Infant planets orbit very young star

Independently, two teams find evidence of planet formation around the same star. Andrew Masterson reports.

An artist's impression of the protoplanet (top, centre) disturbing the flow of gas in the cicumstellar disc.
An artist's impression of the protoplanet (top, centre) disturbing the flow of gas in the cicumstellar disc.
Monash University

Astrophysicists from Australia’s Monash University have made a world-first discovery, identifying for the first time a newly formed planet moving inside a protoplanetary disc.

Protoplanetary, or circumstellar, discs are dense accumulations of gas and dust that catalyse and surround newly formed stars. The star forms because gravity pulls material within the disc closer together. As it does so, dust and grains at the thickening centre heat up and become more compressed, eventually coalescing into the core.

Parts of the rotating disc further out from the centre go through churning motions, gradually flattening and compressing with all the components travelling in the same direction. Eventually, matter within the cloud starts to clump together, a process that continues as multiple clumps aggregate, forming first an embryonic planet – known as a planetesimal – and later a full-size one.

The theory is well supported by circumstantial evidence and modelling, but observation until now has been lacking – or, at least, equivocal.

In a paper published in the journal Astrophysical Journal Letters researchers led by astronomers Christophe Pinte and Daniel Price note that at least 10 studies have claimed to have directly imaged planets forming within discs, but all have been subsequently challenged.

“The quest continues,” the authors note.

Other astronomers have claimed to have found the indirect signatures of newly formed planets by looking at disturbances within protoplanetary discs. However, in each case, other explanations that do not require the presence of planets are equally plausible.

Using data gathered from the Atacama Large Millimetre/submillimetre Array (ALMA) observatory in Chile, Pinte, Price and colleagues now appear to have produced unambiguous evidence.

The target of their observations was a young, four-million-year-old star dubbed HD 163296, about 330 light-years from Earth. The ALMA imaging revealed that in one area the flow of gas within the cloud surrounding the star behaved differently – forming a kink or bump.

Intrigued, the researchers compared the observatory readings to simulations predicting the flow of gas around the star.

“We compared the observations with computer models to show that the ‘something else’ fits beautifully with predictions for the flow pattern around a newborn planet,” Pinte explains.

“Even though we do not see the planet directly, we have a very strong indication that it is there, and that it is still interacting and feeding from its disc, just like a newborn baby feeding from its mother.”

The infant planet, Pinte and his colleagues calculated, has a mass about twice that of Jupiter, and orbits 39 billion kilometres from its host planet.

The paper is archived in full on the website of the European Southern Observatory (ESO).

Interestingly, the Australian team is not the only group of astrophysicists to propel HD 163296 into the news. On the same day that Pinte and Price’s paper was published, the same journal contained a paper from a team led by Richard Teague of the University of Michigan, US.

Also using ALMA data and disturbances to gas flow, the researchers report the discovery of two more young planets, both about the mass of Jupiter, orbiting at 12 billion and 21 billion kilometres from host star.

The relevant paper is also archived on the ESO site.

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