Two backward orbiting exoplanets in a triple stellar system prove protoplanetary disk-tilt mechanism

K2-290 is a new exoplanetary system composed of two exoplanets orbiting "backwards" their parent star. HARPS-N made the first observations of this system and they have been pivotal for this exciting discovery.

Why exciting? It is widely assumed that a star and its protoplanetary disk are initially aligned, with the stellar equator parallel to the disk plane, as occurred for the Solar System. The case of K2-290 and other two previously known indicate that this initial alignment can be modified by an external mechanism. Differently than in the other two cases, K2-290 consists in a triple stellar system. The primary K2-290A is a late-type F-star, the secondary K2-290B an M-dwarf star at a projected separation of 113 astronomical units, and the tertiary K2-290C another M-dwarf star located further away. It is K2-290A which harbors the two exoplanets in a retrograde orbit with respect to the star's rotation. They have the size of a sub-Neptune (K2-290b, orbital period 9.2 d) and of a warm-Jupiter (K2-290c, 48.4 d). The K2-290 system provides a robust proof of evidence that stars and protoplanetary disks can become grossly misaligned due to the gravitational torque from neighbouring orbiting companions.

The retrograde orbit of K2-290c was discovered by measuring the Rossiter-McLaughlin (RM) effect with both HARPS-N on 25 April 2019 and the High Dispersion Spectrograph (HDS, Subaru telescope, Hawaii) on 12 June 2019. ESPRESSO (VLT, ESO, Chile) measured the RM effect on the radial velocity curve of K2-290b on 20 July 2019.

This impressive result was obtained by a group of researchers led by PhD Maria Hjorth from Stellar Astrophysical Centre, Aarhus, Denmark. It is noteworthy that observing time at TNG was allocated in the framework of the TNG-NOT agreement.

More details in the paper "A backward-spinning star with two coplanar planets" published in the esteemed periodical "Proceedings of the National Academy of Sciences".

Mass-radius diagram

This artist illustration depicts the exoplanetary system in K2-290. It shows the main star K2-290 A, its two planets, and in the background the smaller companion K2-290 B. The unique aspect about the K2-290 system is that the planetary host star (K2-290 A) rotates backwards with respect to the coplanar orbital motion of its two planets. When it was only a few million years old, the disk had a much larger extent than the orbits of the two planets, forming inside. The gravitational torque from K2-290 B led to a precession of the disk's rotation away from initial alignment with its host star. (Credits to Christoffer Grønne)

Planet portrait

The Rossiter-Mc Laughlin effect for K2-290b (right, ESPRESSO observations) and K2-290c (left, HARPS-N and HDS observations). The dashed-line curves are the expected effects in case of orbits in the same sense of the stellar rotation. The fact that they are actually reversed tells us that the planets are orbiting backward.
(Figure from Hjorth et al., link below)

Link to the papers: