According to a group of astronomers led by Dr Daniel Tamayo of the University of Toronto Scarborough’s Centre for Planetary Science and the Canadian Institute for Theoretical Astrophysics, circular gaps in a disk of dust and gas swirling around the young star HL Tauri (HL Tau) are made by forming planets.
This image shows the protoplanetary disc surrounding the young star HL Tauri. Image credit: ALMA / ESO / NAOJ / NRAO.
The HL Tau system is located in the constellation Taurus, approximately 450 light-years away. It is less than 100,000 years old and about 17.9 billion km in radius.
HL Tau’s protoplanetary disk is unusually massive and bright, which makes it an excellent place to search for signs of forming planets.
The image of HL Tau, taken in October 2014 by the Atacama Large Millimeter/submillimeter Array (ALMA), sparked a flurry of scientific debate.
While those who observed the original image claimed that planets were most likely responsible for carving the gaps, some remained skeptical.
It had been suggested that the gaps, especially the outer three, could not represent forming planets because they are so close together.
It was argued that planets massive enough to carve such gaps should be scattered violently by the force of gravity and ejected from the system early on in its development.
But a new study by Dr Tamayo and co-authors is the first to suggest the gaps are evidence of planetary formation because the gaps are separated by amounts consistent with what’s called a special ‘resonant configuration.’ In other words, these planets avoid violent collisions with each other by having specific orbital periods where they miss each other, similar to how Pluto has avoided Neptune for billions of years despite the two orbits crossing one another.
“HL Tau likely represents the first image taken of the initial locations of planets during their formation. This could be an enormous step forward in our ability to understand how planets form,” said Dr Tamayo, the lead author of the paper accepted for publication in the Astrophysical Journal (arXiv.org preprint).
“The system can be much more stable in a resonant configuration and it’s a natural state for planets in the HL Tau system to migrate to,” he added.
“While the HL Tau system remains stable in its relatively young age, over billions of years it will act as a “ticking time bomb.” Eventually the planets will scatter, ejecting some and leaving the remaining bodies on elliptical orbits like the ones found around older stars.”
“Our Solar System does not seem to have undergone such a dramatic scattering event.”
Future observations could also go a long way in determining whether our Solar System is typical or an oddity ideally suited for life.
“If further observations show these to be the typical starting conditions around other stars, it would reveal our Solar System to be a remarkably special place,” Dr Tamayo said.
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Daniel Tamayo et al. 2015. Dynamical Stability of Imaged Planetary Systems in Formation: Application to HL Tau. ApJ, accepted for publication; arXiv: 1502.05099
