An international team of astronomers, including researchers from the University of Cambridge, has made the most detailed image of the ring of dusty debris surrounding a young star and found that the ice content of colliding comets within it is similar to comets in our own solar system.

The chemical kinship may indicate a similarity in comet formation conditions between the outer reaches of this planetary system and our own.

Luca Matra

Earlier observations of the star, known as Fomalhaut and located 25 light years from Earth, were taken in 2012 by astronomers using the Atacama Large Millimetre/submillimetre Array (ALMA), located in Chile. The 2012 results were gathered when the telescope was still under construction, and while they only revealed about half of the debris disc, the observations provided hints about the nature and possible origin of the disc.

The new observations offer a far more complete view of this glowing band of debris, a band of rubble resulting from comets smashing together near the outer edges of the planetary system. The gases observed within the ring by the team suggest that there are chemical similarities between its icy contents and comets in our own solar system.

“We can finally see the well-defined shape of the disc, which may tell us a great deal about the underlying planetary system responsible for its highly distinctive appearance,” said Meredith MacGregor, an astronomer at the Harvard-Smithsonian Center for Astrophysics, and lead author on one of two papers accepted for publication in the Astrophysical Journal describing these observations.

Fomalhaut is a relatively nearby star system and one of only about 20 in which planets have been imaged directly. The entire system is approximately 440 million years old, or about one-tenth the age of our solar system. As revealed in the new ALMA image, a brilliant band of icy dust about two billion kilometres wide has formed approximately 20 billion kilometres from the star.

Debris discs are common features around young stars and represent a dynamic and chaotic period in the history of a solar system. Astronomers believe they are formed by the ongoing collisions of comets and other solid objects, known as planetesimals, in the outer reaches of a recently formed planetary system. The leftover debris from these collisions absorbs light from its central star and re-radiates that energy as a faint glow that can be studied with ALMA.

Using the new ALMA data and detailed computer modelling, the researchers could calculate the precise location, width, and geometry of the disc. These parameters confirm that such a narrow ring is likely produced through the gravitational influence of planets in the system.

The new observations are also the first to definitively show “apocenter glow,” a phenomenon predicted in a 2016 paper by Margaret Pan, a scientist at the Massachusetts Institute of Technology and co-author on the new papers. Like all objects with elongated orbits, the dusty material in the Fomalhaut disc travels more slowly when it is farthest from the star. As the dust slows down, it piles up, forming denser concentrations in the more distant portions of the disc. These dense regions can be seen by ALMA as brighter millimetre-wavelength emission.

Using the same dataset, but focusing on distinct millimetre-wavelength signals naturally emitted by molecules in space, the researchers also detected vast stores of carbon monoxide gas in precisely the same location as the debris disc.

“These data allowed us to determine that the abundance of carbon monoxide plus carbon dioxide around Fomalhaut is about the same as found in comets in our own solar system,” said Dr Luca Matrà of Cambridge’s Institute of Astronomy, and lead author of the team’s second paper. “This chemical kinship may indicate a similarity in comet formation conditions between the outer reaches of this planetary system and our own.” Matrà and his colleagues believe this gas is either released from continuous comet collisions or the result of a single, large impact between ‘supercomets’ hundreds of times more massive than Hale-Bopp.

The presence of this well-defined debris disc around Fomalhaut, along with its curiously familiar chemistry, may indicate that this system is undergoing its own version of the Late Heavy Bombardment, a period approximately four billion years ago when the Earth and other planets were routinely struck by swarms of asteroids and comets left over from the formation of our solar system.

“Twenty years ago, the best millimetre-wavelength telescopes gave the first fuzzy maps of sand grains orbiting Fomalhaut. Now with ALMA’s full capabilities the entire ring of material has been imaged,” said Paul Kalas, an astronomer at the University of California at Berkeley and principal investigator on these observations. “One day we hope to detect the planets that influence the orbits of these grains.”

Adapted from a National Radio Astronomy Observatory press release.

Meredith A. MacGregor et al. 'A Complete ALMA Map of the Fomalhaut Debris Disk.' arXiv:1705.05867 
L. Matrà et al. 'Detection of exocometary CO within the 440 Myr-old Fomalhaut belt: a similar CO+CO2 ice abundance in exocomets and Solar System comets.' arXiv:1705.05868

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