Here is a first. Astronomers see a moon forming around an exoplanet

Astronomers have discovered, for the first time, moons forming in the debris disk around a large exoplanet. Astronomers have long suspected that this is how large planets, like Jupiter in our own solar system, take their moon. Everything happens around a very young star named PDS 70, about 370 light years from the constellation Centaurus.

"For the first time, we can conclusively see the telltale signs of a circumplanetary disk …"

Andrea Isella, Senior Author, Rice University

The accepted theory of planet formation is called the nebular hypothesis. It all starts with the formation of a star in a huge gas cloud called Giant Molecular Cloud (GMC). As the star is formed, the cloud is transformed into a rotating disk of flattened gas and dust, called a protoplanetary disk or circumstellar disk. The material begins to fuse into bouquets in this disc, and these bunches turn into planets.

If the mass of a planet that forms in the disc exceeds about 10 Earth masses, something else happens. Due to its mass, this planet opens a gap in the protoplanetary disk. When materials cross this space, they can get close enough to the planet so that its gravity dominates that of the host star. This material is then trapped in a circumplanetary disk (CPD) rotating around the planet, like a disk in a disk.

Much of the material contained in a circumplanetary disk is accumulated in the planet in formation. But not at all. The same forces that created planets from the circumstellar disk go to work. They can create moons from the rotating material in the disc around the planet.

Now, a team of astronomers has spotted for the first time this circumplanetary disk and moons forming there.

The main author of the study highlighting these findings is Andrea Isella, an astronomer at Rice University in Houston, Texas. The results were published in letters from The Astrophysical Journal and are titled "Detection of submillimeter emissions in the continuum associated with candidate protoplanets".

"The 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 while Jupiter was very young."

Everything is happening around the star PDS 70. This star was in the news a year ago when astronomers captured the very first image of a planet being formed in a circumstellar disk. This planet is called PDS 70b. This discovery was great news at the time, for good reason.

PDS 70b is not the only planet orbiting the star. There is another planet, PDS 70c, also in orbit, and they are two gas giants. Both planets were detected by the European Southern Observatory (ESO) very large telescope (VLT) in optics and infrared. The warm glow of hydrogen accumulating in the pair of planets is what gave them.

The team combined the VLT observations with the new radio observations from the Atacama Large Millimeter / Sub-Millimeter Array (ALMA) network. The result is convincing evidence of a protoplanetary disk around the outermost star, PDS 70c.

"For the first time, we can conclusively see the telltale signs of a circumplanetary disk, which supports many of the current theories of planet formation," said Andrea Isella, lead author.

"By comparing our observations to high-resolution infrared and optical images, we can clearly see that an otherwise enigmatic concentration of tiny dust particles is actually a dust disk that surrounds the planet, the first characteristic of this type ever observed. conclusively, "he said. According to the researchers, this is also the first time that a planet is clearly visible in these three distinct light bands.

One question answered, another asked

PDS 70b and c have different characteristics, and the team responsible for this study does not know exactly what it means.

"What it is and what it means for this planetary system is not yet known."

Andrea Isella, Senior Author, Rice University

PDS 70c, the outermost star of the pair, is as far from its star as Neptune from the sun. It's exactly in the same place as a knot of dust evident in the ALMA data. Since this planet shines so brightly in the bands of infrared and hydrogen light, astronomers can argue convincingly that a fully formed planet is already in orbit. The infrared light bands and the hydrogen bands show that the gas nearby is still welcomed on the surface of the planet, thus ending its growth spurt in adolescence.

Astronomers estimate that the PDS 70c represents about 1 to 10 times the mass of Jupiter. "If the planet is in the widest part of this estimate, it is quite possible that planet-sized moons form around it," Isella said.

But the PDS 70b has something else to do. This planet, which is about the same distance from its star as Uranus in the sun, has a mass of dust trailing behind it like a tail. And astronomers do not know how it fits.

"What it is and what it means for this planetary system is not yet known," Isella said. "The only conclusive thing we can say is that the planet is far enough apart to be an independent element."

Astronomers are pretty sure that the process they see around PDS 70c is identical to the one that created the moons of Jupiter. It should be noted, however, that the other gas giant in our solar system is distinct from Jupiter. The moons of Saturn were probably created as a result of a circumplanetary disk, but its rings of ice were probably created by comets and other rocky bodies that crash.

These exoplanetary systems are notoriously difficult to observe in optical and infrared light. The energy of the star in these parts of the spectrum drowns the light of the planets. But not for ALMA.

ALMA focuses on radio waves and stars emit only weakly radio waves. The team says it can continue to observe the PDS 70 system with ALMA to monitor its evolution and evolution.

"This means we can return to this system at different times and better map the orbit of the planets and the concentration of dust in the system," Isella concluded. "This will give us unique insights into the orbital properties of solar systems in their very early stages of development."

The discovery of this circumplanetary disk and the likely moons that form it are interesting, but the way the team discovered the disk is also promising for the future. While others have been found, this study is the most compelling.

"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."

In the conclusion of their article, the authors state: "We affirm that optical, NIR and (sub) millimeter observations are very complementary because they explore various aspects of the planet's accretion processes and are affected by various systematic errors. ". They also note that ALMA alone can not do the job. By combining the different observations, they opened these exoplanets and their disks for a more detailed study.

According to the study: "While ALMA and the existing optical telescopes are reaching all their imaging capabilities, the next near circumstellar disk observations characterized by cavities and voids like those observed in PDS 70 may reveal more newborn planets interacting with their natural disk. These observations are fundamental to study the processes responsible for the formation of planetary systems. "

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