Exoplanets Have Been Caught In The Act Of Forming Moons For The First Time

This artist's illustration shows two gas giant exoplanets orbiting the young star PDS 70. J. Olmsted (STScI)

Two independent teams of astronomers have found evidence that a circumplanetary disk surrounds two nearby exoplanets. This is believed to be a fundamental stage in the evolution of gas giant planets that could lead to the formation of moons.

Both studies used the incredible capabilities of the Very Large Telescope, although they employed different instruments. The first paper, published in The Astrophysical Journal Letters, used the SINFONI instrument. The second paper, published in Nature Astronomy, employed the MUSE instrument. 

The system is known as PDS 70 and is located 370 light-years from Earth. It is extremely young at only 6 million years old, a bit over a thousandth of the age of our own Solar System. Planet PDS 70 b was discovered last year, a giant object weighing between four and 17 times Jupiter's mass. Astronomers were eager to find out more about this object and how it’s changing in its early years.

“We found the first evidence for a disc of gas and dust around it – known as a circumplanetary disc,” lead author of the first study Dr Valentin Christiaens, from Monash University, said in a statement. “We think the large moons of Jupiter and other gas giants were born in such a disc, so our work helps to explain how planets in our Solar System formed.”

The second team not only saw the circumplanetary disk, but they also discovered that PDS 70 b is not alone. A second planet, PDS 70 c, exists further away from the star and with a slightly smaller mass, between one and 10 times that of Jupiter.

Through a combination of adaptive optics and data processing, astronomers were able to cancel out the light from the central star (marked by a white star) to reveal two orbiting exoplanets. ESO and S. Haffert (Leiden Observatory)

These two planets are only the second multi-planet system to be directly photographed. The team tracked the hydrogen swirling about in the system, a technique not common in observations of exoplanets.

“This new observing mode was developed to study galaxies and star clusters at higher spatial resolution. But this new mode also makes it suitable for exoplanet imaging, which was not the original science driver for the MUSE instrument,” stated Sebastiaan Haffert of Leiden Observatory, lead author of the second paper. “We were very surprised when we found the second planet.”

The discovery of the second planet gives even more insight into what the early solar system might have been like. The two gas giants are in a 2:1 resonance, meaning that for every two orbits of PDS 70 b, PDS 70 c completes one. This leads credence to the hypothesis that Jupiter first migrated inward towards the Sun and then outward towards Saturn, before settling in its current orbit.

“This is the first unambiguous detection of a two-planet system carving a disk gap,” said Julien Girard of the Space Telescope Science Institute in Baltimore, Maryland, a co-author on both studies.

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