We have identified a seven-planet system

The astronomical find of the summer: a star system comprising at least 5 and most probably 7 planets orbiting a Sun-like star has been identified by astronomers from the European Southern Observatory (ESO).

 

HD 10180 is situated 127 light years away from us. It is a yellow star (type G1) of the principal sequence, fairly similar to our Sun, in the Hydrus Constellation (near to the south celestial pole, not to be confused with the Hydra, which stretches along the celestial equator).

HD 10180, a star fairly similar to the Sun. Credit: ESO

By studying it with the help of the HARPS spectrograph attached to the 3.6 m telescope belonging to the La Silla Observatory in Chile, it was possible to highlight rocking movements linked to the presence of five planets of a mass comparable to that of Neptune. Two other planets, an equivalent to Saturn and a “Super-Earth”, could complete the system but are still to be definitely confirmed. These seven planets appear to have almost circular orbits that are regularly spaced in accordance with a progression which brings to mind the Titus-Bode law governing the spacing of the eight planets of the Solar System.

On the edges of the Atacama Desert in Chile, the dome of the 3.6 m telescope belonging to the La Silla Observatory to which Geneva University’s HARPS spectrograph is attached. Credit: ESO

The planet nearest to the star is also the smallest ever detected outside of the Solar System. This would be a telluric planet with a mass equal to 1.4 times that of our Earth which completes its orbit at a mere 3.3 million km from its star in 28.3 hours. This hot rocky planet is therefore about 20 times closer to its Sun than our Mercury is and must be an inferno.

Planets nos.2 to 6 are small gaseous planets with their respective masses corresponding to 13, 12, 25, 24 and 21 times that of the Earth. They are spaced out in the equivalent of our inner Solar System (inside the orbit of Mars) in orbits that they complete in 6, 16, 50, 123 and 601 days. As for the last planet (64 times the Earth), it takes 6 years to complete an orbit on a par with that of our main asteroid belt.

An artist’s impression of the HD 10180 planet system, which appears to be spaced much closer together than our own as the furthest planet currently detected is orbiting at a distance equal to that of our great asteroid belt. Credit: ESO

This system is noticeably lacking in a gaseous giant like Jupiter. According to Christophe Lovis, from Geneva University Observatory, who presented these results during the symposium entitled “Detection and dynamics of exoplanets in transit” which is currently taking place in the Haute Provence Observatory near to Forcalquier, France, such systems of planets with low masses like those around HD 10180 could be fairly common. “But their formation history remains a puzzle” he said. However, it could be possible to find a relationship between the mass of the systems discovered and the properties of the stars. All the systems of massive planets have been detected around massive, metal-rich stars, whereas the light planet systems detected to date have been around less massive, metal-poor stars. This concurs with theoretical models but has still to be confirmed for a greater number of observations to ensure that this pattern is not linked to detection methods. It is, in fact, easier to detect a light planet near to a low-mass star than near to a massive star on which its influence is negligible.

Up until now, the biggest known multi-planet system outside of our own was that of 55 Cancri A, a yellow dwarf (G8) that is part of a double system and endowed with five gaseous planets, including two giants, discovered in 2007.