon Europa

Figure 2: Europa, an icy moon of Jupiter, is surprisingly crater free. The impact rates given by this article might help explain why. [NASA/JPL-Caltech/SETI Institute]

Based on their finding that inner moons are less frequently hit than previously thought, the authors draw exactly this conclusion about Europa, an icy moon of Jupiter suspected to harbor an ocean under its outer shell (Figure 2). They posit that its surface is somewhere between 45 and 105 million years old — an ever-so-slight upward shift compared to previous best age estimates of 40–90 million years.

That’s a big uncertainty! And the calculation to find that range, like many others in today’s seemingly innocuous, number-updating article, is finicky. Especially considering its overlap with the previous range, this is more interesting as a proof of concept for the way models like this can impact our understanding of seemingly tangential topics, like the internal processes of icy moons.

But as observations of the small bodies in the outer solar system continue to improve, modeling efforts like this one can help us understand exactly what’s going on out there — letting us know what to look for in future observations, and beginning the cycle anew.

Original astrobite edited by Benjamin Cassese.

About the author, Mark Dodici:

Mark is a first-year PhD student in astronomy and astrophysics at the University of Toronto. His space-based interests include planetary systems, from their births to their varied deaths, as well as the dynamics of just about anything else. His Earth-based interests include coffee, photography, and a little bit of singing now and again. You can follow him on Twitter @MarkDodici.

Photograph of a meteor streaking across the night sky

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: On the Proposed Interstellar Origin of the USG 20140108 Fireball
Authors: Peter G. Brown and Jiří Borovička
First Author’s Institution: University of Western Ontario
Status: Published in ApJ

Since the unambiguous identification of ʻOumuamua, the first interstellar visitor to our solar system, popular interest in astronomy and astrobiology has exploded. Theories (however unlikely) that ʻOumuamua represented some artifact of an extraterrestrial intelligent civilization not only turned significant public attention to the study of small solar system bodies, but also that of so-called Unidentified Anomalous Phenomena (UAPs). Long speculated to be sightings of alien spacecraft, the study of UAPs is now being tackled by several groups, ranging from the Pentagon, to NASA, to private institutions. Perhaps the best-advertised of these is the Galileo Project, headed by Dr. Avi Loeb, lately Chair of Harvard’s Department of Astronomy, originator of the ʻOumuamua derelict spaceship theory, and second author of an article claiming the archival detection of an interstellar meteor (USG 20140108) from US government (USG) monitoring data.

ʻOumuamua’s Predecessor?

Most meteors we see come from orbits bound to the solar system. However fast these bolides (fiery meteors) appear and disappear in their starry streaks, they typically don’t fly fast enough to indicate unbound, extrasolar origins. USG 20140108 was originally detected by classified government sensors and released as part of the Center for Near-Earth Object Studies (CNEOS) Fireball Catalog, and its interstellar nature was proposed by Siraj and Loeb (2022) based on the government-measured velocity, about 45 km/s. The CNEOS data don’t come with uncertainties, so Siraj and Loeb rely on a Department of Defense statement that “the velocity estimate reported to NASA is sufficiently accurate to indicate an interstellar trajectory.” Armed with this letter, Siraj and Loeb confidently claimed the object to be interstellar in origin, and Loeb’s Galileo Project chartered an expedition to attempt to recover fragments of the bolide from the bottom of the ocean.

Independent Estimations

Rather than try to hunt down the original, classified source of the 2014 event, the authors of today’s article try to estimate the uncertainty on the bolide’s measured speed from other, independently observed fireballs. Aside from the inflammatory claims regarding its origin, USG 20140108 does stand out in the CNEOS catalog, with the second highest speed and sixth lowest altitude of peak brightness out of nearly a thousand recorded events. Independent estimates should help inform the precision of the CNEOS data and determine whether a proposed interstellar origin is likely.

