Some solitary planets wander through interstellar space, offering unexpected surprises about their formation and evolution.
Astronomers recently observed a particularly remarkable case in the constellation of Chamaeleon, approximately 620 light-years from Earth. This isolated world, designated as Cha 1107-7626, has the particularity of being surrounded by a disk of matter and displaying intense accretion activity. Using the Very Large Telescope of the European Southern Observatory, researchers were able to measure that this solitary planet absorbs nearly six billion tons of gas and dust every second, a growth rate never before observed for a planetary-mass object.
Artist's impression of exoplanet Cha 1107-7626 showing its accretion disk
Credit: ESO/L. Calçada/M. Kornmesser
Spectroscopic analysis reveals that this accretion process doesn't occur regularly but through violent bursts, similar to the growth spurts observed in young stars. The international team, led by Víctor Almendros-Abad from the Italian National Institute for Astrophysics, detected that the planet's magnetic field plays a role in these episodes of intense accretion. During active phases, the chemical composition of the surrounding disk changes, with the temporary appearance of water vapor that disappears when accretion slows down.
This discovery challenges the traditional boundary between planets and stars. As Aleks Scholz from the University of St Andrews explains, the origin of these rogue planets remains enigmatic: do they form like low-mass stars or are they giant planets ejected from their original stellar system? Combined observations from the Very Large Telescope and the James Webb Space Telescope show that Cha 1107-7626 has a mass between five and ten times that of Jupiter, making it one of the lightest known free-floating planetary-mass objects to host a disk and display active accretion.
The prospects for studying these solitary worlds look promising with future astronomical instruments. The Extremely Large Telescope, equipped with the largest mirror ever built for astronomical observation, should enable the detection of many other examples of these rogue planets. Belinda Damian, an astronomer at the University of St Andrews, emphasizes that this discovery offers us a unique glimpse into the early formation phases of isolated planets, revealing processes that were thought to be reserved for stars.
Planetary accretion: how worlds grow in space
Accretion refers to the process by which celestial bodies accumulate surrounding matter to increase their mass. This phenomenon occurs when an object's gravity attracts gas and dust present in its immediate vicinity.
In the case of traditional planets, accretion occurs mainly during their formation in protoplanetary disks around young stars. Dust particles gradually clump together to form planetesimals, which continue to capture matter until they become full-fledged planets.
What makes Cha 1107-7626 exceptional is that it continues this accretion process well after its initial formation, and does so without the presence of a central star. The observed rate of six billion tons per second is particularly remarkable because it far exceeds what is usually observed in classical planetary systems.