Weighing up the birth of planets with heavy molecule discovery

The discovery of a heavy molecule of carbon monoxide in a protoplanetary disc of gas and dust could solve the mystery of planet formation.

Using the Atacama Long Millimeter Array (ALMA)radio telescope in Chile, a team of astronomers led by researchers from the University of Leeds have discovered a rare, heavy molecule lurking in a ring of gas and dust surrounding a young star 330 light-years from Earth.

The observation made in the protoplanetary disc surrounding the star — HD 163296 — could change our understanding of how planets form around stars. In particular, it could answer a conundrum that has bothered astronomers for years, namely how these discs contain enough material to form planets in the first place.

The detection of the rare form of ‘heavy’ carbon monoxide — ¹³C¹⁷O— in what the researchers call the most accurate measurement of mass in a protoplanetary disc to date, indicates that protoplanetary discs, where planets are created, could be more massive than previously believed.

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

“This is an important finding in terms of planetary systems in discs,” Booth explains. “If they contain more gas, then they have more building material to form more massive planets.”

“The mass of a disc is its most fundamental property as this sets the material available to form planets.”

A Discovery not to be taken lightly.

The discovery is a fortunate one as observations of protoplanetary discs have, until now, confused scientists as they did not seem to contain enough gas and dust to form planets.

“Since we have found this extra mass, it brings into question if other discs are hiding mass,” Booth says. “Currently the masses of discs are not high enough to explain the number of exoplanets detected, a problem for theories of how these planets form.”

The inner red regions represent the dust in the disc, thought to be shaped into rings by forming planets. The wider blue region is the carbon monoxide (CO) gas in the disc. The inner green region shows the rarer 13C17O gas that the researchers have detected for the first time. (Credit: ALMA (ESO/NAOJ/NRAO), Booth and colleagues, University of Leeds)

Despite this good fortune, the discovery took Booth and her team somewhat by surprise and is a testament to the power and sensitivity of the European Space Agency’s ALMA telescope — originally designed to study some of the coldest objects in the Universe.

As its role is an observer of the ‘cold Universe’ ALMA is able to view light invisible to the naked eye, giving it a significant advantage over visible-light based telescopes.

“The original goal of our observations was to look for the signatures of water molecules in the disc, but we didn’t find any,” she explains. “We didn’t set out to find ¹³C¹⁷O but the ALMA telescope is incredibly sensitive and our observations also covered the frequency of the ¹³C¹⁷O signal.”

Dr John Ilee is also a researcher at Leeds involved in the study, he adds that if other discs are hiding amounts of mass comparable to those the team measured in HD 163296 then it would indicate that astronomers have been underestimating the mass of protoplanetary discs for some time.

Thus, effectively solving the mystery of the missing mass in planet formation.

Ilee explains how spotting molecules in protoplanetary discs allows researchers to weigh discs of gas and dust: “We can measure disc mass 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.”

This says Ilee, could explain why we’ve been underestimating the mass of gas in protoplanetary discs thus far.

He continues: “The study used a technique to observe the much rarer ¹³C¹⁷O molecule — allowing us to peer deep inside the disc and find a previously hidden reservoir of gas.”

Materials like ¹³C¹⁷O are referred to as isotopologues — rare isotopes which one atom has a varying number of neutrons within its nucleus. Booth points out that isotopologues are key for measuring gas mass with ALMA.

“Because ¹³C¹⁷O is so rare we require a lot of observing time with ALMA,” she continues. “We need to push the telescope to its technical limits in order to search for its faint signals.”

Booth predicts that the next significant breakthroughs for this research will come via observations of the gas and dust in protoplanetary discs conducted with the Square Kilometer Array — due to begin construction in Australia and South Africa in 2020. Hopefully, finally discovering how dust grains, the seeds of planets clump together and grow.

The researchers are not waiting for 2027 when the Square Kilometer Array’s first results are due to come in though, they are currently planning the next steps in their research, with the help of ALMA.

“The next step is to do a survey of lots of discs to look for rare carbon monoxide molecules like ¹³C¹⁷O,” Booth says. “We suspect that ALMA will enable us to look for these forms of carbon monoxide in other discs.”

“By doing that, we can more accurately measure the mass of these discs and determine if we have been systematically underestimating the mass they contain,” concludes Booth.