To do this, the authors examined 17 other fireball events, all present in the CNEOS catalog but also independently observed by non-classified space- and ground-based cameras (a full list of these events and their references is present in Table 3 of the article). By comparing these independent events to the CNEOS data, the authors identified two critical parameters — speed and radiant direction (from which the meteor came) — that may be particularly unreliable in the CNEOS data. The 17 events span a decade in time, 20 km/s in speed, and were roughly evenly distributed across the sky.

As we see in Figure 1, as the bolide’s USG-reported speed increases, so does the difference between the USG-reported speed and independently measured speed. No clear speed-dependent trend is apparent in the radiant data, but the USG-reported and independently measured radiants can differ widely. Following these data, the authors predict that for the fireball’s measured speed of 45 km/s, there should be a measurement uncertainty of 10–15 km/s. They also note that given the range of radiant discrepancies, there may be up to a 30-degree error in the apparent radiant.

plots comparing speed and radiant reported by the USG and the difference between those quantities and the ground-based quantities.

Figure 1: Speed (left) and radiant (right) differences between USG and independent sensing of 17 bolides plotted against USG-reported speed. [Brown and Borovička 2023]

Interstellar or Intrastellar?

So, armed with their new uncertainties, the authors compare USG 20140108 to other bolides in the catalog on supposedly unbound orbits. As seen in Figure 2, assuming the other CNEOS measurements have uncertainties comparable to the trends identified in the article, all the unbound bolides have significant chances of actually being on bound orbits. USG 20140108 is again the most extreme of these, but given the velocity and radiant errors estimated in the article, the authors conclude that the simplest answer is that USG 20140108 originated within our own solar system.

Radiant versus velocity plot

Figure 2: Radiant elongation versus velocity plot showing putatively unbound bolides in the CNEOS catalog. USG 20140108 is shown as the red cross. Unbound orbits are found to the right of the line and bound orbits are to the left of the line. [Brown and Borovička 2023]

Hard Rock or Heavy Metal?

While the reliability of the government measurements is the main focus of the article, the authors also used their analysis to reexamine the meteorite’s composition. Siraj and Loeb claimed that in order to produce the observed flashes and flares, the meteor must have had a metallic composition of anomalous strength, possibly indicating extrasolar material. If, in fact, the meteor’s speed was much slower (as the new wider uncertainties would allow for), then a stony meteorite is capable of producing the observed light curve.

Without invoking exotic materials or alien destinations, it appears USG 20140108 may just be yet another simple shooting star, burning brightly yet briefly. Articles like this exist in a long tradition of academic discourse, but few other topics draw as much public and private interest (both scientific and sensational). While we’ll almost certainly never know the bolide’s true provenance, the recently reported recovery of supposed fragments from the meteor (possibly in contravention of international treaties) means this result will definitely be critical to interpreting any new claims about its nature.

Original astrobite edited by Evan Lewis.

About the author, Yoni Brande:

I’m a fourth-year PhD candidate at the University of Kansas, working on exoplanet discovery and characterization. I primarily work with TESS transit data and Hubble Space Telescope exoplanet transmission spectroscopy data, and I’m also interested in enabling more collaborative science with open source astronomical software tools. When I’m not doing research or writing Astrobites, I can be found in a sci-fi streaming binge, running, lifting, cooking, or on Twitter @YoniAstro.

Globular cluster Liller 1

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Demographics of Hierarchical Black Hole Mergers in Dense Star Clusters
Authors: Giacomo Fragione and Frederic A. Rasio
First Author’s Institution: Northwestern University
Status: Published in ApJ

Observations of gravitational waves from binary black hole mergers made by LIGO, Virgo, and KAGRA have revolutionized our understanding of the demographics of compact objects like neutron stars and black holes. At the range detectable by these instruments, for example, they have shed light on the distribution of black hole masses, which gives us a glimpse into the late-stage evolution of massive stars. In particular, stellar evolution models predict a dearth of black hole remnants with masses between fifty and a few hundred times the mass of our Sun, because of the so-called pair-instability process. However, LIGO–Virgo–KAGRA observations have indicated the existence of black hole binaries where one member of the binary is in this pair-instability range, meaning that the simple stellar evolution picture doesn’t fully explain the observed population!

