'Hot Jupiter' exoplanets may have formed very rapidly

In the Solar System, rocky planets like the Earth and Mars are found near the Sun, whereas gas giant planets such as Jupiter and Saturn are further away. Hence the surprise of Michel Mayor and Didier Queloz when they discovered the very first exoplanet, exactly twenty years ago. This turned out to be a gas giant like Jupiter, but orbiting twenty times closer to its host star than the Earth does to the Sun.

Since then, astronomers have shown that these future 'hot Jupiters' form in the outer regions of the protoplanetary disc, the cloud of dust and gas from which the central star and its surrounding planets are born, and then migrate inwards. It is when such gas giants get close to their star that they heat up and become hot Jupiters, unlike our own Jupiter, a cold gas giant which is five times further from the Sun than the Earth. But just when do these hot Jupiters migrate close to their host star? Until now, astronomers hypothesized two possible scenarios: the process might take place at a very early stage, when the young planets are still forming within the original disc, or else much later, once a number of planets have formed and interact in a choreography so unstable that some of them are flung inwards to the immediate vicinity of the central star.

Now, an international team of astrophysicists, including several French researchers and led by Jean-François Donati from the Institut de Recherche en Astrophysique et Planétologie (IRAP, CNRS/Université Toulouse III-Paul Sabatier), may have found evidence supporting the first of these scenarios. Using ESPaDOnS, a spectropolarimeter built by teams at IRAP for the Canada-France-Hawaii Telescope (CFHT), they observed stars in formation in a stellar nursery located in the constellation Taurus, some 450 light-years from the Earth. One of them, V830 Tau, exhibits signatures similar to those caused by a planet 1.4 times more massive than Jupiter, but orbiting 15 times closer to its star than the Earth does to the Sun. This discovery suggests that hot Jupiters may be extremely young and potentially far more frequently found around stars in formation than around mature stars like the Sun.

Young stars are a mine of information about planetary formation. Due to their very intense activity and magnetic fields, they are covered with sunspots hundreds of times bigger than those on the Sun. These therefore generate perturbations in the star's spectrum that are much greater than those caused by planets, making them much harder to detect, even when they are hot Jupiters. To overcome this problem, the team initiated the MaTYSSE survey aimed at mapping the surface of these stars and detecting the presence of any hot Jupiters.

By monitoring these young stars as they rotate and using tomographic techniques inspired from medical imaging, it is possible to reconstruct the distribution of dark and bright features on their surfaces, as well as the topology of the magnetic field. Modeling also makes it possible to correct the perturbations caused by this activity and thus detect the potential presence of hot Jupiters. In the case of V830 Tau, the authors were able to use this new technique to discover a hitherto hidden signal hinting at the presence of a giant planet. Although more data are required to validate the signal, this promising first result clearly demonstrates that the method used by the team can be the key to solving the puzzle of how hot Jupiters form.

SPIRou, the new instrument being built by teams at IRAP for the CFHT and scheduled for first light in 2017, will push back the limits of this method, thanks to its ability to observe at infrared wavelengths, where young stars are much brighter. This will make it possible to explore the formation of stars and exoplanets in even greater detail.