NASA’s TESS mission discovers third exoplanet
NASA’s Transiting Exoplanet Survey Satellite has spotted its third exoplanet. Credit: NASA Goddard Space Flight Center, edited by MIT News

The Transiting Exoplanet Survey Satellite (TESS), a space telescope launched in 2018 that searches for planets orbiting distant stars, just found its third and slowest-moving planet yet. This world, between the sizes of Earth and Neptune, is likely a gaseous planet with a surface temperature of only about 150°C, researchers estimate. Called HD 21749b, it orbits its host star—an orange star cooler than the Sun—in 36 days, which makes it the longest-period planet spotted so far by TESS.

A Planetary Oddball

It’s the longest-period transiting planet within 100 light years.

This new discovery follows on the heels of two other planets spotted by TESS last year. But HD 21749b, spotted in the Southern Hemisphere constellation of Reticulum at a distance of roughly 53 light years, is a bit of an oddball given its relatively large and long orbit. “It’s the longest-period transiting planet within 100 light years,” said Chelsea Huang, an astronomer at the Massachusetts Institute of Technology and a member of the TESS team.

TESS scientists also announced the discovery of a second transiting planet candidate orbiting the same star. The recorded signal of this world, which orbits in a period of 8 days, reveals a planet “almost exactly the size of Earth,” said Huang.

Huang and her colleagues announced these results this week at the 233rd Meeting of the American Astronomical Society, held in Seattle, Wash. The findings will also appear in a paper submitted for publication to Astrophysical Journal Letters.

TESS is rapidly making good on its design goal of finding small planets orbiting bright stars.

These new observations of HD 21749b are an in-depth look at a new world, said Gregory Laughlin, an astronomer at Yale University not involved in the research. “TESS is rapidly making good on its design goal of finding small planets orbiting bright stars.”

The next year and a half of TESS’s planned mission is expected to reveal thousands more exoplanets, team members predict. “The torrent of data is starting to flow,” said George Ricker, principal investigator of TESS. “The best is yet to come.”

—Katherine Kornei (; @katherinekornei), Freelance Science Journalist


Kornei, K. (2019), NASA space telescope spots its third planet, Eos, 100, Published on 09 January 2019.

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