The Habitability of GJ 357D: Possible Climate and Observability

The GJ 357 system harbors three planets orbiting a bright, nearby M2.5V star at 9.44 pc. The innermost planet, GJ 357b (TOI-562.01), is a hot transiting Earth-sized planet with Earth-like density, which receives about 12 times the irradiation Earth receives from the Sun, and was detected using data from TESS. Radial velocities discovered two more planets in the system at 9.12 (GJ 357 c) and 55.6 days (GJ 357 d), with minimum masses of 3.59 ± 0.50 and 6.1 ± 1 Earth masses, and an irradiation of 4.4 and 0.38 Earth's irradiation, respectively. GJ 357 d receives slightly less stellar irradiation than Mars does in our own solar system, which puts it in the Habitable Zone (HZ) for its host star. GJ 357 d could not have been detected with TESS and whether it transits remains an open question. Here we model possible conditions within which GJ357 d could sustain surface habitability and present planetary models as well as synthetic transmission, reflection, and emission spectra for a range of models for GJ 357 d from water worlds to Earth-like models. With Earth-analog outgassing rates, GJ 357 d would be a frozen rocky world; however, with an increased CO2 level, as would be expected if a geological cycles regulates CO2 concentration like on Earth, the planet models show temperate surface conditions. If we can detect a transit of GJ 357 d, it would become the closest transiting, potentially habitable planet in the solar neighborhood. Even if GJ 357 d does not transit, the brightness of its star makes this planet, in the HZ of a close-by M star, a prime target for observations with Extremely Large telescopes as well as future space missions.