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radio images of eight protoplanetary disks

Protoplanetary disks, the sites of planet formation around young stars, are rich and complex objects. An in-depth investigation of 15 protoplanetary disks has recently been published in a Focus Issue of the Astrophysical Journal Letters. This investigation leveraged the exquisite resolution and sensitivity of the Atacama Large Millimeter/submillimeter Array (ALMA), a moveable network of 66 radio dishes in the Atacama Desert in Chile.

Featuring high-resolution observations of multiple gas species, updated image processing techniques, and state-of-the-art modeling, the exoALMA project examines the early stages of planet formation, when protoplanets embedded within dusty disks induce rings, gaps, spirals, and other structures.

This Monthly Roundup consists of quick snippets describing each of the currently published research articles in the exoALMA Focus Issue of the Astrophysical Journal Letters. Each snippet links to the corresponding research article, and the Focus Issue landing page is linked at the bottom of this post.

Introducing exoALMA

exoALMA disks

The full exoALMA sample. All 15 disks show evidence of substructure in their gas emission. [Teague et al. 2025]

First up, exoALMA project PI Richard Teague (Massachusetts Institute of Technology) and collaborators introduced the survey, the disk sample, and the science goals. The team mapped the dust and gas distributions of 15 protoplanetary disks that were selected for their brightness, large size, lack of obscuration, and low inclination angles. The three key science goals were to 1) detect protoplanets hidden within the disks, 2) study the disks’ dynamical structures, especially as these structures relate to instabilities that can perturb the disks and determine their lifetimes, and 3) determine the disks’ density and temperature structure.

Interpreting interferometric data from ALMA requires special care, as Ryan Loomis (National Radio Astronomy Observatory) and coauthors show in their overview of the data calibration and imaging pipeline. The team described their methods of processing the data, including a recently developed alignment method that greatly reduces the number of artifacts that could be mistaken for planet-induced velocity perturbations.

plot of emitting surface of J1615

Emitting height from images processed using the standard method (CLEAN) and using regularized maximum likelihood (RML). [Adapted from Zawadzki et al. 2025]

Although extreme care was taken not to introduce spurious signals through the calibration process, it’s critical to perform additional tests to confirm that detected signals are real, rather than artifacts. To that end, Brianna Zawadzki (Wesleyan University) and coauthors presented exoALMA images processed with a procedure called regularized maximum likelihood. The regularized maximum likelihood–processed images showed the same non-Keplerian features as the images processed with the standard algorithm, suggesting that these features are real.

Analysis Methods and Toolkit Testing

Analyzing observations of protoplanetary disks is challenging, motivating Thomas Hilder (Monash University) and collaborators to take the first step toward addressing issues like beam smearing, the creation of probabilistic data products, and incorporating a realistic noise model. Using their newly developed methods, the team analyzed several disks in the exoALMA sample, finding velocity substructures at greater velocities than existing methods.

Vertical structure and Keplerian rotation of the exoALMA sample

Vertical structure and Keplerian rotation of the exoALMA sample as inferred from modeling. Click to enlarge. [Izquierdo et al. 2025]

Andrés Izquierdo (University of Florida; Leiden University; European Southern Observatory) and collaborators described their methodology for investigating gas structure in the exoALMA sample. The team’s fitting method allowed them to separate the contribution of a disk’s front and back sides from the rest of its emission, leading to estimates of the orientation and vertical profile of each disk.

Jaehan Bae (University of Florida) and collaborators drew attention to the tools astronomers use to interpret observations of protoplanetary disks — namely, forward models. Bae’s team used five different hydrodynamics models to simulate a protoplanetary disk harboring an embedded giant planet. Then, they used two radiative transfer models to simulate carbon monoxide channel maps and calculate the temperature of the disk. Finally, they extracted the location of the synthetic planet. This investigation ultimately showed good agreements between the model outputs, showing that any combination of the models tested in this work is suitable.

plots of nonaxisymmetry of the exoALMA sample

Residual signal-to-noise ratio for the exoALMA sample, ordered from least to most nonaxisymmetric. Rings and gaps are marked with solid and dashed ellipses, respectively. Click to enlarge. [Curone et al. 2025]

Seeking Structure

A team headed by Pietro Curone (University of Milan; University of Chile) examined the rich ALMA dataset for signs of structure within the disks. The team modeled each disk with an axisymmetric model, then subtracted off the best-fitting model to reveal substructures and asymmetries in the disks. Several features were revealed, including shadows, offsets between inner and outer disks, spiral structures, and crescent-shaped asymmetries. Though one disk, PDS 66, was apparently without any internal structure, this investigation showed that structures are common in this sample of large, bright disks. Further work is needed, however, to explore whether these features are common in the disk population as a whole.

Maria Galloway-Sprietsma (University of Florida) and collaborators identified the disk surfaces that emit particular spectral lines. The team found that 12CO traced the upper atmosphere of the disks, while 13CO — an isotopologue of the more common molecule 12CO — and CS (carbon monosulfide) probed deeper regions. In addition, nearly all of the disks showed evidence of localized substructure, highlighting the need for theoretical work to explain the wide variety of disk structures and behaviors.

