Weird ‘sub-Neptune’ exoplanet found by TESS
“HD 21749b seems to be made of denser stuff than the gaseous worlds we're used to”
The Transiting Exoplanet Survey Satellite (TESS) has discovered a planet orbiting the star HD 21749, which lies about 53 light years from Earth in the faint constellation Reticulum, scientists have announced.
The world, known as HD 21749b, is a weirdo - at least by the standards of our own Solar System.
It's a ‘sub-Neptune’ about three-times bigger than Earth, which means it's likely gaseous rather than rocky. But HD 21749b seems to be made of denser stuff than the gaseous worlds we're used to, because it's 23-times more massive than Earth.
“We think this planet wouldn't be as gaseous as Neptune or Uranus, which are mostly hydrogen and really puffy,” discovery team leader Diana Dragomir of the Massachusetts Institute of Technology's Kavli Institute for Astrophysics and Space Research said. “The planet likely has a density of water, or a thick atmosphere.”
HD 21749b completes one orbit of its host star, which is nearly as bright as our Sun, every 36 Earth days. The exoplanet likely has an average surface temperature around 150 degrees Celsius (300 degrees Fahrenheit), discovery team members said.
The researchers also detected hints of another, smaller planet in the system, a planet that would have an orbital period of 7.8 days. If this world is confirmed it will be the first roughly Earth-size planet found by TESS.
TESS launched to Earth orbit in April 2018 on a mission to find alien worlds circling stars in our Solar System's neighbourhood. Like
NASA's recently deceased Kepler space telescope, which is responsible for about 70 per cent of all known exoplanet discoveries to date, TESS uses the transit method. That is, it looks for tiny dips in starlight caused when planets cross their host stars' faces from the satellite's perspective.
This shifting focus makes it tough for TESS to find planets that lie far from their host stars and therefore take a long time to complete one orbit. Indeed, HD 21749b is very far-flung for
TESS; two other smallish worlds found by the mission have orbital periods of 11 hours and 6.3 days respectively.
“It's the coolest small planet that we know of around a star this bright,” Dragomir said. “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. Here we were lucky and caught this one and can now study it in more detail.”