Star System With a Record 9 Planets Found



The sun-like star, called HD 10180, located approximately 127 light-years away in the constellation Hydrus, is home to a record nine planets, making it  the most populated system of extrasolar planets yet found. In a previous study that was published in August 2010, astronomers identified five confirmed alien worlds and two planetary candidates. If confirmed, the planetary system around 'HD 10180' would be the richest-ever discovered. 

The new study confirms both previous candidates in the HD 10180 system, and also suggests that two more planets could be orbiting the star said lead author Mikko Tuomi, at the Centre for Astrophysics Research at the University of Hertfordshire in Hatfield, UK. “Now that Pluto is not a planet, this system is likely more planet-rich than the Solar System,” says Tuomi.

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.

Since the newly detected candidates are still unconfirmed, more research is needed to determine if they are bona fide planets, and not erroneous signals.

Tuomi used a Bayesian framework, which evaluates many possible scenarios with the aim of seeing which is most consistent with the data in total. He  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.

"While the existence of the larger of these two is well supported by the data, the signal corresponding to the smaller one exceeds the detection threshold only barely, which gives it a very small but non-eligible probability of being a false positive," Tuomi said.

He 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.”

"They are certainly not in the habitable zone and likely have no prospects for hosting life," Tuomi told "However, one of the Neptune-sized planets in the system with an orbital period of 600 days is actually in the middle of the habitable zone, which makes it an interesting target when the better detection methods enable us to observe moons orbiting exoplanets in the future."

As instruments and observatories become more sophisticated, and as astronomers hone planet-hunting techniques, densely populated systems similar to HD 10180 and our own solar system could be discovered in greater numbers.

"This certainly tells our methods are sufficient for detecting richly populated planetary systems," Tuomi said. "Just how common they are, we do not know based on only two examples. My guess would be that they are very common, though, because they are very hard to detect and we already have one when the precision of our instruments enables the detection of these systems only barely."

The finding also suggests that similar planetary systems could be more common throughout the universe than was previously thought.

"Scientifically this would not be of much significance because it has been suspected for a long time that such populous planetary systems exist in the universe," Tuomi said. "Philosophically, though, it shows that our very own solar system is not special in this respect either — systems with great numbers of planets are very likely common throughout the universe and it is only a matter of time when we find even richer systems."

Image at the top of the page is an artist's illustration of the star HD 10180 and a planet that could be one of the smallest. Credit: ESO/L. Calçada

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