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coronal loops over a sunspot group

What happens just as a solar flare begins to produce X-rays? New research leverages data from an Earth-orbiting X-ray telescope to examine the early stages of solar flares.

image of the sun's surface

The Solar Dynamics Observatory spacecraft captured this image of a large solar flare in 2014. [SDO/NASA]

A Common Occurrence

With the help of Sun-gazing observatories in space and on Earth, researchers have witnessed thousands of solar flares. Solar flares are powered by the explosive rearrangement of magnetic fields, accelerating charged particles to breakneck speeds and generating a brilliant flash of X-rays.

While the overall picture of flare production is clear, the details are still under scrutiny. To understand how solar flares happen, researchers must study each stage of the process closely. Today’s article examines the onset phase, which happens just before a flare ramps up to maximum brightness.

plot of counts per second during different phases of a solar flare

The counts per second (CPS) measured by DAXSS during several phases of a flare. [Adapted from Telikicherla et al. 2024]

A Three-Phase Process

Solar flares tend to happen in three stages:

  1. The precursor phase, in which pent-up magnetic energy is released and the first signs of the flare appear in the form of non-thermal particle motions
  2. The impulsive phase, in which charged particles get accelerated to extremely high energies, triggering the release of radio waves, hard (high-energy) X-rays, and gamma rays
  3. Finally, the decay phase, in which soft (low-energy) X-rays gradually brighten and then fade

Anant Telikicherla (Laboratory of Atmospheric and Space Physics, University of Colorado Boulder) and coauthors focused on the onset phase, which refers to the moments just before the impulsive phase, when the first soft X-rays are emitted but before the hard X-ray emission begins. Using data from the Dual-zone X-ray Solar Spectrometer (DAXSS), the team analyzed the soft X-ray spectra of six solar flares. The sampled flares took place in 2022 and had a range of characteristics, from average-intensity C-class flares to more powerful M-class flares. Most of the flares in the sample were eruptive, meaning they were accompanied by explosions of plasma into space, while one was confined, lacking a plasma explosion.

Flare and Back Again

The observations revealed that during the onset phase, when the first soft X-rays are produced, the solar plasma is already extremely hot, in the range of 10–15 million kelvin. The temperature then dips before rising again during the impulsive phase. In this way, the onset phase parallels the rising and falling temperatures of the main flare phase, leading Telikicherla’s team to suspect that the onset phase acts as “preconditioning” for the main event.

image of two coronal loops during the onset phase

Example of two coronal loops (indicated with red arrows) forming during the onset phase. [Adapted from Telikicherla et al. 2024]

The authors also examined extreme-ultraviolet images of the flares to understand the underlying motions of the plasma. They saw two general types of behavior, which they call one-loop and two-loop onset flares. For a one-loop onset flare, a single arc of coronal plasma brightens during the onset phase. This coronal loop then interacts with a second loop that brightens during the impulsive phase. During a two-loop onset flare, two coronal loops brighten, merging into a single loop during the impulsive phase.

Remarkably, the observed behavior appears to mimic the evolution of the main phase of a solar flare, suggesting that the onset phase offers a preview of what’s to come. Future work analyzing hard X-ray emission will explore the connection further, helping to understand whether the characteristics of the onset phase can be used to predict the properties of the main flare stage.

Citation

“Investigating the Soft X-Ray Spectra of Solar Flare Onsets,” Anant Telikicherla et al 2024 ApJ 966 198. doi:10.3847/1538-4357/ad37f6

A computer rendering of a spacecraft with a large radio dish against a black background studded with stars.

A little over nine years ago, a plucky robot with a huge radio dish for a head and a heart made of radioactive plutonium completed one of the most daring space missions conceived of to date. After hibernating through nearly a decade of interplanetary travel, the probe, earnestly dubbed New Horizons by its dedicated creators, woke up as it neared the dwarf planet Pluto. By design it was then beelining towards its destinations at over 50,000 miles per hour, far too fast to slow down using its limited remaining propellant. And so, it didn’t. In barely a blink of an eye, New Horizons snapped as many pictures as it could, furiously recorded data from its other instruments, and served as humanity’s first ambassador to the king of the Kuiper Belt before continuing zipping onward into our outer solar system.

The north pole of Pluto, as seen by New Horizons during its 2015 flyby. [NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute]

And then it was over. Though the New Horizons probe was traversing a region never before visited by another mission, there was no planned second destination in mind. Pluto had been the goal, and after the successful flyby, the mission could quite fairly have been declared a triumph and the team could have let New Horizons rest again.

But the probe still had some fuel in its tanks — in a very literal sense. Though New Horizons was traveling on a safe path that wouldn’t take it near any known objects, its controllers could potentially use the onboard thrusters to steer the probe closer to another Kuiper Belt object for a first-of-its-kind investigation. The only flaw in this plan was that when New Horizons left Earth in 2006, astronomers didn’t know of any objects near enough to Pluto to be reachable once the primary flyby was complete. The very first Kuiper Belt objects (besides Pluto) had only been discovered a few years earlier in 1992, and although scientists were beginning to suspect that millions of small planetesimals could inhabit the backwoods of our solar system beyond our planets, they hadn’t found many of them yet.

