Compositions of rocky exoplanets: The interior compositions of small rocky exoplanets cannot be observed directly but are expected to relate to the composition of the host star. Adibekyan et al . analyzed a sample of rocky exoplanets, inferring the planets’ iron fractions by combining their masses and radii with an interior structure model. The iron fractions of the host stars were calculated from stellar elemental abundances. The two iron fractions, that of the planets and that of the stars, correlate with each other, but the slope is steeper than 1, indicating that planet formation processes modify the compositions of rocky planets. —KTS

In recent years, the advent of a new generation of radial velocity instruments has allowed us to detect planets with increasingly lower mass and to break the one Earth-mass barrier. Here we report a new milestone in this context by announcing the detection of the lowest-mass planet measured so far using radial velocities: L 98-59 b, a rocky planet with half the mass of Venus. It is part of a system composed of three known transiting terrestrial planets (planets b–d). We announce the discovery of a fourth nontransiting planet with a minimum mass of 3.06_−0.37^+0.33 M_⊕and an orbital period of 12.796_−0.019^+0.020days and report indications for the presence of a fifth nontransiting terrestrial planet. With a minimum mass of 2.46_−0.82^+0.66 M_⊕and an orbital period 23.15_−0.17^+0.60days, this planet, if confirmed, would sit in the middle of the habitable zone of the L 98-59 system. L 98-59 is a bright M dwarf located 10.6ṗc away. Positioned at the border of the continuous viewing zone of theJames WebbSpace Telescope, this system is destined to become a corner stone for comparative exoplanetology of terrestrial planets. The three transiting planets have transmission spectrum metrics ranging from 49 to 255, which undoubtedly makes them prime targets for an atmospheric characterization with theJames WebbSpace Telescope, theHubbleSpace Telescope, Ariel, or ground-based facilities such as NIRPS or ESPRESSO. With an equilibrium temperature ranging from 416 to 627 K, they offer a unique opportunity to study the diversity of warm terrestrial planets without the unknowns associated with different host stars. L 98-59 b and c have densities of 3.6_−1.5^+1.4and 4.57_−0.85^+0.77g cm⁻³, respectively, and have very similar bulk compositions with a small iron core that represents only 12 to 14% of the total mass, and a small amount of water. However, with a density of 2.95_−0.51^+0.79g cm⁻³and despite a similar core mass fraction, up to 30% of the mass of L 98-59 d might be water.

Transmission spectroscopy during planetary transits, which is based on the measurements of the variations of the planet-to-star radius ratio as a function of wavelength, is a powerful technique to study the atmospheric properties of transiting planets. One of the main limitations of this technique is the effects of stellar activity, which up until now, have been taken into account only by assessing the effect of non-occulted stellar spots on the estimates of the planet-to-star radius ratio. In this paper, we study the impact of the occultation of a stellar spot and plage on the transmission spectra of transiting exoplanets for the first time. We simulated this effect by generating a large number of transit light curves for different transiting planets, stellar spectral types, and different wavelengths. Results of our simulations indicate that the anomalies inside the transit light curve can lead to a significant underestimation or overestimation of the planet-to-star radius ratio as a function of wavelength. At short wavelengths, the effect can reach to a difference of up to 10% in the planet-to-star radius ratio, mimicking the signature of light scattering in the planetary atmosphere. Atmospheric scattering has been proposed to interpret the increasing slopes of transmission spectra toward blue for exoplanets HD 189733b and GJ 3470b. Here, we show that these signatures can be alternatively interpreted by the occultation of stellar plages. Results also suggest that the best strategy to identify and quantify the effects of stellar activities on the transmission spectrum of a planet is to perform several observations during the transit epoch at the same wavelength. This will allow for identifying the possible variations in transit depth as a function of time due to stellar activity variability.

Stellar-activity features such as spots can complicate the determination of planetary parameters through spectroscopic and photometric observations. The overlap of a transiting planet and a stellar spot, for instance, can produce anomalies in the transit light-curves that may lead to an inaccurate estimation of the transit duration, depth, and timing. These inaccuracies can for instance affect the precise derivation of the planet radius. We present the results of a quantitative study on the effects of stellar spots on high-precision transit light-curves. We show that spot anomalies can lead to an estimate of a planet radius that is 4% smaller than the real value. Likewise, the transit duration may be estimated about 4%, longer or shorter. Depending on the size and distribution of spots, anomalies can also produce transit-timing variations (TTVs) with significant amplitudes. For instance, TTVs with signal amplitudes of 200 s can be produced when the spot is completely dark and has the size of the largest Sun spot. Our study also indicates that the smallest size of a stellar spot that still has detectable effects on a high-precision transit light-curve is around 0.03 time the stellar radius for typical Kepler Telescope precision. We also show that the strategy of including more free parameters (such as transit depth and duration) in the fitting procedure to measure the transit time of each individual transit will not produce accurate results for active stars.

Context. The search for planets orbiting metal-poor stars is of utmost importance for our understanding of planet formation models. However, no dedicated searches have been conducted so far for very low mass planets orbiting such objects. Only a few cases of low-mass planets orbiting metal-poor stars are thus known. Amongst these, HD 41248 is a metal-poor, solar-type star on the orbit of which a resonant pair of super-Earth-like planets has been announced. This detection was based on 62 radial velocity measurements obtained with the HARPS spectrograph (public data).Aims. We present a new planet search program that is using the HARPS spectrograph to search for Neptunes and super-Earths that orbit a sample of metal-poor FGK dwarfs. We then present a detailed analysis of 162 additional radial velocity measurements of HD 41248, obtained within this program, with the goal of confirming the existence of the proposed planetary system.Methods. We analysed the precise radial velocities, obtained with the HARPS spectrograph, together with several stellar activity diagnostics and line profile indicators.Results. A careful analysis shows no evidence for the planetary system. One of the signals, with a period of ~25 days, is shown to be related to the rotational period of the star, and is clearly seen in some of the activity proxies. We were unable to convincingly retrieve the remaining signal (P ~ 18 days) in the new dataset.Conclusions. We discuss possible causes for the complex (evolving) signals observed in the data of HD 41248, proposing that they might be explained by the appearance and disappearance of active regions on the surface of a star with strong differential rotation, or by a combination of the sparse data sampling and active region evolution.