Young star provides a glimpse of how planets are made


Two recent studies offer insight into the mystery of planet formation. Andrew Masterson reports.


V1247 Orionis, a young, hot star surrounded by a dynamic ring of gas and dust, known as a circumstellar disc. Is the dark ring evidence of a planet forming?
V1247 Orionis, a young, hot star surrounded by a dynamic ring of gas and dust, known as a circumstellar disc. Is the dark ring evidence of a planet forming?
Stefan Kraus

High powered telescopes aimed at a distant young hot star called V1247 Orionis have yielded some intriguing clues to one of astronomy’s most enduring mysteries: how planets form.

The star – which is being monitored by the Atacama Large Millimetre Array (ALMA) telescope in Chile for – can be found in the background of Orion’s Belt. Like most young, hot stars, it is surrounded by a large disc of gas and dust.

Astronomers believe that these discs are intimately bound up with the process of planet formation, but the mechanism is poorly understood.

Dust existing at the outer edges of the disc is likely to drift away into space, while that in the inner regions has been shown to be subjected to drag from the surrounding gas. As a result, it tends to fall inwards into the star in a process known as “radial drift”.

Now, however, a team led by Stefan Kraus from the University of Exeter in the UK has analysed data from ALMA and identified a large anomaly in the make-up of the disc.

In images compiled from the telescope a clear, wide division can be seen, separating an internal ring, close to the star itself, from a less well defined outer arc.

In a paper published in Astrophysical Journal Letters, the scientists contend that the section in between – which shows as a dark loop – is evidence of a planet being formed.

The dark strip, says Kraus, is likely to be a vortex. For millions of years, as the planet has been slowly growing, its moving mass has generated areas of high pressure around it.

The effect, he says, is a little like bow waves created by a moving ship. In the space between them, the vortex serves to protect dust from both the drag inwards of the star and the pull outwards of space, allowing particles to progressively clump together, forming a proto-planet.

"The exquisite resolution of ALMA allowed us to study the intricate structure of such a dust-trapping vortex for the first time,” says Kraus.

“The crescent in the image constitutes a dust trap that formed at the outer edge of the dark strip.

“Dust trapping is one potential solution to a major stumbling block in our theories of how planets form, which predicts that particles should drift into the central star and be destroyed before they have time to grow to planetesimal sizes.”

This is not the only time in recent years that the dust disc around V1247 Orionis has been studied by astronomers.

In 2016, a team led by Yurina Ohta of Osaka University in Japan used data from a different instrument to determine that the disc exhibited “extreme asymmetry”. The team noted that the strange geometry might indicate the disc was spiral-shaped rather than elliptical, but suggested that it might also indicate “ the presence of unseen companions and/or planet-forming processes”.

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Andrew Masterson is news editor of Cosmos.
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