Unable to let the opportunity to study one of these likely ancient, preserved relics from the formation of our solar system slip away, an international team of astronomers took it upon themselves to search for a second target for New Horizons. Incredibly, the bulk of this work took place while the spacecraft was actively flying towards its eventual destination. The New Horizons team had thrown their hats over the wall, and with limited time before their robot drifted into interstellar space or succumbed to any one of the large number of hazards of the void, they needed to discover a new object quickly.

To spoil the ending: this industrious team was ultimately successful. They discovered a strange, 32-km-long, snowman-shaped object called Arrokoth, and New Horizons performed its second flawless Kuiper Belt flyby on New Year’s Day, 2019. However, though this end result is well known, the effort that went into this search was previously less well documented. This has been recently remedied with a narrative-like article published in The Planetary Science Journal, which details the decade of frantic, build-the-plane-while-flying-it type of work that led to Arrokoth’s discovery, along with the creation of an entire set of tools and best practices that guide many of today’s solar-system mapping efforts.

Early Days

The astronomers’ work began in 2004, in the heady early days of Kuiper Belt research when all of the models were constantly being revised as new discoveries poured in. How many Kuiper Belt objects were out there? How big were they? What orbits did they travel on? These were all uncertain foundational questions, but armed with the Subaru Telescope on Mauna Kea and optimism for quick discoveries, the team set out to answer them.

A photograph of a large telescope with an open rectangular covering.

The Subaru Telescope, a major component of the search efforts. [NAOJ]

From 2004 to 2005, the researchers took deep images of wide swaths of the sky as they searched for just the right object along New Horizons’ eventual path. With every new telescope pointing, they imaged stars that had yet to be recorded in any catalog and likely had never been observed by any human before them. While thrilling, this bounty of side-effect discoveries actually complicated their search. There were so many stars in their images, it was exceedingly difficult to spot the faint, moving Kuiper Belt objects that darted among them.

Painstaking manual inspection of all of the images did eventually yield 24 new objects, but none were close enough to New Horizons’ path to be feasible destinations. After dabbling with some citizen science efforts and a handful of new software routines to automate a portion of their workflow, the team regrouped and devised a new strategy for their search.

Heating Up

Between 2011 and 2015, the astronomers began a more intensive observing campaign. Several factors had emerged in the intervening years that tipped the odds of a suitable detection in their favor. For one, new instruments had been designed and installed on several large telescopes, which accelerated the search. For another, the area of the sky they had to sweep was shrinking thanks to the shifting arrangement of the Earth, Pluto, and New Horizons.

Adding to their advantages, this time around the researchers incorporated complex software tools into their analysis from the very start. The enormous quantity of data anticipated from this new search would place immense pressures on the old method of carefully comparing two images by eye to look for the moving objects; it was now time for computers to take the lead.

These new instruments and analysis techniques, along with yet more manual vetting, produced more than 50 additional discoveries. Again though, further analysis on all of these objects confirmed that they were just out of New Horizons’ reach. The probe could wave as it sailed by, but it wouldn’t get close enough to take any useful images of the surfaces. By now, as the mission team approached the actual Pluto flyby and an impending deadline for any final course corrections, time was running short.

Big Guns, Big Future

The first images of Arrokoth, taken with the Hubble Space Telescope. Click to enlarge. [Buie et al. 2024]

Since “…we absolutely had to find an object in 2014,” (Buie et al. 2024), it was time to leave the ground-based telescopes behind and bring in a closer: the most famous telescope in (and off of) the world, the Hubble Space Telescope. The team was ultimately awarded over 200 orbits’ worth of observing time in 2014, and finally, they were rewarded for their efforts with a handful of viable potential targets. One of these would eventually come to be known as Arrokoth, and the rest is history.

This decade of surveying, searching, waiting, and re-scheming ultimately left the planetary science community with another flyby and 80 new Kuiper Belt Objects, yes, but also with techniques less tangible than these discoveries. As astronomers push ever deeper into the night sky and search more and more crowded regions of stars, they often use the observing plans, algorithms, and sometimes the actual code developed as a part of this search. It was a tremendous effort that paid off handsomely, and as we near the 10-year anniversary of our robotic arrival at the Kuiper Belt, a reflection and celebration of how we have learned to study this strange place is in order.

Citation

“The New Horizons Extended Mission Target: Arrokoth Search and Discovery,” Marc W. Buie et al 2024 Planet. Sci. J. 5 196. doi:10.3847/PSJ/ad676d

JWST image of Wolf–Rayet star WR 140

Instead of fading smoothly, some supernova light curves take a bumpy road from brilliance to obscurity. Can unusual binary systems containing a rapidly spinning, wind-emitting magnetar and a stellar companion explain these light curves?

Light Curve Wiggles

illustration of a magnetar

Artist’s impression of a highly magnetized stellar remnant called a magnetar. [ESO/L.Calçada; CC BY 4.0]

When a star explodes, researchers record the light curve from its final moments and attempt to understand its life and death. Certain supernovae show bumps and wiggles in their light curves, the cause of which is not yet agreed upon. Researchers suspect that some of these light-curve bumps crop up when the expanding shock wave of the supernova slams into gas and dust surrounding the star. Other brightness increases might occur when the explosion leaves behind a magnetar — an extremely dense, city-size stellar remnant that spins rapidly and has a strong magnetic field — that injects energy into its surroundings.

