TOI-700 d

NASA’s TESS satellite scans almost the whole sky for planets crossing nearby stars. On 3 January 2020 a team led by Emily Gilbert announced its first Earth-sized planet in a habitable zone: TOI-700 d, about 100 light-years away in the southern constellation Dorado. NASA’s Spitzer Space Telescope then confirmed the signal.

Found after a correction

The planet came to light only after a catalogue error was fixed. The TESS database first listed TOI-700 as more Sun-like than it is, which made its planets look larger and hotter than they really are. Several researchers caught the mistake, among them Alton Spencer, a high school student working with the TESS team. Once the star’s figures were corrected, the outermost planet shrank to about Earth’s size and fell inside the habitable zone.

Spitzer then caught TOI-700 d crossing its star on the predicted schedule. Its data improved the estimate of the planet’s orbital period by 56 percent and of its size by 38 percent, and ruled out a smaller, dimmer companion star as the source of the signal.

A quiet star

TOI-700 is a red dwarf with about 40 percent of the Sun’s mass and size and about half its surface temperature. Many red dwarfs are violent flare stars that could strip the air from their planets, but TOI-700 is calm: in eleven sectors of TESS observations it showed no white-light flares at all. That makes its planets better candidates for keeping an atmosphere.

TOI-700 d is the outermost of four known planets. The innermost, TOI-700 b, is almost exactly Earth’s size and orbits in about 10 days; TOI-700 c is 2.6 times Earth’s size, probably dominated by gas, with a 16-day orbit. TOI-700 d circles its star every 37 days and receives about 88 percent of the energy Earth gets from the Sun.

What its climate could be

Its mass and atmosphere have not been measured. Climate models published alongside the discovery explored many possibilities, from an Earth-like world to a dry one, and found only a small chance of a runaway greenhouse effect turning it into a Venus. The planet is thought to be tidally locked, with one side in permanent daylight, so its clouds and winds may look very different from Earth’s. Of 20 versions modelled at NASA’s Goddard Space Flight Center, one is an ocean world under a thick carbon-dioxide atmosphere, like the one early Mars may have had, with a deep cloud layer on the side facing the star. Another is a cloudless, all-land copy of modern Earth, where winds blow away from the night side and meet at the point directly beneath the star. A 2024 study argued it could keep a secondary atmosphere rich in heavier gases. If it has any atmosphere at all, its surface is likely warmer than its equilibrium temperature of about 269 K (-4 °C).

A second one

In 2023 Gilbert’s team announced TOI-700 e, a second Earth-sized planet in the same star’s habitable zone. It is 0.95 times Earth’s radius and orbits every 27.8 days, inside the orbit of planet d, in the wider “optimistic” habitable zone. Finding it took 21 sectors of TESS data from the mission’s first and third years.