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.

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).

Isolated planetary-mass objects share their mass range with planets but do not orbit a star. They lack the necessary mass to support fusion in their cores and thermally radiate their heat from formation as they cool, primarily at infrared wavelengths. Many isolated planetary-mass objects show variations in their infrared brightness consistent with nonuniform atmospheric features modulated by their rotation. SIMP J013656.5+093347.3 is a rapidly rotating isolated planetary-mass object, and previous infrared monitoring suggests complex atmospheric features rotating in and out of view. The physical nature of these features is not well understood, with clouds, temperature variations, thermochemical instabilities, and infrared-emitting aurora all proposed as contributing mechanisms. Here we report JWST time-resolved low-resolution spectroscopy from 0.8 to 11 μm of SIMP J013656.5+093347.3, which supports the presence of three specific features in the atmosphere: clouds, hot spots, and changing carbon chemistry. We show that no single mechanism can explain the variations in the time-resolved spectra. When combined with previous studies of this object indicating patchy clouds and aurorae, these measurements reveal the rich complexity of the atmosphere of SIMP J013656.5+093347.3. Gas giant planets in the solar system, specifically Jupiter and Saturn, also have multiple cloud layers and high-altitude hot spots, suggesting these phenomena are also present in worlds both within and beyond our solar system.

The Earth sits inside a 300 pc-wide void that was carved by a series of supernova explosions that went off tens of millions of years ago, pushing away interstellar gas and creating a bubble-like structure. The ⁶⁰Fe peak deposits found in the deep-sea crust have been interpreted by the imprints left by the ejecta of supernova explosions occurring about 2–3 and 5–6 Myr ago. It is likely that the ⁶⁰Fe peak at about 2–3 Myr originated from a supernova occurring in the Upper Centaurus Lupus association in Scorpius Centaurus (≈140 pc) or the Tucana-Horologium association (≈70 pc), whereas the  ≈5–6 Myr peak is likely attributed to the solar system's entrance into the bubble. In this Letter, we show that the supernova source responsible for synthesizing the ⁶⁰Fe peak deposits  ≈2–3 Myr ago can consistently explain the cosmic-ray spectrum and the large-scale anisotropy between 100 TeV and 100 PeV. The cosmic-ray knee could then potentially be attributed entirely to a single nearby "PeVatron" source. Matching the intensity and shape of the cosmic-ray spectrum allows us to place stringent constraints on the cosmic-ray energy content from the supernova as well as on the cosmic-ray diffusion coefficient. Making use of such constraints, we provide a robust estimate of the temporal variation of terrestrial ionizing cosmic radiation levels and discuss their implications in the development of early life on Earth by plausibly influencing the mutation rate and, as such, conceivably assisting in the evolution of complex organisms.

We present the first results of JWST Cycle 1 and 2 observations of Sgr A* using NIRCam taken simultaneously at 2.1 and 4.8 μm for a total of ~48 hr over seven different epochs in 2023 and 2024. We find correlated variability at 2.1 and 4.8 μm in all epochs, continual short-timescale (a few seconds) variability, and epoch-to-epoch variable emission implying long-term (~days to months) variability of Sgr A*. A highlight of this analysis is the evidence for subminute, horizon-scale time variability of Sgr A*, probing inner accretion disk size scales. The power spectra of the light curves in each observing epoch also indicate long-term variable emission. With continuous observations, JWST data suggest that the flux of Sgr A* is fluctuating constantly. The flux density correlation exhibits a distinct break in the slope at ~3 mJy at 2.1 μm. The analysis indicates two different processes contributing to the variability of Sgr A*. Brighter emission trends toward shallower spectral indices than the fainter emission. Cross-correlation of the light curves indicates for the first time a time delay of 3–40 s in the 4.8 μm variability with respect to 2.1 μm. This phase shift leads to loops in plots of flux density versus spectral index as the emission rises and falls. Modeling suggests that the synchrotron emission from the evolving, age-stratified electron population reproduces the shape of the observed light curves with a direct estimate of the magnetic field strengths in the range between 40 and 90 G and an upper cutoff energy, E_c, between 420 and 720 MeV.

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.