In a recent research article, Jin-Ping Zhu (Monash University) and collaborators expanded on the latter possibility, pairing a powerful magnetar with an unlucky companion star to explain bumpy features in the light curves of certain supernovae.

diagram illustrating the stages of the magnetar–star binary engine model

A diagram illustrating the stages of the magnetar–star binary engine model. Click to enlarge. [Zhu et al. 2024]

What Goes Bump in a Supernova Light Curve

The proposed theory starts with an ordinary star and a massive star in a close binary system. As the massive star evolves, it sheds its outer layers through rapid rotation and fierce winds, exposing its super-hot core and becoming a rare Wolf–Rayet star. As the Wolf–Rayet star continues to evolve, tidal interactions between the stars in the binary system spin the Wolf–Rayet star up to high speeds. It eventually explodes in a core-collapse supernova, leaving behind a rapidly spinning magnetar.

Other models have invoked magnetars to explain bumpy supernova light curves, but this theory goes a step further, giving the companion an important role to play. As the newborn magnetar and the companion star swing around each other on their tight orbits, the magnetar’s powerful particle wind collides with the other star, evaporating some of the unlucky companion. The evaporated stellar material is then heated and accelerated by the magnetar wind, producing a bump in the light curve.

A Fitting Theory

plot of supernova light curves and model results

Example of multi-band light curves for a supernova that is well fit by the authors’ model. [Adapted from Zhu et al. 2024]

That’s the theory — how does it compare to observations? Zhu and collaborators applied their magnetar–star binary engine model to the light curves of supernovae with a single bump after maximum brightness. They found that the model generally fits the observations well, with the best-fitting results implying that a significant chunk — about 25–60% — of the companion star gets evaporated.

Zhu and collaborators suspect that their model may apply to light curves with multiple bumps, as well. If the companion star remains bound to the magnetar after the supernova explosion but is kicked into a new, highly eccentric orbit, a bump could be created each time the stars draw close to one another on their orbits.

The team notes that there isn’t yet firm observational or theoretical evidence that rapidly rotating massive stars leave behind magnetars, and it’s not clear whether a magnetar embedded within a supernova remnant can sustain a magnetar wind, as is required here. Future work may shore up the needed evidence, and in the meantime, this work provides a new way to interpret bumpy light curves.

Citation

“Bumpy Superluminous Supernovae Powered by a Magnetar–Star Binary Engine,” Jin-Ping Zhu et al 2024 ApJL 970 L42. doi:10.3847/2041-8213/ad63a8

photograph of spiral and elliptical galaxies

plot of a signal from a nearby fast radio burst

Example of a signal from a nearby fast radio burst. The top panel shows the overall intensity over time, while the bottom panel shows the frequency of the burst over time. [Adapted from Bhardwaj et al. 2024]

Where do fast radio bursts come from? New research shows that the nearby host galaxies of these fleeting flashes have something in common, which may help researchers understand the origins of fast radio bursts.

Mysterious Bursts

Fast radio bursts are powerful, milliseconds-long flashes of radio waves of unknown origin. Since the first fast radio burst was discovered in 2007, astronomers have detected roughly a thousand of these mysterious signals. The source of these bursts is still up for debate, with supernovae, magnetars, colliding objects, and other energetic phenomena tapped as candidates.

To understand the origin of these bursts, it helps to know what kind of galaxies they happen in. If fast radio bursts emerge from spiral galaxies with active star formation, it could mean that the bursts are linked to “prompt” formation channels such as the deaths of short-lived massive stars. If instead bursts come from elliptical galaxies with little or no star formation, that could imply that bursts come from “delayed” channels like the slowly progressing mergers of stellar remnants. So, what kind of galaxies do fast radio bursts tend to come from?

In the Neighborhood

Mohit Bhardwaj (Carnegie Mellon University and McGill University) and collaborators turned to data from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) to answer this question. Their aim was to study fast radio bursts in the local universe since a sample of nearby bursts is less likely to be affected by observational biases. To find nearby bursts, Bhardwaj’s team searched the first CHIME fast radio burst catalog for signals with low dispersion measure. On average, the farther the source, the larger the dispersion measure — essentially, because there’s more stuff in between the source and Earth to disperse the radio signal.

visible-light images of candidate host galaxies

Visible-light images of the localization regions of the four fast radio bursts in this study. The red boxes indicate the most likely host galaxy for each burst. Note that FRB 20181223C has four potential host galaxies (red and cyan boxes in the upper-left image), but only one that satisfied the source’s maximum redshift limit. Click to enlarge. [Bhardwaj et al. 2024]

The team found four cataloged bursts with dispersion measure excess (the amount left over after the contribution from the Milky Way is subtracted off) less than 100 parsecs per cubic centimeter, which corresponds to a distance of about 1.3 billion light-years. After searching the localization regions of these bursts in deep optical images from the Panoramic Survey Telescope and Rapid Response System survey, the authors found only one plausible host galaxy for each burst.

