The Nancy Grace Roman Space Telescope, set to launch into orbit, aims to explore dark energy and dark matter, potentially addressing the unresolved Hubble Tension. This discrepancy highlights conflicting measurements of the universe's expansion rate. Some scientists believe Roman may confirm flaws in the standard cosmological model, leading to new physics.

Astronomers have been measuring comets for centuries. But in December of last year, researchers collected a unique set of comet observations: they recorded some of the highest-quality data of one of the weirdest known comets while it was far from the Sun.

Simultaneous Spectra and Images

A few months after the famous interstellar comet 3I/ATLAS rounded the Sun and began its journey back into the space between the stars, a group of astronomers led by Nathan Roth (NASA Goddard Space Flight Center) aimed JWST in the direction of its retreat to watch it fade away. Using a special observing mode that leveraged the Integral Field Unit (IFU) in JWST’s Near Infrared Spectrometer, these observations didn’t result in just a single image of the comet, but a full stack of images, each corresponding to a different wavelength of light.

Using the IFU allowed the observers to combine the best of both regular imaging, which reveals spatial structure, and standard spectroscopy, which reveals wavelength dependencies. As a result, they could make spatially resolved maps of how the emission from the comet’s fuzzy atmosphere of gas and dust, or coma, changed with distance from the comet’s icy nucleus and angle to the Sun. Since different molecules radiate at different wavelengths, these maps in turn reveal the composition and geometry of 3I/ATLAS’s coma.

Mapping the Coma

These maps, and the spectrum created by summing all of the spatial pixels together, reveal the comet’s coma in gorgeous detail, including hundreds of individual spectral lines that can be attributed to specific molecules. These maps revealed two unique features.

First, in contrast with normal solar system comets where water is the most abundant molecule, water was actually a secondary player here: there was significantly more carbon monoxide, or CO, than water in 3I/ATLAS’s coma. The researchers note that this might not be representative of the true distribution of the two species, but instead be the result of the timing of these observations. When observing a standard solar system comet, astronomers usually examine their targets when the object in question is within 2 au of the Sun. At these close distances, solar radiation is strong enough that water ice dominates the outgassing and activity in a coma.

However, these IFU measurements were taken when 3I/ATLAS was about 2.4 au from the Sun, or right in the transition region where water doesn’t sublimate as easily as CO or carbon dioxide (CO₂). It’s likely that as the comet approaches the so-called “ice line” where water will naturally freeze out, CO will only increase its domination of the coma.

Every Molecule for Itself

Second, each molecule seemed to be outgassing in its own unique direction from the coma. CO is leaving the nucleus in two different jets, one aimed directly back at the Sun and one perpendicular to it; CO₂ and methane (CH₄) are both leaking away in the anti-Sun direction; and water and methanol (CH₃OH) are sublimating in nearly spherically symmetric shells. The authors speculate that these geometries may be caused by different “active sites” on the comet, and that had they observed the comet for one full rotational period, they might have seen these jets pointing in different directions.

Although 3I/ATLAS is now too far and faint for observations like these, this study reveals just how much we can learn in the brief time interstellar visitors spend in our solar system. When the fourth interstellar comet inevitably wanders through the inner solar system, astronomers will doubtless be ready to use this observing mode again.

Citation

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Exactly how the universe became reionized after cooling to form stars and galaxies remains an open question. A recent study investigates how a merging set of galaxies may have spurred an onslaught of ionizing photons into the intergalactic medium.

Lyman Continuum Leakers and Merging Galaxies

After its chaotic and blistering beginning, the universe cooled and began to coalesce, stars and galaxies flickering on to set the universe alight. From a few hundred million to about a billion years after the Big Bang, galaxies pumped high-energy ionizing Lyman continuum (LyC) photons into the intergalactic medium during what is known as the epoch of reionization. The exact mechanisms that produced and allowed LyC emission to escape galaxies during this era remain elusive.

One promising driver of LyC leakage is galaxy mergers — strong gravitational interactions and interstellar medium mixing can induce bursts of star formation, creating the hot, young stars that emit LyC photons. In particular, galaxy mergers can create tidal tails, streams of wispy gas, that can sustain brief but intense bouts of star formation that quickly use up surrounding gas. With no interstellar gas left to absorb radiation, the fresh stars in these streams can easily pump their high-energy photons into the intergalactic medium, contributing to cosmic reionization.

