A team of astronomers at the Massachusetts Institute of Technology (MIT) has discovered one of the closest multi-planet systems to our own. Two super-Earths orbit around the cool M-dwarf star HD 260655 and the whole system is located 10 parsecs, or about 33 light-years, away.
The first planet, HD 260655 b, orbits the star every 2.8 days and is about 1.2 times as big as the Earth but about twice as massive. HD 260655 c is further out. It orbits the star in 5.7 days, has a mass of about three Earths, and is 1.5 times bigger. The discovery was presented at the 240th Meeting of the American Astronomical Society.
Despite the fact that the dwarf star is much cooler than our Sun, the planets are still too close to it, so they are very hot, making them outside the "habitable zone". The innermost planet is estimated to have scorching temperatures of 437°C (818°F), while the outer planet is around 287°C (548°F).
“We consider that range outside the habitable zone, too hot for liquid water to exist on the surface,” Michelle Kunimoto, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research and one of the discovery’s lead scientists, said in a statement.
The planets might not be habitable but their relative closeness to us and the brightness of their stars will allow astronomers to study them in detail and maybe even work out the properties of any atmosphere they might possess.
“Both planets in this system are each considered among the best targets for atmospheric study because of the brightness of their star,” explained Kunimoto. “Is there a volatile-rich atmosphere around these planets? And are there signs of water or carbon-based species? These planets are fantastic test beds for those explorations.”
And that’s not all. Maybe these two worlds are not alone.
“But there might be more planets in the system,” co-author Avi Shporer added. “There are many multiplanet systems hosting five or six planets, especially around small stars like this one. Hopefully, we will find more, and one might be in the habitable zone. That’s optimistic thinking.”
The system was first identified using NASA’s planet-hunter, the Transiting Exoplanet Survey Satellite (TESS), which identifies dips in brightness as planets pass regularly in front of their star. To confirm that the planets were actually there, the team had to use an independent method. They looked at observations of star HD 260655 to determine potential wobbles due to the gravitational pull of the planets.
“Every planet orbiting a star is going to have a little gravitational pull on its star,” Kunimoto explains. “What we’re looking for is any slight movement of that star that could indicate a planetary-mass object is tugging on it.”
They found evidence for it in data collected by the High-Resolution Echelle Spectrometer (HIRES), an instrument that operates as part of the Keck Observatory in Hawaii and by CARMENES, which was a private data collection from the Calar Alto Observatory in Spain. The planets are really there.