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illustration of a planetary 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: Can Cold Jupiters Sculpt the Edge-of-the-multis?
Authors: Nicole Sobski and Sarah Millholland
Authors’ Institutions: Wellesley College and Massachusetts Institute of Technology
Status: Published in ApJ

The Kepler mission revealed thousands of transiting exoplanets. Through this unique data set, we have come to understand much more about exoplanet demographics and the occurrence rates of different kinds of planets. In particular, we now understand that the most common type of planetary system is what we refer to as a “compact multi-planetary system.” That is to say, a system of multiple small planets (less than 4 Earth radii each) where each planet orbits in similar, short timespans (usually less than about 50 days). Studies of the full Kepler data set show that these “compact multis” are ubiquitous across the galaxy. Furthermore, these systems are shown to have a high rate of intra-system uniformity, sometimes called “peas-in-a-pod” architecture. Planets within a compact multi often have similar radii and similar spacing in their orbital periods.

But in a system of multiple transiting exoplanets of such uniformity, what causes the system to “end”? That is, what causes the pod to be filled up with peas? A new study looks into one possible force that may “sculpt” the edge of a compact multi-planet system: cold Jupiters. A cold Jupiter is a planet that has the mass of our own solar system’s Jupiter and is widely separated from its host star. The authors explore if and how the existence of a cold Jupiter at wide separation from an inner compact multi arrangement might determine how the pod gets filled with peas.

To test this, the authors needed a data set of known compact multis. They turned to the Kepler data set and selected the systems with at least four transiting planets. This left them with 279 planets in 64 systems. Next, they compiled the previously measured masses of these planets; only 60 of 279 had masses, so for the remainder they used a well-described mass–radius relationship to estimate planet masses. Planet mass is a crucial input for this study as mass is the primary parameter needed for dynamical studies, which inherently rely on gravitational forces.

To determine if and how a cold Jupiter could sculpt the edge of the inner compact system, the authors devised a dynamical study. Dynamical studies test how planets interact with each other through their gravitational influence on one another; often a researcher will test to see if a certain configuration of planets is stable or if it is unstable. In an unstable system, some or all of the planets get gravitationally kicked out of the system. Using a simulation software called the Stability of Planetary Orbital Configurations Klassifier (SPOCK), which takes in a list of planet masses and a few basic orbital parameters, the authors were able to compute the stability of the system over a specified time span. The authors added a simulated cold Jupiter (with parameters drawn from random distributions) to a planetary system with real planet masses and orbits and then determined the system’s stability over 1 billion years. For each of the 64 systems in the sample, they performed this test 10,000 times, noting the probability that the system was stable or not over that time span.

Each test resulted in one of three outcomes: unstable, fully stable, or metastable. Within the context of this study, metastable meant that the system was stable in about half of the simulations. Unstable simulations mean that the system will be ripped apart by gravitational interactions. The injected cold Jupiters that result in unstable configurations are not considered plausible edge-sculptors, then, because we do see planets in these systems; if the presence of a cold Jupiter made the system unstable, we wouldn’t see planets in the system. Next, the stable simulations represent injected cold Jupiters that had no effect on the inner system. These cold Jupiters exist at too wide a separation from the inner system for gravity to play a role in sculpting the edge of the pea pod. Therefore, these planets were also not considered plausible edge-sculptors. The metastable simulations, in which the odds of the system falling apart were between 30% and 70% (above 70% was considered fully stable), are the most interesting. These simulations correspond to systems in which the injected cold Jupiter had a real effect on the inner compact system and could potentially sculpt the edge of the inner compact system, as shown in Figure 1.

Plot of probability of stability as a function of perturber orbital distance and perturber mass

Figure 1: An example of the results of the main dynamical experiment for one multi-planet system. Each point is one of the 10,000 simulations where a cold Jupiter with randomized orbital parameters (y axis) and mass (x axis) is injected into the dynamical simulation along with the real planets. The dots are color coded by the probability of a stable configuration after 1 billion years: purple is unlikely to be stable, yellow is very likely stable. The metastable region is defined by the shading and red line. Because this particular system has a very small metastable region, it is highly unlikely that any cold Jupiter, if it exists, would play the role of the “edge sculptor.” [Adapted from Sobski & Millholland 2023]

However, looking at the mass and orbital parameters of the injected cold Jupiters that resulted in metastable simulations revealed a problem for the authors’ hypothesis: If these planets really existed in the Kepler systems tested, then observers should have been able to detect them in real data sets. These planets are large enough in radius that even accounting for transit probabilities, they likely would have been detected at appropriate rates in the Kepler data set. Similarly, they are massive enough and close enough to the inner system that radial-velocity surveys should easily detect them, but real radial-velocity data sets do not. (Radial-velocity surveys rely on measuring the Doppler shift of light due to a planet tugging on its host star.)

