Exoplanets Research Articles

HADES RV Programme with HARPS-N at TNG. XVI. A super-Earth in the habitable zone of the GJ 3998 multi-planet system

The low masses of M dwarfs create attractive opportunities for exoplanet radial-velocity (RV) detections. These stars, however, exhibit strong stellar activity that may attenuate or mimic planetary signals. We present a velocimetric analysis of one such M dwarf, GJ 3998 (d= parsec ), with two published short-period super-Earths: GJ 3998 b and GJ 3998 c. We use additional data from the HARPS-N spectrograph to confirm these two planets and to look for more. We carry out joint modelling of: (i) RV planetary signals, (ii) stellar rotation in RV and activity indicators through Gaussian processes, and (iii) long-term trends in RV and activity indicators. We constrain the rotational period of GJ 3998 to P_ rot =30.2± 0.3 day and discover long-term sinusoidal imprints in RV and the full width at half maximum with a period of P_ cyc day . We confirm GJ 3998 b and GJ 3998 c, and detect a third planet: GJ 3998 d, whose signal had previously been attributed to stellar activity. GJ 3998 d has an orbital period of $41.78± 0.05 day$ a minimum mass of earthmass$ and a mean insolation flux of earthflux$ . This makes it one of the few known planets receiving an Earth-like insolation flux.

The Once and Future Gas: Methane's Multifunctional Roles in Earth's Evolution and Potential as a Biosignature

Methane (CH4) is a simple molecule that, due to its radiative forcing, wields an outsized impact on planetary heat balance. Methane is formed by diverse abiotic pathways across a range of pressures and temperatures. Biological methanogenesis for anaerobic respiration uses a terminal nickel-containing enzyme and is limited to the archaeal domain of life. Methane can also be produced in aerobic microbes during bacterial methylphosphonate and methylamine degradation and via nonenzymatic reactions during oxidative stress. Abiotic CH4 is produced via thermogenic reactions and during serpentinization reactions in the presence of metal catalysts. Reconstructions of methane cycling over geologic time are largely inferential. Throughout Earth's history, methane has probably been the second most important climate-forcing greenhouse gas after carbon dioxide. Biological methanogenesis has likely dominated CH4 flux to Earth's atmosphere for the past ∼3.5 billion years, during which time CH4 is thought to have generally declined as atmospheric oxygen has risen. Here we review the evolution of the CH4 cycle over Earth's history, showcasing the multifunctional roles CH4 has played in Earth's climate, prebiotic chemistry, and microbial metabolisms. We also discuss the future of Earth's atmospheric CH4, the cycling of CH4 on other planetary bodies in the Solar System (with special emphasis on Titan), and the potential of CH4 as a biosignature on Earth-like extrasolar planets. ▪ Before life arose on Earth, abundant atmospheric CH4 in Earth's early atmosphere was likely key for establishment of habitable conditions and production of organic molecules for prebiotic chemistry. ▪ Biological methanogenesis for anaerobic respiration is only known to exist in some groups of anaerobic archaea, but CH4 can also be produced via enzymatic and nonenzymatic biological pathways that are not directly coupled to energy conservation. The relative importance of each of these pathways to the global CH4 cycle is a topic of active research, but archaeal methanogenesis dominates all other biological pathways for CH4 generation. ▪ As atmospheric O2 rose over Earth history, models suggest that atmospheric CH4 declined; in the distant deoxygenated future, atmospheric CH4 is predicted to rise again. ▪ Future missions to Titan will aid in understanding the complex organic chemistry on the only other planetary body in our Solar System with an active methane cycle.

The Dynamical History of HIP-41378 f—Oblique Exorings Masquerading as a Puffy Planet

Abstract The super-puff HIP-41378 f represents a fascinating puzzle due to its anomalously low density on a far-out orbit in contrast with other known super-puffs. In this work, we explore the hypothesis that HIP-41378 f is not in fact a low-density planet, but rather hosts an opaque ring system. We analyze the dynamical history of the system and show that convergent migration is necessary to explain the system's long-term stability. We then show that this same migration process plausibly captures HIP-41378 f into spin–orbit resonance and excites the planetary obliquity to high values. This tilts the surrounding ring and is a plausible explanation for the large transit depth. In the end, we also briefly comment on the likelihood of other super-puff planets being in high-obliquity states. We show that the existence of a tilted extensive ring around a high obliquity planet can serve as an explanation for puffy planets, particularly in multiplanetary systems at far distances from their host stars.

