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Despite the thousands of planets in orbit around stars known to date, the mechanisms of planetary formation, migration, and atmospheric loss remain unresolved. In this work, we confirm the planetary nature of a young Saturn-size planet transiting a solar-type star every 8.03 d, TOI-1135 b. The age of the parent star is estimated to be in the interval of 125-1000 Myr based on various activity and age indicators, including its stellar rotation period of 5.13 ± 0.27 days and the intensity of photospheric lithium. We obtained follow-up photometry and spectroscopy, including precise radial velocity measurements using the CARMENES spectrograph, which together with the TESS data allowed us to fully characterise the parent star and its planet. As expected for its youth, the star is rather active and shows strong photometric and spectroscopic variability correlating with its rotation period. We modelled the stellar variability using Gaussian process regression. We measured the planetary radius at 9.02 ± 0.23R_⊕(0.81 ± 0.02R_Jup) and determined a 3σupper limit of < 51.4M_⊕(< 0.16M_Jup) on the planetary mass by adopting a circular orbit. Our results indicate that TOI-1135 b is an inflated planet less massive than Saturn or Jupiter but with a similar radius, which could be in the process of losing its atmosphere by photoevaporation. This new young planet occupies a region of the mass-radius diagram where older planets are scarse, and it could be very helpful to understanding the lower frequency of planets with sizes between Neptune and Saturn. 

Chromospheric Ca II activity cycles are frequently found in late-type stars, but no systematic programs have been created to search for their coronal X-ray counterparts. The typical time scale of Ca II activity cycles ranges from years to decades. Therefore, long-lasting missions are needed to detect the coronal counterparts. The XMM-Newton satellite has so far detected X-ray cycles in five stars. A particularly intriguing question is at what age (and at what activity level) X-ray cycles set in. To this end, in 2015 we started the X-ray monitoring of the young solar-like star ɛ Eridani, previously observed on two occasions: in 2003 and in early 2015, both by XMM-Newton . With an age of 440 Myr, it is one of the youngest solar-like stars with a known chromospheric Ca II cycle. We collected the most recent Mount Wilson S-index data available for ɛ Eridani, starting from 2002, including previously unpublished data. We found that the Ca II cycle lasts 2.92 ± 0.02 yr, in agreement with past results. From the long-term XMM-Newton lightcurve, we find clear and systematic X-ray variability of our target, consistent with the chromospheric Ca II cycle. The average X-ray luminosity is 2 × 10 28 erg s −1 , with an amplitude that is only a factor of 2 throughout the cycle. We apply a new method to describe the evolution of the coronal emission measure distribution of ɛ Eridani in terms of solar magnetic structures: active regions, cores of active regions, and flares covering the stellar surface at varying filling fractions. Combinations of these three types of magnetic structures can only describe the observed X-ray emission measure of ɛ Eridani if the solar flare emission measure distribution is restricted to events in the decay phase. The interpretation is that flares in the corona of ɛ Eridani last longer than their solar counterparts. We ascribe this to the lower metallicity of ɛ Eridani. Our analysis also revealed that the X-ray cycle of ɛ Eridani is strongly dominated by cores of active regions. The coverage fraction of cores throughout the cycle changes by the same factor as the X-ray luminosity. The maxima of the cycle are characterized by a high percentage of covering fraction of the flares, consistent with the fact that flaring events are seen in the corresponding short-term X-ray lightcurves predominately at the cycle maxima. The high X-ray emission throughout the cycle of ɛ Eridani is thus explained by the high percentage of magnetic structures on its surface. 

