Astronomers Discover Unusual New Exoplanet

The Transiting Exoplanet Survey Satellite (TESS), NASA’s space telescope mission launched in 2018 to replace the retiring Kepler telescope, has discovered a new, unusual planet. The findings were announced by researchers on 7 January, marking the eighth confirmed find for the new planet-hunting mission.  


Known as HD 21749b, the new, unusual planet is three times bigger than Earth, and yet 23 times more massive. Located 53 light years away from Earth in the Reticulum constellation, the exoplanet orbits a star close to the size of the sun every 36 days. Researchers also say the exoplanet likely has an average surface temperature of 150 C. "It's the coolest small planet that we know of around a star this bright," said MIT’s Diana Dragomir, lead discovery researcher. 


Because HD 21749b is a sub-Neptune, it is likely more gaseous than rocky, but it seems to be made of denser particles. "We think this planet wouldn't be as gaseous as Neptune or Uranus, which are mostly hydrogen and really puffy," Dragomir said. "The planet likely has a density of water, or a thick atmosphere."  


There are hints of another planet in the same system the researchers have found. With an orbital period of 7.8 days, this will be the first Earth-sized planet TESS has located – if its presence is confirmed. 


The latest discoveries are promising as the TESS mission is continues to survey the entire sky during its two-year mission. Launched in April 2018, it uses the transit method – looking for small dips in starlight that are caused by planets orbiting their host stars. Every overlapping sector of the sky is studies for 27 days at a time. That way, TESS should be able to find planets that are further away from their host stars and thus have longer orbits.  Kepler, its predecessor, is responsible for about 70% of all discovered exoplanet, and TESS has already contributed by discovering three small exoplanets, with many more discoveries expected in the future. 


"We know a lot about atmospheres of hot planets, but because it's very hard to find small planets that orbit farther from their stars and are therefore cooler, we haven't been able to learn much about these smaller, cooler planets," Dragomir explained. "But here, we were lucky and caught this one and can now study it in more detail."  


This wasn’t the first time astronomers studied the HD 21749 system. A different group did so a decade ago using the High Accuracy Radial velocity Planet Searcher (HARPS), and while they had detected a signal, they couldn’t conclusively attribute it. Dragomir and her team combined TESS observations with HARPS research to confirm that the signal. The astronomers then relied on the Magellan II Telescope, located in Chile to confirm the discovery and calculate HD 21749b’s mass and orbital parameters. 


"There was quite some detective work involved, and the right people were there at the right time," Dragomir said. “But we were lucky and we caught the signals, and they were really clear." 


Dragomir and her team have submitted their findings to the Astrophysical Journal Letters. As for TESS, it is predicted that it will confirm the presence of thousands of exoplanets when its mission is complete. As scientific instruments continue to improve – such as NASA’s James Webb Space Telescope, scheduled to be launched in 2021 – astronomers will find out a lot more about the new worlds that are located close to Earth.

Image: NASA's Goddard Space Flight Center, edited by MIT News