Evidence for Planets

exoALMA images of disks with velocity kinks due to the presence of a planet

From top to bottom: velocity kinks associated with an embedded planet; 12CO observations with the identified velocity kinks; filtered ALMA observations, showing the location of the planetary wake; exoALMA continuum emission; and polarimetric images. The blue circles show the locations of the purported planets. Click to enlarge. [Pinte et al. 2025]

Christophe Pinte (University Grenoble Alpes; Monash University) and collaborators analyzed substructures revealed by 12CO emission. These images revealed arcs, spiral arms, and kinks in 13 out of the 15 disks in the sample. Of these, six were consistent with wakes forming due to planets with masses between 1 and 5 Jupiter masses orbiting at 80–310 au.

Charles Gardner (Rice University; Los Alamos National Laboratory) and coauthors focused on LkCa 15, a disk that appears markedly different in dust and gas emission. The disk’s dust continuum emission shows a 40-au-wide region depleted of dust, while the gas emission, traced by CO, shows no depletion of gas in the same region. Though previous infrared observations have suggested that this dust-depleted region could host massive planets, Gardner’s team concluded that a chain of small planets or processes unrelated to planet formation likely excavated the region instead.

A team led by Jochen Stadler (Côte d’Azur University) searched for deviations from Keplerian rotational velocity — potential evidence for embedded protoplanets. The team discovered deviations up to 15% from the background velocity, with deviations appearing at both large and small radial scales. Rings and gaps visible in the dust emission tended to align with gas pressure maxima and minima, respectively. However, the team also found gas pressure structures in the outer disk that were beyond the dust emission and were not accompanied by rings or gaps.

Comparison of kinematic and emission-line-derived masses for the exoALMA sample

Comparison of kinematic and emission-line-derived masses. Click to enlarge. [Trapman et al. 2025]

Masses and Abundances

A team led by Cristiano Longarini (University of Cambridge; University of Milan) put the disks of the exoALMA sample on the scale. By modeling the rotation curves of 12CO and 13CO, the team constrained the masses of 10 disks and their central stars. Leon Trapman (University of Wisconsin-Madison) and collaborators also considered the problem of disk masses. The team used CO and N2H+ emission to measure the gas masses of 11 disks in the exoALMA sample and compared the results to previous kinematic measurements. The two sets of measurements tend to agree within a factor of three.

Giovanni Rosotti (University of Milan) and collaborators introduced a new model that connects the emitting height of CO molecules to the gas surface density and temperature. Unlike existing models, the team’s new model can be applied to optically thick observations. Comparing the results of this new method with dynamical estimates relying on interstellar medium abundances of CO yields lower gas masses, suggesting that the CO abundance in the protoplanetary disks studied is depleted relative to the interstellar medium.

Turbulence, Traps, and Vortices

Protoplanetary disks are expected to experience turbulence, large-scale gas motions, and instabilities. Marcelo Barraza-Alfaro (Massachusetts Institute of Technology) and collaborators investigated whether a program like exoALMA could detect signs of these processes at work. The team used 3D numerical simulations to explore the observational signatures of the vertical shear instability, the magnetorotational instability, and the gravitational instability. They found that rings, arcs, and spirals can arise from instabilities. While spirals present in the disks in the exoALMA sample could feasibly be due to the magnetorotational instability or the gravitational instability, ring-like and arc-like features due to the vertical shear instability were not detected.

Tomohiro Yoshida (National Astronomical Observatory of Japan; The Graduate University for Advanced Studies) and collaborators reported on their detection of pressure-broadened emission-line wings in RX J1604.3−2130 A. This detection allowed the team to constrain the disk’s gas surface density — a critical property related to the mass available to form planets. They also clearly showed a dust ring coinciding with a gas pressure maximum, proving that gas pressure maxima can create dust traps. The gas-to-dust surface density ratio at the location of this dust trap suggests that the disk has already birthed protoplanets or the dust trapping efficiency is low.

dust crescents in four protoplanetary disks

Locations of crescent-shaped dust clumps in the four disks from this study. Click to enlarge. [Adapted from Wölfer et al. 2025]

Vortices are theorized to shepherd dust grains into crescent-shaped traps, providing an environment for dust grains to clump together and grow into planetesimals and eventually planets. Lisa Wölfer (Massachusetts Institute of Technology) and collaborators searched for kinematic signatures of dust-trapping vortices in CO line emission from four disks that exhibit crescent-shaped concentrations of dust. None of the four disks exhibited the clear signature of a vortex, and higher-resolution data or observations of emission lines that trace motions closer to the midplane of the disk may be necessary to probe this signal further.

The full list of articles in this Focus Issue can be found here.

Citation

“exoALMA. I. Science Goals, Project Design, and Data Products,” Richard Teague et al 2025 ApJL 984 L6. doi:10.3847/2041-8213/adc43b

Illustration of a white dwarf accreting gas from a companion star, a scenario that could lead to the white dwarf's accretion-induced collapse

Do neutron stars form solely through core-collapse supernovae, or is there another way? New research explores theorized processes in which a white dwarf shrinks down to become a neutron star.

