NASA Goddard Space Flight Center

‘Mini-Neptune’ found orbiting distant star

A warm, gaseous planet about three times the diameter of Earth circles an orange dwarf star about 53 light-years away, astronomers reported today at the annual meeting of the American Astronomical Society in Seattle, Washington. The planet, dubbed HD21749b (depicted above in an artist’s representation), is one of three small exoplanets discovered by one of NASA’s newest satellites.

HD21749b has an estimated density about that of water. That means it’s unlikely to be a rocky planet like Earth, though it may have some rocky parts. It’s also a lot hotter than our home planet, orbiting its sun—HD21749—at about half the distance from which Mercury orbits our star. Data suggest the planet has a relatively toasty cloud-top temperature of 149°C, somewhat cooler than Mercury because the host star HD21749 is somewhat smaller and cooler than our sun.

The planet was first discovered by NASA’s Transiting Exoplanet Survey Satellite, which was launched in April 2018. That probe is designed to look for the minieclipses that occur when planets pass in front of their host stars as seen from Earth. Subsequent analyses of old data gathered by ground-based telescopes helped the scientists calculate the planet’s 36-day-long “year.”

In an as-yet-unconfirmed finding, submitted to The Astrophysical Journal Letters, the team also reports detecting an Earth-size planet that orbits HD21749 once every 7.8 days or so. Because that planet orbits its host star even closer than HD21749b does, it would likely have a surface temperature that’s much hotter.