KOBE-1: The first planetary system from the KOBE survey

Context. K-dwarf stars are promising targets in the exploration of potentially habitable planets. Their properties, falling between G and M dwarfs, provide an optimal trade-off between the prospect of habitability and ease of detection. The KOBE experiment is a blind-search survey exploiting this niche, monitoring the radial velocity of 50 late-type K-dwarf stars. It employs the CARMENES spectrograph, with an observational strategy designed to detect planets in the habitable zone of their system. Aims. In this work, we exploit the KOBE data set to characterize planetary signals in the K7 V star HIP 5957 (KOBE-1) and to constrain the planetary population within its habitable zone. Methods. We used 82 CARMENES spectra over a time span of three years. We employed a generalized Lomb–Scargle periodogram to search for significant periodic signals that would be compatible with Keplerian motion on KOBE-1. We carried out a model comparison within a Bayesian framework to ensure the significance of the planetary model over alternative configurations of lower complexity. We also inspected two available TESS sectors in search of planetary signals. Results. We identified two signals: at Pb = 8.5 d and Pc = 29.7 d. We confirmed their planetary nature through ruling out other non-planetary configurations. Their minimum masses are 8.80 ± 0.76 M⊕ (KOBE-1 b), and 12.4 ± 1.1 M⊕ (KOBE-1 c), corresponding to absolute masses within the planetary regime at a high certainty (>99.7%). By analyzing the sensitivity of the CARMENES time series to additional signals, we discarded planets above 8.5 M⊕ within the habitable zone. We identified a single transit-like feature in TESS, whose origin is still uncertain, but still compatible within 1σ with a transit from planet c. Conclusions. The KOBE-1 multi-planetary system, consisting of a relatively quiet K7-dwarf hosting two sub-Neptune-minimum- mass planets, establishes the first discovery from the KOBE experiment. We have explored future prospects for characterizing this system, concluding that Gaia DR4 will be insensitive to their astrometric signature. Meanwhile, nulling interferometry with the Large Interferometer For Exoplanets (LIFE) mission could be capable of directly imaging both planets and characterizing their atmospheres in future studies.

Eight New Candidate Multiplanet Systems among TESS Objects of Interest

Abstract We present a search of the TESS Object of Interest (TOI) Catalog for new multiplanet systems. We perform a Box Least Squares search on light curves prepared by the Quick-Look Pipeline to recover known candidates and perform a targeted search for additional candidates in each system. Following vetting and manual inspection, we present eight new planet candidates in multiplanet systems. Highlighted systems include TOI-2037 and TOI-6543, each displaying a nearness to a 2:1 orbital resonance. Four candidates in the TOI-5624 system, including our candidate TIC 53498154.02, have gained TOI status. If confirmed, the TOI-5624 system becomes the 29th known five-planet system. Our identification of candidate TIC 302305400.02 in the TOI-5487 system presents an interesting case study for the formation of hot Jupiter systems, potentially marking the first known system harboring a close-in giant outer companion to an existing hot Jupiter.

A One-Dimensional Energy Balance Model Parameterization for the Formation of CO2 Ice on the Surfaces of Eccentric Extrasolar Planets.