Alternate Pathways

illustration of a magnetar, a type of magnetized neutron star that may form through accretion-induced collapse

Accretion-induced collapse and merger-induced collapse are potential formation pathways for magnetized neutron stars called magnetars, illustrated here. [NASA/Swift/Sonoma State University/A. Simonnet]

Neutron stars are known to form when high-mass stars explode in core-collapse supernovae. Theory suggests these stars can also form through accretion-induced collapse and merger-induced collapse. These scenarios involve a white dwarf, the remnant core of a low- to intermediate-mass star. If a white dwarf becomes too massive to support itself, either by accreting gas from a companion or colliding with another white dwarf, it could, under certain conditions, collapse into a neutron star.

So far, researchers have yet to definitively associate any transient signal with the creation of a neutron star from a collapsing white dwarf, though there are several candidates. To advance the search for these signals, researchers must understand the details of the collapse and predict its electromagnetic signature.

Collapsing Further

Eirini Batziou (Max Planck Institute for Astrophysics and Technical University of Munich) and collaborators investigated the burst of nucleosynthesis that might follow a white dwarf’s collapse, potentially powering an electromagnetic transient. The team performed two-dimensional hydrodynamic simulations of six white dwarfs on the cusp of collapsing into neutron stars. Each simulation run varied the mass, central density, rotation rate, and angular momentum profile of the white dwarf. Two of the modeled white dwarfs had masses very close to the Chandrasekhar limit, the mass above which a white dwarf can hypothetically no longer support itself through electron degeneracy pressure. The remaining synthetic stars had higher masses that were bolstered by rapid rotation.

Each of the modeled white dwarfs began to collapse when the crushing pressure of gravity forced the star’s sea of electrons to invade atomic nuclei, removing the support of electron degeneracy pressure. The white dwarfs’ cores shrank and condensed into protoneutron stars, while their outer layers fell inward and rebounded.

simulation results for a white dwarf undergoing accretion-induced collapse

Simulation snapshots showing the evolution of ejected material over time. The top row shows a non-rotating white dwarf and the bottom row shows a rapidly rotating white dwarf. The left side of each panel shows the mass density and the right side shows the radial velocity. Note that the times and the values represented by the color bars are different between the two rows. Click to enlarge. [Adapted from Batziou et al. 2025]

Setting the Stage for Element Creation

The simulations show that rotating and non-rotating white dwarfs have different outcomes when they collapse. Non-rotating white dwarfs cast off material in all directions, while rapidly rotating white dwarfs tend to lose material through wide outflows at their poles. The equator of the rapidly rotating white dwarf is ringed by a torus of material that feeds the neutron star as it forms.

These divergent outcomes affect the amount of ejected material and the production of neutrinos. Ultimately, these differences result in opposite behavior: in the non-rotating case, the ejected material is initially rich in neutrons before transitioning to a proton-rich outflow, while the rotating case starts out with proton-rich ejecta that transitions to a neutron-rich outflow. This suggests that elements created through the rapid capture of neutrons, like gold, can form when white dwarfs collapse into neutron stars, potentially powering an observable electromagnetic signal.

These simulations by Batziou and coauthors are the first to extend substantially beyond the moment the outer layers of the collapsing white dwarf bounce off the central protoneutron star, illuminating the ejection of material, production of neutrinos, and nucleosynthesis. Future work will dive deeper into the details of element creation, predict the luminosity of these events, and explore the role of magnetic fields.

Citation

“Nucleosynthesis Conditions in Outflows of White Dwarfs Collapsing to Neutron Stars,” Eirini Batziou et al 2025 ApJ 984 197. doi:10.3847/1538-4357/adc300

disk of hot gas swirling around a black hole

When JWST peered back into the early universe, its keen eyes revealed an unexpected population of galaxies nicknamed “little red dots” for their compact size and bright red hue. A new research article proposes that tidal disruption events could be responsible for the surprising appearance of these galaxies.

Galactic Surprises

galaxies known as little red dots

Six “little red dot” galaxies seen by JWST. [NASA, ESA, CSA, STScI, Dale Kocevski (Colby College)]

Using JWST, researchers have discovered more than 300 little red dots. Most of these galaxies are concentrated around when the universe was 600 million years old, though they’ve been spotted out to about 1.6 billion years after the Big Bang. But why do these galaxies look the way they do? Do they host enormous, active black holes, as many researchers suspect? Do they get their characteristic red color from a population of old, evolved stars? Or could rapid, dust-shrouded star formation be the cause?

A successful theory of little red dot identity must account for several observed properties: “V”-shaped spectra, broad H-alpha lines, and little or no emission at X-ray wavelengths. In a recent research article, Jillian Bellovary (Queensborough Community College; CUNY Graduate Center; American Museum of Natural History) has theorized that little red dots get their curious characteristics from stars being torn apart by a young and growing black hole.

A New Hypothesis

In this scenario, little red dots arise in the extremely dense star clusters created in the early universe. Because of their extraordinary density, these star clusters are vulnerable to gravitational collapse, causing the packed-together stars to crash into one another. The colliding stars combine to form a supermassive star that collapses, leaving behind an intermediate-mass black hole — a seed that could someday grow into a supermassive black hole.

