Disc forming the moon discovered around the distant planet

By using the most powerful network of radio telescopes on earth, astronomers have made the first observations of a circumplanetary disk of gas and dust similar to that which would have given birth to the moons of Jupiter.

The discovery, reported online today to Letters from the Astrophysical Journal, adds to the intriguing story of the planet PDS 70 C, a gas giant in formation about 370 light-years from Earth, revealed for the first time last month in images with visible light.

Astronomer Andrea Isella and his colleagues at Rice University, using the huge ALMA (Atmama Large Millimeter / Submilleter Array) at 66 antennas in Chile, collected millimeter-wave radio signals revealing the presence of dust particles in the star system, where PDS 70c and its sister planet, PDS 70b, are still in formation.

"Planets form from gas and dust disks around newly formed stars.If a planet is large enough, it can form its own disk when it collects materials around the planet. star, "said Isella. "Jupiter and its moons are a small planetary system in our solar system, for example, and it is thought that the moons of Jupiter were formed from a circumplanetary disk when Jupiter was very young."

But most of the planet formation models show that the circumplanetary disks disappear after about 10 million years, which means that circumplanetary disks no longer exist in our solar system since more than 4 billion d & rsquo; # 39; years. To search for them elsewhere and gather evidence to test the theories of planet formation, Isella and her colleagues are looking for very early star systems where they can directly observe the discs and planets that are still forming at the planet. 39; inside. In the new study, Isella and her colleagues analyzed the observations made by ALMA in 2017.

"There are a handful of candidate planets that have been detected in the discs, but it's a whole new field, and they're all still being debated," Isella said. . "(PDS 70b and PDS 70c) are among the most robust because there have been independent observations with different instruments and techniques."

PDS 70 is a dwarf star representing about three quarters of the mass of the sun. Its two planets are 5 to 10 times larger than Jupiter, and the deepest, PDS 70b, is orbiting about 1.8 billion kilometers from the star, about the distance from the sun to Uranus. The PDS 70c is one billion kilometers away, in an orbit the size of Neptune.

The PDS 70b was first revealed in 2018 in infrared images of a planet-finding instrument called SPHERE at the Very Large Telescope (VLT) of the European Southern Observatory. In June, astronomers used another VLT instrument called MUSE to observe a wavelength of visible light called H-alpha, which is emitted when hydrogen falls on a star or planet and becomes ionized.

"H-alpha gives us more confidence that they are planets, because it suggests that they always suck gas and dust and that they grow up," Isella said.

The millimeter wave length observations provided by ALMA provide even more evidence.

"This completes the optical data and provides a completely independent confirmation that there is something there," he said.

According to Isella, direct observation of planets with circumplanetary disks could allow astronomers to test theories of planet formation.

"There are many things we do not understand about planet formation, and we finally have the tools to make direct observations and start answering questions about the formation of our solar system and the formation of solar systems. other planets. "

Isella is Assistant Professor of Physics and Astronomy and Earth Sciences, Environment and Planets at Rice, and Co-Investigator of the CLEVER Planets project, funded by NASA and based at Rice.