NASA's New Planet Hunter Has Found Its First Earth-Sized World

Artist's conception of HD 21749c's system. Robin Dienel, courtesy of the Carnegie Institution for Science.

Just one year after launch, NASA’s newest planet-hunter, the Transiting Exoplanets Survey Satellite (TESS), has found its first Earth-sized exoplanet. This world, HD 21749c, is located 53 light-years from us and orbits its star in just eight days. The planet is also not alone, as TESS has discovered a sub-Neptune orbiting the same star.

The signals collected by TESS were followed up by the Planet Finder Spectrograph (PFS) on the Magellan II telescope at the Las Campanas Observatory in Chile. PFS has the ability to provide astronomers with estimates for the mass of exoplanets. As revealed in The Astrophysical Journal Letters, the sub-Neptune HD 21749b weighs around 23 Earths and has 20 times the volume of our planet.

This warm giant planet orbits its star every 36 days. While this is a small number compared to the planets in the Solar System (Mercury, for example, orbits the Sun in 88 days), HD 21749b's is the longest orbital period among the planets detected by TESS. Most of the planets found by this mission are expected to have orbital periods of fewer than 10 days, so the analysis of this object was actually more complex than expected.

“There was quite some detective work involved, and the right people were there at the right time,” lead author Diana Dragomir of MIT’s Kavli Institute for Astrophysics and Space Research, said in a statement. “But we were lucky, and we caught the signals, and they were really clear.”

HD 21749c’s mass is yet to be estimated but it has a radius 0.9 times Earth’s own. Similarities with Earth seems to end there, though. It’s certainly not a mild world. It has an orbital period of eight days around a star 80 percent the mass of our Sun, so it is most likely scorched.

“Measuring the exact mass and composition of such a small planet will be challenging, but important for comparing HD 21749c to Earth,” co-author Sharon Wang, from the Carnegie Institute for Science added. “Carnegie’s PFS team is continuing to collect data on this object with this goal in mind.”

TESS was launched in April 2018 and its primary mission is to look for planets around 500,000 stars including the 1,000 closest red dwarfs. Between 500 and 1,000 Earth-sized planets are expected to be discovered by the spacecraft, and about 20 of these could be in the habitable zone around their stars.

“For stars that are very close by and very bright, we expected to find up to a couple dozen Earth-sized planets,” said Dragomir. “[T]his 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.”

TESS's official science mission started on July 25, 2018, and for its first year of operation, it is monitoring the southern sky. Its primary mission is designed to last two years, and it will switch to the northern sky during its second year.

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