Eccentric planets may spend a significant portion of their orbits at large distances from their host stars, where low temperatures can cause atmospheric CO2 to condense out onto the surface, similar to the polar ice caps on Mars. The radiative effects on the climates of these planets throughout their orbits would depend on the wavelength-dependent albedo of surface CO2 ice that may accumulate at or near apoastron and vary according to the spectral energy distribution of the host star. To explore these possible effects, we incorporated a CO2 ice-albedo parameterization into a one-dimensional energy balance climate model. With the inclusion of this parameterization, our simulations demonstrated that F-dwarf planets require 29% more orbit-averaged flux to thaw out of global water ice cover compared with simulations that solely use a traditional pure water ice-albedo parameterization. When no eccentricity is assumed, and host stars are varied, F-dwarf planets with higher bond albedos relative to their M-dwarf planet counterparts require 30% more orbit-averaged flux to exit a water snowball state. Additionally, the intense heat experienced at periastron aids eccentric planets in exiting a snowball state with a smaller increase in instellation compared with planets on circular orbits; this enables eccentric planets to exhibit warmer conditions along a broad range of instellation. This study emphasizes the significance of incorporating an albedo parameterization for the formation of CO2 ice into climate models to accurately assess the habitability of eccentric planets, as we show that, even at moderate eccentricities, planets with Earth-like atmospheres can reach surface temperatures cold enough for the condensation of CO2 onto their surfaces, as can planets receiving low amounts of instellation on circular orbits.

Discovery of a cold giant planet and mass measurement of a hot super-Earth in the multi-planetary system WASP-132

Hot Jupiters generally do not have nearby planet companions, as they may have cleared out other planets during their inward migration from more distant orbits. This gives evidence that hot Jupiters more often migrate inward via high-eccentricity migration due to dynamical interactions between planets rather than more dynamically cool migration mechanisms through the protoplanetary disk. Here we further refine the unique system of WASP-132 by characterizing the mass of the recently validated 1.0-day period super-Earth WASP-132c (TOI-822.02), interior to the 7.1-day period hot Jupiter WASP-132b. Additionally, we announce the discovery of a giant planet at a 5-year period (2.7 AU). We also detected a long-term trend in the radial velocity data indicative of another outer companion. Using over nine years of CORALIE radial velocities (RVs) and over two months of highly sampled HARPS RVs, we determined the masses of the planets from smallest to largest orbital period to be Mc = 6.26−1.83+1.84 M⊕, Mb = 0.428−0.015+0.015 MJup, and Md = sin i 5.16−0.52+0.52 MJup, respectively. Using TESS and CHEOPS photometry data, we measured the radii of the two inner transiting planets to be Rc = 1.841−0.093+0.094 R⊕ and Rb = 0.901−0.038+0.038 RJup. We find a bulk density of ρc = 5.47−1.71+1.96 g cm−3 for WASP-132c, which is slightly above the Earth-like composition line on the mass-radius diagram. WASP-132 is a unique multi-planetary system in that both an inner rocky planet and an outer giant planet are in a system with a hot Jupiter. This suggests it migrated via a rarer dynamically cool mechanism and helps to further our understanding of how hot Jupiter systems form and evolve.

Absorption spectra of PS in the ultraviolet and infrared region.

The line list is essential for accurately modeling various astrophysical phenomena, such as stellar photospheres and atmospheres of extrasolar planets. This paper introduces a new line database for the PS molecule spanning from the ultraviolet to the infrared regions, covering wavenumbers up to 45000cm-1 and containing over ten million transitions between 150,458 states with total angular momentum J<160. Accurate line intensities for rotational, vibrational and electronic transitions are generated by using the general purpose variational code DUO. Before applying the DUO program, the potential energy curves (PECs), the electric dipole transition moments (EDTMs) and the spin-orbit coupling (SOC) matrix need to be provided, which are computed by an ab initio method at the icMRCI level. The DUO program is executed to generate two files - a state file and a transition file, which are provided to the ExoCross program which supply cross sections, intensities, partition functions, lifetimes and quantum number assignments. The cross-sections and intensities for the observed transitions are necessary for the correct modeling of the spectra. Tests of the line lists show that our predictions not only provide a favorable comparison with the experimental spectrum but also a cornerstone for various astronomical observations.