A Spiral Sample

Including the four bursts with newly identified host galaxies from this work, researchers have now localized the positions of 18 nearby fast radio bursts. What ties these burst-hosting galaxies together, if anything? As it turns out, they’re all spiral galaxies. That’s an intriguing result, but does it necessarily mean that fast radio bursts are more likely to come from spiral galaxies, or is it just easier to detect fast radio bursts from spiral galaxies?

images of the 18 fast radio burst host galaxies

The host galaxies of all 18 local universe fast radio bursts used in this study. Click to enlarge. [Bhardwaj et al. 2024]

Bhardwaj and collaborators explained that their sample selection is actually biased against bursts from spiral galaxies because these galaxies tend to have more signal-dispersing material than elliptical galaxies do, making spiral galaxies more likely to be eliminated by the dispersion measure cutoff. Given the typical ages of stars in spiral galaxies, the team suggests that the dominant formation pathway for fast radio bursts in the local universe is through core-collapse supernovae, which mark the explosive end of stars more than about eight times the mass of the Sun.

Bhardwaj’s team noted that this doesn’t mean all fast radio bursts must come from supernovae; because a small number of known bursts have arisen in unusual locations like globular clusters and non-star-forming spiral galaxies, delayed pathways like mergers of stellar remnants may be responsible for certain bursts.

Citation

“Host Galaxies for Four Nearby CHIME/FRB Sources and the Local Universe FRB Host Galaxy Population,” Mohit Bhardwaj et al 2024 ApJL 971 L51. doi:10.3847/2041-8213/ad64d1

Image of the Sharpless 2-106 star-forming region

Editor’s Note: Lexi Gault is a fourth-year graduate student at Indiana University who was recently selected as the 2024–2025 AAS Media Fellow. We’re excited to welcome Lexi to the team and look forward to featuring her writing on AAS Nova regularly!

Once thought to be a pure descendant of one the universe’s first stars, new research on star J1010+2358 uncovers a more complex history.

The First Stars and Their Violent Ends

During the universe’s debut into star formation, only hydrogen, helium, and some lithium existed. Without metals (i.e., elements heavier than helium) to efficiently cool the gas, the first generation of stars, called Population III stars, likely formed with much higher masses than any subsequent generation. Based on theoretical modeling, Pop III stars may have had masses hundreds of times the mass of the Sun; however, their true mass distribution is still under investigation.

Some of the most massive Pop III stars likely ended their lives in pair-instability supernovae (PISNe), a type of supernova so energetic and violent it completely rips the star apart, leaving behind no stellar remnant. Metals that formed inside these stars and within their explosions are released into the interstellar medium, creating a newly enriched cosmic soup that fuels the next generation of star formation. Finding stars born out of PISN material, and particularly descendants whose chemical signatures trace back to a single Pop III star, is no simple task, but this detection would provide powerful information about the first stars in the universe.

Measuring Chemical Abundances

With a low metallicity and unique chemical abundance pattern, the star J1010+2358 was initially suggested to have been formed from the gaseous remains of a single 260-solar-mass Pop III star. However, more recent analysis suggests that the star’s origins are more ambiguous, perhaps obtaining only 10% of its metals from such a PISN.

Plot of chemical abundance pattern

Measured chemical abundance pattern of J1010+2358 (red stars). Multiple model predictions are overlaid; the dashed gray line shows a 100% 260-solar-mass PISN contribution, indicating an ill fit to the data. Click to enlarge.
[Skúladóttir et al. 2024]

To more confidently determine J1010+2358’s ancestry, a team led by Ása Skúladóttir (University of Florence) used the Ultraviolet and Visible Echelle Spectrograph on the European Southern Observatory’s 8.2-meter Very Large Telescope to derive a more detailed abundance pattern of the star. They found the carbon and aluminum abundances to be significantly higher than predicted for pure PISN descendants. The team also remeasured a number of other elements, and the results further signal that J1010+2358 is not a descendant of a single, massive Pop III star as previously claimed.

Likely Origins of J1010+2358

How, then, did J1010+2358 obtain its interesting chemical composition? Through applying theoretical models for various progenitor combinations, Skúladóttir’s team found a best fit where the star’s metals come from a combination of a 13-solar-mass second-generation star that underwent a core-collapse supernova and a 39-solar-mass Pop III core-collapse supernova. They considered multiple scenarios with the star obtaining some of its metals from a 260-solar-mass PISN, but all plausible fits have PISN contributions too low to further characterize the mass distribution of first-generation stars.

Quality of fit results for J1010+2358 progenitors

Quality of fit results for J1010+2358 containing two progenitors, with one being a PISN of a given mass and fraction of metal contribution and the other being a 13-solar-mass second-generation star that underwent a core-collapse supernova. Values 2 and below are best quality fits and most plausible progenitor scenarios. Click to enlarge. [Skúladóttir et al. 2024]

Stars have complex histories stored within their chemical DNA, and the difficulty of identifying a star with a single first-generation predecessor underscores the importance of careful chemical abundance analyses. Although J1010+2358 is likely not a true PISN descendant, the lessons learned through this star will be critical in the quest to uncover the properties of the universe’s first stars, and upcoming high-resolution spectroscopic surveys may reveal more promising candidates in the near future.

