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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

DEM L 190 supernova remnant

New data suggest that the second-brightest star in the constellation Sagittarius is not one star but two. The two stars will someday merge to form a single star with a mass of 12 solar masses, and at just 228 light-years away, the resulting star will be the nearest core-collapse supernova progenitor to Earth.

One Star of Two?

The star Sigma Sagittarii, formally named Nunki by the International Astronomical Union, has been under scrutiny for decades. Various studies published in the last half-century have found Nunki to be either two stars in a tight binary system or a single star, depending on the methods and instruments used.

Paranal Observatory

Three of the Very Large Telescope Interferometer’s 1.8-meter Auxiliary Telescopes, which can be maneuvered into different positions. [Y. Beletsky (LCO)/ESO; CC BY 4.0]

Now, a team led by independent researcher Idel Waisberg has performed an interferometric investigation of the star, aiming to discern its single or binary status. The team used the GRAVITY instrument on the Very Large Telescope Interferometer (VLTI) at Paranal Observatory in Chile.

The VLTI encompasses eight telescopes, including four 8.2-meter telescopes that are fixed in place and four 1.8-meter telescopes that can travel between different stations, changing the resolving power of the interferometer. For this search, Waisberg and collaborators positioned the smaller telescopes so that the greatest distance between them was 130 meters, achieving a resolution of just 3.5 milliarcseconds.

Future Site of a Supernova

The VLTI data clearly show that Nunki is a close binary star system. The two stars have nearly the same mass — 6.5 and 6.3 solar masses — and are separated by just 0.60 au, orbiting one another every 50 days. Stellar isochrone modeling suggests that the stars are roughly 30 million years old.

illustration of a star filling its Roche lobe

Diagram of a binary star system in which one star has filled its Roche lobe. The blue shaded areas indicate the stars, while the black lines outline the two stars’ Roche lobes. [Philip D. Hall; CC BY 4.0]

Though the new data have shown that Nunki is in fact two stars, further analysis showed that this configuration is only temporary. As the two stars evolve, the more massive of the two will be the first to expand into a red giant. In 20 million years, the growing star will spill out of its Roche lobe — the volume in which gas is gravitationally bound to that star — and donate gas to its smaller companion. Unstable mass transfer will eventually lead the stars to merge, forming a single star of about 12 solar masses.

At 12 solar masses, Nunki will be massive enough to undergo core collapse, and at just 228 light-years away, it is the nearest core-collapse supernova progenitor system to Earth. Be sure to mark your calendar for tens of millions of years in the future — such a close supernova promises to be spectacular!

Citation

“Hidden Companions to Intermediate-Mass Stars. XXVI. Uncovering Nunki = Sigma Sagittarii as a 6.5 M⊙ + 6.3 M⊙, 0.60 au Binary,” Idel Waisberg et al 2025 Res. Notes AAS 9 71. doi:10.3847/2515-5172/adc739

Titan sunglint

Earthlike yet alien, Titan is the only solar system body other than Earth known to host persistent liquid on its surface. The glint of sunlight on this surface liquid provides a means to study the complex interplay between Titan’s seas, shorelines, and atmosphere.

When the Sun Hits the Sea as Seen by Cassini, That’s a Mare

specular reflection on Titan

The first image of specular reflection of sunlight off Titan’s hydrocarbon seas, taken on 8 July 2009 by Cassini. [NASA/JPL/University of Arizona/DLR]

In 2009, the Saturn-orbiting Cassini spacecraft spotted the glare of sunlight reflecting off a sea of liquid on the surface of Titan, Saturn’s largest moon. This observation of specular (i.e., “mirror-like”) reflection, or sunglint, confirmed the presence of liquid on Titan’s surface — what researchers now know to be oily lakes and seas of liquid hydrocarbons like methane and ethane.

A recent research article from Michael Heslar and Jason Barnes (University of Idaho) yields new insight into the behavior of Titan’s seas and coastal regions in Punga Mare. Punga Mare, located nearly at the north pole, is the smallest of Titan’s named seas and holds the distinction of being the first place where extraterrestrial waves were definitively identified.

