Evidence for young planets in stellar discs

Astronomers may have potentially solved a longstanding question as to why gaps appear in some dusty discs that surround young stars.

An artist's impression of the central gap in a disc surrounding a young star, with dust in brown and gas in blue. The illustration shows a planet creating the gap as it orbits.
Credit: ESO/M. Kornmesser

The discs that form around young stars contain dust and gas from which planets will later be formed, indicating the very early beginnings of new planetary systems.

But there is a class of discs called ‘transitional discs’ that contain large gaps in the dust surrounding the young star.

Two theories so far have been put forward to explain these gaps: either that radiation and stellar winds are blowing away and destroying the material, or that young planets forming around the star are clearing away the dust as they orbit.

Because these discs are relatively small and far from Earth, a team of astronomers have used ESO’s Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to observe the process up close.

Nienke van der Marel from Leiden Observatory led the team, who used ALMA to map the distribution of gas and dust in four of these discs.

New images show that there is a significant amount of gas and dust still present within the gaps around the star, but also show that there are gaps in the gas itself.

The team concluded this could only mean that newly formed planets are indeed clearing the gas as they orbit the star, trapping the dust particles that are located further out.

“Previous observations already hinted at the presence of gas inside the dust gaps,” says van der Marel. “But as ALMA can image the material in the entire disc in much greater detail than other facilities, we could rule out the alternative scenario. The deep gap points clearly to the presence of planets with several times the mass of Jupiter, creating these caverns as they sweep through the disc.”

Further observations will now be required to enable astronomers to learn more about the planetary formation process.

“All the transitional discs studied so far that have large dust cavities also have gas cavities. So, with ALMA, we can now find out where and when giant planets are being born in these discs, and compare these results with planet formation models,” says Ewine van Dishoeck of Leiden University and the Max Planck Institute for Extraterrestrial Physics.

“Direct planetary detection is just within reach of current instruments, and the next generation telescopes currently under construction, such as the European Extremely Large Telescope, will be able to go much further. ALMA is pointing out where they will need to look.”

Front image: Artist's impression of a transitional disc around a young star.
Credit: ALMA (ESO/NAOJ/NRAO)/M. Kornmesser
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