The sub-Neptune frontier has opened a new window into the rich diversity of planetary environments beyond the solar system. The possibility of hycean worlds, with planet-wide oceans and H₂-rich atmospheres, significantly expands and accelerates the search for habitable environments elsewhere. Recent JWST transmission spectroscopy of the candidate hycean world K2-18 b in the near-infrared led to the first detections of the carbon-bearing molecules CH₄ and CO₂ in its atmosphere, with a composition consistent with predictions for hycean conditions. The observations also provided a tentative hint of dimethyl sulfide (DMS), a possible biosignature gas, but the inference was of low statistical significance. We report a mid-infrared transmission spectrum of K2-18 b obtained using the JWST MIRI LRS instrument in the ∼6–12 μm range. The spectrum shows distinct features and is inconsistent with a featureless spectrum at 3.4σ significance compared to our canonical model. We find that the spectrum cannot be explained by most molecules predicted for K2-18 b, with the exception of DMS and dimethyl disulfide (DMDS), also a potential biosignature gas. We report new independent evidence for DMS and/or DMDS in the atmosphere at 3σ significance, with high abundance (≳10 ppmv) of at least one of the two molecules. More observations are needed to increase the robustness of the findings and resolve the degeneracy between DMS and DMDS. The results also highlight the need for additional experimental and theoretical work to determine accurate cross sections of important biosignature gases and identify potential abiotic sources. We discuss the implications of the present findings for the possibility of biological activity on K2-18 b.

The search for habitable environments and biomarkers in exoplanetary atmospheres is the holy grail of exoplanet science. The detection of atmospheric signatures of habitable Earth-like exoplanets is challenging owing to their small planet–star size contrast and thin atmospheres with high mean molecular weight. Recently, a new class of habitable exoplanets, called Hycean worlds, has been proposed, defined as temperate ocean-covered worlds with H₂-rich atmospheres. Their large sizes and extended atmospheres, compared to rocky planets of the same mass, make Hycean worlds significantly more accessible to atmospheric spectroscopy with JWST. Here we report a transmission spectrum of the candidate Hycean world K2-18 b, observed with the JWST NIRISS and NIRSpec instruments in the 0.9–5.2 μm range. The spectrum reveals strong detections of methane (CH₄) and carbon dioxide (CO₂) at 5σ and 3σ confidence, respectively, with high volume mixing ratios of ∼1% each in a H₂-rich atmosphere. The abundant CH₄ and CO₂, along with the nondetection of ammonia (NH₃), are consistent with chemical predictions for an ocean under a temperate H₂-rich atmosphere on K2-18 b. The spectrum also suggests potential signs of dimethyl sulfide (DMS), which has been predicted to be an observable biomarker in Hycean worlds, motivating considerations of possible biological activity on the planet. The detection of CH₄resolves the long-standing missing methane problem for temperate exoplanets and the degeneracy in the atmospheric composition of K2-18 b from previous observations. We discuss possible implications of the findings, open questions, and future observations to explore this new regime in the search for life elsewhere.

The origin of heavy elements synthesized through the rapid neutron capture process (r-process) has been an enduring mystery for over half a century. J. Cehula et al. recently showed that magnetar giant flares, among the brightest transients ever observed, can shock heat and eject neutron star crustal material at high velocity, achieving the requisite conditions for an r-process. A. Patel et al. confirmed an r-process in these ejecta using detailed nucleosynthesis calculations. Radioactive decay of the freshly synthesized nuclei releases a forest of gamma-ray lines, Doppler broadened by the high ejecta velocities v ≳ 0.1c into a quasi-continuous spectrum peaking around 1 MeV. Here, we show that the predicted emission properties (light curve, fluence, and spectrum) match a previously unexplained hard gamma-ray signal seen in the aftermath of the famous 2004 December giant flare from the magnetar SGR 1806–20. This MeV emission component, rising to peak around 10 minutes after the initial spike before decaying away over the next few hours, is direct observational evidence for the synthesis of ∼10⁻⁶ M_⊙ of r-process elements. The discovery of magnetar giant flares as confirmed r-process sites, contributing at least ∼1%–10% of the total Galactic abundances, has implications for the Galactic chemical evolution, especially at the earliest epochs probed by low-metallicity stars. It also implicates magnetars as potentially dominant sources of heavy cosmic rays. Characterization of the r-process emission from giant flares by resolving decay line features offers a compelling science case for NASA's forthcoming COSI nuclear spectrometer, as well as next-generation MeV telescope missions.

