NASA’s Transiting Exoplanet Survey Satellite (TESS) finds its first Earth-size planet orbiting in the habitable zone of its star.

This new-found exoplanet, named TOI 700 d, lies 100 light-years away. The Earth-size planet orbits in the habitable zone of its parent star, meaning it could hold liquid water.

TOI 700 d is one of only five Earth-size planets astronomers have ever found in a star’s habitable zone.

“TESS was designed and launched specifically to find Earth-sized planets orbiting nearby stars,” Paul Hertz, astrophysics division director at NASA Headquarters in Washington, said in a statement.

“Planets around nearby stars are easiest to follow-up with larger telescopes in space and on Earth. Discovering TOI 700 d is a key science finding for TESS. Confirming the planet’s size and habitable zone status with Spitzer is another win for Spitzer as it approaches the end of science operations this January.”

The exoplanet is part of a multi-planet system around TOI 700, a small, cool M-dwarf star in the Dorado constellation. The star is only about 40% of our sun’s mass and size, with about half its surface temperature.

The researchers previously thought the star was more similar to our Sun. So, they thought the planets are larger and hotter than they really are. But several researchers, including Alton Spencer, a high school student working with members of the TESS team, identified the error.

“When we corrected the star’s parameters, the sizes of its planets dropped, and we realized the outermost one was about the size of Earth and in the habitable zone,” said Emily Gilbert, a graduate student at the University of Chicago. “Additionally, in 11 months of data, we saw no flares from the star, which improves the chances TOI 700 d is habitable and makes it easier to model its atmospheric and surface conditions.”

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TESS found three different planets circling the star TOI 700. The innermost world, TOI 700 b, is about the size of Earth and completes one orbit every 10 Earth days. The center planet, TOI 700 c, is 2.6 times bigger than our planet, meaning it’s likely a gassy “mini-Neptune,” and zips around TOI 700 every 16 days.

But the outermost planet, TOI 700 d, is the most intriguing one. It’s just 20% larger than Earth and completes one orbit every 37 days. The alien world receives 86% of the stellar energy that Earth gets from the sun, putting TOI 700 d in the habitable zone, the research team said.

All of the planets are probably tidally locked, which means one side always faces their parent star.

Astronomers confirmed their discovery using NASA’s Spitzer Space Telescope. They also modeled the planet’s potential environment to further investigate its habitability.

“Given the impact of this discovery—that it is TESS’s first habitable-zone Earth-size planet —we really wanted our understanding of this system to be as concrete as possible,” Rodriguez said. “Spitzer saw TOI 700 d transit exactly when we expected it to. It’s a great addition to the legacy of a mission that helped confirm two of the TRAPPIST-1 planets and identify five more.”

In the future, missions like NASA’s James Webb Space Telescope could determine if the planets have atmospheres and their compositions.

The researchers presented the findings at the 235th meeting of the American Astronomical Society in Honolulu.

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