The Hubble Space Telescope has taken a direct picture of a Jupiter-like protoplanet, which is forming right now in a very unusual, intense and rapid way that differs radically from the way the vast majority of planets in the Universe form. It is the first direct proof of the long-standing hypothesis of the possible formation of a Jupiter-like planet through a process called "protoplanetary instability".
The discovered protoplanet at the formation stage is still confined inside the protoplanetary disk, a disk of dust and gas surrounding a young star up to 2 million years old. According to the leading theory on planet formation, almost all planets grow from small formations that, as material accumulates, acquire their own gravity and attract more matter, whose particles collide, heat up and fuse. In the end, the formed nucleus slowly starts attracting gas from the surrounding space, gradually turning into a gas giant.
In contrast to the above-described way of forming planets, the process of "protoplanetary disk instability" is much faster on a cosmic time scale. The large protoplanetary disk around the star cools and, under the influence of the star's gravity and its own gravitational forces, it disintegrates into several massive fragments. The fragment, which is in the right conditions, then continues to shrink and eventually becomes a planet.
The planet in question at the start is called AB Aurigae b, it is now nine times the size of Jupiter and orbits its star at a distance of 13.84 billion kilometres, more than twice the distance between the Sun and Pluto. At that distance, a planet would take much longer to form through the traditional growth process to gain its current mass. And given this fact, scientists have concluded that the planet AB Aurigae b formed through a process of "protoplanetary disk instability".
The protoplanet AB Aurigae b was discovered and studied using two Hubble Space Telescope Imaging Spectrograph and the Near Infrared Camera and Multi-Object Spectrograph instruments. The scientists also used a data package from Japan's Subaru 8.2-metre telescope in Hawaii's SCExAO instrument. A contributing factor is that the planet's protoplanetary disk is nearly perpendicular to Earth.
Observations of the protoplanet AB Aurigae b at different wavelengths, plus 13 years of observations by the Hubble telescope, have also contributed to the discovery and study of this planet's orbit.
"This discovery provides strong evidence that some of the gas giants may form through the mechanism of 'protoplanetary disk instability'" - The researchers write, "The main aspect of this process is the gravitational forces, the effects of which will cause all the remnants of the young star formation process to turn into planets after a while."