Two alien worlds with perpendicular orbits, unique case

155 light years from Earth, around the star Hd 3167, there are two planets moving along transverse orbits, placed at right angles to each other: a unique case among the known planets outside the Solar System, discovered thanks to the observations of the Espresso spectrograph, mounted at the Very Large Telescope of the Southern European Observatory (Eso), and of Harps-N, at the Galileo National Telescope of the National Institute of Astrophysics (Inaf). The results are published in the journal Astronomy & Astrophysics by an international team which also includes researchers from INAF.

The study, led by Vincent Bourrier of the University of Geneva, examined three planets revolving around the star Hd 3167: Hd 3167b is a super-Earth of about 5 Earth masses, with an orbital period of just 0.96 days; Hd 3167c is a mini-Neptune of 9.8 Earth masses, with an orbital period of approximately 30 days; Hd 3167d has a mass of approximately 7 land masses. The observations show that Hd 3167b has an equatorial orbit, while Hd 3167c is on a practically polar orbit, inclined with respect to the former by about 90 degrees.

This complex geometry, observed for the first time in an exoplanetary system, is the consequence of a different dynamic evolution of the two planets, with HD 3167b retaining the primordial orbit of the protoplanetary disk from which it formed, having undergone few interactions during its evolution, while the orbit of Hd 3167c was perturbed by gravitational interactions.

"The architecture of the planetary system of HD 3167 testifies to a complicated evolution of the system", explains to Media Inaf one of the authors of the study, Giusi Micela of INAF of Palermo.

"The contemporary existence of planets with aligned and non-aligned orbits is a unique case among known exoplanets and probably a rare combination suggesting a complex dynamic history, possibly due to interaction with another star. Harps' combined use -N for the mini-Neptune and Espresso for the super-Earth, at the limit of the instrumentation available today, has proved to be an effective strategy for the study of such a complex system ".