While observational studies have increasingly pointed to galaxy mergers as considerable drivers of LyC escape, only a small number have explicitly considered the role of mergers in the epoch of reionization due to spectral and spatial resolution limitations. Leveraging the high-resolution capabilities of JWST, Shengzhe Wang (University of Chinese Academy of Sciences; National Astronomical Observatories, Chinese Academy of Sciences) and collaborators performed detailed analyses of two LyC-leaker candidates to better understand the role of galaxy mergers in heating up the universe.

Confirming (and Refuting) Merger-Driven Leaks with JWST

Using candidate LyC leakers first identified in the LymAn Continuum Escape Survey (LACES) carried out with the Hubble Space Telescope, Wang and team obtained high-resolution JWST spectroscopy for two galaxies at redshift z = 3.1 (about 2 billion years after the Big Bang), LACES94460 and LACES104037. After carefully identifying and measuring emission lines in each candidate, the authors confirmed LACES104037 as a merger-driven LyC leaker. However, the apparent LyC emission from LACES94460 was determined to be contamination from a low-redshift object.

Based on the JWST spectroscopy, LACES104037 is an early-stage merger system with two primary interacting galaxies (LACES104037-bulk and LACES104037s) with a tidal-tail structure connecting them (LACES104037-LyC) where the LyC emission originates. Using model predictions based on the JWST spectroscopy and photometry from previous studies, the team found that LACES104037-LyC is a star-forming clump in a tidal tail with a stellar age of about 5 million years and an extreme LyC photon escape fraction of approximately 0.99. This points to merger-induced star formation being an extremely efficient way to dump ionizing photons into the intergalactic medium.

LACES104037-LyC represents the first confirmed LyC leaker of its kind: an external clump of star formation in a tidal tail. This discovery aids in our understanding of how mergers can drive high-energy photon emission, and highlights the critical role galaxy mergers may play in the epoch of reionization. Future high-resolution observations will hopefully reveal more knots of LyC-leaking stars within merging galaxies and provide a deeper understanding of this important evolutionary phase of the universe.

Citation

“Confirmation and Refutation of Lyman Continuum Leakers at z ~ 3 with JWST NIRSpec IFU,” Shengzhe Wang et al 2026 ApJL 1006 L17. doi:10.3847/2041-8213/ae8523

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Ultra-hot Jupiter WASP-121b is once again puzzling astronomers. New JWST observations have revealed aspects of this superheated planet’s atmosphere that models cannot yet explain.

An Extreme Planet

Among the more than 6,000 known exoplanets, there are many types of worlds that aren’t represented in our solar system. The most exotic and unfamiliar planets might be the ultra-hot Jupiters: giant planets nestled so close to their host stars that they are tidally locked, with temperatures on their permanently starlit sides climbing to thousands of degrees.

WASP-121b, which circles its F-dwarf host star every 30.5 hours, is one of the best-studied ultra-hot Jupiters. Previous work has started to reveal the complexity of its atmosphere, and now, data from JWST have opened a new window onto WASP-121b’s behavior.

New View from JWST

To learn more about WASP-121b’s atmosphere, Robert C. Frazier (University of Michigan) and collaborators analyzed JWST Near Infrared Imager and Slitless Spectrograph (NIRISS) observations covering more than one full orbit of the planet. These observations allowed them to construct the system’s phase curve, which describes the total amount of emitted and reflected light from the planet and its host star over an entire orbit.

The team used two 3D general circulation models to predict the planet’s atmospheric structure and extract synthetic spectra and phase curves to compare against the JWST observations. Using nine model configurations in total, the team varied several inputs, including sources of atmospheric opacity, sources of atmospheric drag, and assumptions about whether clouds are able to form on the planet’s cooler nightside.

Left with a Mystery

Frazier and collaborators found two aspects of WASP-121b’s phase curve that their models couldn’t explain: the planet emits less light overall than predicted, and the location of its phase-curve offset changes with wavelength.