Summering at Punga Mare

In the Cassini observation selected for this study, it’s summertime in the northern hemisphere, with balmy temperatures hovering just above 90K (−298℉/−183℃). Cassini viewed Punga Mare from an altitude of roughly 6,000 kilometers and at an oblique angle of 126 degrees. From this angle, Titan’s landmasses appear dark and the sea appears bright.

diagram of the geometry of specular reflection and sun glitter

The geometry of specular reflection (sunglint) and sun glitter. The incidence and emission angles, i and e, are equal. Click to enlarge. [Heslar et al. 2020]

Several anomalously bright features, collectively called specular features, appear in and near Punga Mare. These features are examples of sunglint and sun glitter. Sunglint occurs when sunlight reflects off a smooth surface as light reflects off a mirror, with the angle of the incoming sunlight equal to the observer’s viewing angle. Sun glitter is a related phenomenon that occurs outside the sunglint zone, where localized changes in the angle of the surface — from waves, for example — reflect sunlight toward the observer.

Coasts, Islands, and Inlets

Punga Mare sunglint and sun glitter

Cassini Visual and Infrared Mapping Spectrometer observation of Punga Mare at wavelengths of 5.0 (red), 2.8 (green), and 2.0 (blue) μm. Click to enlarge. [Adapted from Heslar & Barnes 2025]

Heslar and Barnes identified specular features along the coastline, bordering a chain of islands, out at sea, and in coastal inlets.

The bright areas along the coast appear to overlap both the sea surface and the land surface. While it’s not yet clear exactly what causes this brilliant coastal outline, Heslar and Barnes speculated that it may be evidence for waves along the coastline; rough seas may also be the cause of the sun glitter illuminating the Hawaiki island chain and out at sea.

The specular features associated with inlets may mark where rivers flow into or out of Punga Mare. Rather than wave activity, these bright areas might mark where nitrogen has bubbled up to the surface.

Altogether, this study demonstrated the variety and depth of information that can be gleaned from a single sunglint observation — and the authors closed by noting how effective an orbiter would be for studying ocean behavior on Titan. NASA’s upcoming Dragonfly mission may also be able to capture sunglint as the spacecraft leapfrogs across Titan’s surface.

Citation

“Sea Surface and Hydrological Activity Observed in Titan’s Punga Mare,” Michael F. Heslar and Jason W. Barnes 2025 Planet. Sci. J. 6 74. doi:10.3847/PSJ/adbc9e

illustration of a spider pulsar

In the vast menagerie of cosmic critters, perhaps none is as impressively deadly as the spider pulsar. Spider pulsars are a subset of millisecond pulsars: tiny, extraordinarily fast-spinning remnants of dead massive stars. Like other types of pulsars, spider pulsars emit narrow beams of radio emission that can sweep across our field of view as the pulsar spins, generating the characteristic radio pulses from which pulsars get their name. Locked in a close embrace with a low-mass stellar companion, a spider pulsar roasts its companion with high-energy radiation and strips away the companion’s atmosphere with powerful particle winds. Eventually, like the ill-fated mates of certain spiders, the companion star may be entirely destroyed.

Just as there are many species of spiders, so too does it appear that there are different “species” of spider pulsars. So far, two categories are well established, and a third has started to emerge. These categories are based on the mass of the pulsar’s companion star and the orbital period of the system. Black widows have the lowest-mass companions, weighing in at just 5% of the mass of the Sun, and orbital periods less than one day. Redbacks, named after black-and-red spiders also known as Australian black widows, are similarly closely orbiting but have companions with masses ranging from 0.1 to 1.0 solar mass. Finally, the emerging class of huntsman pulsars, named for a large, long-legged type of spider, have low-mass red giant companions and relatively long orbits of 5–10 days.

Today, the Monthly Roundup will introduce three recent investigations of spider pulsars.

Discovery of a Second Huntsman

In 2016, researchers reported the discovery of a ~2-solar-mass millisecond pulsar in a 5.4-day orbit with a 0.33-solar-mass red giant companion stripped of its outer atmosphere. This pulsar — with its evolved stellar companion and long orbital period — was unlike any other spider pulsar known at the time and is now the prototype of a new class of spider pulsars nicknamed “huntsman” pulsars. A year later, a second system with similar properties was found, though the pulsar was not directly detected and remains a candidate.