Citation

“On the Pair-instability Supernova Origin of J1010+2358,” Ása Skúladóttir et al 2024 ApJL 968 L23. doi:10.3847/2041-8213/ad4b1a

Radar images of the near-Earth object Apophis

As you read this, an asteroid named 99942 Apophis is spiraling along a trajectory that will bring it uncomfortably close to Earth in about five years’ time. Thankfully, astronomers are very confident that this 340-meter ball of rock will miss us and instead will squeak between Earth and the Moon on 13 April 2029 before continuing on its way through the solar system. But such a close brush with disaster inspires reflections on worst-case scenarios. Even though everything we know about the laws of physics tells us that Apophis won’t collide with Earth on its current trajectory, could anything happen between now and the flyby that would change that?

Unlikely Odds

Illustration of different hypothetical trajectories Apophis could be on as it approaches Earth. An animated version can be seen here. [Wiegert 2024]

Recently, Paul Wiegert, The University of Western Ontario, took up a specific version of this question: if a separate smaller object crashed into Apophis, could that impact nudge the asteroid off its current path and onto a catastrophic collision course?

Luckily, almost definitely not. But, unsurprisingly given a situation as complicated as orbital dynamics and city-destroying impacts, there are caveats. While Wiegert estimates that the odds of Apophis stumbling into an asteroid larger than 3.6 meters across — large enough to divert Apophis into Earth — are about two in a billion, the odds of it encountering a smaller asteroid are higher. He estimates there’s about a one in a million chance that Apophis could strike an asteroid bigger than 60 centimeters, which in theory would be large enough to nudge Apophis onto an orbit that would still miss us in 2029, but would then lead to an impact later (in 2036, for example). We’d have to be doubly unlucky for this to happen, since even in this unlikely event, most small impacts leave Apophis on paths that take it farther from Earth in the future, not towards it. So, there’s certainly no need for alarm, but it will be worth monitoring Apophis in the coming years to make sure it hasn’t strayed from its safe path.

Sneaky Asteroid

There lies an unfortunate twist to this story, however: we can’t actually observe Apophis until early 2027, at best, so we have no way to know if it’s been jostled onto a dangerous trajectory until then. The asteroid has been lurking too close to the Sun for telescopes to safely see it since 2021, so our next look at it will come after a significant gap in time.

Illustration of the angle between Apophis and the Sun, as seen from Earth. The yellow band denotes geometries where telescopes cannot observe the asteroid. [Wiegert 2024]

Wiegert, acknowledging both the extreme unlikeliness of a deflection but also the dire consequences of misplaced complacency, also calculates how we might tell if Apophis is on a new path once it finally reappears in the night sky. He finds that if the asteroid is even a few tenths of an arcsecond away from its predicted position, we should immediately attempt an intensive observing campaign to check its new trajectory. A deviation wouldn’t automatically spell danger, but it would indicate that something happened while we waited for Apophis to emerge, and that there’s a chance that we could be in trouble.

When it comes to something as serious as asteroid impacts, it’s good to double-check every assumption and to investigate even the most unlikely scenarios. Thankfully Earth is almost certainly safe from an asteroid impact in 2029, though up until then and beyond, astronomers will be sure to keep checking for signs of danger.

Citation

“On the Sensitivity of Apophis’s 2029 Earth Approach to Small Asteroid Impacts,” Paul Wiegert 2024 Planet. Sci. J. 5 184. doi:10.3847/PSJ/ad644d

illustration of a hot Jupiter exoplanet orbiting close to its host star

Are hot Jupiters lonely, or do they have nearby planetary companions? The answer may depend on how these close-in planets form. A recent discovery adds to the small number of known systems containing a hot Jupiter and an inner planet companion and provides clues to hot Jupiter formation.

ALMA image of the disk around the young star TW Hydrae

Planets form in protoplanetary disks, like the one pictured here around the star TW Hydrae. Interactions between young planets and the gas of the disk can cause planetary migration. [S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)]

The Origins of Hot Jupiters

Hot Jupiters are among the strangest creatures in the exoplanet zoo. These worlds, which are roughly the mass of Jupiter, orbit extremely close to their host stars, achieving temperatures far hotter than the giant worlds in our solar system.

It’s not yet clear how hot Jupiters come to be. Do they form in place, vacuuming up massive amounts of gas in the blazing heat of their host stars? Do they get their start in cooler regions and then migrate to warmer pastures as they form? Or do they form far from their stars, then get kicked closer by gravitational interactions that dramatically change their orbits? To learn more, researchers are collecting all the information on hot Jupiters that they can — and they’re especially interested in the company that hot Jupiters keep.