The James Webb Space Telescope (JWST) recently measured the transmission spectrum of K2-18b, a habitable-zone sub-Neptune exoplanet, detecting CH₄ and CO₂ in its atmosphere. The discovery paper argued the data are best explained by a habitable "Hycean" world, consisting of a relatively thin H₂-dominated atmosphere overlying a liquid water ocean. Here, we use photochemical and climate models to simulate K2-18b as both a Hycean planet and a gas-rich mini-Neptune with no defined surface. We find that a lifeless Hycean world is hard to reconcile with the JWST observations because photochemistry only supports <1 part-per-million CH₄ in such an atmosphere while the data suggest about ∼1% of the gas is present. Sustaining percent-level CH₄ on a Hycean K2-18b may require the presence of a methane-producing biosphere, similar to microbial life on Earth ∼3 billion years ago. On the other hand, we predict that a gas-rich mini-Neptune with 100× solar metallicity should have 4% CH₄ and nearly 0.1% CO₂, which are compatible with the JWST data. The CH₄ and CO₂ are produced thermochemically in the deep atmosphere and mixed upward to the low pressures sensitive to transmission spectroscopy. The model predicts H₂O, NH₃, and CO abundances broadly consistent with the nondetections. Given the additional obstacles to maintaining a stable temperate climate on Hycean worlds due to H₂ escape and potential supercriticality at depth, we favor the mini-Neptune interpretation because of its relative simplicity and because it does not need a biosphere or other unknown source of methane to explain the data.

Among the atmospheric gases that have been proposed as possible biosignatures in exoplanetary atmospheres, organosulfur gases are currently considered one of the more robust indicators of extant life. These gases include dimethyl sulfide (DMS; CH₃SCH₃), carbonyl sulfide (OCS), and carbon disulfide (CS₂), which are predominantly secondary metabolic products of living organisms on Earth. Here we present results that challenge this interpretation and provide constraints on the robustness of organosulfur gases as biosignatures. Through laboratory photochemical experiments, we show the abiotic production of organosulfur gases, including DMS, OCS, methane thiol (CH₃SH), ethane thiol (C₂H₅SH), CS_2, and ethyl methyl sulfide (CH₃CH₂SCH₃) via photochemistry in analog atmospheres. Gas-phase products of H₂S/CH₄/N₂ haze photochemistry, with or without CO₂, were collected and analyzed using gas chromatography equipped with sulfur chemiluminescence detection. Depending on the starting conditions, we estimate that DMS, OCS, CH₃SH, CH₃CH₂SH, CS₂, and CH₃CH₂SCH₃ are produced in mixing ratios >10⁻¹ ppm_v. We further demonstrate that as the mixing ratio of CO₂ increases, so does the relative importance of OCS compared to DMS. Although our results constrain the robustness of common organosulfur gases as biosignatures, the presence of these compounds may serve as an indicator of metabolic potential on exoplanets.

