Habitable planets common around red dwarf stars
Sen—An international team have discovered eight more planets in our local neighbourhood of the Galaxy, at least three of which are potentially habitable. What’s more, the finds lead them to believe that most if not all red dwarf stars have planets in orbit around them.
The scientists were led by Mikko Tuomi, of the UK’s University of Hertfordshire. His team found the planets by analysing archived data from two high-precision planet surveys made with the Ultraviolet and Visual Echelle Spectrograph (UVES) and High Accuracy Radial velocity Planet Searcher (HARPS), both operated by the European Southern Observatory (ESO) in Chile.
The research also suggests that habitable-zone super-Earth planets orbit around at least a quarter of the red dwarfs in the Sun’s own neighborhood.
ESO's two instruments do not see the planets directly. They find the new worlds by measuring how much the stars wobble as they are affected by the gravity of a planet in orbit around it.
Describing the technique, Dr Tuomi said: “We were looking at the data from UVES alone, and noticed some variability that could not be explained by random noise. By combining those observations with data from HARPS, we managed to spot this spectacular haul of planet candidates.
“We are clearly probing a highly abundant population of low-mass planets, and can readily expect to find many more in the near future – even around the very closest stars to the Sun.”
An artist’s representation of how the new potentially habitable worlds might appear compared to some in our own Solar System. Credit: PHL@UPR Arecibo, NASA
All the new planets orbit red dwarf stars, with Gliese (GJ) labels, that lie between 15 and 80 light years from the Sun, making them relatively close neighbours. Of the stars with planets in the habitable zone, Gliese 180 is 38 light years away, Gliese 442 is 41 light years away, and Gliese 682 is at a distance of 17 light years.
The eight planets take between two weeks and nine years to complete each orbit, which puts them at a distance from their stars of between 6 and 600 million km.
Dr Tuomi reckons that three of his new discoveries are in the so-called “habitable zone” of their stars, where water could exist as a liquid and so support life, and just a little more massive than the Earth. Colleagues at the Planetary Habitability Laboratory (PHL) in Puerto Rico are even more optimistic and suggest that four might be habitable, with Gliese 180 having two such worlds.
Explaining the discrepancy, Dr Tuomi told Sen: “Well, that is because we use a more conservative definition for a HZ (habitable zone) planet. The PHL adds some sort of an “extended HZ”, which I, frankly, do not know how it’s calculated, but that adds some areas of potential habitability to the inner and outer edges of the HZ as we have defined it. They included the inner companion of the GJ 180 system (planet b) that we consider too hot to be potentially habitable.”
Because the planets lie so close to our own Solar System, it is being suggested that some of the eight new planets would be prime candidates for study by NASA’s next-generation observatory the James Webb Space Telescope, when it launches in 2018.
Dr Tuomi told Sen: “Certainly we should try to see the the candidates in this sample with the longest orbital periods and thus the largest separations from the host stars, such as GJ 229 b, although they would not be habitable but cold planets the size of Neptune or Saturn.
“Such observations would be extremely valuable in telling us the orbital orientations, sizes, and atmospheric properties of the planets enabling us to test composition models in detail.
“The sad bit is that, although several hundred planets have been observed, we know frustratingly little of their chemistry, physical properties, and atmospheric conditions, which makes labeling any of the “habitable”, as we have done here based on certain assumptions, equivalent to educated guessing.”
The discoveries are reported in the journal Monthly Notices of the Royal Astronomical Society. The team also plan to follow up a further ten weaker signals that they believe they have detected.
Another visualisation of the new planetary systems. Credit: Guillem Anglada-Escudé, Queen Mary University of London