Close Companions

So far, astronomers know of only a handful of hot Jupiters that have planetary companions whose orbits lie between the hot Jupiter and the host star. Certain theories of hot Jupiter formation, like high-eccentricity migration, predict that inner planets will be ejected or destroyed when a giant planet migrates from a distant orbit to a close-in one, becoming a hot Jupiter. To understand whether high-eccentricity migration is the main way that hot Jupiters form, as current observations suggest, researchers must understand how common these close-in companions are.

light curves for two exoplanets around TOI-1408

Normalized transit signals from the newly discovered inner planet TOI-1408 c (top) and the previously known hot Jupiter TOI-1408 b (bottom). [Adapted from Korth et al. 2024]

Judith Korth (Lund University) and collaborators used observations from the Transiting Exoplanet Survey Satellite (TESS) and ground-based telescopes to search for close-in planets around TOI-1408. TOI-1408 is known to host a hot Jupiter (TOI-1408 b) with a period of 4.42 days. The team searched TOI-1408 b’s transit signals for transit timing variations — changes in the time between transits — and found a clear signal at 2.2 days.

Searching through the TESS light curves, the team found evidence for a planet with that orbital period. They suspect that the automated TESS transit search pipeline missed the planet because of its large transit timing variations. Modeling suggests a mass of 7.6 Earth masses and a radius of 2.22 Earth radii, making TOI-1408 c a mini-Neptune planet.

Far-Out Findings

TOI-1408 b has joined the small but growing ranks of hot Jupiters with low-mass inner planet companions. This finding suggests that TOI-1408 b couldn’t have formed through high-eccentricity migration. Instead, in-situ formation or disk migration must be responsible.

The data analyzed in this work held another surprise: TOI-1408 c might not be the only new member of the TOI-1408 family! Korth’s team used radial velocity modeling to search for other planets and found clear evidence for another object in the system with a period of thousands of days. While more follow-up observations are needed to narrow in on the precise properties of this object, Korth’s team suggests it has a mass of roughly 14.6 Jupiter masses and an orbit lasting nearly 7 years.

Citation

“TOI-1408: Discovery and Photodynamical Modeling of a Small Inner Companion to a Hot Jupiter Revealed by Transit Timing Variations,” Judith Korth et al 2024 ApJL 971 L28. doi:10.3847/2041-8213/ad65fd

supernova

A kilometer below Japanese ground lies a massive cylindrical tank, its steel walls lined with more than 10,000 photomultiplier tubes that await the arrival of neutrinos. Could recent upgrades to the Super-Kamiokande neutrino detector improve our ability to spot and assess supernova explosions in real time?

JWST image of SN 1987A

JWST image of SN 1987A. The optical signal of this explosion was preceded by the arrival of a burst of neutrinos, a handful of which were observed by detectors on Earth. [NASA, ESA, CSA, Mikako Matsuura (Cardiff University), Richard Arendt (NASA-GSFC, UMBC), Claes Fransson (Stockholm University), Josefin Larsson (KTH)]

Memo from a Dying Star

In 1987, 25 tiny messengers arrived at Earth following a tremendous explosion 168,000 light-years away. This signal marked the first time that these near-massless messengers — neutrinos — had ever been directly observed from a Type II supernova, the core collapse of a massive star.

The benefit of observing these particles is clear: because neutrinos so rarely interact with matter, they arrive at Earth carrying untouched information about the death of the star that produced them. And because neutrinos escape the collapsing star more readily than photons do, they arrive before the visible light from the supernova. This means that if scientists can detect and localize a supernova neutrino burst, they can notify observatories about the imminent optical signal from the supernova. Combining the neutrino and electromagnetic observations could then provide valuable insight into long-standing questions, like the mechanism of the star’s explosion.

The catch? We only expect a handful of nearby (i.e., in our galaxy) supernova explosions every century — and what’s more, observing neutrinos is no easy feat! In the nearly four decades since those 25 messengers heralded the supernova SN 1987A, we haven’t detected any other neutrinos linked to supernovae. But that doesn’t mean we haven’t been preparing.

A Salty Upgrade for Super-K

Super-Kamiokande scale model

A scale model showing the (empty) tank of the underground Super-K neutrino detector. A recent upgrade saw a gadolinium salt added to the water that fills the tank. [Adapted from Wikipedia user Motokoka; CC BY-SA 4.0]

The Super-Kamiokande (Super-K) neutrino detector has recently undergone an upgrade: scientists have dissolved a gadolinium salt in the 55,000 tons of ultrapure water that fills its underground tank. The addition of this rare-earth metal improves the detector’s ability to differentiate between neutrinos and antineutrinos, thereby enabling scientists to more accurately localize the supernova that produced an incoming burst of neutrinos.

In a recent article led by Yuri Kashiwagi (Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo), Super-K scientists have analyzed how this upgraded observatory — and a corresponding real-time alert system developed to notify optical astronomers of the explosion and tell them where to point their telescopes — will respond to a hypothetical supernova within our galaxy.

Passing the Message On

Using multiple different supernova models, Kashiwagi and collaborators simulated the neutrino burst that would be produced by a supernova exploding roughly 33,000 light-years away. Through further simulations, the team then explored how successfully Super-K would detect neutrinos from the burst, how well the supernova’s location could be identified from Super-K’s detections, and how quickly this information could be broadcast to astronomical observatories via the alerting system.

supernova localization

The blue contours on this sky map show an example of the reconstructed supernova position that Super-K neutrino observations should provide for a supernova that occurs 33,000 light-years away. In this example, the supernova’s location can be identified to within 3°. [Adapted from Kashiwagi et al. 2024]

The authors found that the simulated supernova’s location could be rapidly identified to within 3–7° on the sky, and that information could be sent to optical observatories within minutes. In many cases, this response would be sufficient for wide-field telescopes like the upcoming Vera Rubin Observatory to catch the rise of the optical signal from a supernova — and the neutrino data captured by Super-K could even help distinguish between different supernova models.

