The Oblique Orbit of the Super-Neptune HAT-P-11b

TL;DR

This study measures the highly inclined orbit of the Neptune-sized exoplanet HAT-P-11b, revealing a significant spin-orbit misalignment that supports a specific planetary migration scenario.

Contribution

First measurement of spin-orbit alignment for a super-Neptune, showing a highly inclined orbit using spectroscopic Rossiter-McLaughlin observations.

Findings

HAT-P-11b's orbit is highly inclined with a sky-projected obliquity of about 103 degrees.

The high obliquity supports a migration scenario involving few-body interactions and tidal effects.

Starspot-crossing anomalies in Kepler data are predicted to recur once per stellar rotation period.

Abstract

We find the orbit of the Neptune-sized exoplanet HAT-P-11b to be highly inclined relative to the equatorial plane of its host star. This conclusion is based on spectroscopic observations of two transits, which allowed the Rossiter-McLaughlin effect to be detected with an amplitude of 1.5 m/s. The sky-projected obliquity is 103_{-10}^{+26} degrees. This is the smallest exoplanet for which spin-orbit alignment has been measured. The result favors a migration scenario involving few-body interactions followed by tidal dissipation. This finding also conforms with the pattern that the systems with the weakest tidal interactions have the widest spread in obliquities. We predict that the high obliquity of HAT-P-11 will be manifest in transit light curves from the Kepler spacecraft: starspot-crossing anomalies will recur at most once per stellar rotation period, rather than once per orbital period as they would for a well-aligned system.