Keywords

The Kepler mission has discovered thousands of exoplanets around various stars with different spectral types (M, K, G, and F) and thus different masses and effective temperatures. Previous studies have shown that the planet occurrence rate, in terms of the average number of planets per star, drops with increasing stellar effective temperature (T_eff). In this paper, with the final Kepler Data Release (DR25) catalog, we revisit the relation between stellar effective temperature (as well as mass) and planet occurrence, but in terms of the fraction of stars with planets and the number of planets per planetary system (i.e., planet multiplicity). We find that both the fraction of stars with planets and planet multiplicity decrease with increasing stellar temperature and mass. Specifically, about 75% late-type stars (T_eff < 5000 K) have Kepler-like planets with an average planet multiplicity of ∼2.8, while for early-type stars (T_eff > 6500 K) this fraction and the average multiplicity fall down to ∼35% and ∼1.8, respectively. The decreasing trend in the fraction of stars with planets is very significant with ΔAIC > 30, though the trend in planet multiplicity is somewhat tentative with ΔAIC ∼ 5. Our results also allow us to derive the dispersion of planetary orbital inclinations in relationship with stellar effective temperature. Interestingly, it is found to be similar to the well-known trend between obliquity and stellar temperature, indicating that the two trends might have a common origin.

Seeing oceans, continents, quasi-static weather, and other surface features on exoplanets may allow for detecting and characterizing life outside the solar system. The Proxima b exoplanet resides within the stellar habitable zone, possibly allowing for liquid water on its surface, as on Earth. However, even the largest planned telescopes will not be able to resolve its surface features directly. Here we demonstrate an inversion technique to indirectly image exoplanet surfaces using observed unresolved reflected light variations over the course of the exoplanet's orbital and axial rotation: ExoPlanet Surface Imaging (EPSI). We show that the reflected light curve contains enough information to detect both longitudinal and latitudinal structures and to map exoplanet surface features. We demonstrate this using examples of solar system planets and moons, as well as simulated planets with Earth-like life and artificial structures. We also describe how it is possible to infer the planet and orbit geometry from light curves. Then, we show how albedo maps of Proxima b can be successfully reconstructed for tidally locked, resonance, and unlocked axial and orbital rotation. Such albedo maps obtained in different wavelength passbands can provide "photographic" views of distant exoplanets. We estimate the signal-to-noise ratio necessary for successful inversions and analyze telescope and detector requirements necessary for the first surface image reconstructions of Proxima b and other nearby exoplanets using EPSI. This is a significant challenge, but the success of such measurements depends heavily on large-aperture diffraction-limited telescope performance—a feat that may be achieved on the ground before it is in space.

Every Sun-like star will eventually evolve into a red giant, a transition which can profoundly affect the evolution of a surrounding planetary system. The timescale of dynamical planet evolution and orbital decay has important implications for planetary habitability, as well as post-main-sequence star and planet interaction, evolution, and internal structure. Here, we investigate these effects by estimating planet occurrence around 2476 low-luminosity red giant branch (LLRGB) stars observed by the NASA K2 mission. We measure stellar masses and radii using asteroseismology, with median random uncertainties of 3.7% in mass and 2.2% in radius. We compare this planet population to the known population of planets around dwarf Sun-like stars, accounting for detection efficiency differences between the stellar populations. We find that 0.49% ± 0.28% of LLRGB stars host planets larger than Jupiter with orbital periods less than 10 days, tentatively higher than main-sequence stars hosting similar planets (0.15% ± 0.06%). Our results suggest that the effects of stellar evolution on the occurrence of close-in planets larger than Jupiter are not significant until stars have begun ascending substantially up the red giant branch (≳5–6 R_⊙).

