Super-Earths are planets more massive than Earth, but substantially less massive than the ice giants of our solar system, Uranus and Neptune. The newly discovered super-Earths could be rocky worlds, and lie close to the “habitable zone” of this red dwarf star – the zone where water could exist in liquid form on a planetary surface.
“The exciting thing about these planets are that they orbit a star so close to our sun, and so very bright,” said UNSW Professor Chris Tinney, a co-author of the paper. “We now know of thousands planets of Super-Earth-mass, or smaller. But most of those planets orbit distant and faint stars. Planets orbiting nearby stars are key for searches with future telescopes for both exoplanetary atmospheres, and eventually evidence for life.”
The team of astronomers monitored the red dwarf, using the HARPS spectrograph at the European Southern Observatory in Chile, and then combined that data with data from the Anglo-Australian Planet Search (using the Anglo-Australian Telescope near Coonabarabran), the Planet finder Spectrograph (on in Chile) and the HIRES instrument on the Keck telescopes on Maunakea, Hawaii.
Using a technique known as “Doppler wobble”, they measured the tiny back and forth motion of Gliese 887 caused by the gravitational pull of its planets. The regular signals correspond to orbital periods of 9.3 and 21.8 days, which indicates the presence of two super-Earths –Gliese 887b and Gliese 887c. The team estimates the surface temperature of the outer planet (Gliese 887c) to be around 70oC.
At 11 light years away, Gliese 887 is one of the closest stars to our sun. It is much dimmer, and about half the size. This means its habitable zone is closer to Gliese 887, than our sun’s habitable zone.
The team discovered a few more things about Gliese 887 that are good news for both the newly discovered planets, and for astronomers studying them.
“The red dwarf has little in the way of starspots or other magnetic activity,” says Professor Brad Carter, from the University of Southern Queensland. “If Gliese 887 were as active as many other red dwarf stars, its stellar winds and activity would likely significantly erode any planetary atmospheres. So given the star is relatively inactive the newly discovered planets could be expected to retain their atmospheres”.
“The star Gliese 887 is also very stable”, says Professor Rob Wittenmyer, also from USQ. “The brightness of Gliese 887 is really very constant. This will make it much easier to detect the atmospheres of these super-Earths using coming space-based facilities like the James Webb Space Telescope – the successor to the Hubble Telescope, which is due to be launched soon.”
“In the era of space-based exoplanet-hunting telescopes, like NASA’s Kepler and TESS, this result shows that astronomy from the ground continues to play a crucial role in our understanding of planets in our local neighbourhood,” says co-author Dr Simon O’Toole, from Macquarie University/Australian Astronomical Optics.
Lead author of the study, Dr Sandra Jeffers, from the University of Göttingen, says: “These planets will provide the best possibilities for more detailed studies, including the search for life outside our Solar System.”
