Which star has the most planets surrounding it? It’s no longer the Sun, according to an astronomer who says he can identify nine planets orbiting the bright, nearby star HD 10180 (pictured). “Now that Pluto is not a planet, this system is likely more planet-rich than the Solar System,” says Mikko Tuomi, a postdoctoral researcher at the Centre for Astrophysics Research at the University of Hertfordshire in Hatfield, UK.
Tuomi’s analysis, accepted for publication on 6 April by the journal Astronomy & Astrophysics, is a re-interpretation of 190 measurements made between 2003 and 2009 by the High Accuracy Radial Velocity Planet Searcher (HARPS), a spectrograph on the 3.6-metre La Silla telescope in Chile, which looks for periodic wobbles in a star caused by the tug of its planets. The 2010 announcement from the HARPS team identified five planets, with suggestions of a sixth and seventh.
But that study was based on a classical, or frequentist, approach, which asks whether the data can support or reject narrow, sequential hypotheses: is there a seventh planet? Is there an eighth planet? Tuomi applied a more computationally demanding Bayesian framework, which evaluates many possible scenarios with the aim of seeing which is most consistent with the data in total. And Tuomi finds that the most likely scenario, one with a 99.7% probability, includes eighth and ninth planets with masses 5.1 and 1.9 times that of the Earth.
As a check, Tuomi performed a simple stability analysis and found that the orbits of the two new planets are gravitationally stable to the first order. “It’s quite a coincidence,” he says. He hopes that other astronomers will undertake a full dynamical analysis to see if the orbits could persist over the lifetime of the star, which is estimated to be 4.3 billion years. He also expects that HARPS data collected on HD 10180 since 2009 could resolve remaining doubts. He adds, “We will find equally rich and even richer systems in the near future, I have no doubt.”
Eric Ford, an astronomer at the University of Florida in Gainesville, says that Tuomi’s analysis is statistically “more rigorous” than the HARPS team’s. “He’s taken the published data and gone as far as you can go with it,” he says. “At the same time, I’d say that caution is needed.” Based on Tuomi’s analysis, Ford says that one could use different adjectives for the record setting planets: evidence for the seventh planet is “strong”; evidence for the eighth is “significant”; and evidence for the ninth, for now, is just “suggestive”.
Ford says that the study is neither particularly controversial nor surprising. Astronomers are rapidly becoming accustomed to the ubiquity of multiple planet systems. It’s just a question of how big solar systems can be. “Most of the time, when you find one planet, you find another,” says Ford. “Where does it stop?”
Credit: ESO and Digitized Sky Survey 2. Acknowledgment: Davide De Martin