Situated at the center of a dense star cluster, an intermediate-mass black hole would be poised to ensnare stars in its gravitational web, pulling the stars apart with its powerful tidal forces and stealing their mass. These star-shredding events are known as tidal disruption events.

predicted number density of intermediate-mass black holes and observed density of little red dots

Predicted number density of intermediate-mass black holes (blue and orange lines). The symbols show the observed number densities of little red dots as a function of redshift. [Bellovary 2025]

Simulations predict that in the era during which little red dots have been observed, a cubic megaparsec should contain about 0.3–1 intermediate-mass black holes. (These intermediate-mass black holes could arise through the cluster collapse scenario outlined above, or through other pathways like the collapse of the first stars in the universe.) Observations show roughly 10,000 fewer little red dots in this same volume.

Because tidal disruption events tend to remain bright for about a year, this implies that a tidal disruption event rate of one per 10,000 years is necessary for tidal disruption events to explain little red dots.

Matching Predictions

predicted tidal disruption event rates

Modeled tidal disruption event (TDE) rates for two different models and various values of velocity dispersion, σ, and stellar density, nstar. [Adapted from Bellovary 2025]

Using analytic and numerical models, Bellovary showed that this tidal disruption event rate is reasonable for intermediate-mass black holes with masses between 1,000 and 100,000 solar masses. But would tidal disruption events provide a match for other little red dot characteristics? Tidal disruption events tend to be faint at X-ray wavelengths and sport broad H-alpha lines, much like little red dots. Another plus is that unlike in the active black hole scenario, the H-alpha emission from a tidal disruption event doesn’t scale with the mass of the black hole; this means that black holes categorized as over-massive based on the strength of their H-alpha emission may not be over-massive after all. One drawback of the tidal disruption event scenario is that it doesn’t necessarily predict the characteristic red color of a little red dot; this color might come from the galaxy’s stars instead.

As Bellovary notes, little red dots could have multiple causes. Some might be due to tidal disruption events, while others could contain active black holes, ancient populations of red stars, or dusty starburst galaxies, as others have suggested.

To verify the tidal disruption event hypothesis, astronomers would need to find evidence for time variability at rest-frame ultraviolet wavelengths, as well as a characteristic decrease in brightness over time. These changes are expected to unfold over the course of years or decades, meaning that while observations may settle this question in due time, there will be plenty of time to ponder the mysteries of little red dots while we wait.

Citation

“Little Red Dots Are Tidal Disruption Events in Runaway-Collapsing Clusters,” Jillian Bellovary 2025 ApJL 984 L55. doi:10.3847/2041-8213/adce6c

globular cluster

JWST observations have begun uncovering some of the oldest star clusters in the universe, but their formation mechanisms are still uncertain. A recent study uses high-resolution simulations to understand early star formation and how local globular clusters may have originated. 

Supersonically Induced Gas Objects

Our leading cosmological model of the universe suggests that stars and galaxies coalesce within dark matter halos, eventually forming the larger structures we see today. However, some of the oldest astronomical objects in the universe — massive collections of stars called globular clusters — contain little to no dark matter, making their formation mechanisms somewhat mysterious. How do large clusters with thousands of stars form outside of dark matter halos?

supersonically induced gas objects

Visualizations of the gas density in the simulated star-forming regions in this study. Click to enlarge. [Lake et al 2025]

Recent studies have suggested that as the dark matter and baryons (i.e., normal matter) in the early universe began to clump together and form slightly denser areas, the pristine gas and dark matter began to move relative to one another. Baryons, still tightly coupled to photons, did not have the same freedom to collect in dense regions that the dark matter did, creating supersonic (about five times the speed of sound) relative motions that separated gas clumps and early dark matter halos. These separate, dark-matter-free gas clumps formed baryon-enriched structures known as supersonically induced gas objects.

Over the past decade, simulations and theory have shown that supersonically induced gas objects can form star clusters, which may be the origin story of many of the local globular clusters seen today. However, these simulations have been limited by their resolution. Unable to resolve individual star formation and not including key aspects like feedback, these simulations have left open questions about these dark-matter-free objects.  

High-Resolution Simulations

Seeking to more deeply understand supersonically induced gas objects, William Lake (University of California, Los Angeles; Dartmouth College) and collaborators perform high-resolution simulations that include detailed physics and can track the formation of individual stars. The simulations varied the metallicity and presence of protostellar jets, properties that are both known to impact the star formation process in gas clouds.

imf

Mass distribution for the stars formed in the simulated supersonically induced gas objects. The simulated clusters form more high-mass stars than what is seen in local star-forming regions. Click to enlarge. [Lake et al 2025]

From the simulations, the authors find that star formation occurs naturally in these early systems. Both feedback from jets and the metallicity of the gas affect star-formation outcomes — jets tend to have a minor impact on the star formation efficiency in low-metallicity clouds, but disruption from the jets can cause low-mass stars to form in metal-enriched systems. However, in general, the authors conclude that supersonically induced gas clouds tend to form higher-mass stars than local star clusters, which agrees with what is expected for the universe’s first stellar population. 

Given that these objects likely form very high-mass stars, these primordial star clusters will have extremely high stellar mass surface densities and brightnesses compared to what is observed locally. With such enhanced brightness, these objects may be bright enough for JWST to observe them. Further high-redshift observations of star clusters will allow for further constraints on the formation of the universe’s first stars and will yield more information about the possible origins of the globular clusters we see in the local universe.