We present high-definition observations with the James Webb Space Telescope (JWST) of >1000 Cepheids in a geometric anchor of the distance ladder, NGC 4258, and in five hosts of eight Type Ia supernovae, a far greater sample than previous studies with JWST. These galaxies individually contain the largest samples of Cepheids, an average of >150 each, producing the strongest statistical comparison to those previously measured with the Hubble Space Telescope (HST) in the near-infrared (NIR). They also span the distance range of those used to determine the Hubble constant with HST, allowing us to search for a distance-dependent bias in HST measurements. The superior resolution of JWST negates crowding noise, the largest source of variance in the NIR Cepheid period–luminosity relations (Leavitt laws) measured with HST. Together with the use of two epochs to constrain Cepheid phases and three filters to remove reddening, we reduce the dispersion in the Cepheid P–L relations by a factor of 2.5. We find no significant difference in the mean distance measurements determined from HST and JWST, with a formal difference of −0.01 ± 0.03 mag. This result is independent of zero-points and analysis variants including metallicity dependence, local crowding, choice of filters, and slope of the relations. We can reject the hypothesis of unrecognized crowding of Cepheid photometry from HST that grows with distance as the cause of the "Hubble tension" at 8.2σ, i.e., greater confidence than that of the Hubble tension itself. We conclude that errors in photometric measurements of Cepheids across the distance ladder do not significantly contribute to the tension.

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.

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.

Detections of fast X-ray transients (FXTs) have accrued over the last few decades. However, their origin has remained mysterious. Rapid progress is now being made thanks to timely discoveries and localizations with the Einstein Probe mission. Early results indicate that FXTs may frequently, but not always, be associated with gamma-ray bursts (GRBs). Here, we report on the multiwavelength counterpart of FXT EP240414a, which has no reported gamma-ray counterpart. The transient is located 25.7 kpc in projection from a massive galaxy at z = 0.401. We perform comprehensive photometric and spectroscopic follow-up. The optical light curve shows at least three distinct emission episodes with timescales of ~1, 4, and 15 days and peak absolute magnitudes of M_R ∼ −20, –21, and –19.5, respectively. The optical spectrum at early times is extremely blue, inconsistent with afterglow emission. It may arise from the interaction of both jet and supernova (SN) shock waves with the stellar envelope and a dense circumstellar medium, as has been suggested for some luminous fast blue optical transients (LFBOTs). At late times, the spectrum evolves to a broad-lined Type Ic SN, similar to those seen in collapsar long GRBs. This implies that the progenitor of EP240414a is a massive star creating a jet-forming SN inside a dense envelope, resulting in an X-ray outburst with a luminosity of ~10⁴⁸ erg s⁻¹ and the complex observed optical/IR light curves. If correct, this argues for a causal link between the progenitors of long GRBs, FXTs, and LFBOTs.

We present new astrometric constraints on the stochastic gravitational-wave background and construct the first astrometric Hellings–Downs curve using quasar proper motions. From quadrupolar vector spherical harmonic fits to the Gaia proper motions of 1,108,858 quasars, we obtain a frequency-integrated upper limit on the gravitational-wave energy density, ${h}_{70}^{2}{\Omega }_{\mathrm{GW}}\leqslant 0.023$ (95% confidence limit), for frequencies between 11.2 nHz and 3.1 × 10⁻⁹ nHz (1.33/t₀). However, from the astrometric Hellings–Downs curve that describes the correlated proper motions between 2,104,609,881 quasar pairs as a function of their angular separation, we find a stronger constraint: a characteristic strain of h_c ≤ 2.7 × 10⁻¹² for f_ref = 1 yr⁻¹ and ${h}_{70}^{2}{{\rm{\Omega }}}_{{\rm{GW}}}\leqslant 0.0096$ at 95% confidence. We probe down to ±0.005 μas² yr⁻² in correlated power and obtain the lowest astrometric limit to date. This is also the first time that optical wavelength astrometry surpasses limits from radio-frequency interferometry. This astrometric analysis does not yet reach the sensitivity needed to detect the pulsar timing–based red gravitational-wave spectrum extrapolated to the quasar gravitational-wave sensitivity window, assuming that the turnover in the spectrum occurs at ∼1 nHz for massive black hole binaries. The limits presented here may exclude some exotic interpretations of the stochastic gravitational-wave background.

