TOI-700 is a red dwarf star located in the constellation of Dorado and about 101.5 light-years away from our solar system. The planetary system that orbits this star includes the planet TOI-700 d, the first exoplanet of a size comparable to the terrestrial one discovered within a conservative habitable zone by the Transiting Exoplanet Survey Satellite (TESS). At the time of the discovery of this planet, in January 2020, TOI-700 became the brightest known star that hosts around it a transiting terrestrial exoplanet located within the so-called habitable zone.
TOI-700, whose name is an acronym for “Transiting Exoplanet Survey Satellite Object of Interest,” is a red dwarf of spectral class M, having 40% of the mass and radius of the Sun and a temperature of about 3 500 K. The star seems to be in a moment of low activity, so much so that in the 11 months of observation by TESS no flare was reported; moreover, its low rotation speed is due to its sluggish movement
Thanks to the TESS satellite, three exoplanets orbiting around TOI-700 have weights, all three probably rotating around it synchronously, which is always showing the same face.
In three different articles the description of the discovery of the planetary system, the system itself has and the history of the observations of the planet TOI 700d carried out with the Spitzer space telescope and the characterization of this planet, from these studies it appears that the composition of the planets identified as TOI-700 b and TOI-700 d, the innermost and most external to the orbit, is almost rocky, and therefore similar to that of Earth, while that of the planet TOI-700 c is more similar to that of Neptune, to be precise, it could be a mini-Neptune. It was assumed that the two innermost planets, TOI 700 b, and TOI 700 c, have grown faster thanks to the accretion of essential quantities of gas and dust, while the outermost planet would have become more slowly, incorporating less gas. The innermost would later lose its gas content by photoevaporation. Another explanation for the arrangement of densities in this system could instead be that of an orbital migration operated by TOI 700 c, which would have moved towards an internal orbit. However, this scenario will become more plausible if further studies show that TOI 700 c has a significantly higher mass than the other two, while for now, the data on masses, also useful for improving measurements on densities, are still coarse.
Of the three planets discovered, the most interesting, precisely because of its positioning within a habitable zone, is TOI-700 d. This planet receives from its star several photons in the extreme ultraviolet 35 times greater than that which the earth receives from the sun, but in any case 50 times less than that obtained by the planet TRAPPIST-1 and TRAPPIST-1, and 86% of the energy that Earth receives from its star; under these conditions, the atmosphere of a planet with pressure similar to that of Earth should last for over a billion years. Several simulations of possible environmental situations present on the TOI-700 d, which include, among others, a planet entirely covered by oceans, with a dense atmosphere saturated with carbon dioxide and a thick layer of clouds on the face facing its star, or a planet of all cloudless and formed only of drylands, with strong winds that from the face in shadow sweep the planet heading towards the illuminated front, have shown how TOI-700 d could be an essential candidate for the title of “habitable planet” and terrestrial analog. Precisely for this reason, the TOI-700 planetary system has been placed at the top of the list of objectives to be observed with the new James Webb space telescope, which is expected to be launched in 2021.