The super-Earth HD 3167b is close to orbiting within the stellar equatorial plane, while the mini-Neptune HD 3167c orbits above the poles of the host star, and the orbits of the two planets are nearly perpendicular (mutual inclination 102.3 degrees), according to a team of astronomers led by the Observatoire Astronomique de l’Universite de Geneve.HD 3167 is a bright K0-type star some 149 light-years away in the constellation of Pisces.
Also known as EPIC 220383386 and 2MASS J00345752+0422531, it has a radius and a mass roughly 86% that of the Sun, and is approximately 8 billion years old.
HD 3167 hosts at least three exoplanets: HD 3167b, c and d.
The innermost planet, HD 3167b, is a super-Earth on an ultrashort period of 0.96 days and the outermost one, HD 3167c, is a mini-Neptune on a 29.84 days orbit.
While HD 3167b and HD 3167c transit, this is not the case for HD 3167d, which orbits in between them on a 8.51 days orbit.
“Theories of the origin of planetary systems predict that planets form in the equatorial plane of their star and continue to evolve there, unless disturbed by special events,” said Dr. Vincent Bourrier from the Observatoire Astronomique de l’Universite de Geneve and colleagues.“This is not the case in the Solar System, where our planets lie close to the solar equatorial plane. In this case, the planets are said to be aligned with their star.”
“However, a 2019 study showed that two of the three planets around HD 3167 are not aligned with it. HD 3167c and HD 3167d actually pass over the star’s poles, nearly 90 degrees from its equatorial plane.”
In the new study, the astronomers observed the HD 3167 system with the ESPRESSO spectrograph on ESO’s Very Large Telescope and the HARPS-N spectrograph on the 3.58-m Telescopio Nazionale Galileo.
Using the Rossiter-McLaughlin effect Revolutions (RMR) technique, they were able to measure the spin-orbit angles of HD 3167b and HD 3167c.
“The two planets are on perpendicular orbits,” the researchers said.
“This could be explained by HD 3167b being strongly coupled to the star and retaining its primordial alignment, whereas HD 3167c would have been brought to a nearly polar orbit via secular gravitational interactions with an outer companion.”
“Follow-up observations of the system and simulations of its dynamical evolution are required to search for this companion and explore the likelihood of this scenario.”
“HD 3167b is the smallest exoplanet with a confirmed spectroscopic Rossiter-McLaughlin signal.”
“The RMR technique opens the way to determining the orbital architectures of the super-Earth and Earth-sized planet populations.”
The team’s paper was published in the journal Astronomy & Astrophysics.
