Scientists have discovered an Earth-sized planet, called TOI 700 e, orbiting within its star’s habitable zone. This planet was found using data from NASA’s Transiting Exoplanet Survey Satellite. It is 95% the size of Earth and probably rocky.

Three planets in this system, known as TOI 700 b, c and d, were previously found by astronomers. The habitable zone is where planet d also orbits. However, it took scientists another year of TESS observations to find TOI 700 e.

TOI 700 is a small, cool M dwarf star about 100 light-years away in the southern constellation of Dorado. The planet d with the size of the Earth and the habitable zone is in a 37-day orbit along with two other worlds. The innermost planet, TOI 700 b, orbits the star every ten days and is nearly 90% the size of Earth. The orbit of TOI 700 c, which is more than 2.5 times the size of Earth, lasts 16 days.

The planets are likely tidally, meaning they rotate once per circle, so one side always faces the star, just as the moon always faces Earth from one side of its orbit. TOI 700 e, which may also be tidally locked, takes 28 days to orbit its star and places planet e between planets c and d in the so-called optimistic habitable zone.

Emily Gilbert, a postdoctoral fellow at NASA’s Jet Propulsion Laboratory in Southern California who led the work, said: “This is one of the few systems with multiple, minor planets with a habitable zone that we know of. That makes the TOI 700 system an exciting prospect for additional tracking. Planet e is about 10% smaller than planet d, so it system also shows how additional TESS observations help us find ever smaller worlds.”

Ben Hord, a doctoral student at the University of Maryland, College Park, and a graduate researcher at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said: “If the star was a little closer or the planet a little bigger, we might have seen TOI 700 e in the first year of TESS data. But the signal was so weak that we needed the extra year of transit observations to identify it.”

Gilbert said, “Follow-up studies of the TOI 700 system with space and ground observatories are underway and may provide further insights into this rare system.”

Allison Youngblood, a research astrophysicist and TESS’s adjunct project scientist at Goddard, said: “TESS has just completed its second year of northern sky observations. We look forward to the other exciting discoveries hidden in the mission’s wealth of data.”

Gilbert presented the result on behalf of her team at the 241st meeting of the American Astronomical Society in Seattle. A paper on the newly discovered planet was accepted by The Astrophysical Journal Letters.