Citation

“The Stellar Initial Mass Function of Early Dark Matter–Free Gas Objects,” William Lake et al 2025 ApJL 985 L6. doi:10.3847/2041-8213/add347

protoplanetary disk HH 30

New JWST observations provide a detailed look at the jets of four young stars, revealing shocks, mass loss, and wiggly behavior that hints at a hidden binary companion.

Taurus star-forming region

A portion of the Taurus star-forming region. [ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin; CC BY 4.0]

From Cloud to Star to Planetary System

The transformation from a turbulent cloud of hydrogen gas to a star circled by planets is complicated. As stars coalesce from their natal clouds, they gather gas from their surroundings and flatten it into a dense, dusty disk. While feeding on the gas from this disk, young stars launch powerful, narrow jets and broad, slower-moving winds. As accretion slows, planets begin to form, getting their start from clumps of dust grains.

In a recent research article, JWST observations give insight into the details of this process, illuminating the winds and jets of the disks surrounding young stars.

JWST’s View

Naman Bajaj (Lunar and Planetary Laboratory, The University of Arizona) and collaborators investigated four protoplanetary disks with JWST’s Near Infrared Spectrograph (NIRSpec). The four disks — Tau 042021, HH 30, FS Tau B, and IRAS 04302 — reside in the Taurus star-forming region, which is 1–2 million years old and roughly 450 light-years away.

JWST images of four protoplanetary disks

JWST images of the four disks as seen in a selection of emission lines. Green contour lines show the location of continuum emission. [Adapted from Bajaj et al. 2025]

Each of these disks displays narrow jets that emerge perpendicularly above and below the disk, nested within broad, cone-shaped winds. The disks were selected for their edge-on appearance, which highlights the jets and winds that emerge from the disk.

Bajaj’s team identified more than 40 emission lines for each disk, allowing them to determine the properties of the jets, such as the density and shock speed. One important aspect that can be gleaned from these observations is an estimate of the mass carried away by the jets. Using three independent methods, the team found that jet mass-loss rates for the four disks was on average a billionth of a solar mass per year.

The Wiggly Jet of Tau 042021

Observed locations of the center of the redshifted and blueshifted jets (circles) as well as a fit to a binary orbit model (green dashed line). [Bajaj et al. 2025]

Though appearing to jut straight out from the disk, each of the jets studied showed signs of side-to-side wiggles. Tau 042021’s jets are especially interesting, displaying mirror-symmetric wiggling, in which the redshifted and blueshifted wiggles mirror one another. At present, the only explanation for these synchronized wiggles is a binary companion. By modeling the jet wiggles as emanating from a star in a binary system, the authors concluded that Tau 042021 likely contains a 0.33-solar-mass star and a 0.07-solar-mass star in a 2.5-year orbit with a separation of 1.35 au.

Bajaj and coauthors presented a rich dataset that illuminates the behavior of jets from young stars, and their work isn’t yet done; this is the second research article the team has produced from these data, and more are in the works.

Citation

“Class I/II Jets with JWST: Mass-Loss Rates, Asymmetries, and Binary-Induced Wigglings,” Naman S. Bajaj et al 2025 AJ 169 296. doi:10.3847/1538-3881/adc73c

Artistic representation of the merger of a neutron star and a black hole

When a black hole consumes a neutron star, it’s typically thought to do so without an electromagnetic belch. New research explores the conditions under which a black hole’s neutron-star feast produces an observable electromagnetic signal.

Signaling a Collision

In the past few years, gravitational-wave detectors across the globe have detected a handful of neutron star–black hole mergers. What sorts of electromagnetic signals might accompany these outbursts of gravitational waves is a matter of intense interest.

In rare cases, if the black hole and neutron star are relatively close in mass, their gravitational tussle will rip the neutron star apart. As the shredded stellar material collects in a searingly hot disk around the black hole, it is expected to power a bright transient like a kilonova or a gamma-ray burst.

More commonly, the black hole is much more massive than the neutron star. In this case, the black hole should swallow the neutron star whole and without an electromagnetic trace — unless, as recent research shows, a strong magnetic field surrounds the neutron star.

Detailed Simulations

simulated merger of a neutron star and a black hole

The simulated merger of a neutron star and a black hole, in which the neutron star is swallowed whole. Click to enlarge. [Kim et al. 2025]

Yoonsoo Kim (California Institute of Technology) and collaborators used general relativistic magnetohydrodynamic simulations to explore the electromagnetic signals that might accompany the collision of a black hole and a magnetized neutron star. The simulations follow an 8.0-solar-mass black hole as it merges with a 1.4-solar-mass neutron star with a magnetic field of roughly 1016 Gauss — more than 10 quadrillion times the strength of Earth’s magnetic field.