While there’s still more to learn, it seems likely that Super-K is well prepared for future nearby supernova detections. Now all that’s left is to wait for the next explosion!

Citation

“Performance of SK-Gd’s Upgraded Real-time Supernova Monitoring System,” Y. Kashiwagi et al 2024 ApJ 970 93. doi:10.3847/1538-4357/ad4d8e

JWST photograph of the Cartwheel Galaxy

Today’s Monthly Roundup is a bit of an astrophysical mishmash, highlighting some of the structures that exist in our universe. From galaxies to planets, we’ll explore where these structures come from and the tools researchers use to study them.

A Ring Galaxy

When gamma-ray telescopes like Fermi survey the sky, they see discrete sources of gamma rays as well as a diffuse background glow. Discerning exactly where these gamma rays come from is no easy task, and roughly 30% of known gamma-ray sources haven’t been identified. Many of these unassociated sources are likely active galactic nuclei (AGN) — accreting supermassive black holes that can launch powerful jets — but some may have more unusual origins.

annotated ultraviolet image of Kathryn's Wheel

An ultraviolet image (background color) of the Kathryn’s Wheel system with contours showing the Hα emission. Annotations have been added to show the various components of this system. Click to enlarge. [Adapted from Paliya & Saikia 2024]

In a recent article, Vaidehi Paliya and D. J. Saikia (Inter-University Centre for Astronomy and Astrophysics) matched the gamma-ray source 4FGL J1647.5−5724 to a galaxy called Kathryn’s Wheel. Kathryn’s Wheel is a ring galaxy, consisting of a central gas-poor galaxy surrounded by a ring of star formation. This curious system likely formed when a nearby dwarf galaxy, LEDA 3080069, shot through it like a bullet, kicking gas out of the system and triggering a shock wave that kick-started star formation in a ring around it.

Multiwavelength images of Kathryn’s Wheel show bright Hα and ultraviolet emission, both of which signal the presence of hot, massive young stars. Intense star-forming regions are known to emit gamma rays, in part because of the frequent core-collapse supernovae that shock interstellar gas and accelerate cosmic rays. A closer look at the galaxy revealed that its gamma-ray emission is stronger than expected, suggesting either a population of rapidly spinning stellar remnants called pulsars or — as a more mundane explanation — interference from a foreground Milky Way star.

In addition to triggering star formation, galaxy collisions can also activate AGN. While the current data show no signs of gamma-ray variability or relativistic jets — both of which would indicate an AGN — more observations are needed to look into the possibility further.

High-Energy Bubbles

In 2010, scientists discovered that the Milky Way has been blowing bubbles. Observations with the Fermi Gamma-ray Space Telescope revealed bubbles of gamma-ray emission extending 50 degrees above and below the plane of our galaxy. Ten years later, the eROSITA instrument on the Spectrum-Roentgen-Gamma spacecraft spotted a similar but even more extensive structure in X-rays. The origin of these structures, called the Fermi and eROSITA bubbles, is not yet known. Many researchers have suggested that a past period of AGN activity, in which the Milky Way’s central supermassive black hole accreted matter and shot out powerful jets, could be responsible.

Recently, Po-Hsun Tseng (National Taiwan University) and collaborators approached the AGN activity hypothesis from a new angle. Previous work has tested this theory under the assumption that the AGN jets emerged vertically from the plane of the galaxy, matching the orientation of the bubbles. However, this doesn’t have to be the case — the direction of an AGN’s jets is related to the spin of the black hole, which doesn’t have to be parallel to the galactic disk.

simulated gamma-ray bubbles

Simulated gamma-ray bubbles from jets emerging perpendicular to the disk (top), at a 45-degree angle to the disk (middle), and parallel to the disk (bottom). [Adapted from Tseng et al. 2024]

Using special relativistic fluid dynamics simulations, Tseng’s team examined whether angled AGN jets could produce vertical bubbles. The simulated jets emerged from the plane of the Milky Way at a 45-degree angle, remaining “on” for 120,000 years before shutting off. A key component of the team’s model is the inclusion of a thin, dense, clumpy layer of interstellar gas lying parallel to the galactic disk. When the jet collides with this layer, interactions between cosmic rays in the jet and gas within the layer produce gamma rays.

Ultimately, the team found that under certain conditions, the jets transfer their kinetic energy to the dense layer of gas without plowing through it, and the once-narrow jets instead emerge vertically from the disk as bubbles. While more modeling is needed to understand the origin of the Fermi and eROSITA bubbles, this work shows that the assumption of vertical jets need not apply.