Revisiting the multi-planetary system of the nearby star HD 20794

Context. Close-by Earth analogs and super-Earths are of primary importance because they will be preferential targets for the next generation of direct imaging instruments. Bright and close-by G-to-M type stars are preferential targets in radial velocity surveys to find Earth analogs. Their brightness allows us to achieve the best precision on RV measurements and search for signals with amplitudes of less than 1 m s−1. Aims. We present an analysis of the RV data of the star HD 20794, a target whose planetary system has been extensively debated in the literature. The broad time span of the observations makes it possible to find planets with signal semi-amplitudes below 1 m s−1 in the habitable zone. Methods. We analyzed RV datasets spanning more than 20 years. We monitored the system with ESPRESSO. We joined ESPRESSO data with the HARPS data, including archival data and new measurements from a recent program. We applied the post-processing pipeline YARARA to HARPS data to correct systematics, improve the quality of RV measurements, and mitigate the impact of stellar activity. Results. We confirm the presence of three planets, with periods of 18.3142 ± 0.0022 d, 89.68 ± 0.10 d, and 647.6−2.7+2.5 d, along with masses of 2.15 ± 0.17 M⊕, 2.98 ± 0.29 M⊕, and 5.82 ± 0.57 M⊕ respectively. For the outer planet, we find an eccentricity of 0.45−0.11+0.10, whereas the inner planets are compatible with circular orbits. The latter is likely to be a rocky planet in the habitable zone of HD 20794. From the analysis of activity indicators, we find evidence of a magnetic cycle with a period of ~3000 d, along with evidence pointing to a rotation period of ~39 d. Conclusions. We have determined the presence of a system of three planets orbiting the solar-type star HD 20794. This star is bright (V=4.34 mag) and close (d = 6.04 pc), and HD 20794 d resides in the stellar habitable zone, making this system a high-priority target for future atmospheric characterization with direct imaging facilities.

Two Earth-size Planets and an Earth-size Candidate Transiting the nearby Star HD 101581*

Abstract We report the validation of multiple planets transiting the nearby (d = 12.8 pc) K5V dwarf HD 101581 (GJ 435, TOI–6276, TIC 397362481). This system consists of at least two Earth-size planets whose orbits are near a mutual 4:3 mean-motion resonance, HD 101581 b ( R p = 0.956 − 0.061 + 0.063 R ⊕ , P = 4.47 days) and HD 101581c ( R p = 0.990 − 0.070 + 0.070 R ⊕ , P = 6.21 days). Both planets were discovered in Sectors 63 and 64 TESS observations and statistically validated with supporting ground-based follow-up. We also identify a signal that probably originates from a third transiting planet, TOI-6276.03 ( R p = 0.982 − 0.098 + 0.114 R ⊕ , P = 7.87 days). These planets are remarkably uniform in size and their orbits are evenly spaced, representing a prime example of the “peas-in-a-pod” architecture seen in other compact multiplanet systems. At V = 7.77, HD 101581 is the brightest star known to host multiple transiting planets smaller than 1.5 R ⊕. HD 101581 is a promising system for atmospheric characterization and comparative planetology of small planets.

More Likely Than You Think: Inclination-driving Secular Resonances Are Common in Known Exoplanet Systems

Multiplanet systems face significant challenges to detection. For example, farther-orbiting planets have a reduced signal-to-noise ratio in radial velocity detection methods, and small mutual inclinations between planets can prevent them from all transiting. One mechanism for exciting mutual inclination between planets is secular resonance, where the nodal precession frequencies of the planets align so as to greatly increase the efficiency of the angular momentum transport between planets. These resonances can significantly misalign planets from one another, hindering detection, and typically can only occur when there are three or more planets in the system. Naively, systems can only be in resonance for particular combinations of planet semimajor axes and masses; however, effects that alter the nodal precession frequencies of the planets, such as the decay of stellar oblateness, can significantly expand the region of parameter space where resonances occur. In this work, we explore known three-planet systems, determine whether they are in (or were in) secular resonance due to evolving stellar oblateness, and demonstrate the implications of resonance on their detectability and stability. We show that about 20% of a sample of three-planet transiting systems seem to undergo these resonances early in their lives.