As the simulated neutron star plunges toward the black hole, the neutron star’s magnetosphere — the region of space in which particles bend to the will of the neutron star’s magnetic field — begins to ripple with waves. The waves expand outward at nearly the speed of light, launching an exceptionally massive shock. This monster shock drives the creation of an electromagnetic transient: a flash of radio waves called a fast radio burst.

plot showing the striped, pulsar-like wind structure

The “striped” pulsar-like wind, shown at 7 milliseconds post-merger. The color scale shows the toroidal magnetic field component. [Adapted from Kim et al. 2025]

Striped Winds and a Fleeting Pulsar Phase

A second type of transient arises after the neutron star merges with the black hole. After the neutron star vanishes within the black hole’s waiting maw, its leftover magnetic field rearranges and begins to rotate, dragged along by the spinning black hole. Briefly, the black hole enters a pulsar-like state, surrounded by a “striped” wind created by spiraling magnetic field lines in alternating directions. As this magnetic field dissipates, a “fireball” of magnetized electron–positron plasma is ejected. When these electron–positron pairs annihilate, they briefly release another electromagnetic signal: a burst of X-rays and gamma rays.

Kim and coauthors noted that their simulated neutron star–black hole pair is similar in mass to the colliding pairs spotted by gravitational-wave detectors. This suggests that detectable gravitational-wave events may be accompanied by brief bursts at radio, X-ray, and gamma-ray wavelengths, providing another avenue to learn about these cosmic collisions.

Citation

“Black Hole Pulsars and Monster Shocks as Outcomes of Black Hole–Neutron Star Mergers,” Yoonsoo Kim et al 2025 ApJL 982 L54. doi:10.3847/2041-8213/adbff9

white dwarf WD 1856+534

Using JWST, researchers have spotted a freezing cold 5.2-Jupiter-mass exoplanet orbiting an old white dwarf at a distance of just 0.02 au. With a temperature of 186K (−125℉/−87℃), this is the coldest exoplanet whose light has been directly detected.

The Fate of Sun-Like Stars

Low- to intermediate-mass stars eventually evolve into red giants and then white dwarfs: crystallized, superheated stellar cores that slowly cool and fade over millennia. What happens to the planets around stars that evolve into white dwarfs is an open question, one that can be answered by detecting and characterizing the planets that remain in these systems.

Planets orbiting at radii beyond 2 au are expected to weather their host star’s transition to a red giant, and dedicated searches for white-dwarf exoplanets have revealed a small number of planets at this safe distance. Observations have also begun to hint at planets orbiting white dwarfs more closely — within the “forbidden zone” thought to be scoured out by the host star’s transformation into a red giant — and researchers have now confirmed the presence of a planet eking out an existence extremely close to its white dwarf host.

Companion Detection

plot of infrared excess for WD 1856+534

Excess infrared emission of WD 1856+534b and WD 0310-688b, a planet candidate. The blue lines show the white dwarfs from the MIRI Exoplanets Orbiting WDs (MEOW) survey with no infrared excess. [Limbach et al. 2025]

The white dwarf WD 1856+534 is located roughly 82 light-years away. In 2020, researchers using data from the Transiting Exoplanet Survey Satellite and a handful of ground-based telescopes detected a Jupiter-sized object orbiting WD 1856+534 every 1.4 days at a distance of just 0.02 au — nearly 30 times closer than Mercury is to the Sun.

The original observations couldn’t discern whether the object, cataloged as WD 1856+534b, was a massive exoplanet or a low-mass brown dwarf. Now, a team led by Mary Anne Limbach (University of Michigan) has performed follow-up observations of the system. Using JWST’s Mid-Infrared Instrument (MIRI), Limbach’s team detected WD 1856+534b by subtracting a detailed model of the white dwarf’s flux from the observed flux. The object’s faint thermal glow was detected with an overall statistical significance of 5.7 sigma.

Temperature and radius of WD 1856+534b compared to other exoplanets and solar system planets

Temperature and radius of WD 1856+534b (yellow star) compared to several known exoplanets, solar system planets, and free-floating planets (FFPs). Click to enlarge. [Limbach et al. 2025]

Brown Dwarf or Planet?

By modeling this thermal emission, Limbach and collaborators definitively showed that WD 1856+534b is a planet. Its mass is likely around 5.2 Jupiter masses, though masses between 0.84 and 5.9 Jupiter masses are possible. With an exceedingly chilly temperature of just 186K — only 60K warmer than Jupiter — WD 1856+534b is the coldest exoplanet whose emission has been directly detected.

Because WD 1856+534b couldn’t have survived its host star’s transformation into a red giant at its current position, it must have migrated inward from a more distant orbit. The cause of this migration isn’t yet clear, though common-envelope evolution or gravitational nudges from another planet or star may have played a role. Upcoming JWST observations that probe WD 1856+534b’s atmosphere and search for other planets in the system could provide answers.

Citation

“Thermal Emission and Confirmation of the Frigid White Dwarf Exoplanet WD 1856+534 b,” Mary Anne Limbach et al 2025 ApJL 984 L28. doi:10.3847/2041-8213/adc9ad

Jupiter

Exoplanet transit observations are now sensitive enough to detect when a planet is flattened slightly by its rotation. Using a new code tailor-made for the purpose, researchers have placed constraints on the rotation period of a cold, puffy exoplanet.

(Don’t) Assume a Spherical Planet

Though countless astronomy problem sets have asked students to consider planets to be spheres, not all planets are so perfect. Rapid rotation turns gaseous planets into slightly squished, or oblate, spheroids. Measurements of this subtle departure from a perfect sphere have the potential to reveal the rotation rates of distant exoplanets and the amount of angular momentum they carry, helping to disentangle how these planetary systems formed.

