Moon-forming circumplanetary disc spotted in a distant star system

In a first for astronomers, a disc of gas and dust has been found around an exoplanet. This circumplanetary disc is capable of forming a system of moons.

This is a composite image of PDS 70. Comparing new ALMA data to earlier VLT observations, astronomers determined that the young planet designated PDS 70 c has a circumplanetary disk, a feature that is strongly theorized to be the birthplace of moons. ( ALMA (ESO/NAOJ/NRAO) A. Isella; ESO)

The first-ever observations of a disc of dust and gas surrounding an exoplanet have been made by a group of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA).

Such belts of material are strongly linked by astronomers to the formation of planets and moon systems, such as that hosted by Jupiter.

The circumplanetary disc was discovered around one of the planets in the home of a young star — PDS 70 — located 370 light-years from Earth. Astronomers have recently used the Very Large Telescope (VLT) to confirm the presence of two massive Jupiter-sized exoplanets in the same system. The observation came in the form of the warm glow of hydrogen gas accreting onto the planets.

The ALMA observations captured the faint radio wave emission given off by the particles of dust around the star. The data from ALMA, coupled with earlier optical and infrared VLT observations, suggest that a disc, capable of forming moons, surrounds the outer planet in the system.

A colour-enhanced image of millimetre-wave radio signals from the ALMA observatory in Chile shows a disk of gas and dust (right of centre) around exoplanet PDS 70 c, the first-ever observation of the kind of circumplanetary disk that is believed to have birthed the moons of Jupiter more than 4 billion years ago. ( A. Isella, ALMA (ESO/NAOJ/NRAO))

Andrea Isella, an astronomer at Rice University in Houston, Texas, and lead author on a paper published in the Astrophysical Journal Letters, says: “For the first time, we can conclusively see the telltale signs of a circumplanetary disc, which helps to support many of the current theories of planet formation.

“By comparing our observations to the high-resolution infrared and optical images, we can clearly see that an otherwise enigmatic concentration of tiny dust particles is actually a planet-girding disc of dust, the first such feature ever conclusively observed.”

The researchers say that this is the first time that an exoplanet has been clearly seen in these three distinct bands of light.

Radio astronomers using the Atacama Large Millimeter/submillimeter Array of telescopes in Chile have found a disk of gas and dust (left) around exoplanet PDS 70 c, a still-forming gas giant that was obscured from view in the 2018 infrared image (right) that first revealed its sister planet, PDS 70 b. ( A. Isella, ALMA (ESO/NAOJ/NRAO))

A circumplanetary ring of this nature differs from the rings seen around a planet like Saturn. Whilst that planet’s rings are composed of ice formed by the impact of comets and rocky bodies, a circumplanetary disc is the leftovers from planet-formation.

Data collected by ALMA also revealed key differences between the two newly discovered planets themselves. PDS 70b, which dwells closer to its host star, has a ‘tail’ of dust trailing behind it.

Isella comments: “What this is and what it means for this planetary system is not yet known. The only conclusive thing we can say is that it is far enough from the planet to be an independent feature.”

The second planet PDS 70c occupies the same space as a ‘knot’ of dust. As the planet is shining brightly in the infrared and hydrogen bands of light, astronomers believe it is a fully-formed planet with nearby gas accreting onto its surface — fueling its growth.

PDS 70c is further out from its star — about 5.3 billion km, roughly the same distance as Neptune from our sun — and could be as large as ten times the mass of Jupiter. The researchers point out that if it is at the larger end of this mass estimate, it’s very possible planet-sized moons are forming around it.

These observations come despite the fact that optical studies of planetary systems are incredibly difficult. This is a result of the sheer brightness of the host star, its glare making the dimmer planets incredibly hard to pick out from the noise.

ALMA avoids this difficulty because at the range it studies, millimetre and submillimetre wavelengths, stars don’t emit much light.

Isella concludes: “This means we’ll be able to come back to this system at different time periods and more easily map the orbit of the planets and the concentration of dust in the system.

“This will give us unique insights into the orbital properties of solar systems in their very earliest stages of development.”