Binary black holes are believed to form by two main channels: in isolation, from a binary star system, and dynamically, as depicted in Figure 1. In the former (left), the binary system evolves through a common-envelope phase, which, if the conditions are right, will result in a binary black hole system. In the dynamical pathway (right), interactions between stars in the dense centers of clusters are frequent and can lead to the formation of binary black hole systems. Subsequent interactions can tighten the orbit, leading to mergers.

Cartoon depiction of the two primary channels for binary black hole formation

Figure 1: Cartoon depiction of the two primary channels for binary black hole formation (left and right columns), with time increasing from top to bottom in each column. At left, a binary star system enters a common-envelope phase that shrinks the orbit of the binary. If this results in ejection of the envelope, the system will eventually form a binary black hole system that will ultimately merge. At right, a binary system of a black hole and a star will become a system of binary black holes through a three-body interaction in a dense star cluster. Later interactions will shrink the orbit until the two black holes merge. [Mapelli 2020; CC BY 4.0]

Focusing on the latter channel, today’s authors explore how such a mechanism could produce intermediate-mass black holes (those with masses between 100 and 1,000 solar masses), specifically through hierarchical black hole mergers. Hierarchical black hole mergers are those in which one (or both) of the component black holes is a so-called “later-generation” black hole: the remnant of a previous binary black hole merger in the center of a dense star cluster. (A first-generation black hole is one produced at the end of a star’s life, a second-generation one is formed from two first-generation black holes, and so on.) These sorts of repeated mergers of stellar-mass black holes would yield black holes in the intermediate-mass black hole range, providing a natural answer to the provenance of these black holes. Today’s authors use a new modeling framework to predict the properties of star clusters that can produce a detectable distribution of hierarchical black hole mergers.

Demographics of Cluster Hosts

The primary challenge associated with this channel of intermediate-mass black hole formation is the “recoil kick” that results from asymmetry in the merger process. In some cases, the velocity with which the resulting black hole forms can exceed the escape velocity of the star cluster in which it formed, leading to ejection (and thus limiting the possibility of a subsequent merger). Using the modeling framework developed in a previous work, today’s authors are able to predict the properties of clusters that will be able to retain a binary black hole remnant after the merger, as displayed in Figure 2. From these panels it is clear that hierarchical black holes are likely only produced and retained in the most massive and densest clusters, as these will be the ones with the deepest potential wells and highest rates of interaction.

probability that a merger remnant will survive in the cluster as a function of cluster mass and density

Figure 2: The probability that a merger remnant will survive in the cluster (color scale) as a function of cluster mass and density, going from the merger of two first-generation black holes (left), to the merger of a first- and second-generation black hole (middle), and the merger of two second-generation black holes (right). The points correspond to observations of different types of star clusters from the literature. Notice how the remnant of two first-generation black holes merging is relatively easy to retain, but it becomes progressively more difficult as we introduce later generations. [Adapted from Fragione and Rasio 2023]

In these massive and dense clusters, repeated black hole mergers can eventually result in the formation of a single massive black hole >1,000 times the mass of our Sun that dominates the interactions and binary merger process. It turns out that these massive black holes generally grow as a result of mergers with first-generation black holes, as mergers of two hierarchically produced black holes (i.e., two second- or third-generation black holes) tend to impart a strong kick on the remnant, driving it to escape from the cluster. Therefore, robustly modeling the effect of kicks is crucial to understanding the rates of intermediate-mass black hole formation by this hierarchical merger process.

