Astronomers snap image of moon-forming disk surrounding an alien world
Astronomers have captured the first clear images of a colossal moon-forming debris disk orbiting a distant alien planet. According to the authors of the new study, the disk has enough material to create three astronomical bodies the size of Earth’s Moon.
The discovery was made by scientists using the European Southern Observatory’s (ESO) Atacama Large Millimeter/submillimeter Array (ALMA) to image the star system PDS 70, which sits roughly 400 light-years away from Earth in the constellation Centaurus.
PDS 70 consists of a parent star around which two enormous protoplanets are known to orbit. It is thought that these worlds, which were imaginatively named PDS 70b and PDS 70c, share key characteristics with the gas giants Jupiter and Saturn that orbit in our home solar system.
As the newly formed planets orbited in the dust-choked environment surrounding the young star, their gravitational influence allowed them to collect the material and add it to their mass. This in turn created an enormous, relatively debris-free cavity surrounding the parent star which is easily identifiable in the wide field view of PDS 70.
“More than 4,000 exoplanets have been found until now, but all of them were detected in mature systems,” said Miriam Keppler, co-author of the new study and researcher at the Max Planck Institute for Astronomy in Germany. "PDS 70b and PDS 70c, which form a system reminiscent of the Jupiter-Saturn pair, are the only two exoplanets detected so far that are still in the process of being formed."
Earlier observations of the outermost exoplanet PDS 70c had hinted at the presence of a circumplanetary disk surrounding the growing world. However, it was not until the higher resolution imaging capabilities of ALMA were brought to bear that astronomers were able to reveal the impressive nature of the structure.
“Our work presents a clear detection of a disc in which satellites could be forming,” comments Myriam Benisty, lead author of the study and a researcher at the University of Grenoble, France, and at the University of Chile. “Our ALMA observations were obtained at such exquisite resolution that we could clearly identify that the disc is associated with the planet and we are able to constrain its size for the first time.”
The team behind the study believes that the vast circumplanetary disk seen in the ALMA data is comprised from material that was collected by PDS 70c as it swept the inner solar system soon after its creation. According to the study authors, it is around 500 times larger than Saturn’s rings, and spans a region of space that is roughly the equivalent to the gulf that separates Earth from the Sun. Some of the material in the disk will continue to accrete into the planet, prolonging its growth. However the remainder could also give rise to the formation of new moons.
This formation process echoes the one by which planets form around a star. Tiny collisions in the disk structure give rise to clumps of matter that continue to grow and siphon material from the surrounding environment until they become a fully fledged moon. The authors of the study estimate that the ring surrounding PDS 70c harbors enough material to create three satellites the size of Earth’s Moon.
The new ALMA imagery will allow astronomers to test existing theories on how planets and moons come to form. The system will also present a tantalizing target for the next generation of observatories, including the European 40-meter Extremely Large Telescope, which is currently being constructed in Chile.
The paper has been published online in The Astrophysical Journal Letters.
The video below shows an animation of PDS 70 in motion.