Super-Earths found around nearby star offer clues for studying exoplanet atmospheres and extraterrestrial life
The closest exoplanets to the Sun provide the best opportunities for detailed study, including searching for evidence of life outside the Solar System, say scientists. Now, at roughly 11 light-years away, astronomers have detected GJ 887 or Gliese 887, a red dwarf star about half as massive as the Sun, which hosts one of the closest multi-planet systems to our Solar System. According to the study, the nearby system is home to at least two, and perhaps three, super-Earth sized exoplanets. Their proximity offers a promising opportunity to study exoplanet atmospheres using the soon-to-be-launched James Webb Space Telescope, a successor to the Hubble Telescope.
Super-Earths are planets more massive than the Earth, but substantially less massive than our local ice giants, Uranus and Neptune. In the study led by the University of Göttingen, the RedDots team of astronomers identified the system of super-Earth planets orbiting Gliese 887, the brightest red dwarf star in the sky. The newly discovered super-Earths could be rocky worlds and lie close to the “habitable zone” of this red dwarf star, that is, the zone where water could exist in liquid form on a planetary surface. “These planets will provide the best possibilities for more detailed studies, including the search for life outside our Solar System,” says lead author of the study Dr Sandra Jeffers from the University of Göttingen in the analysis published in Science.
According to the authors, Gliese 887 is the 12th closest star system to the Sun. It is much dimmer and about half the size of our Sun. This means its habitable zone is closer to Gliese 887 than the Sun's habitable zone, in which the Earth orbits. “The exciting thing about these planets is that they orbit a star so close to the Sun, and so very bright. We now know of thousands of 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,” says co-author of the study, Professor Chris Tinney, a University of New South Wales (UNSW)-based planet hunter.
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 3.9m Anglo-Australian Telescope near Coonabarabran), the Planet Finder Spectrograph (on Cerro Las Campanas in Chile) and the HIRES instrument on the Keck telescopes on Maunakea, Hawaii.
A popular method for discovering exoplanets uses Doppler measurements of a star's motion to reveal the gravitational pull of any planets orbiting around it. Known as the “Doppler wobble,” the technique enabled scientists to measure the tiny back and forth wobbles of the star Gliese 887 caused by the gravitational pull of its planets.
Using this approach, the astronomers observed GJ 887 each night for three months. Combining their data with archival measurements of the star, spanning nearly 20 years, they detected the presence of at least two super-Earth sized planets – known as Gliese 887b and Gliese 887c -- tightly circling the nearby red dwarf with orbital periods of 9.3 and 21.8 days. The former is about 4.2 times Earth's mass and orbits just 6.8% of an astronomical unit (AU) from its star (one astronomical unit is the average distance between Earth and the sun), whereas Gliese 887c about 7.6 times Earth's mass and orbits 12% of an AU from the red dwarf. The team estimates the surface temperature of the outer planet (Gliese 887c) to be around 70 degrees Celsius. They also found evidence for a possible third planet farther out, with an orbital period of approximately 50 days. The authors suggest that both of the confirmed planets are likely too hot to maintain liquid water on their surfaces. However, the more distant location of the unconfirmed third planet may place it within GJ 887's so-called “habitable zone.”
RedDots discovered a few more interesting facts about Gliese 887, which turn out to be good news not only for the newly discovered planets but also for astronomers studying them. GJ 887 is less active than other red dwarfs, so the newly discovered worlds may be spared from harmful solar flares common to that type of star. The red dwarf has very few starspots, unlike our Sun. If Gliese 887 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. So given the star is relatively inactive, the newly discovered planets may retain their atmospheres, or have thicker atmospheres than the Earth, and potentially host life, even though GJ 887 receives more light than the Earth.
Another interesting feature the team found is that the Gliese 887 is also very stable and its brightness 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. “If someone had to live around a red dwarf, they would want to choose a quieter star-like GJ 887. If further observations confirm the presence of the third planet in the habitable zone, then GJ 887 could become one of the most studied planetary systems in the Solar neighborhood,” writes Melvyn Davies, professor of astronomy at the Department of Astronomy and Theoretical Physics, Lund University, Sweden, in a related perspective.
In 2016, the RedDots astronomy team found the closest exoplanet to the Sun, which is roughly Earth-mass and orbits Proxima Centauri. This was followed in 2018 with the announcement of a super-Earth orbiting Barnard's star, the second-closest star to the Sun. A system of three planets orbiting the red dwarf star GJ 1061, just slightly further away from us than GJ 887, was also announced by the team in 2019. According to NASA, more than 4,000 exoplanets have been discovered and are considered ‘confirmed.’ However, there are thousands of other ‘candidate’ exoplanet detections that require further observations to say for sure whether or not the exoplanet is real, say experts.