A well aligned orbit for the 45 Myr old transiting Neptune DS Tuc Ab

TL;DR

This study measures the alignment of the Neptune-sized planet DS Tuc Ab's orbit with its young star's equator, revealing a well-aligned orbit and developing a technique to model starspots and planetary transits.

Contribution

It provides the first measurement of the orbital alignment for a young Neptune-sized exoplanet and introduces a new method for modeling starspots during transits.

Findings

Orbit is well aligned with stellar equator (lambda ≈ 2.5°).

Detected large starspots affecting spectral lines.

Developed a technique for simultaneous starspot and transit modeling.

Abstract

DS Tuc Ab is a Neptune-sized planet that orbits around a member of the 45 Myr old Tucana-Horologium moving group. Here, we report the measurement of the sky-projected angle between the stellar spin axis and the planet's orbital axis, based on the observation of the Rossiter-McLaughlin effect during three separate planetary transits. The orbit appears to be well aligned with the equator of the host star, with a projected obliquity of lambda = 2.5 +1.0/-0.9 deg. In addition to the distortions in the stellar absorption lines due to the transiting planet, we observed variations that we attribute to large starspots, with angular sizes of tens of degrees. The technique we have developed for simultaneous modeling of starspots and the planet-induced distortions may be useful in other observations of planets around active stars.