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Context. Most of our current knowledge on planet formation is still based on the analysis of main sequence, solar-type stars. Conversely, detailed chemical studies of large samples of M dwarfs hosting planets are still missing. Aims. Correlations exist between the presence of different types of planets around FGK stars and metallicity, individual chemical abundance, and stellar mass. We aim to test whether or not these correlations still hold for the less-massive M dwarf stars. Methods to determine stellar abundances of M dwarfs from high-resolution optical spectra in a consistent way are still missing. The present work is a first attempt to fill this gap. Methods. We analyse a large sample of M dwarfs with and without known planetary companions in a coherent and homogeneous way. We develop for the first time a methodology to determine stellar abundances of elements other than iron for M dwarf stars from high-resolution optical spectra. Our methodology is based on the use of a principal component analysis and sparse Bayesian methods. We made use of a set of M dwarfs orbiting around an FGK primary with known abundances to train our methods. We applied our methods to derive stellar metalliticies and abundances of a large sample of M dwarfs observed within the framework of current radial-velocity surveys. We then used a sample of nearby FGK stars to cross-validate our technique by comparing the derived abundance trends in the M dwarf sample with those found on the FGK stars. Results. The metallicity distribution of the different subsamples reveals a correlation between the metallicities of M dwarfs and their probability of hosting giant planets. We also find a correlation between this latter probability and stellar mass. M dwarfs hosting low-mass planets do not seem to follow the so-called planet–metallicity correlation. We also find that the frequency of low-mass planets does not depend on the mass of the stellar host. These results appear to be in agreement with those of previous works. However, we note that for giant-planet hosts our metallicities predict a weaker planet–host metallicity correlation but a stronger mass-dependency than corresponding values derived from photometric results. We show for the first time that there seems to be no differences between M dwarfs with and without known planets in terms of their abundance distributions of elements different from iron. Conclusions. Our data show that low-mass stars with planets follow the same metallicity, mass, and abundance trends as their FGK counterparts, which are usually explained within the framework of core-accretion models.

Current theories of planetary evolution predict that infant giant planets have large radii and very low densities before they slowly contract to reach their final size after about several hundred million years 1,2 . These theoretical expectations remain untested so far as the detection and characterization of very young planets is extremely challenging due to the intense stellar activity of their host stars 3,4 . Only the recent discoveries of young planetary transiting systems allow initial constraints to be placed on evolutionary models [5][6][7] . With an estimated age of 20 million years, V1298 Tau is one of the youngest solar-type stars known to host transiting planets; it harbours a system composed of four planets, two Neptune-sized, one Saturn-sized and one Jupiter-sized 8,9 .Here we report a multi-instrument radial velocity campaign of V1298 Tau, which allowed us to determine the masses of two of the planets in the system. We find that the two outermost giant planets, V1298 Tau b and e (0.64 ± 0.19 and 1.16 ± 0.30 Jupiter masses, respectively), seem to contradict our knowledge of early-stages planetary evolution. According to models, they should reach their mass-radius combination only hundreds of millions of years after formation. This result suggests that giant planets can contract much more quickly than usually assumed.V1298 Tau is a relatively bright (V = 10.1), very young K1 star with a mass of 1.170 ± 0.060 M ⊙ (where M ⊙ is the solar mass), a radius of 1.278 ± 0.070 R ⊙ (where R ⊙ is the solar radius), an effective temperature of 5,050 ± 100 K and solar metallicity (Table 1 and Extended Data Fig. 1). It is the physical companion of the G2 star HD 284154. The pair belongs to the Group 29 stellar association 10 and has an age of 20 ± 10 Myr (Extended Data Figs. 1 and 2). V1298 Tau was observed by Kepler's 'Second Light' K2 mission 11 . The analysis of the K2 data revealed the presence of four transiting planets in the system 9 . The three inner planets (b, c and d) were determined to have orbital periods of 24.1396 ± 0.0018, 8.24958 ± 0.00072 and 12.4032 ± 0.0015 days, and radii of 0.916 +0.052 −0.047 , 0.499 +0.032 −0.029 and 0.572 +0.040 −0.035 R Jup (where R Jup is the Jupiter radius). The fourth planet, e, was identified with only a single transit event, with a radius of 0.780 +0.075 −0.064 R Jup and orbital period estimated to be between 40 and 120 days. A previous study constrained the mass of V1298 Tau b to be less than 2.2 M Jup (ref. 12 ) (where M Jup is the Jupiter mass).To measure the planetary masses, we performed an intensive spectroscopic campaign, collecting more than 260 radial velocity (RV) measurements of V1298 Tau using the high-resolution spectrographs HARPS-N, CARMENES, SES and HERMES between April 2019 and April 2020. To monitor its stellar activity variations, we performed contemporaneous V-band photometry using the Las Cumbres Observatory Global Telescope (LCOGT) network 13 .V1298 Tau is a very active star that induces large RV activity variations. To extract the planetary sig...

Context. Observations of young close-in exoplanets are providing initial indications for the characteristics of the population and clues to the early stages of their evolution. Transiting planets at young ages are also key benchmarks for our understanding of planetary evolution via the verification of atmospheric escape models. Aims. We performed radial velocity (RV) monitoring of the 40 Myr old star DS Tuc A with HARPS at the ESO-3.6 m to determine the planetary mass of its 8.14-day planet, which was first revealed by the NASA TESS satellite. We also observed two planetary transits with HARPS and ESPRESSO at ESO-VLT to measure the Rossiter-McLaughlin (RM) effect and characterise the planetary atmosphere. We measured the high-energy emission of the host with XMM-Newton observations to investigate models for atmospheric evaporation. Methods. We employed a Gaussian Processes (GP) regression to model the high level of the stellar activity, which is more than 40 times larger than the expected RV planetary signal. GPs were also used to correct the stellar contribution to the RV signal of the RM effect. We extracted the transmission spectrum of DS Tuc A b from the ESPRESSO data and searched for atmospheric elements and molecules either by single-line retrieval and by performing cross-correlation with a set of theoretical templates. Through a set of simulations, we evaluated different scenarios for the atmospheric photo-evaporation of the planet induced by the strong XUV stellar irradiation. Results. While the stellar activity prevented us from obtaining a clear detection of the planetary signal from the RVs, we set a robust mass upper limit of 14.4 M⊕ for DS Tuc A b. We also confirm that the planetary system is almost (but not perfectly) aligned. The strong level of stellar activity hampers the detection of any atmospheric compounds, which is in line with other studies presented in the literature. The expected evolution of DS Tuc A b from our grid of models indicates that the planetary radius after the photo-evaporation phase will be 1.8–2.0 R⊕, falling within the Fulton gap. Conclusions. The comparison of the available parameters of known young transiting planets with the distribution of their mature counterpart confirms that the former are characterised by a low density, with DS Tuc A b being one of the less dense. A clear determination of their distribution is still affected by the lack of a robust mass measurement, particularly for planets younger than ~100 Myr.