Gliese 667C is a well-studied star lying only 22 light-years from Earth in the constellation of Scorpius, but it appears to have been hiding a pretty significant secret. The star has at least six exoplanets in orbit, three of which orbit within the star’s “habitable zone” — the region surrounding a star that’s not too hot and not too cold for liquid water to exist on a planetary surface.
Where there’s the potential of liquid water, there’s the possibility for life. Therefore, if you were to gamble on the life-giving potential of any given star in the galaxy, Gliese 667C would triple your chances.
This unprecedented discovery is even more impressive as the trio are all “super-Earths” — worlds that are bigger than our planet and have the potential to possess rocky surfaces.
To make this landmark discovery, astronomers analyzed data from the High Accuracy Radial Velocity Planet Searcher (HARPS) instrument at the European Southern Observatory’s (ESO) 3.6-meter telescope in Chile. The instrument analyzes the very slight ‘wobble’ of stars caused by the gravitational tugging of any exoplanets in orbit. The ESO’s Very Large Telescope, the W.M. Keck Observatory and the Magellan Telescopes provided further data to make the discovery.
Astronomers already knew that Gliese 667C had three worlds in orbit, one in the star’s habitable zone, but the finding of three more exoplanets, two of which are also in the habitable zone is a huge discovery. Finding one small planet in a star’s habitable zone is exciting, but finding three is historic.
“The number of potentially habitable planets in our galaxy is much greater if we can expect to find several of them around each low-mass star — instead of looking at ten stars to look for a single potentially habitable planet, we now know we can look at just one star and find several of them,” said Rory Barnes, of the University of Washington, co-author of the study, in an ESO press release Tuesday (June 25).
Gliese 667C is one of three stars that orbit one another as a triple star system. It is a smaller star than our sun at only one third of its mass. Therefore, the energy emitted from the star is lower, forcing the habitable zone closer to the star. As a consequence, the orbital periods of the habitable trio are a lot shorter than what we are accustomed to living in the sun’s larger habitable zone.
But as with any exoplanet discovery, caution is urged to be aware that just because an exoplanet orbits within the habitable zone, it certainly doesn’t mean there is life on these worlds. We know very little else about the true habitability of the trio in Gliese 667C. Also, just because we know their masses, it doesn’t mean we know their physical sizes.
“Their estimated masses range from 2.7 to 3.8 that of the Earth’s,” Mikko Tuomi, of the University of Hertfordshire and co-investigator, told BBC News. “However, we can only estimate the physical sizes by assuming certain compositions that is, well, only educated guessing.
“Their orbital periods are 28, 39, and 62 days, which means that they all orbit the star closer to its surface than Mercury in our own system. Yet, the estimated surface temperatures enable the existence of liquid water on them because of the low luminosity and low mass of the star.”
Although the Gliese 667C system will be very alien when compared to our own solar system, there’s one key comparison that can be made. The solar system also has three planets within our star’s habitable zone — Venus, Earth and Mars — and we know there’s life on one of them. Venus and Mars may be skirting the inner and outer edges, respectively, of the sun’s habitable zone, but scientists believe that even Mars once had the conditions ripe for basic lifeforms to evolve.
There’s little doubt that Gliese 667C will capture the imaginations of the world and it will certainly be the focus of follow-up studies to help build a picture of the three planets’ composition and whether or not they have atmospheres. Sadly, they do not pass in front of their host star and therefore will not be present in Kepler transit data. So we may have to wait for a next-generation space telescope to spy directly into the system or hope, that one day, we might send a probe to investigate.
In cosmic distances, 22 light-years is a mere stone’s throw across the galactic pond.
Image: Artist’s impression of the Gliese 667C system. Credit: ESO