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We report on precise Doppler measurements of L231-32 (TOI-270), a nearby M dwarf (d  = 22 pc, M ⋆ = 0.39 M⊙, R ⋆ = 0.38 R⊙), which hosts three transiting planets that were recently discovered using data from the Transiting Exoplanet Survey Satellite (TESS). The three planets are 1.2, 2.4, and 2.1 times the size of Earth and have orbital periods of 3.4, 5.7, and 11.4 d. We obtained 29 high-resolution optical spectra with the newly commissioned Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) and 58 spectra using the High Accuracy Radial velocity Planet Searcher (HARPS). From these observations, we find the masses of the planets to be 1.58 ± 0.26, 6.15 ± 0.37, and 4.78 ± 0.43 M⊕, respectively. The combination of radius and mass measurements suggests that the innermost planet has a rocky composition similar to that of Earth, while the outer two planets have lower densities. Thus, the inner planet and the outer planets are on opposite sides of the 'radius valley' – a region in the radius-period diagram with relatively few members – which has been interpreted as a consequence of atmospheric photoevaporation. We place these findings into the context of other small close-in planets orbiting M dwarf stars, and use support vector machines to determine the location and slope of the M dwarf (T eff < 4000 K) radius valley as a function of orbital period. We compare the location of the M dwarf radius valley to the radius valley observed for FGK stars, and find that its location is a good match to photoevaporation and core-powered mass-loss models. Finally, we show that planets below the M dwarf radius valley have compositions consistent with stripped rocky cores, whereas most planets above have a lower density consistent with the presence of a H-He atmosphere. [ABSTRACT FROM AUTHOR]

In favourable conditions, the density of transiting planets in multiple systems can be determined from photometry data alone. Dynamical information can be extracted from light curves, providing modelling is done self-consistently, i.e. using a photodynamical model, which simulates the individual photometric observations instead of the more generally used transit times. We apply this methodology to the Kepler-138 planetary system. The derived planetary bulk densities are a factor of 2 more precise than previous determinations, and we find a discrepancy in the stellar bulk density with respect to a previous study. This leads, in turn, to a discrepancy in the determination of masses and radii of the star and the planets. In particular, we find that interior planet, Kepler-138b, has a size in between Mars and the Earth. Given our mass and density estimates, we characterize the planetary interiors using a generalized Bayesian inference model. This model allows us to quantify for interior degeneracy and calculate confidence regions of interior parameters such as thicknesses of the core, the mantle, and ocean and gas layers. We find that Kepler-138b and Kepler-138 d have significantly thick volatile layers and that the gas layer of Kepler-138b is likely enriched. On the other hand, Kepler-138c can be purely rocky. [ABSTRACT FROM AUTHOR]

The masses and radii of extrasolar planets are key observables for understanding their interior, formation and evolution. While transit photometry and Doppler spectroscopy are used to measure the radii and masses respectively of planets relative to those of their host star, estimates for the true values of these quantities rely on theoretical models of the host star which are known to suffer from systematic differences with observations. When a system is composed of more than two bodies, extra information is contained in the transit photometry and radial velocity data. Velocity information (finite speed-of-light, Doppler) is needed to break the Newtonian MR-3 degeneracy. We performed a photodynamical modelling of the two-planet transiting system Kepler-117 using all photometric and spectroscopic data available. We demonstrate how absolute masses and radii of single-star planetary systems can be obtained without resorting to stellar models. Limited by the precision of available radial velocities (38 m s-1), we achieve accuracies of 20 per cent in the radii and 70 per cent in the masses, while simulated 1 m s-1 precision radial velocities lower these to 1 per cent for the radii and 2 per cent for the masses. Since transiting multiplanet systems are common, this technique can be used to measure precisely the mass and radius of a large sample of stars and planets. We anticipate these measurements will become common when the TESS and PLATO mission provide high-precision light curves of a large sample of bright stars. These determinations will improve our knowledge about stars and planets, and provide strong constraints on theoretical models. [ABSTRACT FROM AUTHOR]

Stellar activity, such as starspots, can induce radial velocity (RV) variations that can mask or even mimic the RV signature of orbiting exoplanets. For this reason RV exoplanet surveys have been unsuccessful when searching for planets around young, active stars and are therefore failing to explore an important regime which can help to reveal how planets form and migrate. This paper describes a new technique to remove spot signatures from the stellar line-profiles of moderately rotating, active stars (v sin i ranging from 10 to 50 km s−1). By doing so it allows planetary RV signals to be uncovered. We used simulated models of a G5V type star with differing dark spots on its surface along with archive data of the known active star HD 49933 to validate our method. The results showed that starspots could be effectively cleaned from the line-profiles so that the stellar RV jitter was reduced by more than 80 per cent. Applying this procedure to the same models and HD 49933 data, but with fake planets injected, enabled the effective removal of starspots so that Jupiter mass planets on short orbital periods were successfully recovered. These results show that this approach can be useful in the search for hot-Jupiter planets that orbit around young, active stars with a v sin i of ∼10–50 km s−1. [ABSTRACT FROM AUTHOR]

ABSTRACT A variation of gravitational redshift, arising from stellar radius fluctuations, will introduce astrophysical noise into radial velocity measurements by shifting the centroid of the observed spectral lines. Shifting the centroid does not necessarily introduce line asymmetries. This is fundamentally different from other types of stellar jitter so far identified, which do result from line asymmetries. Furthermore, only a very small change in stellar radius, ∼0.01 per cent, is necessary to generate a gravitational redshift variation large enough to mask or mimic an Earth-twin. We explore possible mechanisms for stellar radius fluctuations in low-mass stars. Convective inhibition due to varying magnetic field strengths and the Wilson depression of starspots are both found to induce substantial gravitational redshift variations. Finally, we investigate a possible method for monitoring/correcting this newly identified potential source of jitter and comment on its impact for future exoplanet searches. [ABSTRACT FROM AUTHOR]