The compact, nonthermal emission in DoAr21 has been studied with the Very Long Baseline Array (VLBA) to investigate the possibility that the residuals of the astrometry fitting are due to the reflex motion induced by a possible companion. We find that the fitting of VLBA astrometric observations of DoAr21 improves significantly by adding the orbital motions of three companions. We obtain an improved distance to the source of 134.6 ± 1.0 pc, and estimate that the central star, DoAr21, has a mass of about 2.04 ± 0.70 M_⊙. We suggest that DoAr21 represents a unique case where two substellar companions, DoAr21b and DoAr21c (m_b ∼ 35.6 ± 27.2 M_jup and m_c ∼ 44.0 ± 13.6 M_jup, respectively), have been found to be associated with a relatively low-mass, pre-main sequence star. In addition, we find that this WTTau star is an astrometric double system, having a low-mass star companion, DoAr21B (m_B ∼ 0.35 ± 0.12 M_⊙), in a relatively eccentric orbit. The orbit of this low-mass stellar companion is compact, while the brown dwarfs are located in external orbits. DoAr21c has the strongest astrometric signature in the periodogram, while DoAr21B has a weak but significant signature. On the other hand, the astrometric signature of DoAr21b does not appear in the periodogram, however, this brown dwarf was directly detected in some of the VLBA observations. The estimated orbital periods of DoAr21B, DoAr21b, and DoAr21c are P_B ∼ 92.92 ± 0.02, P_b ∼ 450.9 ± 3.8, and P_c ∼ 1013.5 ± 25.3 days, respectively. Since the estimated age of this young star is about 0.4–0.8 Myr, the detected brown dwarf companion is among the youngest companions observed to date.

We present here new observations of the eccentric debris ring surrounding the Gyr-old solar-type star HD 202628: at millimeter wavelengths with ALMA, at far-infrared wavelengths with Herschel, and in scattered light with the Hubble Space Telescope (HST). The ring inner edge is found to be consistent between ALMA and HST data. As radiation pressure affects small grains seen in scattered-light, the ring appears broader at optical than at millimeter wavelengths. The best fit to the ring seen with ALMA has inner and outer edges at 143.1 ± 1.7 au and 165.5 ± 1.4, respectively, and an inclination of 574 ± 0.4 from face-on. The offset of the ring center of symmetry from the star allows us to quantify its eccentricity to be $e={0.09}_{-0.01}^{+0.02}$. This eccentric feature is also detected in low resolution Herschel/PACS observations, under the form of a pericenter-glow. Combining the infrared and millimeter photometry, we retrieve a disk grain size distribution index of ∼−3.4, and therefore exclude in situ formation of the inferred belt-shaping perturber, for which we provide new dynamical constraints. Finally, ALMA images show four point-like sources that exceed 100 μJy, one of them being just interior to the ring. Although the presence of a background object cannot be excluded, we cannot exclude either that this source is circumplanetary material surrounding the belt-shaper, in which case degeneracies between its mass and orbital parameters could be lifted, allowing us to fully characterize such a distant planet in this mass and age regime for the very first time.

Wide-field surveys for transiting planets are well suited to searching diverse stellar populations, enabling a better understanding of the link between the properties of planets and their parent stars. We report the discovery of HAT-P-69 b (TOI 625.01) and HAT-P-70 b (TOI 624.01), two new hot Jupiters around A stars from the Hungarian-made Automated Telescope Network (HATNet) survey that have also been observed by the Transiting Exoplanet Survey Satellite. HAT-P-69 b has a mass of ${3.58}_{-0.58}^{+0.58}$ M_Jup and a radius of ${1.676}_{-0.033}^{+0.051}$ R_Jup and resides in a prograde 4.79 day orbit. HAT-P-70 b has a radius of ${1.87}_{-0.10}^{+0.15}$ R_Jup and a mass constraint of $\lt 6.78\,(3\sigma )$ M_Jup and resides in a retrograde 2.74 day orbit. We use the confirmation of these planets around relatively massive stars as an opportunity to explore the occurrence rate of hot Jupiters as a function of stellar mass. We define a sample of 47,126 main-sequence stars brighter than T_mag = 10 that yields 31 giant planet candidates, including 18 confirmed planets, 3 candidates, and 10 false positives. We find a net hot Jupiter occurrence rate of 0.41 ± 0.10% within this sample, consistent with the rate measured by Kepler for FGK stars. When divided into stellar mass bins, we find the occurrence rate to be 0.71 ± 0.31% for G stars, 0.43 ± 0.15% for F stars, and 0.26 ± 0.11% for A stars. Thus, at this point, we cannot discern any statistically significant trend in the occurrence of hot Jupiters with stellar mass.