Merger Rates and Assorted Sundries

With their framework in hand and the properties of the cluster hosts understood, today’s authors then average over the distribution of star clusters of different masses as a function of time to predict merger rates of various generations of hierarchical black holes. In Figure 3, the authors demonstrate that massive clusters (with masses up to 107 solar masses) are necessary to produce later generations of black hole mergers. That is, they find that the rates of hierarchical black hole mergers fall as the maximum cluster mass is lowered from 10 million to 1 million solar masses, as the lower-mass clusters are less likely to hold onto the resulting black holes. In kind, the merger rates for component black holes later than third-generation disappear.

Plots of merger rates as a function of cosmological redshift

Figure 3: The merger rates as a function of cosmological redshift (i.e., over time in the universe’s history) for combinations of merging black holes of different generations (e.g., 1G refers to a first-generation black hole). The left panel is the prediction including star clusters with masses up to 10 million solar masses, while the right panel only includes clusters up to 1 million solar masses. There are no mergers beyond 3G in the right panel, demonstrating that only the most massive clusters can host late-generation black-hole mergers. [Adapted from Fragione and Rasio 2023]

Relatedly, they compare the distribution of merging black hole masses detected with the LIGO–Virgo–KAGRA observatories to those predicted by their model. From such analysis, they demonstrate that within their framework, several of the observed events can only be produced by accounting for hierarchical black hole mergers, with a few appearing to come from mergers of second- and third-generation black holes!

These results, while preliminary, can be extended (by incorporating other metrics, such as the black hole spins) to statistically measure the likelihood of individual gravitational wave events being associated with hierarchical mergers. However, they represent an exciting step towards understanding where these intermediate-mass black holes come from and provide a compelling, natural explanation for how a large population of massive black holes can form and continue to evolve over cosmic time!

Original astrobite edited by Mark Popinchalk.

About the author, Sahil Hegde:

I am an astrophysics PhD student at UCLA working on using semi-analytic models to study the formation of the first stars and galaxies in the universe. I completed my undergraduate at Columbia University, and am originally from the San Francisco Bay Area. Outside of astronomy you’ll find me playing tennis, surfing (read: wiping out), and playing board games/TTRPGs!

three views of Jupiter's moon Io

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Io’s Optical Aurorae in Jupiter’s Shadow
Authors: Carl Schmidt et al.
First Author’s Institution: Boston University
Status: Published in PSJ

Galileo image of Jupiter's moon Io

Figure 1: Jupiter’s innermost Galilean moon, Io. Its yellowish color mainly comes from sulfur on the surface. [NASA/JPL/University of Arizona]

Jupiter’s innermost Galilean moon, Io, has a bunch of really weird properties, in addition to looking like a long-expired pizza (see Fig. 1). Io is being pushed and pulled from all sides, not only by Jupiter itself, but also by the other Galilean moons, Europa, Ganymede, and Callisto. This gives Io some breathtaking tidal effects that make our ocean’s tides look like a few ripples in a pond. These extreme tidal effects cause Io to be very volcanically active, more so than any other object in the solar system. Here on Earth, volcanoes are more or less predictably distributed at the edges of tectonic plates, but they appear randomly over Io’s surface, with no clear patterns. You would think all this volcanic activity would heat up Io’s surface a bit, but no: Io has an average surface temperature of around –130°C. So if you don’t get cooked by randomly erupting volcanoes, the cold won’t be comfortable either. Also, you can’t breathe there, which might be another challenge. Therefore it is best to observe the semi-hellscape surface of Io from afar. There is certainly always enough to see.

Iororae?

In addition to volcanoes, Io has aurorae (not iororae, sadly, missed opportunity there), but they are different from Earth’s. Io’s version is not directly caused by the Sun, but rather by Jupiter’s enormous magnetic field, which is about 20,000 times stronger than Earth’s. Earth’s atmosphere is also very different from Io’s (luckily). The many volcanoes on Io spew out large amounts of sulfur dioxide (SO2), which makes up a large part of the moon’s atmosphere. This molecule gets shredded by a multitude of processes, releasing electrons and ions all around, causing aurorae to appear and helping to form Io’s plasma ring around Jupiter.

