NASA’s planet-hunting observatory found a second exoplanet roughly the same size as Earth and close enough to its star to potentially have liquid water. An exoplanet with similar characteristics has already been found in the same solar system.

“This is one of only a few [solar] systems with multiple, small, habitable-zone planets that we know of,” said Emily Gilbert, a postdoctoral fellow at NASA’s Jet Propulsion Laboratory, in a statement. 

Astronomers named the new planet TOI 700 e. Planet d orbits in the star’s conservative habitable zone, where it is more likely to have an Earth-like atmosphere — and liquid water could exist. 

Planet e is in the optimistic habitable zone, which is where liquid water could exist under the right conditions. This planet was found after collecting a year’s worth of data from NASA’s Transiting Exoplanet Survey Satellite (TESS).

Gilbert said Planet e is about 10 per cent smaller than planet d, “so the system also shows how additional TESS observations help us find smaller and smaller worlds.”

TOI 700 e is 95 per cent the size of Earth and is likely rocky. It has a 28-day orbit around an M dwarf star located 100 light-years away in the constellation Dorado. An M dwarf star is smaller and cooler than our own star. The planet might also be tidally locked, meaning one side always faces its sun.

Astronomers announced in 2020 they had discovered three planets in the TOI 700 system, which are called TOI 700 b, c, and d. These planets are also tidally locked.

Planet b is the system’s innermost planet — about 90 per cent of the Earth’s size, and orbits the star every 10 days. Planet c is more than 2.5 times larger than Earth and has a 16-day orbit. Planet d completes a 37-day orbit, and Planet e is located between planets c and d.

“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 a statement. “But the signal was so faint that we needed the additional year of transit observations to identify it.”

TESS used the extra year of observations to confirm the size of the planets after they were discovered in 2020. The refined data indicates the exoplanets are 10 per cent smaller than originally believed. The atmospheric composition of the planets in the TOI 700 system is not yet known.