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We report the discovery and characterization of a small planet, TOI-1408 c, on a 2.2 day orbit located interior to a previously known hot Jupiter, TOI-1408 b ( P = 4.42 days, M = 1.86 ± 0.02 M _Jup , R = 2.4 ± 0.5 R _Jup ) that exhibits grazing transits. The two planets are near 2:1 period commensurability, resulting in significant transit timing variations (TTVs) for both planets and transit duration variations for the inner planet. The TTV amplitude for TOI-1408 c is 15% of the planet’s orbital period, marking the largest TTV amplitude relative to the orbital period measured to date. Photodynamical modeling of ground-based radial velocity (RV) observations and transit light curves obtained with the Transiting Exoplanet Survey Satellite and ground-based facilities leads to an inner planet radius of 2.22 ± 0.06 R _⊕ and mass of 7.6 ± 0.2 M _⊕ that locates the planet into the sub-Neptune regime. The proximity to the 2:1 period commensurability leads to the libration of the resonant argument of the inner planet. The RV measurements support the existence of a third body with an orbital period of several thousand days. This discovery places the system among the rare systems featuring a hot Jupiter accompanied by an inner low-mass planet.

The hot Neptune desert is one of the most sparsely populated regions of the exoplanet parameter space, and atmosphere observations of its few residents can provide insights into how such planets have managed to survive in such an inhospitable environment. Here, we present transmission observations of LTT 9779 b, the only known hot Neptune to have retained a significant H/He-dominated atmosphere, taken with JWST NIRISS/SOSS. The 0.6–2.85 μ m transmission spectrum shows evidence for muted spectral features, rejecting a perfectly flat line at >5 σ . We explore water- and methane-dominated atmosphere scenarios for LTT 9779 b’s terminator, and retrieval analyses reveal a continuum of potential combinations of metallicity and cloudiness. Through comparisons to previous population synthesis works and our own interior structure modeling, we are able to constrain LTT 9779 b’s atmosphere metallicity to 20–850× solar. Within this range of metallicity, our retrieval analyses prefer solutions with clouds at millibar pressures, regardless of whether the atmosphere is water or methane dominated—though cloud-free atmospheres with metallicities >500× solar cannot be entirely ruled out. By comparing self-consistent atmosphere temperature profiles with cloud condensation curves, we find that silicate clouds can readily condense in the terminator region of LTT 9779 b. Advection of these clouds onto the dayside could explain the high dayside albedo previously inferred for this planet and be part of a feedback loop aiding the survival of LTT 9779 b’s atmosphere in the hot Neptune desert.

We constrain the sky-projected obliquities of two low-density hot Neptune planets, HATS-38 b and WASP-139 b, orbiting nearby G and K stars using Rossiter–McLaughlin (RM) observations with VLT/ESPRESSO, yielding $\lambda =-{108}_{-16}^{+11}$ deg and $-{85.6}_{-4.2}^{+7.7}$ deg, respectively. To model the RM effect, we use a new publicly available code, ironman , which is capable of jointly fitting transit photometry, Keplerian radial velocities, and RM effects. WASP-139 b has a residual eccentricity $e={0.103}_{-0.041}^{+0.050}$ while HATS-38 b has an eccentricity of $e={0.112}_{-0.070}^{+0.072}$ , which is compatible with a circular orbit given our data. Using the obliquity constraints, we show that they join a growing group of hot and low-density Neptunes on polar orbits. We use long-term radial velocities to rule out companions with masses ∼0.3–50 M _J within ∼10 au. We show that the orbital architectures of the two Neptunes can be explained with high-eccentricity migration from ≳2 au driven by an unseen distant companion. If HATS-38 b has no residual eccentricity, its polar and circular orbit can also be consistent with a primordial misalignment. Finally, we perform a hierarchical Bayesian modeling of the true obliquity distribution of Neptunes and find suggestive evidence for a higher preponderance of polar orbits for hot Neptunes compared to Jupiters. However, we note that the exact distribution is sensitive to the choice of priors, highlighting the need for additional obliquity measurements of Neptunes to robustly compare the hot Neptune obliquity distribution to that of Jupiters.