One of the main scientific goals of the TESS mission is the discovery of transiting small planets around the closest and brightest stars in the sky. Here, using data from the CARMENES, MAROON-X, and HIRES spectrographs together with TESS, we report the discovery and mass determination of aplanetary system around the M1.5 V star GJ 806 (TOI-4481). GJ 806 is a bright (V≈ 10.8mag,J≈ 7.3 mag) and nearby (d= 12 pc) M dwarf that hosts at least two planets. The innermost planet, GJ 806 b, is transiting and has an ultra-short orbital period of 0.93 d, a radius of 1.331 ± 0.023R_⊕, a mass of 1.90 ± 0.17M_⊕, a mean density of 4.40 ± 0.45 g cm⁻³, and an equilibrium temperature of 940 ± 10 K. We detect a second, non-transiting, super-Earth planet in the system, GJ 806 c, with an orbital period of 6.6 d, a minimum mass of 5.80 ± 0.30M_⊕, and an equilibrium temperature of 490 ± 5 K. The radial velocity data also shows evidence for a third periodicity at 13.6 d, although the current dataset does not provide sufficient evidence to unambiguously distinguish between a third super-Earth mass (Msini= 8.50 ± 0.45M_⊕) planet or stellar activity. Additionally, we report one transit observation of GJ 806 b taken with CARMENES in search of a possible extended atmosphere of H or He, but we can only place upper limits to its existence. This is not surprising as our evolutionary models support the idea that any possible primordial H/He atmosphere that GJ 806 b might have had would be long lost. However, the bulk density of GJ 806 b makes it likely that the planet hosts some type of volatile atmosphere. With transmission spectroscopy metrics (TSM) of 44 and emission spectroscopy metrics (ESM) of 24, GJ 806 b is to date the third-ranked terrestrial planet around an M dwarf suitable for transmission spectroscopy studies using JWST, and the most promising terrestrial planet for emission spectroscopy studies. GJ 806b is also an excellent target for the detection of radio emission via star-planet interactions. 

Context . The Gl 486 system consists of a very nearby, relatively bright, weakly active M3.5 V star at just 8 pc with a warm transiting rocky planet of about 1.3 R ⊕ and 3.0 M ⊕ . It is ideal for both transmission and emission spectroscopy and for testing interior models of telluric planets. Aims . To prepare for future studies, we aim to thoroughly characterise the planetary system with new accurate and precise data collected with state-of-the-art photometers from space and spectrometers and interferometers from the ground. Methods . We collected light curves of seven new transits observed with the CHEOPS space mission and new radial velocities obtained with MAROON-X at the 8.1 m Gemini North telescope and CARMENES at the 3.5 m Calar Alto telescope, together with previously published spectroscopic and photometric data from the two spectrographs and TESS. We also performed near-infrared interferometric observations with the CHARA Array and new photometric monitoring with a suite of smaller telescopes (AstroLAB, LCOGT, OSN, TJO). This extraordinary and rich data set was the input for our comprehensive analysis. Results . From interferometry, we measure a limb-darkened disc angular size of the star Gl 486 at θ LDD = 0.390 ± 0.018 mas. Together with a corrected Gaia EDR3 parallax, we obtain a stellar radius R * = 0.339 ± 0.015 R ⊕ . We also measure a stellar rotation period at P rot = 49.9 ± 5.5 days, an upper limit to its XUV (5-920 A) flux informed by new Hubble /STIS data, and, for the first time, a variety of element abundances (Fe, Mg, Si, V, Sr, Zr, Rb) and C/O ratio. Moreover, we imposed restrictive constraints on the presence of additional components, either stellar or sub-stellar, in the system. With the input stellar parameters and the radial-velocity and transit data, we determine the radius and mass of the planet Gl 486 b at R p = 1.343 −0.062 +0.063 R ⊕ and M p = 3.00 −0.12 +0.13 M ⊕ , with relative uncertainties of the planet radius and mass of 4.7% and 4.2%, respectively. From the planet parameters and the stellar element abundances, we infer the most probable models of planet internal structure and composition, which are consistent with a relatively small metallic core with respect to the Earth, a deep silicate mantle, and a thin volatile upper layer. With all these ingredients, we outline prospects for Gl 486 b atmospheric studies, especially with forthcoming James Webb Space Telescope ( Webb ) observations.