NASA’s Transiting Exoplanet Survey Satellite (TESS) is on a roll.

Following January’s announcement of a third confirmed exoplanet—HD 21749b—the space telescope has spotted an Earth-sized sibling, HD 21749c.

Both worlds are detailed in a new paper published by The Astrophysical Journal Letters.

“It’s so exciting that TESS, which launched just about a year ago, is already a game-changer in the planet-hunting business,” study co-author Johanna Teske, of the Carnegie Institute for Science, said in a statement.

HD 21749b, revealed early this year by the MIT Kavli Institute for Astrophysics and Space Research, orbits a nearby dwarf star about 53 light years from Earth, in the constellation Reticulum.

It appears to have the longest orbital period of TESS’s first three discoveries.

Compared to the other two sightings—Pi Mensae b, a “super-Earth” with a 6.3-day orbit, and LHS 3844b, a rocky world that speeds around its star in 11 hours—HD 21749b is downright lazy, taking 36 days to complete one rotation.

Exoplanet HD 21749b orbits a star, about the size of the sun, 53 light years away (via NASA/MIT/TESS)

The new planet’s surface reaches about 300℉, which, according to the Kavli Institute, is “relatively cool,” considering the proximity to its star, which is almost as bright as our Sun.

About three times the size of Earth (and a whopping 23 times as massive), HD 21749b is categorized as a “sub-Neptune.” Unfortunately, it is unlikely the gaseous planet is habitable.

On the bright side, TESS has detected its first Earth-sized world, HD 21749c, which takes less than eight days to orbit the host star.

“For stars that are very close by and very bright, we expected to find up to a couple dozen Earth-sized planets,” lead author Diana Dragomir of MIT’s Kavli Institute, said. “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,” she added.

During its initial two-year mission, the Transiting Exoplanet Survey Satellite will observe nearly the entire sky, looking for transit—a phenomenon that occurs when a planet passes in front of its star, causing a dip in the star’s brightness.

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