If we want to know more about this plasma ring and Jupiter’s enormous magnetic field, we need to figure out how the SO2 in Io’s atmosphere behaves and interacts with other molecules and atoms. In daylight, the Sun’s radiation splits up (dissociates) most of the SO2, but what happens to SO2 during night time? There’s no way we can see Io’s night side without sending very expensive spacecraft up there. However, as it so happens, Io passes through Jupiter’s shadow approximately every 42 hours. During the shadow passage, the surface cools down a lot, and the Sun’s radiation will no longer break down molecules. This closely resembles night time on Io, allowing us to see how the rest of the atmosphere reacts at night, if at all.

Turns out, Io has impressive aurorae during the Jupiter eclipse. Now, to see these aurorae we mostly need to be able to observe ultraviolet light, which we can only see with space telescopes. Also, ultraviolet radiation doesn’t tell us everything, and there could be more to see with optical light as well. To view Io’s aurorae in optical light is challenging, though: we can’t directly see Io in Jupiter’s shadow, which makes it hard for telescopes to track the moon’s position. The authors of today’s article managed to see Io’s aurorae in optical light using ground-based telescopes. The authors waited for Io to pass into Jupiter’s shadow as only then are the optical aurorae visible (similar to here on Earth, Io’s aurorae aren’t very visible during the day).

Spotting Io’s Aurorae 101

As you might imagine, seeing Io’s aurorae is even harder than seeing Earth’s. Most of the time when Io is eclipsed by Jupiter, there’s this giant planet in the way blocking our view. But luckily, this is not always the case. When the positions of the Sun, Earth, and Jupiter form a right triangle (fancy astronomer slang: Earth and Jupiter being at quadrature), we can still “see” Io once it enters Jupiter’s shadow (more fancy astronomer slang: at ingress) or just before exit (egress). This is shown in Fig. 2, where a distinction is made between the full shadow (umbra) and the partial shadow (penumbra).

schematics of Io's location relative to Jupiter during different parts of the observing process

Figure 2: Panel A shows where in Io’s orbit around Jupiter you would see panels C and D, as seen from Earth. Panel B shows Io’s orbit from above the ecliptic plane. Panels C and D show the observations of Io when it’s in partial and in full shadow, respectively (the yellow dot is where the instrument thinks Io is). [Schmidt et al. 2023]

All right, we have Io in Jupiter’s shadow. Now we still need to spot some aurorae, and again, Jupiter doesn’t go easy on us; sunlight scattering off Jupiter’s atmosphere further spoils the observation. Luckily, we know quite well what Jupiter looks like, so we can isolate Io’s aurorae, going so far as to even single out some very interesting spectral lines of oxygen and sodium. The intensity of this light shows how much oxygen and sodium are in Io’s atmosphere. Fig. 3 shows the oxygen, sodium, and SO2 trends during three different eclipses.

Plot of disk-averaged brightness as a function of time after ingress

Figure 2: Light intensity of oxygen (red) and sodium (orange) on three different observation nights (circles, diamonds, squares), along with SO2 and SO intensities on one night. [Schmidt et al. 2023]

It seems oxygen doesn’t really care much about the eclipse at all, doing different things on different dates. How come? The authors claim that the amount of visible emission from oxygen depends on the position of Io in the ring of plasma around Jupiter rather than on whether sunlight reaches Io or not.

On the other hand, sodium and SO2 do seem to align, both declining once Io passes into Jupiter’s shadow. To understand why, it’s important to know that the atomic sodium comes mostly from molecules on Io, namely sodium chloride (aka salt). Once the eclipse starts, SO2 gas in the atmosphere freezes out as SO2 ice and subsequently falls down on Io’s surface almost like snow (no snow sculptures here, though; this stuff is very bad for your health). This snow buries the sodium chloride on Io, preventing sodium from showing up in the spectral lines — hence the decline in sodium brightness.

