An exoplanet child, still in full growth


An exoplanet born recently and in full expansion has been identified, almost by chance, in the orbit of a system of binary stars 538 light-years from Earth. The extrasolar planet, perhaps a gas giant, would be less than two million years old and still growing, nourished by the dust disk that seems to surround it.

The initial goal. The discovery of a group of astronomers from the University of Leiden (Holland) took place while the team studied the CS Cha binary star system, in the southern constellation of the Chameleon, using the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument of Very Large ESO Telescope in Chile.

These very small and class T Tauri stars, that is, in the early stages of their evolution, orbit around a common center of gravity and are surrounded by what appears to be a disk of dust and circumbinary gas, which envelops both and extends for 55 astronomical units (55 times the average Earth-Sun distance).

Image Binary stars and the small companion, again infrared: this time, special filters also allow to see the gas and dust disks surrounding the celestial objects. | C. Ginski / SPHERE

Third wheel. Observing the images of the star system, scientists have noticed the presence of a dimmer dot of light outside their circular disk, at about 210 astronomical units away. To verify that it was indeed a celestial object, and not simply "noise" in the surveys, they searched the VLT archives of 11 years ago, and also observed the photos of the system obtained by the Hubble space telescope 19 years ago. The luminous dot was always there.

The object is therefore a feature of the CS Cha system: given the appearance and the position, it could be a brown dwarf (a "missed" star of very reduced mass, too small to trigger the fusion of the hydrogen but too large and hot to be considered a planet) or a gas giant, a sort of "super Jupiter" still growing. The highly polarized light - that is, in which the electromagnetic radiation is in a single plane - coming from the object makes us suspect it is itself surrounded by a disk of gas and dust, which prevents us from determining its mass with precision.

Whatever happens, a success. Whatever its nature, it is still an exciting discovery. The direct images of exoplanets are extremely rare: their presence is usually deduced from the periodic oscillations of brightness of the stars around which they orbit.

Also the direct observations of brown dwarfs are very rare (the first was announced only in 2009); let alone a brown dwarf companion of other stars, and surrounded by its own accretion disk. The star system will be further studied with the Atacama Large Millimetre / submillimetre Array, in Chile.