Enhanced Stability in Planetary Systems with Similar Masses

Abstract This study employs numerical simulations to explore the relationship between the dynamical instability of planetary systems and the uniformity of planetary masses within the system, quantified by the Gini index. Our findings reveal a significant correlation between system stability and mass uniformity. Specifically, planetary systems with higher mass uniformity demonstrate increased stability, particularly when they are distant from first-order mean motion resonances. In general, for nonresonant planetary systems with a constant total mass, non-equal-mass systems are less stable than equal mass systems for a given spacing in units of mutual Hill radius. This instability may arise from the equipartition of the total random energy, which can lead to higher eccentricities in smaller planets, ultimately destabilizing the system. This work suggests that the observed mass uniformity within multiplanet systems detected by Kepler may result from a combination of survival bias and ongoing dynamical evolution processes.

A potential exomoon from the predicted planet obliquity of β Pictoris b

Planet obliquity is the alignment or misalignment of a planet spin axis relative to its orbit normal. In a multiplanet system, this obliquity is a valuable signature of planet formation and evolutionary history. The young β Pictoris system hosts two coplanar super-Jupiters and upcoming JWST observations of this system will constrain the obliquity of the outer planet, β Pictoris b. This will be the first planet obliquity measurement in an extrasolar, multiplanet system. First, we show that this new planet obliquity is likely misaligned by using a wide range of simulated observations in combination with published measurements of the system. Motivated by current explanations for the tilted planet obliquities in the Solar System, we consider collisions and secular spin-orbit resonances. While collisions are unlikely to occur, secular spin-orbit resonance modified by the presence of an exomoon around the outer planet can excite a large obliquity. The largest induced obliquities ( ∼60∘) occur for moons with at least a Neptune-mass and a semimajor axis of 0.03−0.05au ( 40−70 planet radii). For certain orbital alignments, such a moon may observably transit the planet (transit depth of 3−7%, orbital period of 3−7 weeks). Thus, a nonzero obliquity detection of β Pictoris b implies that it may host a large exomoon. Although we focus on the β Pictoris system, the idea that the presence of exomoons can excite high obliquities is very general and applicable to other exoplanetary systems.

An impact-free mechanism to deliver water to terrestrial planets and exoplanets

Context. The origin of water, particularly on Earth, is still a matter of heated debate. To date, the most widespread scenario is that the Earth originated without water and that it was brought to the planet mainly as a result of impacts by wet asteroids coming from further out in space. However, many uncertainties remain as to the exact processes that supplied an adequate amount of water to inner terrestrial planets. Aims. In this article, we explore a new mechanism that would allow water to be efficiently transported to planets without impacts. We propose that primordial asteroids were icy and that when the ice sublimated, it formed a gaseous disk that could then reach planets and deliver water. Methods. We have developed a new model that follows the sublimation of asteroids on gigayear (Gyr) timescales, taking into account the variable luminosity of the Sun. We then evolved the subsequent gas disk using a viscous diffusion code, which leads to the gas spreading both inwards and outwards in the Solar System. We can then quantify the amount of water that can be accreted onto each planet in a self-consistent manner using our code. Results. We find that this new disk-delivery mechanism is effective and equipped to explain the water content on Earth (with the correct D/H ratio) as well as on other planets and the Moon. Our model shows most of the water being delivered between 20 and 30 Myr after the birth of the Sun, when the Sun’s luminosity increased sharply. Our scenario implies the presence of a gaseous water disk with substantial mass for hundreds of millions of years, which could be one of the key tracers of this mechanism. We show that such a watery disk could be detected in young exo-asteroid belts with ALMA. Conclusions. We propose that viscous water transport is inevitable and more generic than the impact scenario. We also suggest it is a universal process that may also occur in extrasolar systems. The conditions required for this scenario to unfold are indeed expected to be present in most planetary systems: an opaque proto-planetary disk that is initially cold enough for ice to form in the exo-asteroid belt region, followed by a natural outward-moving snow line that allows this initial ice to sublimate after the dissipation of the primordial disk, creating a viscous secondary gas disk and leading to the accretion of water onto the exo-planets.

