Observations from a collaboration of telescopes has given a team of astronomers more than they bargained for when not one, but three, super-Earths crammed into the habitable zone of Gliese 667C – the distance from a star where liquid water can exist – sprang into view. What’s more, an additional three, or possibly four, planets might have also been revealed in the system; two of which are thought to be additional super-Earths orbiting on the cusp of the star’s habitable zone.
But Gliese 667C – which is just one member of triple star system, Gliese 667 – is not too unfamiliar to astronomers. Previous studies of the 22 light year distant stellar trio, which rests in the constellation Scorpius, originally revealed three planets with only one of them being in the habitable zone of Gliese 667C, a red dwarf star which, in turn, orbits its brighter orange dwarf companions Gliese 667A and Gliese 667B. “[From the planets’ surface] they would look as painfully bright stars,” says the University of Göttingen’s Guillem Anglada-Escudé who led the study alongside Mikko Tuomi of the University of Hertfordshire. “Similar to all of the [luminosity] of the Moon coming from a point source.”
The finding, which are the fruits of reexamining the system using data retrieved from the High Accuracy Radial velocity Planet Searcher (HARPS) at the ESO La Silla 3.6m telescope, the ESO’s Very Large Telescope, the W M Keck Observatory and the Magellan Telescope, brings the total up to six – and if radial velocity measurements are certain for another planet – even seven worlds.
“This discovery proves that systems with multiple planets in the habitable zone do happen, but one system does not make a trend,” points out Rory Barnes of the University of Washington. “On the other hand, we haven’t found a lot of systems with any potentially habitable planets (because they’re still hard to see), so if one of our first such discoveries includes three (or maybe even five) potentially habitable planets, then that makes me optimistic that we will find more stars that host more than one super-Earth in its habitable zone.”
From our studies of alien worlds and the systems they inhabit, one would think that it was our very own Solar System that was far from the norm. “From what we learned from the Kepler mission and many years of Doppler surveys, it looks like the Solar System is a bit peculiar in the sense it doesn’t have ‘large‘ planets in close-in orbits and its gas giants are very well set in regular or stable orbits,” says Anglada-Escudé.
And, it seems that the formation of the super-Earths – designated Gliese 667Cb, Gliese 667Cd, Gliese 667e – could also be just as peculiar.
“These planets almost assuredly formed in a different manner to those in our Solar System,” says Barnes. “However, it is important to remember that Mars is actually in our habitable zone – it’s just too small to support life (maybe).”
Barnes suggests that the planets in Gliese 667C are likely to have formed at a large distance and were dragged into the habitable zone of the red dwarf very early on in the system’s formation. “This ‘migration’ allows for some compression of the orbits, and hence more planets in the habitable zone,” he tells All About Space. “Most scientists think that the rocky planets in our Solar System formed about where they are today (i.e. no migration). This difference is independent of the planets’ ability to support life.”
However, with limited information on these planets, the team cannot be too certain of what their surfaces hold but they can make an intelligent guess. “We do not know the orientation of the planetary system with respect to the binary,” says Anglada-Escudé. “It is possible that the binary is only visible from some of the hemispheres. The planets are likely to be tidally-locked, meaning they show the same face to the star continuously. That would be a decisive factor in shaping their climates.”
So, time will tell if these planets are indeed friendly to living organisms, implies Anglada-Escudé and his team who have their future research mapped out in their hunt for more distant words. “I am working on improving the data analysis methods to identify even smaller planets and prepare instrumentation for future characterisation efforts,” he says. “That is, preparing methods to study them spectroscopically if they are found to transit.”
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