We report the discovery of BD+05 4868 Ab, a transiting exoplanet orbiting a bright (V = 10.16) K-dwarf (TIC 466376085) with a period of 1.27 days. Observations from NASA's Transiting Exoplanet Survey Satellite reveal variable transit depths and asymmetric transit profiles that are characteristic of comet-like tails formed by dusty effluents emanating from a disintegrating planet. Unique to BD+05 4868 Ab is the presence of prominent dust tails in both the trailing and leading directions that contribute to the extinction of starlight from the host star. By fitting the observed transit profile and analytically modeling the drift of dust grains within both dust tails, we infer large grain sizes (∼1–10 μm) and a mass-loss rate of 10 M_⊕ Gyr⁻¹, suggestive of a lunar-mass object with a disintegration timescale of only several Myr. The host star is probably older than the Sun and is accompanied by an M-dwarf companion at a projected physical separation of 130 au. The brightness of the host star, combined with the planet's relatively deep transits (0.8%–2.0%), presents BD+05 4868 Ab as a prime target for compositional studies of rocky exoplanets and investigations into the nature of catastrophically evaporating planets.

Following the discovery of dimethyl sulfide (DMS; CH₃SCH₃) signatures in comet 67P/Churyumov–Gerasimenko, we report the first detection of this organosulfur species in the interstellar medium during the exploration of an ultradeep molecular line survey performed toward the Galactic center molecular cloud G+0.693-0.027 with the Yebes 40 m and IRAM 30 m telescopes. We derive a molecular column density of N = (2.6 ± 0.3) × 10¹³ cm⁻², yielding a fractional abundance relative to H₂ of ∼1.9 × 10⁻¹⁰. This implies that DMS is a factor of ∼1.6 times less abundant than its structural isomer CH₃CH₂SH and ∼30 times less abundant than its O-analog dimethyl ether (CH₃OCH₃) toward this cloud, in excellent agreement with previous results on various O/S pairs. Furthermore, we find a remarkable resemblance between the relative abundance of DMS/CH₃OH in G+0.693-0.027 (∼1.7 × 10⁻³) and in the comet (∼1.3 × 10⁻³). Although the chemistry of DMS beyond Earth has yet to be fully disclosed, this discovery provides conclusive observational evidence on its efficient abiotic production in the interstellar medium, casting doubt on using DMS as a reliable biomarker in exoplanet science.

Exoplanets orbiting M-dwarfs present a valuable opportunity for their detection and atmospheric characterization. This is evident from recent inferences of H₂O in such atmospheres, including that of the habitable-zone exoplanet K2-18b. With a bulk density between Earth and Neptune, K2-18b may be expected to possess a H/He envelope. However, the extent of such an envelope and the thermodynamic conditions of the interior remain unexplored. In the present work, we investigate the atmospheric and interior properties of K2-18b based on its bulk properties and its atmospheric transmission spectrum. We constrain the atmosphere to be H₂-rich with a H₂O volume mixing ratio of 0.02%–14.8%, consistent with previous studies, and find a depletion of CH₄ and NH₃, indicating chemical disequilibrium. We do not conclusively detect clouds/hazes in the observable atmosphere. We use the bulk parameters and retrieved atmospheric properties to constrain the internal structure and thermodynamic conditions in the planet. The constraints on the interior allow multiple scenarios between rocky worlds with massive H/He envelopes and water worlds with thin envelopes. We constrain the mass fraction of the H/He envelope to be ≲6%; spanning ≲10⁻⁵ for a predominantly water world to ∼6% for a pure iron interior. The thermodynamic conditions at the surface of the H₂O layer range from the supercritical to liquid phases, with a range of solutions allowing for habitable conditions on K2-18b. Our results demonstrate that the potential for habitable conditions is not necessarily restricted to Earth-like rocky exoplanets.

