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The spiralling signatures of planet formation

last modified Apr 27, 2017 09:53 AM
A young star recently observed to be surrounded by spiralling gas and dust could be one of the first to show planet formation ‘in action’ via a mechanism once thought to be unlikely.

Astrophysicists at the University of Cambridge have used theoretical models to determine the origins of the striking large-scale spiral features surrounding a nearby star.

Young stars are surrounded by dense discs of gas and dust, and it is within these discs that planets are assembled. Obscured from our view, the precise details of just how planets form remain difficult to determine from the observations alone.

Last year, astronomers used the extremely sensitive Atacama Large Millimetre Array (ALMA) located in Chile to observe the young, one-million year old star Elias 2-27 (Pérez et al. 2016, Science 353, 1519). The observations were the first to directly resolve the disc around the young star, and showed something very surprising — rather than being a smooth disc, the image showed two prominent spiral arms, each extended to a length about ten times the distance between the Sun and Neptune in our own Solar System.

“These beautiful observations of Elias 2-27 immediately sparked much discussion amongst our research team about what could be causing the spiral arms” said Dr Farzana Meru, of the Institute of Astronomy. Meru and her colleagues set about using their theoretical models to investigate what might be happening around Elias 2-27.

However, this was not an easy task. The investigation involved running many computer simulations to solve the complex calculations of how the gas orbits in the disc and is heated by radiation from the central star. “The simulations we performed would take thousands of hours to run on your average laptop computer” said Dr John Ilee, a co-author on the study. “Fortunately, we were able to use a dedicated supercomputer and some clever tricks to speed up the calculations” added Ilee.

Meru and her collaborators showed two possibilities for the origin of the spiral structures.  The first is that the disc around Elias 2-27 may be so massive that its own gravity naturally causes spirals to form – a so-called ‘self gravitating’ disc.  However, Meru and her colleagues also discovered that the spirals could be formed another way – stirred up by a planet in the outer parts of the disc. 

“At first, we were a little disappointed to discover that no single mechanism was able to produce the spiral structure” said Ilee, leaving the team with further questions.  “But we then found that the mass of the planet required to drive the spirals was huge – nearly 10 times the mass of Jupiter – and that it was very unlikely that the traditional method of planet formation would have been able to form such an object.”

This ‘traditional’ method of planet formation involves the slow, gradual collision and sticking of tiny dust particles within the disc.  Eventually, enough dust particles stick together to form pebbles, and then boulders, and, as the process continues, eventually planet sized objects form in a gradual process known as ‘core-accretion’. 

“Given the young age of Elias 2-27, there simply hasn’t been enough time to create a planet of the required mass by core accretion” said Meru.  “The only way to make such a planet so quickly would be if regions of a self-gravitating disc collapse entirely, creating one or more planets in the process”.

It seems that, whatever the explanation for the spirals, Elias 2-27 could be a smoking gun for planet formation by a process once thought to be rare.

The research paper is published in The Astrophysical Journal Letters.


Artist's impression of the spiral structure in the disc around Elias 2-27.

Credit: Institute of Astronomy - Amanda Smith & Farzana Meru

Simulation image of a protoplanetary disc with a planet that is ten times the mass of Jupiter and is at a distance of 425 astronomical units (i.e. 425 times the distance between the Sun and the Earth).  The interaction between the planet and the disc is causing the large scale spiral structures to form.

Credit: Institute of Astronomy -  Farzana Meru

Simulation image of a protoplanetary disc that is so massive that the gravity within the disc causes the spiral structures to form.  The spirals extend out to approximately 300 astronomical units (i.e. 300 times the distance between the Sun and the Earth).  The disc has been inclined to show what a disc would look like if we look at it from a different angle, just like the Elias 2-27 disc.

Credit: Institute of Astronomy -  Farzana Meru


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