Among Neptunian mass exoplanets (20−50 M _⊕ ), puffy hot Neptunes are extremely rare, and their unique combination of low mass and extended radii implies very low density ( ρ < 0.3 g cm ^−3 ). Over the last decade, only a few puffy planets have been detected and precisely characterized with both transit and radial velocity observations, most notably including WASP-107 b , TOI-1420 b , and WASP-193 b . In this paper, we report the discovery of TOI-1173 A b , a low-density ( $\rho ={0.195}_{-0.017}^{+0.018}$ g cm ^−3 ) super-Neptune with P = 7.06 days in a nearly circular orbit around the primary G-dwarf star in the wide binary system TOI-1173 A/B. Using radial velocity observations with the MAROON-X and HIRES spectrographs and transit photometry from TESS, we determine a planet mass of M _p = 27.4 ± 1.7 M _⊕ and radius of R _p = 9.19 ± 0.18 R _⊕ . TOI-1173 A b is the first puffy super-Neptune planet detected in a wide binary system (projected separation ∼11,400 au). We explore several mechanisms to understand the puffy nature of TOI-1173 A b and show that tidal heating is the most promising explanation. Furthermore, we demonstrate that TOI-1173 A b likely has maintained its orbital stability over time and may have undergone von-Zeipel–Lidov–Kozai migration followed by tidal circularization, given its present-day architecture, with important implications for planet migration theory and induced engulfment into the host star. Further investigation of the atmosphere of TOI-1173 A b will shed light on the origin of close-in low-density Neptunian planets in field and binary systems, while spin–orbit analyses may elucidate the dynamical evolution of the system.

Hot Neptunes, gaseous planets smaller than Saturn (∼3–8 R _⊕ ) with orbital periods less than 10 days, are rare. Models predict this is due to high-energy stellar irradiation stripping planetary atmospheres over time, often leaving behind only rocky planetary cores. Using our TESS full-frame-image pipeline giants in conjunction with Keck/HIRES radial velocity measurements, we present the discovery of TIC365102760 b, a 6.2 R _⊕ (0.55 R _J ), 19.2 M _⊕ (0.060 M _J ) planet transiting a red giant star every 4.21285 days. The old age and high equilibrium temperature yet remarkably low density of this planet ( ${\rho }_{p}={0.58}_{-0.20}^{+0.30}{\rho }_{{\rm{J}}}$ ) suggest that its gaseous envelope should have been stripped by high-energy stellar irradiation billions of years ago. The present-day planet mass and radius suggest the atmospheric stripping was slower than predicted. Unexpectedly low stellar activity and/or late-stage planet inflation could be responsible for the observed properties of this system. Further studies of this system with more precise photometry in multiple passbands will be capable of revealing more details of this planet’s atmosphere.

The hot Saturn population exhibits a boundary in mass–radius space, such that no planets are observed at a density less than ∼0.1 g cm ^−3 . Yet, planet interior structure models can readily construct such objects as the natural result of radius inflation. Here, we investigate the role X-ray and extreme UV irradiation (XUV)-driven mass loss plays in sculpting the density boundary by constructing interior structure models that include radius inflation, photoevaporative mass loss, and a simple prescription of Roche lobe overflow. We demonstrate that planets puffier than ∼0.1 g cm ^−3 experience a runaway mass loss caused by adiabatic radius expansion as the gas layer is stripped away, providing a good explanation of the observed edge in mass–radius space. The process is also visible in the radius–period and mass–period spaces, though smaller, high-bulk-metallicity planets can still survive at short periods, preserving a partial record of the population distribution at formation.

