According to a new study, HD 10180—a sunlike star in the southern constellation Hydrus—may have as many as nine orbiting planets, besting the eight official planets in our solar system.
The star first made headlines in 2010 with the announcement of five confirmed planets and two more planetary candidates.
Now, reanalysis of nearly a decade's worth of data has not only confirmed the existence of the two possible planets but also uncovered the telltale signals of two additional planets possibly circling the star, bringing the total to nine.
"There certainly is, according to my results, strong evidence that this is the most populous planetary system detected—possibly even richer than the solar system," said study leader Mikko Tuomi, an astronomer at the University of Hertfordshire in the U.K.
"But the two new planetary signals I report exceed the detection threshold only just."
Early indications are that both newly detected worlds are super-Earths—planets slightly larger than Earth with rocky surfaces—but more measurements will be needed to confirm their existence.
The planetary system around HD 10180 is too far from Earth for us to see directly. (Related: "First Pictures of Alien Planet System Revealed.")
Instead, astronomers detected the planets by measuring their gravitational tugs on the host star using the High Accuracy Planet Searcher (HARPS) instrument on the European Southern Observatory's 3.6-meter telescope at La Silla, Chile.
Of the two newly confirmed planets, one is about 65 times the mass of Earth, and it orbits farther beyond the main group. The other planet is a super-Earth 1.3 times the mass of our home world that circles very close to the host star.
The two new, unconfirmed planets also have tight orbits: A planet thought to be 1.9 times the mass of Earth completes its orbit in 10 days, while the other world is likely 5.1 Earth masses with an orbit lasting 68 days.
That means, if the planets do exist, they'd be unlikely candidates to host life.
"They are likely hot planets without dense, gaseous atmospheres, because they are just so close to their star," Tuomi said.
The astronomer now hopes to take more measurements and verify the planets are really there.
Tuomi also hopes to scan the skies for other crowded planetary systems like HD 10180. (Also see "'Solar Systems' Common Across the Galaxy, NASA Probe Hints.")
"We have only just started to detect planets, and the known exoplanet systems are but a tip of the iceberg," he said.
"So [our] solar system is only one example among a spectrum of different planetary systems we will find in the near future and [is] definitely not unique."
The new research on the HD 10180 planetary system appears online this week on the website arXiv.org and has been accepted for publication in the journal Astronomy and Astrophysics.