TOI 700 d is the newest planet to join the elite ranks of Earth-sized planets with the potential for liquid surface water – and is the first to be discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS).

TESS monitors large sectors of the sky for 27 days at a time, tracking changes in the brightness from thousands of stars. One such star is the small, cool red dwarf TOI 700, which measures about 40 percent the size of our Sun and is located 100 light-years away in the southern constellation Dorado. As orbiting planets pass TOI 700, TESS monitors changes and records any dimming outlines in front of the star.

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

Three planets were observed by TESS as they orbited TOI 700. TOI 700 b is the innermost planet that is almost exactly Earth-sized, likely rocky, and finishes an orbit every 10 days. The middle planet, TOI 700 c, is 2.6 times larger than Earth, orbits every 16 days, and is largely made of gas.

Then there is TOI 700 d, the only planet within the star’s habitable zone. This planet orbits further out than the other two every 37 days and measures about 20 percent larger than Earth. Its existence was confirmed with the Spitzer telescope. TOI 700 d is one of just a handful of Earth-sized planets that have been discovered in potential habitable zones. Others include the seven-planet system TRAPPIST-1 discovered by NASA’s Kepler Space Telescope.

Scientists have modeled two potential scenarios that may characterize TOI 700 d. One is an ocean-covered world with an atmosphere similar to – but much denser than – early Mars. A second is a dry, cloudless version of today’s Earth. Both of these scenarios suggest different surface temperatures, but further research will be required to confirm or reject such hypotheses. The planet is bright and nearby, making it a good candidate for ground-based telescopes to conduct further analyses.

The findings were presented at the 235th meeting of the American Astronomical Society in Honolulu, Hawaii.