Despite all the difficulties with spotting them, we can nevertheless pull a lot of information from Io’s optical aurorae. Today’s authors demonstrated this by shedding light on the otherwise unobservable night side on Io. The fact that the authors managed to see Io’s aurorae in the optical and from the ground also opens up the possibility to study Io more frequently, which will undoubtedly lead to more interesting results in the future. Io never fails to surprise!

Original astrobite edited by Konstantin Gerbig.

About the author, Roel Lefever:

Roel is a second-year PhD student at Heidelberg University, studying astrophysics. He works on massive stars and simulates their atmospheres/outflows. In his spare time, he likes to hike/bike in nature, swimming, video games, to play/listen to music, and to read (currently The Wheel of Time, but any fantasy/sci-fi really).

multi-wavelength image of a galaxy known as a Lyman-alpha blob

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: UV and Lyα Halos of Lyα Emitters across Environments at z = 2.84
Authors: Satoshi Kikuta et al.
First Author’s Institution: National Astronomical Observatory of Japan and University of Tsukuba
Status: Published in ApJ

Astrophysics is an observational science. At its most basic level, the science we do requires light from the object we’re studying to reach our telescopes. If the light is produced only at a specific wavelength, as is the case for a spectral line, that’s even better, because it can be used to probe the properties of the object we’re trying to study in detail.

A Useful Escapee

Hydrogen, which is the most common atom in galaxies (and in the universe as a whole, by a lot), produces several spectral lines. The one we’re looking at today is produced by an electron transitioning from its first excited state down to its lowest-energy state. This is known as the hydrogen Lyman-alpha (Lyα) transition, and it produces light in the ultraviolet range at a wavelength of 121.6 nanometers in its rest frame. If the transition happens far enough away, however, the expansion of the universe redshifts the light to longer wavelengths in the visible range. Because there’s so much hydrogen in galaxies, this spectral line is very bright, and because many modern-day telescopes are designed to capture visible light, this line is very easy to see.

Let Me OUTTTTT

Based on all this, Lyα should be the perfect type of light to use when studying galaxies! There’s one issue, however — that light still has to reach the telescope. This is harder than it sounds, for the same reason that so much Lyα emission is produced by galaxies in the first place. Hydrogen is very, very common in galaxies, and hydrogen atoms can suck the Lyα light back up (absorb it) just as easily as they emit it in the first place. Because hydrogen is so common, Lyα light has to rattle around for a long time, being absorbed and re-emitted, before it can finally escape the galaxy in which it was produced. This means that by the time it reaches our telescopes, Lyα light isn’t a great tracer of the regions where it was emitted, but it is a very good tracer of the size and shape of the hydrogen gas across the entire galaxy.

Some Very Efficient Observations

In today’s article, the authors investigate a resolved image of the Lyα emission from a still-forming cluster of galaxies at redshift z = 2.84 (light that was emitted 11.4 billion years ago). Because so many galaxies are clumped together in this galaxy cluster, the 3,490 Lyman-alpha-emitting galaxies in the image could all be observed in just 8.5 hours with the Hyper-Supreme Cam on the Subaru telescope. Observations used the g-band optical filter (which shows redshifted ultraviolet emission) and a narrowband filter intended to pick up emission exclusively from the Lyα spectral line. All of the Lyman-alpha-emitting galaxies detected in the image are shown in Figure 1, colored by their equivalent width, which is a measure of the strength of the emission. The dense region at the center of the image contains an extremely bright quasi-stellar object (QSO; a galaxy with a black hole in its center sucking up matter so fast that the matter becomes brighter than the galaxy itself), which is how the cluster was discovered in the first place.

plot of sky positions of galaxies surveyed for this research article

Figure 1: The positions on the sky of the 3,490 Lyman-alpha-emitting galaxies being studied in today’s article. The points are colored by the strength of the Lyα emission from each galaxy, and their size corresponds to the strength of each galaxy’s ultraviolet emission. The gray contours show the regions where the galaxies are most densely grouped together. [Kikuta et al. 2023]