Therefore, the authors conclude that cold Jupiters likely do not help sculpt the edge of compact multi-planet systems. If they could, we would have found them in real data; since we don’t find them in real data, they must not exist in the compact multis studied here. Even though this experiment led to the rejection of the original hypothesis, it is nevertheless a fascinating result that tells us a bit more about the way exoplanet systems may form and evolve.

Original astrobite edited by Lindsay DeMarchi.

About the author, Jack Lubin:

Jack received his PhD in astrophysics from UC Irvine and is now a postdoc at UCLA. His research focuses on exoplanet detection and characterization, primarily using the radial-velocity method. He enjoys communicating science and encourages everyone to be an observer of the world around them.

dusty molecular clouds in the Carina Nebula

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: The Global Structure of Molecular Clouds: I. Trends with Mass and Star Formation Rate
Authors: Nia Imara and John C. Forbes
First Author’s Institution: University of California, Santa Cruz
Status: Published in ApJ

Molecular clouds are stellar nurseries, the birthplace of stars in our universe. These clouds are made up of cold molecular hydrogen, H2, which can clump together and collapse to form stars. Despite being such a vital part of the universe, the physics behind molecular clouds still needs to be better understood. Observations of molecular clouds provide limited information about their three-dimensional structure, making it challenging to study what exactly it is about these clouds that enable them to form stars.

Creating models that resemble molecular clouds is an excellent way to understand them in great detail. Comparing the models to actual observations enables us to tweak the model so that it matches all the observational properties of the object, thus letting us use the properties of the model to study the properties of the actual object.

So, how does one go about building a model of a molecular cloud? A good place to start would be to see how these structures look through a telescope. Observations indicate that most molecular clouds have an elongated shape with more gas concentrated near a central axis. The next step would be to pick a geometrical shape that best resembles the observations, preferably well studied and having well-defined properties. Even though our first instinct is to model the clouds as a sphere (because everything can be assumed to be a sphere!), the authors of today’s article take a more reasonable approach and model the molecular clouds as a cylinder.

Rolling in the Deep (Space)

The next step in building a good model would be to justify why this model is the closest to the molecular cloud. The comparison between the model adopted and an actual molecular cloud is highlighted in Figure 1.

annotated diagram of a cylinder laid atop a molecular cloud

Figure 1: The cylindrical model plotted on top of the observed extinction map of a molecular cloud, with the various parameters defined in red. The pink signifies regions of high extinction. [Imara & Forbes 2023 with annotations by Archana Aravindan]

One physical quantity that the authors are interested in studying from the model is how the cloud’s density (ρ) changes with the distance from the center. This significant quantity directly impacts various aspects of star formation, like when and how many stars can form from the molecular cloud.

How do you know if the model you’ve built is a good one? You test to see how well it replicates the properties of a real molecular cloud! To do this, the authors collected high-resolution observations of a sample of molecular clouds. Density is an important term that needs to be compared to observations, but density measurements are hard to obtain directly from the 2D projections of the clouds. A good approximation would be to get maps of dust extinction, which can be used as a proxy for density. There are well-established relations in place that relate dust extinction to surface density, so the authors use these relations to test their models.

Does the Model Have It All?!

The authors determined that all the clouds in their sample could be represented well by the cylindrical model. They also noted some significant correlations between the model parameters and the observed cloud properties (Figure 2). Clouds with the highest central densities have the lowest mass and star formation rates. High density coincides with high star formation rates since the denser the gas, the higher the chances for stars to form. However, the fact that such clouds have a low total mass seems counterintuitive. The authors determine that the mass of the cloud depends on both the distance along the spine and the perpendicular distance from it. They also find that dense clouds have short depletion times: the time needed to convert all the available molecular gas into stars at the current star formation rate. This gives us a correlation between the structure of the clouds and the timescales on which they would form stars.

