NASA “TESS” Planet Hunter Finds Earth-Size Habitable-Zone World

This illustration of TOI 700 d is based on several simulated environments for an ocean-covered version of the planet. Source: Image credit: NASA Goddard Space Flight Center
This illustration of TOI 700 d is based on several simulated environments for an ocean-covered version of the planet. Source: Image credit: NASA Goddard Space Flight Center

By Jeanette Kazmierczak, NASA JPL

NASA’s Transiting Exoplanet Survey Satellite (TESS) has discovered its first Earth-size planet in its star’s habitable zone, the range of distances where conditions may be just right to allow the presence of liquid water on the surface. Scientists confirmed the find, called TOI 700 d, using NASA’s Spitzer Space Telescope and have modeled the planet’s potential environments to help inform future observations.

TOI 700 d is one of only a few Earth-size planets discovered in a star’s habitable zone so far. Others include several planets in the TRAPPIST-1 system and other worlds discovered by NASA’s Kepler Space Telescope. Said Paul Hertz, astrophysics division director at NASA Headquarters in Washington, DC:

“TESS was designed and launched specifically to find Earth-sized planets orbiting nearby stars. 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.”

TESS monitors large swaths of the sky, called sectors, for 27 days at a time. This long stare allows the satellite to track changes in stellar brightness caused by an orbiting planet crossing in front of its star from our perspective, an event called a transit.

TOI 700 is a small, cool M dwarf star located just over 100 light-years away in the southern constellation Dorado. It’s roughly 40% of the Sun’s mass and size and about half its surface temperature. The star appears in 11 of the 13 sectors TESS observed during the mission’s first year, and scientists caught multiple transits by its three planets. The innermost planet, called TOI 700 b, is almost exactly Earth-size, is probably rocky and completes an orbit every 10 days.  All of the planets are thought to be tidally locked to their star, which means they rotate once per orbit so that one side is constantly bathed in daylight.

Because TOI 700 is bright, nearby, and shows no sign of stellar flares, the system is a prime candidate for precise mass measurements by current ground-based observatories.

Future missions may be able to identify whether the planets have atmospheres and, if so, even determine their compositions.

TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts, and managed by NASA’s Goddard Space Flight Center. Additional partners include Northrop Grumman, based in Falls Church, Virginia; NASA’s Ames Research Center in California’s Silicon Valley; the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope Science Institute in Baltimore. More than a dozen universities, research institutes and observatories worldwide are participants in the mission.

For more information on TESS and TOI 700 d, visit


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