Late last year, astronomers using the ALMA telescope captured an unprecedented image of what appears to be a protoplanetary disc surrounding a young star. Some scientists were skeptical about the claim, but a new simulation run by Canadian astrophysicists is helping bolster its case.
The reason for the skepticism has to do with the positioning of the gaps in the object, known as HL Tau. Some astrophysicists argued that the gaps, especially the outer three, couldn’t possibly be indicative of planet formation because they’re way too close together; anything big enough to form such closely spaced gaps, it was argued, would be thrown straight clear out of the disc owing to gravitational forces.
One of those skeptics was Daniel Tamayo from the University of Toronto-Scarborough’s Centre for Planetary Science. But in attempting prove the gaps weren’t caused by planets, he may have discovered the opposite: that circular gaps in the disc of gas and dust could well be the result of planet formation.
Tamayo’s models showed that the gaps are separated by amounts consistent with what’s known as a resonant configuration. Planets within HL Tau don’t collide into each other because they don’t have criss-crossing orbital periods. In our Solar System, Pluto and Neptune are in such a configuration, which is why they don’t smash into each other despite having crossing orbits.
To show this is possible with HL Tau, Tamayo prepared different simulations involving Saturn-massed objects situated at various points in the gaps (the exact positions of the planets isn’t known, so Tamayo had to play with this a little).
In the simulation shown above, the planets are so close to each other that their mutual gravity results in violently rearranged orbits. This model clearly doesn’t work.
But when the interactions within the disc allows the planets to move into a resonant configuration, it produces a stable system. As he writes at his website:
Our study is therefore the first to show that a planetary interpretation of the gaps is actually viable. Additionally, we argue that if the system indeed hosts resonantly interacting planets, their orbits should be slightly elliptical (while if they were not resonant the disk would make them circular). Excitingly, just two weeks after we posted our article, the ALMA group released an analysis of the data showing the gaps to indeed be eccentric. I was worried that previous estimates of the disk’s and star’s mass implied that the disk would erase the elliptical signature in the gaps, but the new mass estimates of Brogan and collaborators suggest this may be less of a concern. In summary, while it doesn’t prove there are Saturns lurking in that disk—we will likely argue over this for some time yet—I would say planets are currently the leading hypothesis.
Interestingly, Tamayo says this stability isn’t permanent. If the planets are indeed that large, they’ll eventually move into crossing orbits once the disc dissipates. He calls HL Tau system a “ticking time bomb.”
The scientific study is set to appear in an upcoming issue of the Astrophysical Journal, but you can find a preview of it at arXiv: “Dynamical Stability of Imaged Planetary Systems in Formation: Application to HL Tau.”