Astronomers first spot disc-forming satellites around exoplanet

With the help of the Atacama Large Millimeter / Submillimeter Array (ALMA), partnered by the European Southern Observatory (ESO), astronomers for the first time have clearly detected the presence of a disk around the outer planet of our solar system. … These observations provide us with new insights into how moons and planets are formed in young star systems.

“Our work shows a clear detection of a disk on which satellites can form,” said Miriam Benisti, a researcher at the University of Grenoble, France, and the University of Chile, who led this new work, published July 22, 2021 in The Astrophysical Journal Letters. “These observations were obtained ALMA and they have such a resolution that we could clearly determine that the disk is associated with a planet, and we were also able to set limits on its size for the first time, ”he adds.

The disk in question, called the circumplanetary disk, surrounds the exoplanet PDS 70c, one of two Jupiter-like giant planets orbiting the star about 400 light-years from Earth. astronomers already previously discovered keys about the existence of a disk forming a moon around this exoplanet, but since they could not separate the disk from the environment, there was still no way to confirm its presence.

In addition, with the help of ALMA, Benisti and his team found that the diameter of the disc is roughly the same size as the distance between the Earth and the Sun, and enough mass to form up to three satellites the size of our moon.

These findings are important for more than just figuring out how moons are formed. “These new observations are also extremely important to support theories of planet formation that we have not been able to test until now,” explains Jahan Bae, researcher at the Earth and Planets Laboratory at the Carnegie Institute of Science, USA, and one of the authors of the study.

Planets form dusty discs around young stars, forming cavities as they “swallow” material from the circumstellar disc to grow. During this process, the planet can acquire its own circumplanetary disk, which contributes to the growth of the planet by regulating the amount of matter attracted to it. At the same time, gas and dust from the near-planetary disk can collect into larger and larger bodies as a result of multiple collisions, which ultimately leads to the birth of moons in orbit around these planets.

However, astronomers are still not very well versed in these processes. “In short, it is not yet clear when, where and how planets and their moons are formed,” says Stefano Facchini, ESO researcher who is also involved in this research effort.

“At the moment, more than 4,000 exoplanets have been discovered, but they are all part of mature systems. PDS 70b and PDS 70c, which form a system resembling a Jupiter-Saturn pair, are the only two exoplanets discovered so far that are still in the process of forming, ”explains Miriam Keppler, a researcher at the Max Planck Institute for Astronomy, Germany. , and one of the co-authors of this study.

“Thus, this system offers us a unique opportunity to observe and study the formation of planets and satellites,” adds Facchini.

PDS 70b and PDS 70c, the two planets that make up the system, were originally discovered using Very large telescope (VLT) ESO in 2018 and 2019, respectively, and their unique nature means they have been observed later and multiple times with other telescopes and instruments.

These high-resolution observations with ALMA have now allowed astronomers to learn more about the system. In addition to confirming the presence of a circumplanetary disk around PDS 70c and estimating its size and mass, the researchers also found that PDS 70b does not show clear evidence for the existence of such a disk, indicating that its birthplace should be free of dust due to its companion. , PDS 70c.

How Extremely large telescope (ELT) ESO, which is being built at Cerro Armazones in the Chilean Atacama Desert, will help us understand this planetary system even better. “ELT will be critical in this research effort, as with its even higher resolution we will be able to map the system in great detail,” says co-author Richard Teague, researcher at the Center for Astrophysics | Harvard and the Smithsonian, USA. In particular, using the tool METIS (ELT Imager and mid-infrared spectrograph), which will be installed on the ELT, the team will be able to see the movements of the gas surrounding the PDS 70c, thus obtaining a three-dimensional image of the system.