HD 21749b is about 2.8 times Earth’s size, which puts it in the category of a sub-Neptune. Surprisingly, it is also a whopping 23 times as massive as the Earth.
But it is unlikely that the planet is rocky and therefore habitable; it’s more likely made of gas, of a kind that is much more dense than the atmospheres of either Neptune or Uranus.
“We think this planet wouldn’t be as gaseous as Neptune or Uranus, which are mostly hydrogen and really puffy,” said Dr. Diana Dragomir, a postdoctoral researcher at MIT’s Kavli Institute for Astrophysics and Space Research.
“The planet likely has a density of water, or a thick atmosphere.”
HD 21749b orbits HD 21749, a K-type star about 80% the Sun’s mass and located 53 light-years away in the southern constellation Reticulum.
The planet journeys around its parent star in a relatively leisurely 36 days, compared to the two other small planets discovered by TESS: the super-Earth HD 39091c (Pi Mensae c) with a 6.3-day orbit and LHS 3844b, a rocky world that speeds around its star in just 11 hours.
The planet’s surface is likely around 300 degrees Fahrenheit (149 degrees Celsius) — relatively cool, given its proximity to its star, which is almost as bright as the Sun.
“It’s the coolest small planet that we know of around a star this bright,” Dr. 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.”
“But here we were lucky, and caught this one, and can now study it in more detail.”
What’s even more exciting are hints the HD 21749 system holds a second candidate planet about the size of Earth that orbits the star every 8 days. If confirmed, it could be the smallest TESS planet to date.
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