When surrounded by a transparent emission region, black holes are expected to reveal a dark shadow caused by gravitational light bending and photon capture at the event horizon. To image and study this phenomenon, we have assembled the Event Horizon Telescope, a global very long baseline interferometry array observing at a wavelength of 1.3 mm. This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center of the giant elliptical galaxy M87. We have resolved the central compact radio source as an asymmetric bright emission ring with a diameter of 42 ± 3 μas, which is circular and encompasses a central depression in brightness with a flux ratio ≳10:1. The emission ring is recovered using different calibration and imaging schemes, with its diameter and width remaining stable over four different observations carried out in different days. Overall, the observed image is consistent with expectations for the shadow of a Kerr black hole as predicted by general relativity. The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole. We compare our images to an extensive library of ray-traced general-relativistic magnetohydrodynamic simulations of black holes and derive a central mass of M = (6.5 ± 0.7) × 10⁹ M_⊙. Our radio-wave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies and as the central engines of active galactic nuclei. They also present a new tool to explore gravity in its most extreme limit and on a mass scale that was so far not accessible.

We have examined the distribution of the position angle (PA) of the Galactic center filaments with lengths L > 66'' and <66'' as well as their length distribution as a function of PA. We find bimodal PA distributions of the filaments, and long and short populations of radio filaments. Our PA study shows the evidence for a distinct population of short filaments with PA close to the Galactic plane. Mainly thermal, short-radio filaments (<66'') have PAs concentrated close to the Galactic plane within 60° < PA < 120°. Remarkably, the short filament PAs are radial with respect to the Galactic center at l < 0° and extend in the direction toward Sgr A*. On a smaller scale, the prominent Sgr E H ii complex G358.7-0.0 provides a vivid example of the nearly radial distribution of short filaments. The bimodal PA distribution suggests a different origin for two distinct filament populations. We argue that the alignment of the short-filament population results from the ram pressure of a degree-scale outflow from Sgr A* that exceeds the internal filament pressure, and aligns them along the Galactic plane. The ram pressure is estimated to be 2 × 10⁶ cm⁻³ K at a distance of 300 pc, requiring biconical mass outflow rate 10⁻⁴M_⊙ yr⁻¹ with an opening angle of ∼40°. This outflow aligns not only the magnetized filaments along the Galactic plane but also accelerates thermal material associated with embedded or partially embedded clouds. This places an estimate of ∼6 Myr as the age of the outflow.

The central engine of blazar OJ 287 is arguably the most notable supermassive black hole (SMBH) binary candidate that emits nanohertz (nHz) gravitational waves. This inference is mainly due to our ability to predict and successfully monitor certain quasiperiodic doubly peaked high brightness flares with a period of ∼12 yr from this blazer. The use of post-Newtonian accurate SMBH binary orbital description that includes the effects of higher-order gravitational-wave emission turned out to be a crucial ingredient for accurately predicting the epochs of such Bremsstrahlung flares in our SMBH binary central engine description for OJ 287. It was very recently argued that one should include the effects of dynamical friction, induced by certain dark matter density spikes around the primary SMBH, to explain the observed decay of SMBH binary orbit in OJ 287. Invoking binary pulsar timing-based arguments, measurements, and OJ 287's orbital description, we show that observationally relevant SMBH binary orbital dynamics in OJ 287 are insensitive to dark-matter-induced dynamical friction effects. This implies that we could only provide an upper bound on the spike index parameter rather than obtaining an observationally derived value, as argued by M. H. Chan and C. M. Lee.

Gamma-ray bursts (GRBs) are luminous stellar explosions characterized by the ejection of relativistic jets. This work proposes a novel paradigm to study these GRB jets. By analyzing the timing information of prompt pulses and X-ray flares, in conjunction with the multiwavelength afterglow observations, we identify three distinct jets in the extraordinary GRB 060729, with initial bulk Lorentz factors ranging from approximately 20 to 80, smaller than typical values of >100. These three jets undergo two successive collisions, producing the observed pair of X-ray flares. Following these interactions, the system evolves into a fast, narrow jet and a slower, hollow jet that continues to propagate in the circumburst medium, evidenced by the notable twin bumps observed in the X-ray and optical afterglow of GRB 060729. Our findings demonstrate that the timing of the early emission enables us to measure the velocities of the GRB jets. The proposed paradigm enhances our understanding of jet dynamics and shock interactions and serves as a powerful tool for probing the physics of the central engine with the expanded sample in the current golden era of GRB research.