Now, a team led by Jay Strader (Michigan State University) has announced the discovery of the second confirmed huntsman: the pulsar PSR J1947–1120. This source initially came to light in gamma-ray observations by the Fermi Gamma-ray Space Telescope. Millisecond pulsars are known to emit gamma rays, and many pulsars are first discovered in gamma rays. Using new and archival observations from the Neil Gehrels Swift Observatory (X-rays), XMM-Newton (X-rays), Gaia (optical), the Zwicky Transient Facility (optical), the Southern Astrophysical Research telescope (optical), and the Green Bank Telescope (radio), Strader’s team followed up on the gamma-ray detection and identified both the pulsar and its companion.

stellar spectrum

A spectrum of the PSR J1947–1120 system. The spectrum is consistent with a cool K-type star. Click to enlarge. [Strader et al. 2025]

While more observations are needed to pin down the precise properties of the system, current data point to a pulsar with a 2.24-millisecond rotation period in a 10.3-day orbit with a 0.25–0.40-solar-mass red giant star. Modeling suggests that the red giant is highly stripped, retaining only 0.06–0.2 solar mass of envelope material.

Strader’s team outlined a potential formation mechanism for huntsman pulsars that involves a red giant companion in the “red bump” phase. During this phase of stellar evolution, red giants temporarily become smaller and less luminous, resulting in a pileup or “bump” in the Hertzsprung–Russell diagram. Accretion onto the pulsar also ceases during this period, matching what is seen in huntsman systems. Through stellar evolution modeling, the team showed that the properties of the two known huntsman pulsars can be attained by systems containing red giant stars in the red bump phase.

Why, then, have so few huntsman pulsars been found compared to other types of spider pulsars? The red bump phase of red giant evolution is brief, cosmically speaking, lasting on the order of tens of millions of years. Other types of spider pulsars arise in setups that are longer lived, on the order of billions of years, making it simply more likely to find a black widow or redback in the field than a huntsman.

Searching for Spiders That Will Consume Their Companions

Spider pulsars often exhibit eclipses: periods during which the pulsar’s radio signal passes through the ablated material of the companion star, causing the signal to drop out. While radio eclipses are a common feature of spider pulsars, not all spider pulsars experience this phenomenon. The inclination of the system and physical size of the companion star may both influence whether a spider pulsar exhibits eclipses.

The occurrence of radio eclipses may also be related to the mass-loss rate of the companion star, with higher mass-loss rates being associated with eclipses. The rate at which spider pulsars ablate their companions is a crucial piece of information for establishing spider pulsars as the missing link in the creation of solo millisecond pulsars.

Typically, millisecond pulsars like spider pulsars are thought to arise in binary systems. Accretion onto the pulsar from the binary companion spins up the pulsar, helping it achieve its extreme speed. While this mechanism explains the origins of millisecond pulsars in binary systems, it can’t explain solo millisecond pulsars — unless these singletons are spider pulsars that have entirely ablated their companions.

To explore this possibility, astronomers need to study the eclipses and mass-loss rates of a large sample of spider pulsars; so far, none of the known spider pulsars are blowing away their companions quickly enough for their companion stars to disappear within the universe’s current age.

Example of the detected radio eclipses. [Kumari et al. 2025]

In a recent research article, Sangita Kumari (National Centre for Radio Astrophysics, Tata Institute of Fundamental Research) and collaborators used the Giant Metrewave Radio Telescope (GMRT) to search for eclipses in 10 pulsars systems. In all, Kumari’s team observed eclipses for the first time in three systems and constrained the cutoff frequency — crucial to understanding the origin of the eclipses — in a further four systems for which eclipses had previously been observed. Three systems showed no eclipses.

In addition to characterizing the eclipses they detected, the authors calculated the mass-loss rates for two of the pulsar’s companions. The mass-loss rates were too small to fully ablate the companion stars within the age of the universe, as has been the case for other spider pulsars under study. Additionally, the team found no correlation between the pulsar spin-down rates — thought to be related to the mass-loss rate — and the presence of eclipses, suggesting that other factors must play an important role in the creation of eclipses.

