GJ 357 - Nearby warm terrestrial planet transiting an M dwarf from TESS

A planetary system around the nearby M dwarf Gl 357 including a transiting hot Earth-sized planet optimal for atmospheric characterisation
https://arxiv.org/abs/1904.12818

Quote :

We report the detection of a transiting Earth-size planet around Gl 357, a nearby M2.5 V star, using data from the Transiting Exoplanet Survey Satellite (TESS). Gl 357 b (TOI-562.01) is a transiting hot Earth-like planet (Teq=531+-12 K) with a radius of Rb=1.17+-0.06 Re and an orbital period of Pb=3.93 d. Precise stellar radial velocities from CARMENES as well as archival data from HIRES, UVES, and HARPS, also display a 3.93-day periodicity, confirming the existence of the planet and leading to planetary mass of Mb=1.63+-0.30 Me. In addition to the radial velocity signal for Gl 357 b, a second periodicity with Pc=9.12 d indicates the presence of a second, non-transiting planet candidate in the system, Gl 357 c, with a minimum mass of Mc=3.59+-0.50 Me. The star is relatively inactive and exhibits a photometric rotation period of Prot=78+-2 d. Gl 357 b is the second closest transiting planet to the Sun; its brightness makes it a prime target for further investigations, such as atmospheric spectroscopy and is, to date, the best terrestrial transiting planet suitable for atmospheric characterisation with the upcoming JWST and the ground-based ELTs.

Given the value of the system, I went ahead and made a separate thread for it.

GJ 357: A low-mass planetary system uncovered by precision radial-velocities and dynamical simulations
https://arxiv.org/abs/1909.00831

Quote :

We report the detection of a new planetary system orbiting the nearby M2.5V star GJ 357, using precision radial-velocities from three separate echelle spectrographs, HARPS, HiRES, and UVES. Three small planets have been confirmed in the system, with periods of 9.125±0.001, 3.9306±0.0003, and 55.70±0.05 days, and minimum masses of 3.33±0.48, 2.09±0.32, and 6.72±0.94 M⊕, respectively. The second planet in our system, GJ 357c, was recently shown to transit by the Transiting Exoplanet Survey Satellite (TESS; Luque et al. 2019), but we could find no transit signatures for the other two planets. Dynamical analysis reveals the system is likely to be close to coplanar, is stable on Myrs timescales, and places strong upper limits on the masses of the two non-transiting planets b and d of 4.25 and 11.20 M⊕, respectively. Therefore, we confirm the system contains at least two super-Earths, and either a third super-Earth or mini-Neptune planet. GJ 357b & c are found to be close to a 7:3 mean motion resonance, however no libration of the orbital parameters was found in our simulations. Analysis of the photometric lightcurve of the star from the TESS, when combined with our radial-velocities, reveal GJ 357c has an absolute mass, radius, and density of 2.087+0.319−0.315 M⊕, 1.167+0.037−0.036 R⊕, and 2.401+0.451−0.416 gcm−3, respectively. Comparison to super-Earth structure models reveals the planet is likely a predominantly rocky world. The GJ 357 system adds to the small sample of low-mass planetary systems with well constrained masses, and further observational and dynamical follow-up is warranted to better understand the overall population of small multi-planet systems in the solar neighbourhood.

Note the transiting planet is GJ 357 c in this work, whereas it is GJ 357 b in the first work. But that's okay since nothing matters anymore.

Modirrousta-Galian et al. "GJ 357 b: A Super-Earth Orbiting an Extremely Inactive Host Star"
https://arxiv.org/abs/2007.10262

from the conclusion:

Quote :

Using the X-ray luminosity of GJ 357 with our above estimate for the age of the system and the empirical L_x - Age relation from Penz & Micela (2008) we performed a backwards reconstruction of GJ 357 b’s primordial atmospheric mass. We find a theoretically maximum envelope mass of ∼ 38M_⊕. However, it is unlikely that GJ 357 b accreted an envelope this massive, as its small central mass would have hindered its ability to collect gas. Conversely, if one adopts the mass and radius measurements from Jenkins et al. (2019) instead of those from Luque et al. (2019), then GJ 357 b has a density consistent with a compressed remnant core (Mocquet et al. 2014). Its high density could be suggestive of an initially large atmospheric mass.