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
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Our System Has More Than Eight Planets
Nine planets is NOT more than our own solar system has; it’s less. Dwarf planets are planets too, both structurally and compositionally. They are in hydrostatic equilibrium and orbit a star. In fact, Dr. Alan Stern coined the term “dwarf planet” in 1991 to designate a third class of planets, small planets that are very much akin to the larger ones except for their size.
According to the geophysical planet definition, a planet is any non-self-luminous spheroidal body in orbit around a star (thank you, astronomer extraordinaire Al Witzgall for this great definition). An object does not have to gravitationally dominate its orbit to be considered a planet, according to this equally scientific and legitimate definition. It would be a great service if you could acknowledge that the IAU planet definition is just one side in an ongoing debate and not fact.
As Dr. Stern said, our solar system made many more planets than we learned about in school. At minimum, the number of planets in our solar system is 16: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, Eris, and three unnamed dwarf planets discovered in 2011.
As far as exoplanets, we have no way of knowing if any of the more than 700 confirmed gravitationally dominate their orbits.
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This is important since it confirm that systems are individuals but that our system isn’t exceptional in every way.
@ Laurel Kornfield:
Your commentary is totally besides the point of this awesome result. They have potentially observed is more planets than a corresponding observer looking at our system would see, excluding Mercury most likely they would see 6-7 of our 8 planets.
The problem with exoplanet studies referring back to our system is that we are trying to compare observational apples with pears. But most exoplanets will fulfill the planethood dynamic index criteria, which means a planet will be able to eventually clear its neighborhood and up its chances to remain in a stable orbit for long times, since those bodies are the ones we will tend to see.
One can debate whether such a dynamical criteria is useful for our system, but it seems useful elsewhere and in comparison everywhere. Other planet criteria, not so much.
Nitpick:
We do have ways of knowing this absolutely as well as relatively statistically, as I noted above, because a) eventual clearing derives from a dynamical criteria based on orbital and planetary characteristics b) “domination” can be directly observed by some of the same methods used to find the planets in the first place – the “tug-of-war” is how the Kepler team could confirm some dense systems right away, and it is also potentially seen in cases of shepherding (say, Fomalhaut, though that example isn’t decided as of yet).