The rare molecule weighing in on the birth of planets

Astronomers have discovered a rare molecule in the dust and gas disc around a young star – and it may provide an answer to one of the conundrums facing astronomers.

Using one of the most advanced radio telescopes in the Atacama Desert Chile, astronomers have identified a rare molecule in the dust and gas disc of the star called HD 163296, located 330 light years from Earth and formed over the last six million years. The researchers discovered an extremely faint signal showing the existence of a rare form of carbon monoxide - known as an isotopologue (13C17O). 

Following this discovery, a collaboration group of international scientists, led by the University of Leeds, were able to measure the mass of the gas disc formed around a young star – a so-called protoplanetary disc, more accurately than ever before. This is relevant as young planets are born within these discs. According to the researchers’ results, the disc is much heavier – or more ‘massive’ – than previously thought. 

An artist's impression of planets forming in the ring of gas and dust surrounding a star.

An artist's impression of planets forming in the ring of gas and dust surrounding a star.

Alice Booth, a PhD researcher at Leeds who led the study, said: “Our new observations showed there was between two and six times more mass hiding in the disc than previous observations could measure.

“This is an important finding in terms of the birth of planetary systems in discs – if they contain more gas, then they have more building material to form more massive planets.”

The study – The first detection of 13C17O in a protoplanetary disk: a robust tracer of disk gas mass – has been published in Astrophysical Journal Letters.

Dr John Ilee, a researcher at Leeds who was also involved in the study, added: “The disc-exoplanet mass discrepancy raises serious questions about how and when planets are formed.  However, if other discs are hiding similar amounts of mass as HD 163296, then we may just have underestimated their masses until now.”

“We can measure disc masses by looking at how much light is given off by molecules like carbon monoxide.  If the discs are sufficiently dense, then they can block the light given off by more common forms of carbon monoxide – and that could result in scientists underestimating the mass of the gas present. 

“This study has used a technique to observe the much rarer 13C17O molecule – and that’s allowed us to peer deep inside the disc and find a previously hidden reservoir of gas.”

ALMA radio telescope in the Atacama desert in the Andes, Chile.

ALMA radio telescope in the Atacama desert in the Andes, Chile.

The radio telescope used for this discovery - the Atacama Large Millimetre/submillimetre Array (ALMA) – allows the observation of the light that is invisible to the unaided eye, hence astronomers are able to view what is known as the ‘cold universe’ - those parts of space not visible using optical telescopes.

Booth said: “Our work shows the amazing contribution that ALMA is making to our understanding of the Universe. It is helping build a more accurate picture of the physics leading to the formation of new planets.  This of course then helps us understand how the Solar System and Earth came to be.”

The researchers are already planning the next steps in their work. 

She added: “We suspect that ALMA will allow us to observe this rare form of CO in many other discs.  By doing that, we can more accurately measure their mass, and determine whether scientists have systematically been underestimating how much matter they contain.”

Credit images: Top: ALMA (ESO/NAOJ/NRAO),  University of Leeds; middle: ESO/ L. Calçada; bottom: ALMA

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