Two years ago astronomers thought they had found the first evidence of a disk circling a planet from which moons would eventually form. Closer inspection reveals they were wrong, but another planet in the same system really has just such a disk, which has now been observed with stunning clarity. The finding will lead to a better understanding of the formation process for planets, as well as their moons.
PDS 70 is an almost 6 million-year-old star 370 light-years from us with a mass about three-quarters of the Sun's. Two years ago astronomers spotted what they thought might be a moon-forming disk around PDS 70b, its largest planet. However, with plenty of gas and dust still swirling around the system it was hard to distinguish the disk against the background, leaving the find unconfirmed. Certainly, the images were not clear enough to learn anything about the disk, such as its size or mass.
Dr Myriam Benisty of the University of Grenoble gained time on the Atacama Large Millimetre/submillimeter Array (ALMA) to attempt to verify. In The Astrophysical Journal Letters, she reports stunning success. "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," Benisty said in a statement. However, the disk she found is around a smaller planet PFS 70c – the suspected disk around PDS 70b turns out to be an illusion.
If you're imagining a disk the size of Saturn's rings, or even the orbit of the outermost of Jupiter's large moons, you are way off. Instead, the disk has a radius of 86–180 million kilometers (about 53–112 million miles), 0.58–1.2 times the distance from the Earth to the Sun. Benisty and co-authors estimate it has enough material to produce three moons the size of Earths. Unless the disc grows further, PDS 70c will never match Jupiter's moon collection, with the big four having almost twice this mass between them, despite PDS 70c being 1–10 times Jupiter's mass.
PDS 70 is one of the few star systems so young its planets are still hot enough we can see them directly, and particularly unusual in having two planets visible like this. The system has plenty more material further out, some of it distorted in ways consistent with a Saturn-sized world lurking in the outer darkness.
The planetary pair have resonant orbits, with b whizzing around twice in the time it takes c to make one orbit. "These new observations are also extremely important to prove theories of planet formation that could not be tested until now," said co-author Dr Jaehan Bae of the Carnegie Institution for Science.
Astronomers have proposed Jupiter and Saturn formed much further out, and their migration was shaped by their gravitational effect on each other.
Finding two even more massive planets already locked in a resonance like this could improve the understanding of this process. Currently PDS 70c orbits 34au from its star (further than Neptune) while its larger sibling is at 22au, similar to Uranus.