Using data from NASA’s Transiting Exoplanet Survey Satellite, scientists discovered an Earth-size world, TOI 700 e, circling within its star’s habitable zone – the range of distances where liquid water may exist on a planet’s surface. The globe is around 95% the size of Earth and is most likely rocky.

Astronomers have previously detected three planets in this system, designated TOI 700 b, c, and d. Planet D also orbits inside the habitable zone. However, scientists required an extra year of TESS monitoring to identify TOI 700 e.

“This is one of only a few systems with multiple, small, habitable-zone planets that we know of,” said Emily Gilbert, a postdoctoral fellow at NASA’s Jet Propulsion Laboratory in Southern California who led the research. “That makes the TOI 700 system a promising prospect for further investigation. Planet e is approximately 10% smaller than planet d, demonstrating how further TESS observations help us locate smaller and smaller planets.”

Gilbert presented the results on behalf of her team at the American Astronomical Association’s 241st conference in Seattle. The Astrophysical Journal Letters approved a manuscript describing a newly found planet.

TOI 700 is a tiny, cold M dwarf star in the southern constellation Dorado, approximately 100 light-years distant. Gilbert and colleagues revealed in 2020 the finding of the Earth-sized, habitable-zone planet d, which is in a 37-day orbit with two other planets.

The innermost planet, TOI 700 b, is nearly 90% the size of Earth and circles the star every 10 days. TOI 700 c is more than 2.5 times the size of Earth and completes one orbit every 16 days. The planets are most likely tidally locked, which means they rotate just once every orbit, with one side constantly facing the star, similar to how one side of the Moon is always facing Earth.

TESS watches enormous areas of the sky, known as sectors, for around 27 days at a time. These extended looks allow the satellite to follow variations in stellar brightness produced by a planet passing in front of its star from our perspective, a phenomenon known as a transit. Beginning in 2018, the expedition used this technique to investigate the southern sky before moving on to the northern sky. In 2020, it returned to the southern sky to conduct more observations. The additional year of data allowed the scientists to improve the original planet sizes, which were around 10% lower than early projections.

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

TOI 700 e, which may also be tidally locked, orbits its star in 28 days, placing it between planets c and d in the optimistic habitable zone.

Scientists describe the optimistic habitable zone as the range of distances from a star where liquid surface water may have existed at some stage in a planet’s history. This region stretches on either side of the conservative habitable zone, which is the range where experts believe liquid water might remain for the majority of the planet’s lifespan. TOI 700 d circles this area.

Finding additional systems with Earth-sized planets in this region allows planetary scientists to understand more about the evolution of our own solar system.

A follow-up investigation of the TOI 700 system using satellite and ground-based observatories is under underway, according to Gilbert, and it may provide more insights into this unique system.

“TESS just completed its second year of northern sky observations,” said Allison Youngblood, a research astrophysicist and Goddard deputy project scientist. “We’re looking forward to the other exciting discoveries hidden in the mission’s treasure trove of data.”

TESS is a NASA Astrophysics Explorer project led and controlled by the Massachusetts Institute of Technology in Cambridge, Massachusetts, with management from NASA’s Goddard Space Flight Center. Northrop Grumman, based in Falls Church, Virginia, NASA’s Ames Research Center in California’s Silicon Valley, the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts, MIT’s Lincoln Laboratory, and the Space Telescope Science Institute in Baltimore are among the additional partners. The mission involves over a dozen colleges, research institutes, and observatories from across the world.