A Spider on the Dividing Line

J1908+2105 is a spider pulsar that was discovered in a search for counterparts to unidentified gamma-ray sources. The pulsar rotates every 2.56 milliseconds, and it’s in a 3.51-hour orbit with a companion star with a minimum mass of 0.055 solar mass, placing the pulsar between black widows and redbacks. As one of only a few known pulsars that may sit upon the dividing line between black widows and redbacks, studying J1908+2105 may help researchers understand whether these pulsars are evolving from one class to the other.

average radio pulses

Average radio pulses at three different frequencies, showing the pulse behavior when not eclipsed. [Ghosh et al. 2025]

Ankita Ghosh (National Centre For Radio Astrophysics, Tata Institute of Fundamental Research) and collaborators used the Giant Metrewave Radio Telescope (GMRT) and the Parkes radio telescope to study J1908+2105’s radio eclipses. The observations covered several frequency bands, allowing the team to search for frequency-dependent changes in the eclipse profile and duration, as well as to determine the magnetic-field properties of the system and the mass-loss rate of the companion.

The team detected J1908+2105’s eclipses at frequencies up to 4 gigahertz, making this one of only a few pulsars with observable eclipses at such high frequency. They explored several possible sources for the eclipses, such as reaching the plasma frequency cutoff — the frequency at which radio waves are unable to travel through a plasma — and determined that synchrotron absorption is the likeliest cause.

Ghosh’s team measured the mass-loss rate from observations made at several frequencies, finding in the highest frequency band a mass-loss rate large enough for the companion to be ablated within 3 billion years. The authors caution that changes in the orbital properties of the system, spurred by the companion’s mass loss, likely mean that the rate will slow over time, and it may not be possible for the companion to be fully evaporated.

Citation

“PSR J1947−1120: A New Huntsman Millisecond Pulsar Binary,” Jay Strader et al 2025 ApJ 980 124. doi:10.3847/1538-4357/ada897

“Unveiling Low-Frequency Eclipses in Spider Millisecond Pulsars Using Wideband GMRT Observations,” Sangita Kumari et al 2025 ApJ 979 143. doi:10.3847/1538-4357/ad93ba

“Exploring Unusual High-Frequency Eclipses in MSP J1908+2105,” Ankita Ghosh et al 2025 ApJ 982 168. doi:10.3847/1538-4357/adb8e0

Tycho crater's peak

While humans have not set foot on the Moon in over 50 years, multiple spacecraft and rovers have surveyed our closest companion in great detail for decades. Since 2009, the Lunar Reconnaissance Orbiter has orbited the Moon, collecting data in order to map the Moon’s surface and identify potential landing sites and resources for future lunar missions. A recent study focuses on the Moon’s north pole, using data from the orbiter to create new highly detailed maps.

LRO

Artist’s impression of the Lunar Reconnaissance Orbiter orbiting above the surface of the Moon. Click to enlarge. [NASA/GSFC]

Navigating the Moon’s Surface

When making the 238,900-mile journey to the surface of the Moon, it’s important to be able to stick the landing. After being pelted with rogue solar system rocks for billions of years, the Moon’s surface is home to thousands of craters, making peaks and valleys that vary significantly depending on where you land. Thus, having a detailed understanding of the Moon’s surface is imperative to choosing where to aim and knowing what can be accomplished once we get there. 

In recent years, researchers have created detailed maps of the Moon’s south pole to identify regions with rough terrain, boulders, steep edges of craters, and flatter areas safer for landings. In addition to mapping geological features, these studies have found candidate water-ice sources that will be targeted in future missions. With continued observations, higher-resolution studies can reveal more detailed information about other regions on the Moon as well.

Mapping the North Pole

Recently, Michael K. Barker (NASA Goddard Space Flight Center) and collaborators used data from the Lunar Orbiter Laser Altimeter (LOLA) on the Lunar Reconnaissance Orbiter to produce high-quality topographical maps of the Moon’s north pole. LOLA uses a laser to measure variations in the Moon’s surface, and from this information, the authors computed new maps of the surface height, slope, and roughness — how bumpy or uneven the terrain is — in the north polar region. The new surface height and slope maps provide information regarding regions of high and low elevation as well as how rapidly those changes in elevation occur. Additionally, the authors created a new high-resolution map of permanently shadowed regions — areas never hit with direct sunlight, keeping any possible traces of water ice cool and intact.