 

A Chain of Planets

When planets form in the dusty recesses of a protoplanetary disk, their motions within the disk are thought to align the planets in a resonant chain: a setup in which the orbital periods of the planets are integer multiples of one another. For example, a three-planet system with 8-, 16-, and 32-day orbits would be in a resonant chain. If planets do link up in resonant chains when they first form, something — gravitational nudges from passing stars, for example — must break those chains, as only 1% of known planetary systems are in this configuration. It’s critical to identify the small percentage of systems with intact resonant chains since they show the initial state of a planetary system before it’s disrupted.

plot of the cumulative fraction of unstable systems over time

Cumulative fraction of simulated six-planet systems becoming unstable as a function of time. Nearly all non-resonant systems are unstable by 25 million years, while only a small fraction of six-planet resonant chains go unstable in that same stretch of time. Click to enlarge. [Lammers & Winn 2024]

Caleb Lammers and Joshua Winn (Princeton University) investigated a potential resonant chain in the HD 110067 system. If confirmed, HD 110067 would be just the third known resonant chain containing six or more planets. HD 110067 has six known transiting planets with orbital periods between 9.1 and 55 days. The planets’ orbital periods are very nearly in ratios of 3:2 and 4:3 — highly suggestive of a resonant configuration.

Lammers and Winn performed N-body simulations to assess the likelihood that the system is arranged in a resonant chain. They found that in order for HD 110067’s six planets to be dynamically stable, the planets almost certainly must be in a chain. Simulated non-resonant systems become unstable within 25 million years — just 0.3% of the current age of HD 110067’s planetary system — and even systems with as many as five planets linked in a chain are unlikely to survive in that configuration to the present day.

Citation

“A γ-Ray-Emitting Collisional Ring Galaxy System in Our Galactic Neighborhood,” Vaidehi S. Paliya and D. J. Saikia 2024 ApJL 967 L26. doi:10.3847/2041-8213/ad4999

“Can the Symmetric Fermi and eROSITA Bubbles Be Produced by Tilted Jets?” Po-Hsun Tseng et al 2024 ApJ 970 146. doi:10.3847/1538-4357/ad50c5

“The Six-Planet Resonant Chain of HD 110067,” Caleb Lammers and Joshua N. Winn 2024 ApJL 968 L12. doi:10.3847/2041-8213/ad50d2

TW Hydrae

A new analysis of archival data reveals shocked gas in the protoplanetary disk surrounding the young star TW Hydrae. This discovery hints at the presence of a 4-Earth-mass planet and gives researchers a rare opportunity to study the earliest stages of planet formation.

Outflows from Planetary Offspring

Baby planets form in disks surrounding young stars, but the details of this process remain unclear — especially because the planets are often blanketed with dusty gas, hiding them from view. Massive gas planets like Jupiter and Saturn are thought to form by accreting gas onto rocky cores that gradually carve out lanes in the disk.

How can we tell if accretion is happening in a protoplanetary disk? As growing planets collect gas and dust, they also launch material into their surroundings in the form of outflows. As outflowing gas pummels its surroundings, shocks form, triggering the formation of molecules like sulfur monoxide (SO). That gives researchers an in — the planet might be hidden, but emission from these shock-formed molecules can announce its position.

location of SO emission relative to continuum emission of the protoplanetary disk

Integrated intensity of SO emission (orange and green contours) overlaid on a continuum image of the disk. [Adapted from Yoshida et al. 2024]

An Archival Search

This tells us how to potentially find baby planets, but where to look? One of the best places to search for signs of planet formation is around TW Hydrae, an 8-million-year-old star less than 200 light-years away. TW Hydrae possesses the nearest known protoplanetary disk, which from our vantage point appears nearly face on, with concentric light and dark rings like a bullseye. Researchers previously found two gaps in this disk, at 26 and 42 au, that could be explained by two roughly 4-Earth-mass planets. In addition, a clump of emission at 52 au hinted at the presence of a circumplanetary disk feeding gas to a growing planet.

Tomohiro Yoshida (National Astronomical Observatory of Japan) and collaborators analyzed archival data from the Atacama Large Millimeter/submillimeter Array (ALMA) to search for signs of outflows from a baby planet in the TW Hydrae disk. The team spotted an arc of emission from SO molecules originating from a gap 42 au from the star — exactly where a planet is purported to be.

Shocking Evidence

plot of best-fitting outflow trajectories

Best-fitting outflow trajectory (orange line) from the ballistic outflow modeling. [Adapted from Yoshida et al. 2024]

What does modeling say about the origin of this emission? The authors used ballistic outflow modeling to show that the SO outflow could be explained by a growing planet with a mass of 4 Earth masses. Combining estimates of the mass-accretion and mass-loss rates, the team finds an overall rate for the growth of the planet that matches theoretical expectations for a 4-Earth-mass planet.

With evidence for outflows already in hand, Yoshida’s team plans to continue the search, conducting further observations to look for evidence of the outflow in emission from other promising molecules, like silicon monosulfide. Overall, this work solidifies another line of evidence for the presence of a planet in the 42-au gap of TW Hydrae, and we can expect future observations to illuminate this growing planetary family further!

Citation

“Outflow Driven by a Protoplanet Embedded in the TW Hya Disk,” Tomohiro C. Yoshida et al 2024 ApJL 971 L15. doi:10.3847/2041-8213/ad654c

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