At 10.7 light-years away, Lacaille 9352 is the 12th closest star system to the Sun.
Also known as Gliese 887, GJ 887 and HD 217987, this red dwarf lies in the southern constellation of Piscis Austrinus.
The star is much dimmer and about half the size of our Sun, which means that the habitable zone is closer to Lacaille 9352 than Earth’s distance from the Sun.
University of Göttingen astronomer Sandra Jeffers and her colleagues from the RedDots team monitored Lacaille 9352 using the HARPS (High Accuracy Radial Velocity Planet Searcher) spectrograph on ESO’s La Silla 3.6-m telescope.
“We used a technique known as Doppler wobble, which enables us to measure the tiny back and forth wobbles of the star caused by the gravitational pull of the planets,” they explained.
Monitoring Lacaille 9352, they detected periodic signals, indicating the presence of two planets on orbits with periods of 9.3 and 21.8 days.
The planets, named Lacaille 9352b (Gliese 887b) and Lacaille 9352c (Gliese 887c), have minimum masses of 4.2 and 7.6 Earth masses, respectively.
Both planets are interior to, but close to the inner edge of, the liquid-water habitable zone.
They have surface temperature of 195 degrees Celsius (383 degrees Fahrenheit) and 79 degrees Celsius (174 degrees Fahrenheit), respectively.
Dr. Jeffers and co-authors also detected an unconfirmed signal with a period of 50 days, which could correspond to a third super-Earth, Lacaille 9352d (Gliese 887d), in a more temperate orbit.
“We also discovered two interesting facts about Lacaille 9352, which turn out to be good news not only for the newly-discovered planets but also for astronomers,” they said.
“The first is that the red dwarf has very few starspots, unlike our Sun.”
“If Lacaille 9352 was as active as our Sun, it is likely that a strong stellar wind — outflowing material which can erode a planet’s atmosphere — would simply sweep away the planets’ atmospheres. This means that the newly-discovered planets may retain their atmospheres, or have thicker atmospheres than the Earth, and potentially host life, even though Lacaille 9352 receives more light than the Earth.”
“The other interesting feature we discovered is that the brightness of Lacaille 9352 is almost constant. Therefore, it will be relatively easy to detect the atmospheres of the super-Earth system, making it a prime target for the James Webb Space Telescope, a successor to Hubble.”
The discovery is described in a paper in the journal Science.