Finding transiting planets is no longer a surprise, and we can expect a host of transits from the CoRoT mission, which has the advantage of observing from a space-based platform. Moreover, CoRoT will, in the course of its lifetime, survey as many as 120,000 stars for up to five months. Driving home the advantage is the announcement of a new CoRoT planet known as CoRoT-Exo-4b.
We’re dealing with another Jupiter-sized planet orbiting in close proximity to its star, but this one has a unique claim to distinction: Its host star is rotating at the same pace as the planet’s orbit.
Image: Fixated upon a star: An artists impression of the satellite CoRoT in orbit around the Earth. Credit: CNES.
Moreover, for a transiting world, CoRoT-Exo-4b is a relatively long-period planet, orbiting its F-class primary in 9.2 days. Thus far most transiting worlds have had orbits below about five days, two major exceptions being HD 147506b and HD 17156b, the latter with a period of 21.2 days — both of these planets are in highly eccentric orbits. Working out what’s going on in this system is going to prove interesting, and doubtless illuminating when it comes to other planetary systems. Says Suzanne Aigrain (University of Exeter), who discussed the find at the Cool Stars, Stellar Systems, and the Sun conference being held in St. Andrews:
“We don’t know if CoRoT-Exo-4b and its star have always been rotating in synch since their formation about 1 billion years ago, or if the star became synchronized later. CoRoT will no doubt find many more transiting planets, and by systematically measuring their host stars’ rotation periods we will gain valuable insight into how stars interact with their planets.”
CoRoT’s observations are backed by ground telescopes, but the lack of atmospheric distortion that the spacecraft experiences means that we may be finding interesting worlds not all that much larger than the Earth. Moreover, in the case of larger worlds like CoRoT-Exo-4b, the continuous coverage CoRoT can bring has allowed scientists to make precise studies of the host star between transits, thus providing a way to measure the star’s rotation by tracking dark spots on its surface. So we’ve moved beyond planetary detections into the science of planetary interactions with the primary, as noted in the paper on this work:
This system clearly warrants further observational and theoretical investigation to pin down its tidal and rotational evolution status. For example, more detailed analysis of the out-of-transit light curve should enable the active regions on the stellar surface to be mapped in a time-resolved fashion… to search for signs of star-planet magnetic interaction.
The paper is Aigrain et al., “Transiting exoplanets from the CoRoT space mission IV. CoRoT-Exo-4b: A transiting planet in a 9.2 day synchronous orbit,” now available on the arXiv site and accepted for publication in Astronomy & Astrophysics. A news release from St. Andrews is here.