100 light-years away from Earth in the Dorado constellation is a planetary system called TOI 700, where TESS, the planet-hunting satellite, has discovered TOI 700d, the first habitable planet the size of the Earth.
The habitability zone of a star is the distance range where conditions may be appropriate to allow the presence of liquid water on the surface. Scientists have confirmed the discovery using NASA’s Spitzer Space Telescope.
TOI 700d is one of the few planets the size of the Earth discovered so far in the habitable zone of a star. There are also planets in the TRAPPIST-1 system and other worlds discovered by NASA’s Kepler space telescope. Although other similar planets have been found, it is the first to be discovered by TESS, which was launched in 2018.
"TESS was designed and launched specifically to find Earth-sized planets orbiting nearby stars. Planets around nearby stars are easier to follow with larger telescopes in space and on Earth. Discovering TOI 700d is a key scientific finding for TESS. Confirming the size of the planet and the state of the habitable area with Spitzer is another victory for Spitzer as the end of scientific operations approaches in January" stated Paul Hertz, director of the astrophysics division at NASA headquarters in Washington.
The planet, 100 light-years away, is part of a multi-planet system around TOI 700, a small, cold dwarf star M in the constellation of Dorado, which is visible from our southern hemisphere. By comparison, the nearest star to us, Proxima Centauri, is 4.2 light-years from Earth. The star has approximately 40% of the mass and size of our sun, with half the surface temperature.
TOI 700d is the outermost of the three planets, completing a single orbit around the star every 37 Earth days. It is 20% larger than Earth and its smallest star, and the planet receives about 86% of the energy our sun supplies to Earth. And, apparently, scientists think that the planet is blocked by tides, so that one side of the world would always be in daylight.
In a simulation, NASA explained that the planet is covered with oceans with a "dense atmosphere dominated by carbon dioxide similar to the one scientists suspect surrounding Mars as a young world”.
Despite the relatively large number of small discoveries of exoplanets to date, most stars are between 600 and 3,000 light-years away from Earth, which means they’re too far away for a detailed follow-up observation.
While we don’t know the exact conditions in TOI 700d, NASA scientists say they can use what we do know, like the size of the planet and the type of star it orbits, to make predictions about what is possible on this newly discovered planet.
A summary of the new planetary system
The innermost planet, called TOI 700b, is practically the size of the Earth, is probably rocky and completes one orbit every 10 days.
The central planet, TOI 700c, is 2.6 times larger than Earth, orbits every 16 days and is likely to be a gas-dominated world.
TOI 700d, the outermost planet known in the system and the only one in the habitable zone, is 20% larger than Earth, orbits every 37 days and receives from its star 86% of the energy that the Sun provides to Earth.