Unique case in space: discovered two alien worlds with perpendicular orbits

Their peculiarity would be the result of a complex evolution. At 155 light years from Earth there are two planets that move along transverse orbits.

In space, outside the solar system, there is a unique case between planets: around the star Hd 3167, 155 light years from Earth, there are two worlds that move along perpendicular orbits. The discovery was made by an international team involving researchers of Inaf, the National Institute of Astrophysics, thanks to the observations of the spectrograph Espresso, mounted at the Very Large Telescope of the European Southern Observatory (Eso), and Harps-N, at the National Telescope Galileo, just Inaf. The results were then published in the journal Astronomy & Astrophysics.

The discovery of two alien worlds

The object of the study, led by Vincent Bourrier of the University of Geneva, were three planets that revolve around the star Hd 3167. The three worlds were named Hd 3167b, Hd 3167c and HD 3167d, respectively. The first is a super-Earth of about 5 Earth masses, with an orbital period of just 0.96 days; on the second is a mini-Neptune of 9.8 Earth masses, with an orbital period of about 30 days; and the third has a mass of about 7 Earth masses. Observations revealed that Hd 3167b has an equatorial orbit, while Hd 3167c is on a polar orbit, inclined from the first by about 90°.

Because the two have perpendicular orbits

The complex geometry, particularly of Hd 3167b and Hd 3167c, represents a unique case for science, observed for the first time in an exoplanetary system. It is the consequence of a different dynamic evolution of the two planets, with the former 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," explained one of the authors of the study, Giusi Micela of Inaf in Palermo. "The simultaneous existence of planets with aligned and unaligned orbits is unique among known exoplanets and probably a rare combination that suggests a complex dynamical history, probably due to interaction with another star," she added.

In the meantime, researchers in Cambridge have discovered a new class of planets, while another study has identified a planet that survived the death of its star.

Stefania Bernardini