Two articles to be published in the journal “Astronomy & Astrophysics” describe the discovery and characterization of a planet still in its formation phase orbiting the young star PDS 70. Two teams of astronomers used the SPHERE instrument installed on ESO’s VLT to obtain for the first time images of a planet while it’s forming in what is still more or less a disk of gas and dust around the star. Called PDS 70b, the planet is a gas giant that could be larger than expected for its age.
About 370 light years away from Earth, the star PDS 70 has a mass that is about 80% of the Sun’s for an estimated age of about ten million years, which means that it’s very young in astronomical terms. In 2006 a protoplanetary disk was detected around the star with a radius about 140 times the distance between the Earth and the Sun. In September 2012 an article published in “The Astrophysical Journal Letters” described the large gap within that disk, attributed to a process of planetary formation.
In subsequent years, astronomers discovered that there are diverse regions within the protoplanetary disk with an asymmetry and differences in the size of the dust grains. This was attributed to the formation of different planets. The use of the SPHERE instrument installed on the VLT (Very Large Telescope) allowed to obtain clear photos of one of those planets, still in formation.
The SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) instrument was tested for the first time in June 2014 and its main task is exactly to discover and examine giant planets that orbit stars that are nearby in astronomical terms. Unlike other instruments, it allows to get direct images of the planets by eliminating the light of their stars and that’s why in the image of the PDS 70 system you see a black circle in the center in the star’s place.
Miriam Keppler, who led the team that discovered the planet PDS 70b, explained that so far only a handful of observations could find traces of newborn planets around young stars. The problem is that so far most of the candidates could be just some features in the protoplanetary disk.
Not surprisingly, PDS 70b is the first planet in formation confirmed within two surveys on 600 young stars based on the use of SPHERE called SHINE (SpHere INfrared survey for Exoplanets) and DISK (sphere survey for circumstellar DISK). In this case, the planet is clearly visible in the bright circle immediately at the right of its star’s position, obscured by SPHERE.
Because of the importance of this discovery, André Müller already led another team that studied the planet PDS 70b to try to understand its characteristics. The estimate of its size is still vague, between 1.4 and 3.7 times the planet Jupiter’s and if the real measure was close to the greatest one it would be bigger than the astronomers expect for a planet so young, with an age estimated at around 5 million years.
There’s still a lot of work to do to understand the characteristics of the planet PDS 70b, which isn’t an easy job as it’s a state of the art study. Observation strategies must be carefully planned and the models used to analyze the data are continuously updated. All the data collected bring new advances in the knowledge of new planets and in the understanding of the processes of planetary formation.