Unravelling sub-stellar magnetospheres

At the sub-stellar boundary, signatures of magnetic fields begin to manifest at radio wavelengths, analogous to the auroral emission of the magnetised solar system planets. This emission provides a singular avenue for measuring magnetic fields at planetary scales in extrasolar systems. So far, exoplanets have eluded detection at radio wavelengths. However, ultracool dwarfs (UCDs), their higher mass counterparts, have been detected for over two decades in the radio. Given their similar characteristics to massive exoplanets, UCDs are ideal targets to bridge our understanding of magnetic field generation from stars to planets. In this work, we develop a new tomographic technique for inverting both the viewing angle and large-scale magnetic field structure of UCDs from observations of coherent radio bursts. We apply our methodology to the nearby T8 dwarf WISE J062309.94-045624.6 (J0623) which was recently detected at radio wavelengths, and show that it is likely viewed pole-on. We also find that J0623’s rotation and magnetic axes are misaligned significantly, reminiscent of Uranus and Neptune, and show that it may be undergoing a magnetic cycle with a period exceeding 6 months in duration. These findings demonstrate that our method is a robust new tool for studying magnetic fields on planetary-mass objects. With the advent of next-generation low-frequency radio facilities, the methods presented here could facilitate the characterisation of exoplanetary magnetospheres for the first time.

A Fourth Planet in the Kepler-51 System Revealed by Transit Timing Variations

Kepler-51 is a ≲1 Gyr old Sun-like star hosting three transiting planets with radii ≈6–9 R ⊕ and orbital periods ≈45–130 days. Transit timing variations (TTVs) measured with past Kepler and Hubble Space Telescope (HST) observations have been successfully modeled by considering gravitational interactions between the three transiting planets, yielding low masses and low mean densities (≲0.1 g cm−3) for all three planets. However, the transit time of the outermost transiting planet Kepler-51d recently measured by the James Webb Space Telescope 10 yr after the Kepler observations is significantly discrepant from the prediction made by the three-planet TTV model, which we confirmed with ground-based and follow-up HST observations. We show that the departure from the three-planet model is explained by including a fourth outer planet, Kepler-51e, in the TTV model. A wide range of masses (≲M Jup) and orbital periods (≲10 yr) are possible for Kepler-51e. Nevertheless, all the coplanar solutions found from our brute-force search imply masses ≲10 M ⊕ for the inner transiting planets. Thus, their densities remain low, though with larger uncertainties than previously estimated. Unlike other possible solutions, the one in which Kepler-51e is around the 2:1 mean motion resonance with Kepler-51d implies low orbital eccentricities (≲0.05) and comparable masses (∼5 M ⊕) for all four planets, as is seen in other compact multiplanet systems. This work demonstrates the importance of long-term follow-up of TTV systems for probing longer-period planets in a system.

Planet Mass Function around M Stars at 1–10 au: A Plethora of Sub-Earth Mass Objects

Small planets (≲1 M ⊕) at intermediate orbital distances (∼1 au) represent an uncharted territory in exoplanetary science. The upcoming microlensing survey by the Nancy Grace Roman Space Telescope will be sensitive to objects as light as Ganymede and unveil the small planet population at 1–10 au. Instrumental sensitivity to such planets is low, and the number of objects we will discover is strongly dependent on the underlying planet mass function. In this work, we provide a physically motivated planet mass function by combining the efficiency of planet formation by pebble accretion with the observed disk mass function. Because the disk mass function for M dwarfs (0.4–0.6 M ⊙) is bottom heavy, the initial planet mass function is also expected to be bottom heavy, skewing toward Ganymede and Mars mass objects, more so for heavier initial planetary seeds. We follow the subsequent dynamical evolution of planetary systems over ∼100 Myr varying the initial eccentricity and orbital spacing. For initial planet separations of ≥3 local disk scale heights, we find that Ganymede and Mars mass planets do not grow significantly by mergers. However, Earth-like planets undergo vigorous merging and turn into super-Earths, potentially creating a gap in the planet mass function at ∼1 M ⊕. Our results demonstrate that the slope of the mass function and the location of the potential gap in the mass function can probe the initial architecture of multiplanet systems. We close by discussing implications on the expected difference between bound and free-floating planet mass functions.