Stacking Up the Evidence

The authors of this article wanted to investigate how various galaxy properties change the way Lyα photons escape from a galaxy. To do this, they split the full sample of Lyman-alpha-emitting galaxies into subsamples based on other galaxy properties, such as the luminosity of the Lyα and ultraviolet emission, the equivalent width of the Lyα line, or the distance between the galaxy and the central QSO. They then stacked together the images of all the galaxies in each subsample to produce a single average image with a much better signal-to-noise ratio for each subsample. From these averaged images, shown in Figure 2, the authors can calculate the amount of Lyα emission as a function of distance from the center (a radial profile, like the ones shown in Figure 3) to calculate the average size and shape of the Lyα emission from the included galaxies. They can thus deduce how Lyα escape is affected by the galaxy properties that change between subsamples.

Stacked Lyman-alpha images of galaxies in the authors' sample

Figure 2: Stacked images of the Lyα emission from the subsamples of Lyman-alpha-emitting galaxies defined by the authors to explore the effects of various galaxy properties on Lyα emission. From top to bottom, the subsamples are split based on: the magnitude of the ultraviolet emission, the Lyα luminosity, the Lyα equivalent width, and the distance from the central QSO. Contours are provided to show the Lyα shape more clearly. [Adapted from Kikuta et al. 2023]

Factors Affecting Escape

The authors found that Lyα escape matched their expectations well in general: galaxies with more ultraviolet or Lyα luminosity, or with lower equivalent widths (all indicators that a galaxy is making new stars quickly), had much more extended Lyα emission than their lower-luminosity counterparts. This makes sense — bigger galaxies form more stars, so the ones forming stars the fastest should have the most extended hydrogen gas, and thus the most extended Lyα emission.

Trends in the subsamples split up by the galaxy’s environment, however, weren’t quite so well behaved. The size of the Lyα emission didn’t change much as the galaxies got more distant from the central QSO, but the shape (the spatial distribution) of the emission definitely did. Galaxies far from the QSO had a lot of Lyα escape in the center of the galaxy, but the amount dropped off very fast towards the outskirts (the blue and green lines in Figure 3). Galaxies close to the QSO, however, didn’t have as steep a drop-off (the red line in Figure 3).

Plot of radio profiles of Lyman-alpha emission for the galaxy subsamples

Figure 3: The average radial profiles (amount of emission as a function of distance from the center) of the different subsamples, split based on distance from the QSO. Note that, in the circled region, there’s more emission from the close-in galaxies (the red line) than the distant ones (the blue and green lines). [Adapted from Kikuta et al. 2023]

The authors believe that these galaxies have the same amount of hydrogen gas surrounding them as the ones farther away from the QSO, but that gas has been excited by the energetic photons emitted by the QSO. This would change how the hydrogen atoms absorb, and let the Lyα escape more easily even far away from the center of the galaxy. This trend is subtle, and the authors do note that it could be due to an observational bias where galaxies moving quickly in relation to the QSO are missed by the narrowband filter they use.

What’s Next?

High-redshift protoclusters like the one studied here are very important to understanding galaxy evolution, because they will evolve into galaxy clusters like the one we live in now. They are very difficult to study effectively, however, because they’re so far away. It’s therefore critical to develop techniques like the one being used by the authors here, that study many galaxies at once using a minimum amount of telescope time. In the future, the authors plan to do similar analyses on different clusters using different telescopes (such as JWST) to really nail down the factors affecting Lyα escape.

Original astrobite edited by Karthik Yadavalli.

About the author, Delaney Dunne:

I’m a PhD student at Caltech, where I study how galaxies form and evolve by mapping their molecular gas! I do this using COMAP, a radio-frequency Line Intensity Mapping experiment based in California’s Owens Valley.

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