Plots of mass and depletion times as a function of cloud density


Figure 2: The correlation between the mass (left) and depletion times (right, indicated as Mass/Star formation rate) on the y axis with the density (given as ρo/cos i to account for the inclination of the cloud) on the x axis for the observed sample of molecular clouds. As the density increases, both the mass of the cloud and the depletion times decrease. [Imara & Forbes 2023]

Such correlations would have been hard to derive with just observations alone, and this highlights the importance of building models to help us study such intricate properties. This current work extends to a small fraction of the molecular clouds in the local universe, but the models can be developed to represent clouds with other, different geometrical shapes. It can also be extended to explain the characteristics of clouds at various redshifts. With JWST opening the windows to peer at the early universe, building reliable models can be a powerful tool to understand the molecular clouds that formed all the stars, including the very first ones that brightened our universe!

Original astrobite edited by William Balmer.

About the author, Archana Aravindan:

I am a third-year PhD student at the University of California, Riverside, where I study black hole activity in small galaxies. When I am not looking through some incredible telescopes, you can usually find me reading, thinking about policy, or learning a cool language!

Simulation of a supermassive black hole binary

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: Uncovering Hidden Massive Black Hole Companions with Tidal Disruption Events
Authors: Brenna Mockler et al.
First Author’s Institution: The Observatories of the Carnegie Institution for Science & Department of Physics and Astronomy at the University of California, Los Angeles
Status: Published in ApJ

Two Is Company

Today, astronomers believe that nearly every galaxy hosts a supermassive black hole at its center. In addition, galaxies are thought to grow through mergers, in a process known as hierarchical growth. Essentially, smaller galaxies smash together to form a larger galaxy, and this process repeats many times as the universe evolves. When two galaxies hosting supermassive black holes merge, the black holes should sink to the center of the new galaxy rather rapidly, where they could start orbiting each other as a supermassive black hole binary. These binaries are therefore a natural consequence of this picture of hierarchical galaxy evolution and should be a relatively common occurrence in the universe.

However, finding supermassive black hole binaries has been rather difficult with current instrumentation and technology. A supermassive black hole makes itself known when it accretes gas from its surroundings, becoming a luminous active galactic nucleus. As two accreting black holes get closer and closer together, our telescopes become incapable of resolving them as two individual active galactic nuclei. There are other ways to infer that a binary exists when the black holes are close together, but these methods can be tricky — either the signals could also be produced by some other astrophysical phenomenon, or they take decades to confirm. The next generation of gravitational wave detectors, like the Laser Interferometer Space Antenna, will surely help, but we’d still like to be able to look for supermassive black hole binaries in the next decade or more before these detectors are built!

Introducing the Star of the Show

One of the best ways to observe something we can’t see is by looking for its interactions with things we can see. Today’s authors study the interplay of a supermassive black hole binary with stars in the centers of galaxies, highlighting this as a potential way to uncover these binaries. To start, let’s consider just a single supermassive black hole and throw a star at it. Most of the time, this star will orbit the black hole, just like our planets orbit the Sun. However, in some cases, when the orbit is eccentric enough, the star can get just a bit too close to the supermassive black hole, leading to the star’s demise. This measure of “too close” is set by the distance at which the star’s self-gravity can no longer hold itself together against the tidal forces of the black hole, and the star gets ripped to shreds. We call this phenomenon a tidal disruption event, and these events release a huge amount of energy from a previously quiet black hole.

Okay, but how do we get stars onto these elliptical orbits so that they’re disrupted? And how often does this happen? Many research articles have investigated these questions (check out some of the many Astrobites written on tidal disruption events), both from a theoretical and observational perspective. It turns out that one way to get stars onto these highly elliptical orbits is to scatter them off of other stars (through a process called two-body relaxation). This process is relatively rare; both theory and observations agree that the rate for tidal disruption events around single black holes is somewhere around one every 104–105 years (per galaxy).

But what happens when we deposit these stars around a supermassive black hole binary? The authors of today’s article investigate this very question. In particular, they investigate the interaction of stars around the smaller of the two black holes (see Figure 1 for a schematic of this set up).

Cartoon showing the setup involving tidal disruption events happening around the smaller of two black holes in a binary system

Figure 1: Cartoon schematic of the setup considered in today’s article. We have two supermassive black holes with masses m1 and m2, with m1 < m2. The authors investigate stellar orbits around the smaller black hole (m1). [Mockler et al. 2023]

And Now Three’s a Crowd

To explore the effects of a binary supermassive black holes on the rate of tidal disruption events, the authors perform dynamical simulations of the three-body problem we just set up above. They focus in particular on the effects of the eccentric Kozai–Lidov (EKL) mechanism, which is a dynamical effect in a three-body system that allows the eccentricity and inclination of the outer binary (i.e., the star and the lower-mass black hole) to oscillate. EKL oscillations can lead to extreme eccentricities, which is a great way to make tidal disruption events happen! To explore the effects of EKL on the system, the authors test different combinations of binary masses and stellar density profiles. There’s a large range of possible parameters in this problem, so they limit their tests to those in which the timescale for the EKL mechanism is the shortest dynamical timescale (which leads to EKL being the dominant mechanism driving the system’s evolution).