The detection of transiting exoplanets in time-series photometry requires the removal or modeling of instrumental and stellar noise. While instrumental systematics can be reduced using methods such as pixel level decorrelation, removing stellar trends while preserving transit signals proves challenging. As a result of vast archives of light curves from recent transit surveys, there is a strong need for accurate automatic detrending, without human intervention. A large variety of detrending algorithms are in active use, but their comparative performance for transit discovery is unexplored. We benchmark all commonly used detrending methods against hundreds of Kepler, K2, and TESS planets, selected to represent the most difficult cases for systems with small planet-to-star radius ratios. The full parameter range is explored for each method to determine the best choices for planet discovery. We conclude that the ideal method is a time-windowed slider with an iterative robust location estimator based on Tukey's biweight. This method recovers 99% and 94% of the shallowest Kepler and K2 planets, respectively. We include an additional analysis for young stars with extreme variability and conclude they are best treated using a spline-based method with a robust Huber estimator. All stellar detrending methods explored are available for public use in Wōtan, an open-source Python package on GitHub (https://github.com/hippke/wotan).

We present moderate resolution near-infrared spectra in the H, J, and K band of M-dwarf hosts to candidate transiting exoplanets discovered by NASA's K2 mission. We employ known empirical relationships between spectral features and physical stellar properties to measure the effective temperature, radius, metallicity, and luminosity of our sample. Out of an initial sample of 56 late-type stars in K2, we identify 35 objects as M dwarfs. For that subsample, we derive temperatures ranging from 2870 to 4187 K, radii of 0.09–0.83 R_⊙, luminosities of $-2.67\lt \mathrm{log}L/{L}_{\odot }\lt -0.67$, and [Fe/H] metallicities between −0.49 and 0.51 dex. We then employ the stellar properties derived from spectra, in tandem with the K2 light curves, to characterize their planets. We report 33 exoplanet candidates with orbital periods ranging from 0.19 to 21.16 days, and median radii and equilibrium temperatures of 2.3 R_⊕ and 986 K, respectively. Using planet mass–radius relationships from the literature, we identify seven exoplanets as potentially rocky, although we conclude that probably none reside in the habitable zone of their parent stars.

The technique of transmission spectroscopy allows us to constrain the chemical composition of the atmospheres of transiting exoplanets. It relies on very high signal-to-noise spectroscopic (or spectrophotometric) observations and is thus most suited for bright exoplanet host stars. In the era of the Transiting Exoplanet Survey Satellite, Next Generation Space Telescope, and PLAnetary Transits and Oscillations of stars (PLATO), more and more suitable targets, even for mid-sized telescopes, are discovered. Furthermore, a wealth of archival data is available that could become a basis for long-term monitoring of exo-atmospheres. We analyzed archival High Accuracy Radial velocity Planet Searcher (HARPS) spectroscopic time series of four host stars to transiting bloated gas exoplanets, namely WASP-76b, WASP-127b, WASP-166b, and KELT-11b, searching for traces of sodium (sodium doublet), hydrogen (Hα, Hβ), and lithium (670.8 nm). The archival data sets include spectroscopic time series taken during transits. Comparing in- and out-of-transit spectra we can filter out the stellar lines and investigate the absorption from the planet. Simultaneously, the stellar activity is monitored using the Mg i and Ca i lines. We detect sodium in the atmosphere of WASP-76b at a 7–9σ level. Furthermore, we report also at a 4–8σ level of significance the detection of sodium in the atmosphere of WASP-127b, confirming earlier results based on low-resolution spectroscopy. The data show no sodium nor any other atom at high confidence levels for WASP-166b nor KELT-11b, hinting at the presence of thick high clouds.

Saturn is ringing weakly. Exquisite data from the Cassini mission reveal the presence of f-mode oscillations as they excite density waves in Saturn's rings. These oscillations have displacement amplitudes of order 1 m on Saturn's surface. We propose that they result from large impacts in the past. Experiencing little dissipation inside Saturn on account of its weak luminosity, f-modes may live virtually forever, but the very ring waves that reveal their existence also remove energy from them, in 10⁴ to 10⁷ yr for the observed f-modes (spherical degree 2–10). We find that the largest impacts that arrive during these times excite the modes to their current levels, with the exception of the few lowest-degree modes. To explain the latter, either a fortuitously large impact in the recent past or a new source of stochastic excitation is needed. We extend this scenario to Jupiter, which has no substantial rings. With an exceedingly long memory of past bombardments, Jovian f-modes and p-modes can acquire much higher amplitudes, possibly explaining past reports of radial velocity detections, and are potentially detectable by the Juno spacecraft.