The overflowing atmosphere of WASP-121 b. High-resolution transmission spectroscopy with VLT/CRIRES+

Transmission spectroscopy is a prime method to study the atmospheres of extrasolar planets. We obtained a high-resolution spectral transit time series of the hot Jupiter with CRIRES$^+$ to study its atmosphere via transmission spectroscopy of the triplet lines. Our analysis shows a prominent absorption feature moving along with the planetary orbital motion, which shows an observed, transit-averaged equivalent width of approximately 30\,m a slight redshift, and a depth of about 2\,&lt;!PCT!&gt;, which can only be explained by an atmosphere overflowing its Roche lobe. We carried out 3D hydrodynamic modeling to reproduce the observations, which favors asymmetric mass loss with a more pronounced leading tidal tail, possibly also explaining observational evidence for additional absorption stationary in the stellar rest frame. A trailing tail is not detectable. From our modeling, we derived estimates of $ for the stellar and $5.4 for the planetary mass loss rate, which is consistent with X-ray and extreme-ultraviolet (XUV) driven mass loss in

First comparative exoplanetology within a transiting multi-planet system: Comparing the atmospheres of V1298 Tau b and c

The V1298 Tau system is a multi-planet system that provides the opportunity to perform comparative exoplanetology between planets orbiting the same star. Because of its young age (20-30 Myr), this system also provides the opportunity to compare the planet's early evolutionary properties, right after their formation. We present the first atmospheric comparison between two transiting exoplanets within the same multiple planet system: V1298 Tau b and V1298 Tau c. We observed one primary transit for each planet with the Hubble Space Telescope (HST), using Grism 141 (G141) of Wide Field Camera 3 (WFC3). We fit the spectroscopic light curves using state-of-the-art techniques to derive the transmission spectrum for planet c and adopted the transmission spectrum of planet b obtained with the same observing configuration and data analysis methods from previous studies. We measured the mass of planet b and c ($8_ $ M$_ oplus $; respectively) from the transmission spectrum and found the two planets to have masses in the Neptune or sub-Neptune regime. Using atmospheric retrievals, we measured and compared the atmospheric metallicities of planet b and c (logZ/Z$_ $, logZ/Z$_ $, respectively), and found them to be consistent with the solar or sub-solar, which is low (at least one order of magnitude) compared to known mature Neptune and sub-Neptune planets. This discrepancy could be explained by ongoing early evolutionary mechanisms, which are expected to enrich the atmospheres of such young planets as they mature. Alternatively, the observed spectrum of planet c can be explained by atmospheric hazes, which is in contrast to planet b, where efficient haze formation can be ruled out. Higher haze formation efficiency in planet c could be due to differences in atmospheric composition, temperature and/or higher UV flux compared to planet b. In addition, planet c is likely to experience a higher fraction of mass loss compared to planet b, given its proximity to the host star.

JWST COMPASS: The 3–5 μm Transmission Spectrum of the Super-Earth L 98-59 c

We present a JWST Near-InfraRed Spectrograph (NIRSpec) transmission spectrum of the super-Earth exoplanet L 98-59 c. This small (R p = 1.385 ± 0.085R ⊕, M p = 2.22 ± 0.26 R ⊕), warm (T eq = 553 K) planet resides in a multiplanet system around a nearby, bright (J = 7.933) M3V star. We find that the transmission spectrum of L 98-59 c is featureless at the precision of our data. We achieve precisions of 22 ppm in NIRSpec G395H’s NRS1 detector and 36 ppm in the NRS2 detector at a resolution R ∼ 200 (30 pixel wide bins). At this level of precision, we are able rule out primordial H2–He atmospheres across a range of cloud pressure levels up to at least ∼0.1 mbar. By comparison to atmospheric forward models, we also rule out atmospheric metallicities below ∼300× solar at 3σ (or, equivalently, atmospheric mean molecular weights below ∼10 g mol−1). We also rule out pure methane atmospheres. The remaining scenarios that are compatible with our data include a planet with no atmosphere at all, or higher-mean-molecular-weight atmospheres, such as CO2- or H2O-rich atmospheres. This study adds to a growing body of evidence suggesting that planets ≲1.5 R ⊕ lack extended atmospheres.