Major Findings

  1. HD 10180 may have up to 9 planets. It would be the most populated exoplanet system to date
  2. One of the planets is in the habitable zone of the star. An exomoon of this planet could be suitable to life.

Plurality of Planets

As if detecting one exoplanet per star wasn’t difficult, there have been a total of 554 multiplanetary systems detected to date. In order to constrain a second or third planet, the signal needs to be distinguished from the original planet. Furthermore, the original planet will create variations in orbital periods of other planets, due to gravity. Despite these hurdles, multiplanet systems have been detected up to 13,300 light years away.

Most of these systems have two or three planets. One however, may have as many as nine. HD 10180, a star just 127 light years from Earth, was observed with the HARPS spectrograph. Despite being a large system, HD 10180 bears little resemblance to our Solar System. The innermost planet, designated HD 10180b has a minimum mass 1.4 times that of Earth. Remarkably, the planet orbits in only 1.1 days. Far from hospitable, the surface temperature is likely seven times that of Mercury.

Planets c, d, e, f, g, and h are all gas planets, with minimum masses ranging from 13 times that of Earth to 66. Although the giant worlds themselves may be uninhabitable, planets g resides in the habitable zone of the star. Therefore, any moons of this planet would be able to support liquid water on the surface. Of course, other factors, such as atmospheric density and the presence of a magnetosphere, play a role in the habitability of a planet. Still, an exomoon of planet g could offer the best case for habitability in the system. Such a body would be undetectable with current observational capabilities, but is certainly an interesting future prospect.

Planetary parameters of the HD 10180 system

Unconfirmed Planets

In addition to these 7 confirmed planets, there are two more planet candidates. Planet i is likely a terrestrial planet, with a mass twice that of Earth. Like many of the other planets, the projected period is less than 10 days, removing any hope for habitability. Planet j has a mass around 5 times Earth’s, and is probably a Super-Earth. The period of the planet is only around 70 days, still within the orbital period of Mercury. Both of these new planets would undoubtedly be scorchingly hot.

More Multiplanet Systems?

As instruments becomes more precise, the hunt for multiplanet systems will reveal more about our own Solar System. First off, does the number of planets say anything about their mass or orbital separations? Are large planetary systems like the Solar System common? HD 10180 is only the first step towards finding even richer exoplanet systems.

The full study for the nine planet system was published in the Astrophysical Journal by Mikko Tuomi.