The two-state solar wind paradigm is based on observations showing that slow and fast solar wind have distinct properties like helium abundances, kinetic signatures, elemental composition, and charge-state ratios. Nominally, the fast wind originates from solar sources that are continuously magnetically open to the heliosphere like coronal holes while the slow wind is from solar sources that are only intermittently open to the heliosphere like helmet streamers and pseudostreamers. The Alfvénic slow wind is an emerging third class of solar wind that challenges the two-state fast/slow paradigm. It has slow wind speeds but is highly Alfvénic, i.e., has a high correlation between velocity and magnetic field fluctuations along with low compressibility typical of Alfvén waves, which is typically observed in fast wind. Its other properties are also more similar to the fast than slow wind. From 28 yr of Wind observations at 1 au, we derive the solar wind helium abundance (A_He), Alfvénicity (∣σ_c∣), and solar wind speed (v_sw). Characterizing v_sw as a function of ∣σ_c∣ and A_He, we show that the maximum solar wind speed for plasma accelerated in source regions that are intermittently open is faster than the minimum solar wind speed for plasma accelerated in continuously open regions. We infer that the Alfvénic slow wind is likely solar wind originating from open field regions with speeds below the maximum solar wind speed for plasma from intermittently open regions. We then discuss possible implications for solar wind acceleration. Finally, we utilize the combination of helium abundance and normalized cross helicity to present a novel solar wind categorization scheme that illustrates the transition in observations of solar wind at 1 au from magnetically closed to magnetically open sources.

When a binary of early-type stars from the young stellar populations in the Galactic center (GC) region is scattered to the vicinity of the supermassive black hole (SMBH) Sgr A*, one of the components would be tidally ejected as an early-type hypervelocity star (HVS) and the counterpart would be captured on a tight orbit around Sgr A*. Dozens of B-type HVSs moving faster than the Galactic escape speed have been discovered in the Galactic halo and are produced most likely by the SMBH Sgr A*. However, the velocity distribution and in particular the deficit of the HVSs above 700 km s⁻¹ is seriously inconsistent with the expectations of the present models. Here we show that the high-velocity deficit is due to the deficiency in close interactions of stars with the SMBH Sgr A*, because an orbiting intermediate-mass black hole (IMBH) of about 15,000 Solar mass kicked away slowly approaching stars 50–250 million years ago. The SMBH–IMBH binary formed probably after the merger of the Milky Way with the Gaia-Sausage-Enceladus dwarf galaxy, and coalesced about 10 million years ago, leading to a gravitational recoil of Sgr A* at a velocity of 0.3–0.5 km s⁻¹ and to a change of the HVS ejection scenarios. The SMBH–IMBH binary scenario predicts the formation of the S-star cluster at the GC with the distribution of the orbital size and stellar ages that are well consistent with the observations.

Understanding the nature of compressible fluctuations in a broad range of turbulent plasmas, from the intracluster medium to the solar wind, has been an active field of research in the past decades. Theoretical frameworks for weakly compressible magnetohydrodynamical turbulence in an inhomogeneous background magnetic field predict a linear scaling of the normalized mass density fluctuation (δρ/ρ₀), as a function of the turbulent Mach number (${{ \mathcal M }}_{t}$), $\delta \rho /{\rho }_{0}\propto {{ \mathcal M }}_{t}$. However, so far, the scaling relation has been tested only using moderate to low plasma beta (β ≲ 1) solar wind observational data, where the compressibility is weak δρ/ρ₀ ∼ 0.1. Here, we combine NASA's Magnetospheric Multiscale Mission data in Earth's magnetosheath, where β ∼ 10 is high, and β ∼ 1/8 highly compressible magnetohydrodynamic turbulence simulations at unprecedented resolutions. Both show that $\delta \rho /{\rho }_{0}\propto {{ \mathcal M }}_{t}$ holds across a broad range of δρ/ρ₀, ${{ \mathcal M }}_{t}$, and β, demonstrating that $\delta \rho /{\rho }_{0}\propto {{ \mathcal M }}_{t}$ is a robust compressible turbulence relation, going beyond the asymptotics of the weakly compressible theory. We discuss the findings in the context of understanding the nature of strongly compressible turbulent fluctuations and the driving parameter in astrophysical and space plasmas.