The simulations revealed that there should be a burst of tidal disruption events lasting 1–100 million years, depending on the exact simulation parameters. During this time period, the tidal disruption event rates greatly exceed that expected from two-body relaxation, which is what sets the rates of these events in single supermassive black hole systems. However, if the stars near the black hole are not replenished after this period, either from star formation near the galactic nucleus or some dynamical effects, then the rates of EKL-driven tidal disruption events drop to less than those of two-body relaxation. This is highlighted in Figure 2, which shows the EKL-driven tidal disruption event rate as a function of time in these dynamical simulations. So, our best hope for catching tidal disruption events around the smaller black hole in a binary pair is relatively quickly after it enters the binary.

Plot of the rate of tidal disruption events as a function of time

Figure 2: Rate of tidal disruption events occurring around the smaller supermassive black hole as a function of time in the simulations. The shaded blue regions represent different masses of the smaller supermassive black hole, each of which is 10 times less massive than the larger supermassive black hole. The shaded grey region shows the observed rate of optically selected tidal disruption events, and the grey hashed region denotes the rate of tidal disruption events in “post-starburst” (PSB) galaxies (galaxies seen about a few millions of years after a recent burst of star formation, which is often driven by a merger). Finally, the dashed and dotted lines show the rates of tidal disruption events from two-body relaxation (i.e., ordinary tidal disruption events around a single supermassive black hole). The simulations show a burst of tidal disruption events relative to the two-body relaxation rate for the first 1–100 million years. [Adapted from Mockler et al. 2023]

Finding Supermassive Black Hole Binaries with Tidal Disruption Events

To end, the authors leave us with a potential way to search for supermassive black hole binaries using these tidal disruption events. This method relies upon the fact that the two black holes in the binary will dominate two different observable properties. On one hand, the gravitational potential of the galactic nucleus where these two black holes reside will be dominated by the larger of the two black holes, meaning that host galaxy properties that scale with the galaxy’s central black hole mass will be set by this larger black hole. On the other hand, the light curve from a given tidal disruption event is set by the mass of the black hole that the star is accreting onto, which in this case is the smaller black hole. This means that if we see a tidal disruption event that seems to be coming from a small black hole, but it’s actually happening in a galaxy that’s far too big to host such a black hole, then there’s strong evidence that this could be a supermassive black hole binary system! And so, while three may be a crowd, this unlucky star will actually shed some light on its black hole companions as it leaves the party.

Original astrobite edited by Mark Dodici.

About the author, Megan Masterson:

I’m a 3rd-year PhD student at MIT studying transient accretion events around supermassive black holes, including tidal disruption events and changing-look active galactic nuclei. I primarily use X-ray observations to observe the inner accretion flow of these transients, but I am also interested in multi-wavelength follow-up to get the full picture of these fascinating systems. In my free time, I enjoy hiking and watching soccer.

radio and X-ray image of the Dragonfly pulsar wind nebula

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: Hard X-ray Observation and Multiwavelength Study of the PeVatron Candidate Pulsar Wind Nebula “Dragonfly”
Authors: Jooyun Woo et al.
First Author’s Institution: Columbia Astrophysics Laboratory
Status: Published in ApJ

Pulsar Wind Nebulae: Little Space Animals

Crab Nebula composite image

Figure 1: A multi-wavelength view of the Crab Nebula that shows the X-rays from the pulsar wind nebula (pinkish-white region at the center). [NASA, ESA, NRAO/AUI/NSF and G. Dubner (University of Buenos Aires)]

Pulsar wind nebulae are cosmic particle accelerators found all over the Milky Way (and in other galaxies too!). They’re made by the winds of pulsars — rapidly rotating and highly magnetized neutron stars, which are remnants of massive stars — pushing out winds of particles into the environments around them. The most famous example of a pulsar wind nebula is the Crab Nebula, which can be seen in Figure 1 as the small, pinkish-white, tornado-esque structure located in the larger multicolored supernova remnant left over from the original star’s explosion around a thousand years ago.