However, measuring this subtle signal is hard, and so far oblateness has been measured for only a handful of planets. Transiting planets can reveal themselves to be slightly squished only when crossing in front of the edge of their host star, requiring sensitive, high-time-cadence observations — and the right software to analyze the signal.

greenlantern Lights the Way

comparison of flux during the transit of a spherical planet and an oblate planet

Examples of how the flux during a planetary transit differs between an oblate planet model and a spherical planet model. For all of the examples plotted, the oblate planet and its comparison spherical planet cover the same area of their host star mid-transit. The line color signifies the impact parameter. [Price et al. 2025]

Recently, Ellen Price (University of Chicago) and collaborators introduced new software that models the light curves of oblate planets transiting their host stars. The software, named greenlantern, isn’t the first or only code to tackle the problem of modeling transits of oblate planets. greenlantern‘s advantage comes in its efficiency, which minimizes its computational cost, and its public availability. (You can check it out here!)

After walking through the technical aspects of the model, Price’s team introduced their test subject, the sub-Neptune exoplanet HIP 41378f. HIP 41378f is notable for being the outermost of five known planets in its star system, for having an extremely low bulk density of just 0.09 gram per cubic centimeter, and for having a long, chilly, 542-day orbit around its host star.

Price’s team used their code to fit Kepler space telescope data of HIP 41378f and constrain the planet’s oblateness and, by extension, its rotation rate. They determined that the planet’s rotation period is at minimum 15.3 hours, making it a slower rotator than either Jupiter (10 hours) or Saturn (10.5 hours).

joint probability distribution of flattening and projected obliquity

Joint probability distribution for the flattening and projected obliquity (axial tilt) of HIP 41378f. [Price et al. 2025]

Rotating Through the Possibilities

In addition to constraining the rotation rate, this work also showed that current observations of HIP 41378f are consistent with a broad range of axial tilts. What does this mean for how HIP 41378f formed? Given the planet’s mass — 12 Earth masses, give or take a few — a large axial tilt could arise in a variety of ways, including gravitational interactions between the planets in the system. However, with HIP 41378f orbiting quite far from its siblings, it’s not clear why it would be so tilted, and more data are needed to pin down HIP 41378f’s axial tilt.

Finally, Price’s team interpreted the results in the context of HIP 41378f’s possible ring system. Given the planet’s extremely low bulk density, it’s possible that a system of rings has artificially inflated the planet’s apparent radius. In this case, the measured oblateness would reflect the influence of the ring system and could not be used to constrain the planet’s rotation rate.

Citation

“A Long Spin Period for a Sub-Neptune-Mass Exoplanet,” Ellen M. Price et al 2025 ApJL 981 L7. doi:10.3847/2041-8213/adb42b

hot jupie

Understanding exoplanetary system architectures is crucial to uncovering how planetary systems form and evolve. A recent study homes in on the XO-3 system to explore how planets orbit their host stars and what that may mean for their formation pathways.

Alignment Trends

Observations of exoplanetary systems have unveiled multiple interesting properties and characteristics that have expanded our understanding of planet formation beyond our solar system. One such property is the spin–orbit alignment — the direction a planet orbits its host star with respect to the star’s spin. In our solar system, the planets orbit in the same direction the Sun is spinning, but for many exoplanets, this is not the case.

stellar obliquity diagram

Star–planet systems demonstrating spin–orbit alignment and an example of a 40 degree spin–orbit misalignment. Click to enlarge. [AAS Nova/Lexi Gault]

Over the last few years, researchers have noticed a trend in which systems with high planet-to-star mass ratios tend to be more aligned, while planets with smaller mass ratios tend to exhibit larger spin–orbit misalignment. For example, hot Jupiters orbiting small, cool stars tend to remain aligned, while hot Jupiters orbiting massive, hot stars often are misaligned. 

There’s one exception to this trend: planet XO-3b. Orbiting a hot star and within a high planet-to-star mass ratio system, XO-3b was the first exoplanet observed with a high spin–orbit misalignment and has thus been the subject of multiple studies. However, its orbital angle is still uncertain. If this planet is truly an outlier, this would further complicate our understanding of planet formation mechanisms.

Getting to Know XO-3b 

To confirm whether XO-3b is a true outlier and understand why this planet does not bend to the trend, Jace Rusznak (Indiana University) and collaborators acquired new observations of the XO-3 system with NEID, a radial velocity instrument on the WIYN 3.5-meter telescope. By observing how the star’s radial velocity changes as the planet passes in front of the star, they can precisely determine the spin–orbit alignment of the planet as well as measure the properties of the XO-3 system.