maps

Lunar surface maps of slope (left column) and roughness (right column) across smaller (top) and larger (bottom) spatial scales. Click to enlarge. [Barker et al 2025]

How do these maps of the north pole compare to previously computed maps of the south pole? The authors found that both poles exhibit diverse terrains that have been shaped by impact cratering and smoothed by landslides and slumps. Showing fewer large-scale linear roughness features than the south pole, the north polar region has likely been less affected by the ejecta from recent large impacts. Furthermore, when looking at the relationship between the roughness and the temperature of the Moon’s dusty surface, the authors found that rougher areas tend to exhibit higher temperatures on both poles. The exact reason for this trend is not yet known, but it could be linked to the presence of subsurface icy materials, including water ice. 

Where do these results take us? The differences between the north and south poles motivate further exploration of both locations. Future human and robotic missions will allow for more robust studies of the Moon’s surface, which will test for the relationship between surface features and subsurface ices, explore the geography on much smaller scales, and set the grounds for sustained science on the Moon. 

Citation

“Large-scale Roughness Properties of the Lunar North and South Polar Regions as Measured by the Lunar Orbiter Laser Altimeter (LOLA),” Michael K. Barker et al 2025 Planet. Sci. J. 6 83. doi:10.3847/PSJ/adbc9d

double-faced white dwarf

A recent study brings the number of known double-faced white dwarfs to seven. These rare objects, which feature compositional changes across their surfaces, may arise under the influence of magnetic fields.

The Rise of Two-Faced Stars

Hubble image of the Ring Nebula

When a super-hot white dwarf illuminates the diffuse shells of gas that surround it, we see a glowing planetary nebula. The central white dwarf is visible in this image of the Ring Nebula. [NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration]

At the end of their hydrogen-burning lifetimes, low- to intermediate-mass stars shed their outer layers and bare their cores, evolving into Earth-sized spheres of degenerate electron matter called white dwarfs.

In 2023, researchers reported the discovery of a bizarre white dwarf: one with a surface composed of hydrogen on one side and helium on the other. A handful of other white dwarfs with split or patchy surface compositions have since been discovered, giving rise to the growing class of double-faced white dwarfs.

Many aspects of these strange objects are still unknown, including exactly how they form, how common they are, and what kinds of surface composition distributions are possible. To answer these questions, researchers must find more double-faced white dwarfs — and the hunt is on.

A Closer Look at “Binary” White Dwarfs

Because the composition of a double-faced white dwarf varies across its surface, these objects can present different compositional “faces” as they spin, sometimes resembling one kind of white dwarf and sometimes another. To find white dwarfs with this behavior, Adam Moss (University of Oklahoma) and collaborators investigated six systems earmarked as being possible unresolved binary systems containing one hydrogen-rich white dwarf and one helium-rich white dwarf.

The team suspected that these “binary” systems might instead be double-faced white dwarfs exhibiting features of two types of white dwarfs. One last white dwarf, previously identified as having spectral features from both hydrogen and helium, rounded out the sample.

spectra of a double-faced white dwarf

Spectra of white dwarf J0847+4842 taken at the beginning, middle, and end of the night from Apache Point Observatory. The spectra have been offset vertically for clarity. Vertical blue lines mark the locations of hydrogen lines, and vertical red lines mark the locations of helium lines. [Adapted from Moss et al. 2025]

Analyzing spectra from the Gemini North telescope, Apache Point Observatory, and the MMT Observatory, Moss’s team found spectral variations in two of the seven targets. The five targets with no spectral variations are likely bona fide binary systems, while the two time-varying targets showed clear signs of the surface composition variations that characterize a double-faced white dwarf. The spectra of these two objects were well fit with a model featuring polar caps rich in hydrogen and an equatorial belt rich in helium.

Potential Magnetic Origins

These two newly discovered double-faced white dwarfs bring the number of known objects in this class to seven. What does the current sample tell us about the origins of double-faced white dwarfs?

Four of the seven sample members are known to have magnetic fields, which is a far larger proportion than the general population of white dwarfs. This suggests that magnetic fields play an important role in creating these objects’ patchwork surfaces.

