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.

We present the discovery of Andromeda XXXV, the faintest Andromeda satellite galaxy discovered to date, identified as an overdensity of stars in the Pan-Andromeda Archaeological Survey and confirmed via Hubble Space Telescope imaging. Located at a heliocentric distance of $92{7}_{-63}^{+76}$ kpc and $15{8}_{-45}^{+57}$ kpc from Andromeda, Andromeda XXXV is an extended (${r}_{h}=5{3\,}_{-11}^{+13}$ pc), elliptical ( = 0.4 ± 0.2), metal-poor ([Fe/H] ∼ −1.9) system, and the least luminous (M_V = −5.2 ± 0.3) of Andromeda's dwarf satellites discovered so far. Andromeda XXXV's properties are consistent with the known population of dwarf galaxies around the Local Group, bearing close structural resemblance to the Canes Venatici II and Hydra II Milky Way (MW) dwarf satellite galaxies. Its stellar population, characterized by a red horizontal branch or a red clump feature, mirrors that of other Andromeda satellite galaxies in showing evidence for a spread in age and metallicity, with no signs of younger stellar generations. This age–metallicity spread is not observed in MW satellites of comparable stellar mass, and highlights the persistent differences between the satellite systems of Andromeda and the MW, extending even into the ultrafaint regime.

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 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.

Barnard's Star is an old, single M dwarf star that comprises the second-closest extrasolar system. It has a long history of claimed planet detections from both radial velocities and astrometry. However, none of these claimed detections have so far withstood further scrutiny. Continuing this story, extreme precision radial velocity measurements from the ESPRESSO instrument have recently been used to identify four new sub-Earth-mass planet candidates around Barnard's Star. We present here 112 radial velocities of Barnard's Star from the MAROON-X instrument that were obtained independently to search for planets around this compelling object. The data have a typical precision of 30 cm s⁻¹ and are contemporaneous with the published ESPRESSO measurements (2021–2023). The MAROON-X data on their own confirm planet b (P = 3.154 days) and planet candidates c and d (P = 4.124 and 2.340 days, respectively). Furthermore, adding the MAROON-X data to the ESPRESSO data strengthens the evidence for planet candidate e (P = 6.739 days), thus leading to its confirmation. The signals from all four planets are <50 cm s⁻¹, the minimum masses of the planets range from 0.19 to 0.34 M_⊕, and the system is among the most compact known among late M dwarfs hosting low-mass planets. The current data rule out planets with masses >0.57 M_⊕ (with a 99% detection probability) in Barnard's Star's habitable zone (P = 10–42 days).

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.

Some sub-Neptune planets may host habitable conditions; for example "Hycean" worlds with H₂ envelopes over liquid water oceans can maintain potentially hospitable pressures and temperatures at their surface. Recent JWST observations of K2-18b and TOI-270d have shown that such worlds could be compelling targets for biosignature searches, given their extended scale heights and therefore large atmospheric signatures. Methylated biosignatures, a broad group of gases that can be generated by biological attachment of a CH₃ group to an environmental substrate, have been proposed as candidate signs of life for Earth-like exoplanets. However, methyl halides (CH₃ + halogen) have not yet been robustly examined with self-consistent photochemical and spectral models for planets with H₂-dominated atmospheres. Here we demonstrate that methyl chloride (CH₃Cl), predominantly produced by marine microbes, could be detected using JWST in tens of transits or fewer for Hycean planets, comparable to detection requirements for other potential atmospheric biosignatures. The threshold atmospheric mixing ratio for detectability is ∼10 ppm, which can accumulate with global fluxes comparable to moderately productive local environments on Earth.

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.

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.

One of the most striking manifestations of orderly behavior emerging out of complex interactions in any astrophysical system is the 11 yr cycle of sunspots. However, direct sunspot observations and reconstructions of long-term solar activity clearly exhibit amplitude fluctuations beyond the decadal timescale, which may be termed as supradecadal modulation. Whether this long-term modulation in the Sun's magnetic activity results from nonlinear mechanisms or stochastic perturbations remains controversial and a matter of active debate. Utilizing multimillennial-scale kinematic dynamo simulations based on the Babcock–Leighton paradigm—in the likely (near-critical) regime of operation of the solar dynamo—we demonstrate that this supradecadal modulation in solar activity cannot be explained by nonlinear mechanisms alone; stochastic forcing is essential for the manifestation of observed long-term fluctuations in the near-critical dynamo regime. Our findings substantiate some independent observational and theoretical investigations and provide additional insights into temporal dynamics associated with a plethora of natural phenomena in astronomy and planetary systems arising from weakly nonlinear, nondeterministic processes.

