A study of red dwarf stars has revealed eight suspected new planets – three not much larger than the Earth – and indicated that planetary companions are the norm for this type of star.
Red dwarf stars are very faint, low-mass stars. They put out so little light that even those closest to the Earth are not visible to the naked eye. However, as telescopes have become more powerful astronomers have concluded that three quarters of the stars in the galaxy fall into this category.
Their frequency means there are plenty of red dwarfs in our galactic neighbourhood. The study was done on 41 stars 15-80 light years away, tiny distances compared to most of those examined by the Kepler Space Telescope. The orbital periods discovered range from two weeks to nine years. Longer orbiting planets have not been found, not because they don't exist, but because their smaller and slower effects on the parent star are harder to detect.
The discovery brings the number of planets detected around low-mass stars to 25, including the confirmation of two previously suspected bodies. Allowing for the limitations in observation the results suggest almost all red dwarfs have at least one sizable planet. Several of the stars were found to have multiplanetary systems, and one, GJ 229 now has a large planet to add to the already observed brown dwarf (objects that fall between stars and planets in size).
The findings were made using the Doppler shift method and published in Monthly Notices of the Royal Astronomical Society. Small movements of a star towards or away from the Earth are detected based on shifts in the frequency of light emitted, similar to the change in sound from vehicles as they approach and recede. Regular variation in a star's movement can only be explained by the orbit of a planet, constantly tugging the star to and fro. Red dwarfs are particularly suitable for this technique, since their low mass makes them easy for planets to pull around.
Dr Mikki Tuomi of the Univertity of Herfordshire says, “We were looking at data from the Ultraviolet and Visual Echelle Spectograph (UVES) and noticed some variability that could not be explained by random noise.” Unfortunately, the pattern was not clear enough to identify the sizes and orbits of the planets. However, when the results from the High Accuracy Radial Velocity Searcher, also operated by the European Southern Observatory in Chile, were added to those of UVES patterns emerged that could not have been confirmed in either's data alone.
Planets with masses about ten times that of Earth are hard to detect with this technology – the authors estimate only 10-30% would be found, even of those orbiting close to the parent star. Consequently, it seems likely plenty have been missed.
Particularly exciting is the presence of three planets within the habitable zone – the area around a star where liquid water can exist. Red dwarfs put out so little heat that this territory is small and very close to the star. “Our results also indicate that planets with masses between 3 and 10 Earth masses are common in the stellar habitable zones of M dwarfs with an estimated occurrence rate of 0.21 planets per star,” the paper concludes. Larger planets are thought to occur in this region less frequently.
Besides being numerous, red dwarfs are often very old. Large stars burnout quickly, but smaller ones can last for tens of billions of years. This means that if life could evolve on one of these planets (something which is still very much debated) it would have plenty of time to develop.
TV series aside, the most famous red dwarfs are Proxima Centauri, the nearest star to us besides the sun, and Barnard's star, the closest object not part of a multi-star system. Barnard's star was the first object outside the solar system to be reported to have planets, but the study on which this conclusion was based is now thought to have been flawed.