The models predict, on average, 12–15% more emission from the planet than observed. This discrepancy might be traced back to how the planet’s radius is determined, which is a simple-sounding task that can be deceptively difficult; WASP-121b isn’t perfectly spherical, and its emitting area changes with wavelength, making it challenging to normalize the planet’s radius for use in models.

The phase-curve offset is the difference between a planet’s sub-stellar point (where the star is directly overhead) and the brightest point in the planet’s atmosphere. In general, some degree of offset is expected because planetary winds tend to whisk heat away from the sub-stellar point, placing the brightest point somewhere to the east (usually). For WASP-121b, the offset is unexpectedly small — suggesting a source of drag in the atmosphere that prevents winds from transporting heat away from the sub-stellar point — and it’s larger at shorter wavelengths. This wavelength dependence isn’t predicted by any of the models, and it’s not clear what causes it.

These results demonstrate something remarkable: that our observations of a planet nearly 900 light-years away have become so precise that they’re pushing our already complex models to include more physics — an exceptionally exciting motivation for model improvement!

Citation

“The Days Drag On on WASP-121 b: Interpreting Its NIRISS Spectroscopic Phase Curve with General Circulation Models,” Robert C. Frazier et al 2026 ApJ 1004 102. doi:10.3847/1538-4357/ae69ca

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In 2010, the supernova SN 2010jl illuminated a corner of a small, irregular galaxy 150 million light-years away. More than a decade later, researchers have found evidence that this collapsing massive star has created a significant amount of cosmic dust.

Superluminous Supernova

From high-redshift galaxies to nearby star-forming regions, it’s clear that the universe is a dusty place — but where does all this dust come from? Core-collapse supernovae are one possible source. These cataclysms fling enriched material into interstellar space, where dust grains coalesce out of the cooling ejecta. This is a promising explanation for dust in high-redshift galaxies, which require more rapid dust production than other sources, like evolved low- to intermediate-mass stars, can provide.

SN 2010jl offers an excellent chance to study the dust-forming potential of core-collapse supernovae. This supernova was unusually luminous because of intense interactions between the expanding supernova shock and the dense material surrounding the exploding star, and it’s possible that these types of interactions spur the creation of dust. In the first few years after the supernova was discovered, researchers found evidence that it had produced new dust. What has it been up to since?

Checking Back In

Thirteen years after SN 2010jl was discovered, Nathan Smith (Steward Observatory) and collaborators checked in on its dust production. The team collected new ground-based optical spectra from the Keck I telescope and the MMT Observatory and mid-infrared spectra from JWST. The optical spectra — among the latest collected for any superluminous supernova — show numerous narrow emission lines that indicate that the expanding supernova continues to interact with its surroundings.

In the JWST observations, SN 2010jl is the brightest mid-infrared point source in its host galaxy. Through spectral energy distribution modeling, Smith and coauthors found that the brilliant infrared emission is likely due to a shroud of warm dust totaling roughly 20% of the mass of the Sun. This is about 80 times more dust than inferred from observations made a decade earlier.

Where’s the Dust?

While these data appear to tell the tale of a prolific dust-producing supernova, interpreting these results isn’t straightforward — at a distance of 150 million light-years, it’s difficult to pin down the exact location of the dust. If the dust formed in the supernova ejecta that has not yet withstood the passage of the reverse shock, that dust might be destroyed when it passes through the shock. It’s also possible that the emission comes from preexisting dust in the interstellar medium that was destroyed by the passage of the supernova shock, then re-formed as the material cooled. If instead the dust formed in the post-shock region (between the outward-moving forward shock and the inward-moving reverse shock), it should survive and become incorporated into the interstellar medium.

While it’s not possible to rule out the other options entirely, Smith and collaborators argue that at least some of the dust can be traced to new growth in the post-shock region. Because post-shock dust grains are likely to reach the interstellar medium, this suggests that superluminous, strongly interacting supernovae like SN 2010jl are a viable source of dust in the present-day universe and in high-redshift galaxies alike.

Citation

“JWST Spectra Indicate a Large Mass of Postshock Dust Formed by SN 2010jl,” Nathan Smith et al 2026 ApJ 1006 224. doi:10.3847/1538-4357/ae74c5

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