An illustration of NASA’s Transiting Exoplanet Survey Satellite (TESS) NASA's Goddard Space Flight Center

One hundred light years away, there is a planet that might be just right for life. Called TOI 700 d, it is the first Earth-sized world with moderate temperatures found by NASA’s newest planet-hunting space telescope, the Transiting Exoplanet Survey Satellite (TESS).

TOI 700 d orbits a star that is about 40 per cent the size of the sun and half its temperature. It is fairly close to the star, in what is called the habitable zone – the area around a star that is warm enough for water on a world’s surface to remain liquid but not so hot that it all evaporates. This exoplanet gets 86 per cent of the sunlight that Earth does.

Members of the TESS team presented this research at a meeting of the American Astronomical Society in Hawaii on 6 January. TOI 700 d is one of the best targets for further observations to learn whether it really does have the right conditions for life, said Emily Gilbert at the University of Chicago.


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After TESS spotted TOI 700 d, astronomers also used the Spitzer Space Telescope to take more detailed measurements of its size and orbit. Those observations revealed that it is about 22 per cent bigger than Earth and orbits its star once every 37 days.

Gabrielle Engelmann-Suissa at NASA’s Goddard Space Flight Center in Maryland and her colleagues ran 20 simulations of possible conditions on the surface and in the potential atmosphere of TOI 700 d.

“We find across all our 20 simulations that TOI 700 d is a robust candidate for habitability,” she said. “It retains surface water in all of our simulations, and in none of our simulations does it go into a runaway greenhouse state like Venus.”

On Venus, evaporating water created clouds that held in heat and let carbon dioxide build up in the atmosphere, leading to the hellish temperatures on the planet today.

All the simulations assumed that TOI 700 d is rocky and has an atmosphere. “The question which remains is, is this planet rocky like it would be in our solar system, or is it like a mini-Neptune?” said Joseph Rodriguez at the Harvard-Smithsonian Center for Astrophysics in Massachusetts.

If it were a small Neptune-like planet, it would be mostly gas. But if it is rocky and has a thick enough atmosphere made of gases that we know support life, it might be a good place to look for signs of alien life.

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