An explanation for the strangeness of exoplanets with a large axial tilt

Artist's concept of star system with planets as spinning tops (Image NASA/JPL-Caltech, Sarah Millholland)
Artist’s concept of star system with planets as spinning tops (Image NASA/JPL-Caltech, Sarah Millholland)

An article published in the journal “Nature Astronomy” offers a solution to a mystery concerning a configuration of exoplanets pairs’ orbits discovered over the years by NASA’s Kepler space telescope. According to Sarah Millholland and Gregory Laughlin of the American Yale University, obliquity, which is the inclination between these planets’ axis and their orbit, is a key element to explain why those orbits are just outside the natural points of stability.

NASA decreed the end of the mission of the Kepler space telescope at the end of October 2018, but the thousands of exoplanets discovered and the candidates still under examination have greatly enriched our knowledge on the dynamics existing in star systems while at the same time posing new questions. About 30% of the stars similar to the Sun have planets with sizes between those of the Earth and Neptune with almost circular and coplanar orbits, which mean that they lie on the same plane, with a year less than 100 Earth days.

Perhaps the most curious discovery is that many of these exoplanets exist in pairs that are just outside the natural points of stability that typically occupy due to the gravitational interactions that exist among the various planets and with their stars. This mystery led to years of discussions to understand the mechanisms behind that kind of configuration.

Previous research suggested that tidal effects generated by their stars could lead to that kind of planetary configuration but they’are insufficient. According to Sarah Millholland and Gregory Laughlin of Yale University there’s a connection with the “obliquity tides” generated when a significant tilt between the planets’ axis and their orbit is maintained by a coupling associated with a secular resonance. In those cases the tidal dissipation is greatly increased and this affects the orbits with consequences on their characteristics.

Gregory Laughlin explained that the strong tilt of these exoplanets’ axis has a significant impact on their climate and therefore on weather with much more extreme seasons than planets with a lesser obliquity. Together with Sarah Millholland, he already started working on a follow-up to this study to examine how these exoplanets’ structures respond over time to a pronounced obliquity.

This research offers a solution to a mystery of an “exotic” orbital configuration, but it’s not just a matter of solving a scientific curiosity because the implications concerning the surface conditions of rocky planets can be really important. The extreme seasons and unusual weather phenomena can have serious consequences on the habitability of a rocky planet even in the presence of an atmosphere similar to the Earth’s. This shows why every research on exoplanets is important.

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