Moss and collaborators favor a scenario in which magnetic fields affect the process of convection, which can alter the elemental makeup of a white dwarf’s surface as it cools. In this scenario, stronger magnetic fields at the magnetic poles suppress convection, while weaker fields near the magnetic equator allow convection to proceed. Convection brings helium-rich material to the star’s surface, creating a helium-rich belt sandwiched between hydrogen-rich polar caps. Future modeling will explore how complex magnetic field configurations affect convection and the composition of white-dwarf surfaces, advancing our understanding of this rare class of objects.

Citation

“The Emerging Class of Double-Faced White Dwarfs,” Adam Moss et al 2025 ApJ 983 14. doi:10.3847/1538-4357/adbd3a

artist's impression of a pulsar

Researchers recently discovered an object with regular radio pulses less frequent than those of a pulsar but more frequent than those of a long-period radio transient. What do current observations tell us about what this object might be?

Between Two Extremes

The sky at radio wavelengths is inhabited by many objects that brighten, fade, and emit pulses. The most rapidly and regularly varying radio emission comes from pulsars: rotating remnant cores of massive stars that emit beams of radio waves along their magnetic poles. As the beamed emission from a pulsar sweeps across our field of view, it creates the characteristic pulses that give pulsars their name.

Pulsars have pulse periods from milliseconds to a few seconds. Over the past few years, astronomers have discovered a small but growing number of objects with radio pulses with periods of minutes to hours. Though these long-period radio transients (LPRTs) share some properties with pulsars, it’s not yet clear what powers them. One path to uncovering their identity is discovering objects in the period gap between them and pulsars — a region typically missed in searches for pulsars or LPRTs.

Serendipitous Discovery

In a recent publication, Yuanming Wang (王远明; Swinburne University of Technology and ARC Centre of Excellence for Gravitational Wave Discovery) and collaborators reported their discovery of an object that lies within this gap, PSR J0311+1402.

The discovery was serendipitous, with three of the object’s pulses appearing in two minutes of test observations made with the Australian Square Kilometre Array Pathfinder (ASKAP)’s newly installed Commensal Realtime ASKAP Fast Transient Coherent (CRACO) system. CRACO is sensitive to pulse periods within the gap between pulsars and LPRTs.

Light curve of PSR J0311+1402

Light curve of PSR J0311+1402 from MeerKAT observations. Click to enlarge. [Wang et al. 2025]

The team followed up on this detection with observations from ASKAP, Murriyang, MeerKAT, and the Green Bank Telescope. These observations paint a coherent picture of an object with 0.5-second-long pulses every 41 seconds, placing it squarely in the gap between pulsars and LPRTs.

Pulsar or Long-Period Radio Transient?

Could PSR J0311+1402 be a pulsar with an unusually long spin period? The shape of PSR J0311+1402’s pulses and the spectral index (related to how the pulses vary as a function of frequency) resemble those of typical pulsars. However, the spin-down rate of this object, measured from the minuscule change in pulse period over time, potentially places PSR J0311+1402 below the “death line.” Objects below the death line are theorized to be unable to produce radio emission through electron–positron pair production.

plot of change in spin period over time versus spin period for various pulsars and LPRTs

Change in spin period over time versus spin period for various pulsars and LPRTs. PSR J0311+1402 is shown as a star outlined in red. The dotted, dashed, and solid black lines indicate the death lines for various magnetic field models. PSR J0311+1402 falls below all death lines except for the extreme case of a twisted multipole configuration. Click to enlarge. [Wang et al. 2025]

Is PSR J0311+1402 more likely to be an LPRT with a surprisingly short period? Isolated LPRTs have strongly polarized radio emission and are located at low galactic latitudes, while PSR J0311+1402 has weakly polarized radio emission and is located at a moderate galactic latitude. While LPRTs in binary systems have also been found, and have different properties, no companion has been detected for PSR J0311+1402.

Together, these factors suggest that PSR J0311+1402 is more likely to be a slowly rotating pulsar than a short-period LPRT, though further observations to refine the calculation of its spin-down rate are needed. And where there’s one object, there are usually more, suggesting that future searches attuned to periods within the gap between pulsars and LPRTs may find more of these in-between objects and help to illuminate their properties.

Citation

“The Discovery of a 41 s Radio Pulsar PSR J0311+1402 with ASKAP,” Yuanming Wang et al 2025 ApJL 982 L53. doi:10.3847/2041-8213/adbe61

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