One small star, one small planet... at least!
A group of astronomers from the UK and Chile report the discovery of eight new small planets orbiting nearby red dwarf stars, three of which may be habitable. From this result the scientists, led by Mikko Tuomi of the University of Hertfordshire, estimate that a large fraction of red dwarfs, which make up at least three quarters of the stars in the universe, has associated low-mass planets. The new work appears in the journal Monthly Notices of the Royal Astronomical Society.
European Southern Observatory in Chile. The two instruments are used to measure how much a star is affected by the gravity of a planet in orbit around it.The researchers found the planets by analysing archival 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
As an unseen planet orbits a distant star, the gravitational pull between the two causes the star to move back and forth in space. This periodic wobble is detected in the star's light. By combining the data from UVES and HARPS, the team was able to detect signals that were not strong enough to be seen in the data from either instrument alone.
With this more sensitive technique, the astronomers discovered the eight worlds, three of which are in the so-called ‘habitable zone’ of their stars and only a little more massive than the Earth. Planets in this region, where the temperature is just right for water to be present as a liquid, are more likely to be able to support life.
All the newly discovered planets orbit red dwarf stars between 15 and 80 light years from the Sun, making them relatively close to the Solar system. The eight planets take between two weeks and nine years to complete each orbit, placing them at a distance from their stars of between 6 and 600 million km (equivalent to between 0.04 and 4 times the distance from the Earth to the Sun).
“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”, said Mikko Tuomi. “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.”
The team used novel analysis techniques to squeeze the planetary signals out of the data. In particular, they applied Bayes' rule of conditional probabilities that enables us to answer the question “What is the probability that a given star has planets orbiting it based on the available data?” This approach, together with a technique enabling the researchers to filter out excess noise in the measurements, made the detections possible.
Hugh Jones, also from the University of Hertfordshire, says, “This result is somewhat expected in the sense that studies of distant red dwarfs with the Kepler mission indicate a significant population of small radius planets. So it is pleasing to be able to confirm it with a sample of stars that are among the brightest in their class.”
These discoveries add eight new exoplanet signals to the previous total of 17 already known around such low-mass stars. The team also plan to follow up a further ten weaker signals.
James S. Jenkins
A number of other images are available from the Planetary Habitability Laboratory
The new work appears in “Bayesian search for low-mass planets around nearby M dwarfs – Estimates for occurrence rate based on global detectability statistics”, Mikko Tuomi, Hugh R. A. Jones, John R. Barnes, Guillem Anglada-Escudé and James S. Jenkins, Monthly Notices of the Royal Astronomical Society, Oxford University Press, in press.
Notes for editors
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