TESS identifies its first Earth-sized exoplanet

A team of astronomers led by MIT has confirmed that NASA's unmanned Transiting Exoplanet Survey Satellite (TESS) space telescope has discovered its first Earth-sized exoplanet, called HD 21749c. The smallest planet outside our solar system yet found by TESS, it orbits the orange main-sequence star HD 21749, which is 52 light years away from Earth in the constellation of Reticulum and is home to a sub-Neptune-seized world, HAD21749b.

Thanks to the Kepler mission and its extensions, finding Earth-sized planets outside the solar system isn't news, but HD21749c is different. This is because it not only shows that TESS can find small planets, but also ones that are orbiting stars relatively close to us. This means that they are easier to make follow-up studies of, unlike those found by Kepler, which tend to be farther away.

HD21749c may be "Earth-sized," but it's no picnic spot. The small, rocky planet circles its parent star once every 7.8 days, meaning that it's so close that it has a surface temperature of 800° F (425° C), which is about that of Venus and is high enough to melt lead.

According to MIT, the discovery of HD21749c came from a close examination of TESS data that first uncovered a warm "sub-Neptune" orbiting the same star in early 2019. By re-examining the data, it was possible to find the dip in the star's light curve across 11 transits that indicated the presence of a smaller planet.

Launched on April 18 from Cape Canaveral Air Force Station atop a SpaceX Falcon 9 rocket, TESS sits in an elliptical lunar resonant orbit between 108,000 km (67,000 mi) and 375,000 km (233,000 mi) in altitude and circles the Earth every 13.7 days with an inclination of 37 degrees to the Moon's orbit. This allows the spacecraft to remain stable with minimal corrections that would reduce its sensitivity and lets the telescope's four cameras scan the entire sky over time without glare from the Sun or the Earth.

"For stars that are very close by and very bright, we expected to find up to a couple dozen Earth-sized planets," says TESS team member Diana Dragomir, a postdoc in MIT's Kavli Institute for Astrophysics and Space Research. "And here we are – this would be our first one, and it's a milestone for TESS. It sets the path for finding smaller planets around even smaller stars, and those planets may potentially be habitable."

The findings were published in Astrophysical Journal Letters.