Broadly, solar wind source regions can be classified by their magnetic topology as intermittently and continuously open to the heliosphere. Early models of solar wind acceleration do not account for the fastest, nontransient solar wind speeds observed near-Earth, and energy must be deposited into the solar wind after it leaves the Sun. Alfvén wave energy deposition and thermal pressure gradients are likely candidates, and the relative contribution of each acceleration mechanism likely depends on the source region. Although solar wind speed is a rough proxy for solar wind source region, it cannot unambiguously identify source region topology. Using near-Sun observations of the solar wind's kinetic energy flux, we predict the expected kinetic energy flux near Earth. This predicted kinetic energy flux corresponds to the range of solar wind speeds observed in the fast solar wind and we infer that the solar wind's near-Sun kinetic energy flux is sufficient to predict the distribution of the fastest, nontransient speeds observed near Earth. Applying a recently developed model of solar wind evolution in the inner heliosphere, we suggest that the acceleration required to generate this distribution of the fastest, nontransient speeds is likely due to the continuous deposition of energy by Alfvén wave forcing during the solar wind's propagation through interplanetary space. We infer that the solar wind's Alfvénicity can statistically map near-Earth observations to their source regions because the Alfvén wave forcing that the solar wind experiences in transit is a consequence of the source region topology.

We present joint South Pole Telescope and XMM-Newton observations of eight massive galaxy clusters (0.8–2 × 10¹⁵M_⊙) spanning a redshift range of 0.16–0.35. Employing a novel Sunyaev–Zel'dovich + X-ray fitting technique, we effectively constrain the thermodynamic properties of these clusters out to the virial radius. The resulting best-fit electron density, deprojected temperature, and deprojected pressure profiles are in good agreement with previous observations of massive clusters. For the majority of the cluster sample (five out of eight clusters), the entropy profiles exhibit a self-similar behavior near the virial radius. We further derive hydrostatic mass, gas mass, and gas fraction profiles for all clusters up to the virial radius. Comparing the enclosed gas fraction profiles with the universal gas fraction profile, we obtain nonthermal pressure fraction profiles for our cluster sample at  >0.5R₅₀₀, demonstrating a steeper increase between R₅₀₀ and R₂₀₀ that is consistent with the hydrodynamical simulations. Our analysis yields nonthermal pressure fraction ranges of 8%–28% (median: 15% ± 11%) at R₅₀₀ and 21%–35% (median: 27% ± 12%) at R₂₀₀. Notably, weak-lensing mass measurements are available for only four clusters in our sample, and our recovered total cluster masses, after accounting for nonthermal pressure, are consistent with these measurements.

Infrared observations of the inner disks around very low-mass stars (VLMS; <0.3 M_⊙) have revealed a carbon-rich gas composition in the terrestrial planet-forming regions. Contrary to the typically water-rich T Tauri disk spectra, only two disks around VLMS have been observed to be water-rich among more than 10 VLMS disks observed so far with JWST/MIRI. In this Letter, we systematically search for the presence of water and other oxygen-bearing molecules in the JWST/MIRI spectra of 10 VLMS disks from the MIRI mid-INfrared Disk Survey (MINDS). In addition to the two previously reported detections of water emission in this VLMS sample, we detect water emission in the spectra of three other sources and tentatively in one source, and we provide strong evidence for water emission in the remaining disks in the MINDS sample, most of which have bright emission from carbon-bearing molecules. We show that the C₂H₂ emission is much stronger than that of water for sources with low luminosities, and the hydrocarbons outshine the water emission in such conditions. We propose that the appearance of water-rich versus hydrocarbon-rich spectra is related to the location of the water reservoir in the disk relative to the main hydrocarbon reservoir. Our findings indicate that the terrestrial planet-forming regions in VLMS disks have high carbon-to-oxygen ratios (C/O > 1) but can still harbor ample water, similar to those in the T Tauri disks.