The Crab Nebula isn’t the only pulsar wind nebula with a fun nickname; in fact, most of these nebulae and their associated supernova remnants are named after animals that they (very) vaguely resemble. There’s the Mouse, the Goose, and the Kookaburra, just to name a few — and of course, the topic of today’s article, the Dragonfly (see Figure 2). Besides slightly resembling animals, pulsar wind nebulae are also thought to produce the highest-energy particles we detect on Earth. A new catalog of the highest-energy gamma-rays ever seen (see this bite) either links or tentatively associates many of these energetic systems with pulsars or pulsar wind nebulae.

Radio image of the Dragonfly with X-ray contours overlaid

Figure 2: Radio (colour) and X-ray (contours) image of the Dragonfly pulsar wind nebula. Doesn’t it sort of look like a dragonfly? [Jin et al. 2023]

Looking for the Dragonfly with All Sorts of Different (Wavelength) Eyes!

The authors of today’s article investigate the Dragonfly with multiple different telescopes that detect light across the electromagnetic spectrum to get a full picture of what’s going on with the particles accelerated in and around the nebula. The authors model the multi-wavelength emission to try to figure out if the Dragonfly is capable of accelerating particles (electrons, protons, and other things) up to petaelectronvolt (PeV; that’s a quadrillion electronvolts!) energies that then interact to make gamma rays, which would classify it as a PeVatron (a name that aptly describes any astronomical source that can accelerate particles up to PeV energies). We detect the highest-energy charged cosmic rays up to PeV energies, but we haven’t seen too many sources that emit gamma rays at these energies due to instrumental limitations and other things like photon absorption. Since cosmic rays (usually protons) get deviated in their travels to Earth by the swirling magnetic fields of the Milky Way, we need to search for neutral particles of similar energies, like photons (i.e., gamma rays) to find PeVatrons, since they trace a straight line back from the particle to its source.

Using model fitting, the authors can create and evolve a pulsar and pulsar wind nebula to match the observed data, which gives them information like the nebular age, the expected shape of the nebula’s emission, and whether or not it can be a PeVatron, among many other interesting clues that help narrow down what’s going on with the particles and material in this system.

In particular, one interesting thing the authors notice is that the shape of the Dragonfly is long and asymmetric in soft X-ray wavelengths (and potentially in other wavelengths, but it’s hard to say due to much coarser angular resolution; see Figure 3b). Usually we’d expect to see a more spherical shape, so the explanation for this could be that the pulsar that’s powering the nebula is zooming through space at an unusually high speed or, more likely, that the nebula lives within a supernova remnant that hasn’t been seen yet. The interaction of particles from the pulsar wind nebula with the supernova remnant can cause some funky shapes to appear in the surrounding material. The authors suggest that looking at the Dragonfly with a long exposure in radio wavelengths might be able to pick up signs of a supernova remnant that are overwhelmed in other wavelengths by the bright pulsar wind nebula to confirm this scenario.

The Dragonfly as seen in several wavelength ranges

Figure 3: The observed shape of the Dragonfly in a) radio, b) soft X-ray, c) hard X-ray, and d) very-high-energy gamma rays with X-ray contours in blue. The star or X in each figure marks the pulsar location. [Adapted from Woo et al. 2023]

By looking at the full multi-wavelength picture (see Figure 3), the authors note that the size of the pulsar wind nebula decreases with increasing energy in X-ray wavelengths (this isn’t apparent in Figure 3d, because the instrument isn’t able to resolve small structure and blurs everything out to look bigger than it is), meaning that the the nebula becomes a less efficient particle accelerator as we move to higher energies. By modelling this behaviour, the authors find a maximum particle energy of 1.4 PeV, meaning that the Dragonfly really can be a PeVatron.

Maybe a PeVatron? We’ll Have to Wait and See!

There’s still more work to do to figure out if we can actually see gamma rays at energies beyond a PeV from the Dragonfly and to figure out how particles are being transported around the nebula to get the weird asymmetric shape that today’s authors observed. More observations using existing radio, X-ray, and other instruments as well as future ultra-high-energy gamma-ray telescopes (like SWGO and CTAO-South) can help answer these questions and help us get an even more full picture of the Dragonfly.

Original astrobite edited by Lucie Rowland.

About the author, Samantha Wong:

I’m a graduate student at McGill University, where I study high energy astrophysics. This includes studying all sorts of extreme environments in the universe like active galactic nuclei, pulsars, and supernova remnants with the VERITAS gamma-ray telescope.

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.

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