The demographics of young exoplanets can shed light on their formation and evolution processes. Exoplanet properties are derived from the properties of their host stars. As such, it is important to accurately characterize the host stars since any systematic biases in their derivation can negatively impact the derivation of planetary properties. Here we present a uniform catalog of photometrically derived stellar effective temperatures, luminosities, radii, and masses for 4865 young (

We present an atmospheric analysis of LTT 9779 b, a rare planet situated in the hot-Neptune desert, that has been observed with Hubble Space Telescope (HST)/WFC3 with G102 and G141. The combined transmission spectrum, which covers 0.8–1.6 μ m, shows a gradual increase in transit depth with wavelength. Our preferred atmospheric model shows evidence for H _2 O, CO _2 , and FeH with a significance of 3.1 σ , 2.4 σ , and 2.1 σ , respectively. In an attempt to constrain the rate of atmospheric escape for this planet, we search for the 1.083 μ m helium line in the G102 data but find no evidence of excess absorption that would indicate an escaping atmosphere using this tracer. We refine the orbital ephemerides of LTT 9779 b using our HST data and observations from TESS, searching for evidence of orbital decay or apsidal precession, which are not found. The phase-curve observation of LTT 9779 b with JWST NIRISS should provide deeper insights into the atmosphere of this planet and the expected atmospheric escape might be detected with further observations concentrated on other tracers such as Ly α .

Photoevaporation is a potential explanation for several features within exoplanet demographics. Atmospheric escape observed in young Neptune-sized exoplanets can provide insight into and characterize which mechanisms drive this evolution and at what times they dominate. AU Mic b is one such exoplanet, slightly larger than Neptune (4.19 R _⊕ ). It closely orbits a 23 Myr pre-main-sequence M dwarf with an orbital period of 8.46 days. We obtained two visits of AU Mic b at Ly α with Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph. One flare within the first HST visit is characterized and removed from our search for a planetary transit. We present a nondetection in our first visit, followed by the detection of escaping neutral hydrogen ahead of the planet in our second visit. The outflow absorbed ∼30% of the star’s Ly α blue wing 2.5 hr before the planet’s white-light transit. We estimate that the highest-velocity escaping material has a column density of 10 ^13.96 cm ^−2 and is moving 61.26 km s ^−1 away from the host star. AU Mic b’s large high-energy irradiation could photoionize its escaping neutral hydrogen in 44 minutes, rendering it temporarily unobservable. Our time-variable Ly α transit ahead of AU Mic b could also be explained by an intermediate stellar wind strength from AU Mic that shapes the escaping material into a leading tail. Future Ly α observations of this system will confirm and characterize the unique variable nature of its Ly α transit, which, combined with modeling, will tune the importance of stellar wind and photoionization.

We report the confirmation of three exoplanets transiting TOI-4010 (TIC-352682207), a metal-rich K dwarf observed by the Transiting Exoplanet Survey Satellite in Sectors 24, 25, 52, and 58. We confirm these planets with the High Accuracy Radial velocity Planet Searcher for the Northern Hemisphere radial velocity observations and measure their masses with 8−12% precision. TOI-4010 b is a sub-Neptune ( P = 1.3 days, ${R}_{p}={3.02}_{-0.08}^{+0.08}\,{R}_{\oplus }$ , ${M}_{p}={11.00}_{-1.27}^{+1.29}\,{M}_{\oplus }$ ) in the hot-Neptune desert, and is one of the few such planets with known companions. Meanwhile, TOI-4010 c ( P = 5.4 days, ${R}_{p}={5.93}_{-0.12}^{+0.11}\,{R}_{\oplus }$ , ${M}_{p}={20.31}_{-2.11}^{+2.13}\,{M}_{\oplus }$ ) and TOI-4010 d ( P = 14.7 days, ${R}_{p}={6.18}_{-0.14}^{+0.15}\,{R}_{\oplus }$ , ${M}_{p}={38.15}_{-3.22}^{+3.27}\,{M}_{\oplus }$ ) are similarly sized sub-Saturns on short-period orbits. Radial velocity observations also reveal a super-Jupiter-mass companion called TOI-4010 e in a long-period, eccentric orbit ( P ∼ 762 days and e ∼ 0.26 based on available observations). TOI-4010 is one of the few systems with multiple short-period sub-Saturns to be discovered so far.