Combining the new radial velocity measurements with previous Transiting Exoplanet Survey Satellite (TESS) data for XO-3b, the authors determine that the planet has a spin–orbit misalignment of 40.2 degrees. To confirm that this planet is truly an outlier, the team constructs a sample of single-star exoplanetary systems using the NASA Exoplanet Archive. They find a statistically significant difference in the alignment of systems with planet-to-star mass ratios above and below 2×10-3. With a high planet-to-star mass ratio of 9×10-3, the XO-3 system is unusually misaligned compared to other systems with similar planet-to-star mass ratios. 

spin orbit misalignment

Spin–orbit angle distribution for cool-star systems (top) and hot-star systems (bottom) as a function of planet-to-star mass ratio. The location of the XO-3 system is labeled, showing its high misalignment compared to other systems of similar planet-to-star mass ratios. Click to enlarge. [Rusznak et al 2025]

Undetected Companion

Now left with the conundrum of how XO-3b wound up misaligned, the authors consider the possibility of an undetected stellar binary companion that could have pulled the planet out of line. Previous observations of XO-3 did not directly observe a physically associated stellar companion, but errors in the Gaia astrometry of the star are atypically high for a single-star system. These errors could be the result of an undetected stellar companion gravitationally interacting with both XO-3 and its planet. 

Further observations of the XO-3 system are required to confirm that the system is actually a stellar binary, but if a second star is found, XO-3b would no longer be an outlier in the spin–orbit alignment versus planet-to-star mass ratio trend. This would increase the significance of the emerging relationship that, for single-star systems, planets with high planet-to-star mass ratios tend to be aligned even around hot stars, suggesting that planets in such systems are born aligned and stay that way.

Citation

“From Misaligned Sub-Saturns to Aligned Brown Dwarfs: The Highest Mp/M* Systems Exhibit Low Obliquities, Even around Hot Stars,” Jace Rusznak et al 2025 ApJL 983 L42. doi:10.3847/2041-8213/adc129

Messier 87 jet

Upgraded interferometers will give researchers a never-before-seen view of the jets of active supermassive black holes. By modeling what might be seen when these instruments come online, researchers have discovered a new way to measure black hole spin.

Black Holes and Relativistic Jets

Closeup of Messier 87's relativistic jet

Closeup of Messier 87’s relativistic jet. [NASA and the Hubble Heritage Team (STScI/AURA)]

Galaxies across the universe harbor supermassive black holes. Determining the properties of these black holes — their masses and spins — is key to understanding the formation and evolution of supermassive black holes and how they shape the evolution of their host galaxies.

Some supermassive black holes produce relativistic particle jets that are thought to be powered by the black hole’s spin. This means that precise observations of black hole jets could provide a potential way to measure the spin of a black hole.

Planned and proposed interferometers will stretch observing baselines to great distances — even into space — to attain the high resolution necessary for this sensitive measurement. Building on the successes of the Event Horizon Telescope, a planet-spanning interferometer that has revealed images of the supermassive black holes at the center of the giant elliptical galaxy Messier 87 and the Milky Way, observatories like the Next-Generation Event Horizon Telescope and the Black Hole Explorer will advance our understanding of supermassive black holes and relativistic jets.

Tracing Rays from Modeled Jets

To learn what might be gleaned from future images of black holes and black hole jets, Zachary Gelles (Princeton University) and collaborators developed a model of a nearly face-on relativistic black hole jet, much like the jet from Messier 87’s black hole. The team’s model incorporates both general relativistic magnetohydrodynamics, which describes a magnetized fluid subjected to the rules of Einstein’s General Theory of Relativity, and force-free electrodynamics, which focuses on the dynamics of the system’s electromagnetic fields.

ray-traced polarized image of a relativistic jet

Ray-traced polarized image of a collimated black hole jet. The white bars show the direction of polarization, while the color scale shows the normalized intensity. Several abrupt changes in the polarization direction as a function of radius are visible. Click to enlarge. [Adapted from Gelles et al. 2025]

With this model in hand, the team used ray tracing — following the paths that photons would take through the modeled jet — to predict how the jet would appear in polarized light. Examining the results for jets with varying degrees of narrowness, or collimation, the team noted that the polarization of the most collimated jet behaved strangely, with the polarization angle changing dramatically with position.

Gelles and coauthors demonstrated that one of these sudden polarization changes happens at the black hole’s light cylinder, or the radius at which the jet becomes relativistic and the magnetic field switches from being mostly poloidal to mostly azimuthal. Because the position of the light cylinder is dependent upon the spin of the black hole, measuring the location of this polarization swing allows for a measurement of the black hole’s spin.

The Promise of Polarization

This method has several potential advantages over other methods. Unlike the current leading method for measuring black hole spin, X-ray spectroscopy, this method applies to low-luminosity active black holes, which are thought to be common throughout the universe. And while the model includes a number of simplifications, the team asserts that incorporating features of more realistic jets, such as asymmetry, is unlikely to change the outcome.

plot of polarization angle as a function of impact parameter

Polarization direction as a function of impact parameter, showing the locations and causes of the abrupt changes in polarization angle. Click to enlarge. [Gelles et al. 2025]

Gelles’s team also showed that the location of the polarization flip for slowly spinning black holes is 10 times farther out than it is for rapidly spinning black holes. What this means in practice is that determining whether a black hole is slowly or rapidly spinning doesn’t require extraordinarily high resolution, just a general sense of where the flip happens.

Looking forward, Gelles’s team plans to continue their simulations, shoring up their predictions until they can be tested when future interferometers come online.

Citation

“Signatures of Black Hole Spin and Plasma Acceleration in Jet Polarimetry,” Z. Gelles et al 2025 ApJ 981 204. doi:10.3847/1538-4357/adb1aa

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