Switchbacks, characterized by large-angle deflections of the local interplanetary magnetic field relative to the background, are frequently observed throughout the heliosphere and play a crucial role in the solar wind dynamics. Recent studies have highlighted the importance of local generation mechanisms, such as expanding waves, turbulence, velocity shear, and footpoint motion, in the formation of switchbacks. Utilizing nearly two decades of data from the WIND spacecraft near 1 au, we conducted a detailed investigation into the differences between switchbacks and their surrounding environment, focusing on the influence of solar wind conditions on their occurrence rate and deflection degree. Our findings indicate that switchbacks are embedded within Alfvén waves, and their occurrence rate and maximum deflection degree are significantly enhanced during large-amplitude Alfvén waves. Specifically, the occurrence rate of switchbacks during these periods shows no significant correlation with solar wind velocity, suggesting that Alfvén wave activity is the primary driver of switchback formation. Our results further indicate that the evolution of switchbacks is closely related to the evolution of Alfvén waves, and that velocity shear and footpoint motion do not play an additional role in their evolution. These findings provide valuable insights into the dynamics of switchbacks and their relationship with solar wind conditions, contributing to a deeper understanding of the complex interactions within the heliosphere.

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.

Thorne–Żytkow objects (T$\dot{{\rm{Z}}}$Os) have been predicted to form when a neutron star is engulfed by a diffuse, convective giant envelope. Accretion onto a neutron star at a rate that is larger than 10⁻⁴M_⊙ yr⁻¹ is expected to lead to significant emission of neutrinos of all flavors with energy of 1–100 MeV. Since the neutrino signal is expected to largely vary in time (from milliseconds to thousands of years), we outline detection strategies tailored to the signal duration. We find that neutrino detection from T$\dot{{\rm{Z}}}$Os up to the Small Magellanic Cloud is within the reach of current- and next-generation neutrino observatories, such as Super- and Hyper-Kamiokande, the IceCube Neutrino Observatory, and JUNO. Interestingly, if targeted searches for neutrinos from T$\dot{{\rm{Z}}}$O candidates (e.g., VX Sgr in our Galaxy as well as HV 2112 and HV 11417 in the Small Magellanic Cloud) should lead to positive results, neutrinos could positively identify the nature of such sources and their accretion rate. Furthermore, the diffuse supernova neutrino background may be able to rule out extreme scenarios for the formation and accretion rates of TŻOs. Our findings should serve as motivation for establishing dedicated searches for neutrino emission from T$\dot{{\rm{Z}}}$Os. This is especially timely since it is challenging to detect TŻOs via electromagnetic radiation unambiguously, and the T$\dot{{\rm{Z}}}$O gravitational-wave signal could be probed with next-generation detectors for sources within our Galaxy only.

Using solar cycle–long helioseismic measurements of meridional and zonal flows in the near-surface shear layer (NSSL) of the Sun, we study their spatiotemporal variations and connections to active regions. We find that near-surface inflows toward active latitudes are part of a local circulation with an outflow away from them at depths around 0.97 R_⊙, which is also the location where the deviations in the radial gradient of rotation change sign. These results, together with opposite signed changes, over latitude and depth, in the above quantities observed during the solar minimum period, point to the action of the Coriolis force on large-scale flows as the primary cause of changes in rotation gradient within the NSSL. We also find that such Coriolis force mediated changes in near-surface flows toward active latitudes only marginally change the amplitude of zonal flow and hence are not likely to be its driving force. Our measurements typically achieve a high signal-to-noise ratio (>5σ) for near-surface flows but can drop to 3σ near the base (0.95 R_⊙) of the NSSL. Close agreements between the depth profiles of changes in rotation gradient and in meridional flows measured from quite different global and local helioseismic techniques, respectively, show that the results are not dependent on the analysis techniques.