Institute of Astronomy

Astronomers detect gas released in a giant planetary collision

Published on 20/10/2021 

An international team of astronomers from MIT, the National University of Ireland at Galway, University of Cambridge, and elsewhere have discovered evidence of a giant impact that occurred in a nearby star system, just 95 light years from Earth. The study represents the first detection of a planetary atmosphere that was vaporised by a giant impact.

The formation of terrestrial planets like Earth takes place over the first tens of millions of years of a star’s lifetime. Theories of planet formation predict that in the latest formation stages, planets grow by colliding with each other in so-called giant impacts. As well as larger planets, these massive collisions can lead to the formation of satellites like the Moon, but also produce debris that can be observed by telescopes on Earth. However, to date, there is little observational evidence of giant impacts in planetary systems around other stars.

In this discovery, a rare glimpse comes from the nearby young star HD172555, which is visible with the naked eye from dark sites in the Southern Hemisphere. This star has been an object of intrigue among astronomers because of the large amount of dust orbiting within its terrestrial planet region. The dust’s unusual composition indicated that it most likely represents debris from the aftermath of a planetary impact - akin to the one that led to the formation of the Moon in our Solar System.

The team detected Carbon Monoxide gas around this young HD172555 planetary system (23 million years old, compared to the age of Earth and the Solar System which is about 4.6 billion years). The gas was found orbiting in large amounts in a region analogous to the outer terrestrial planet region of the Solar System, adding further evidence of a giant impact. The team determined such an impact likely occurred between a roughly Earth-sized terrestrial planet and a smaller impactor that collided at least 200,000 years ago, and at speeds of 10 kilometres per second, or more than 22,000 miles per hour.

Crucially, the detected gas indicates that the high-speed impact likely blew away part of the larger planet’s atmosphere — a dramatic event that would explain the observed gas and dust around the star. The findings, appearing today in Nature, represent the first detection of a planetary atmosphere that was vaporized by a giant impact. The observations confirm the predictions of planet formation models, as this planetary system is precisely in the age range where terrestrial planet formation through giant impacts is expected to happen. 

Prof. Mark Wyatt from the IoA, who was a co-author in the study, said: ”This discovery opens up a new window into processes that were ongoing in the infancy of our own Solar system. Importantly it shows that as planets are growing part of their atmospheres are also being lost to space in these giant impacts. And these observations are telling us what the atmospheres on those newly formed planets are made up of. In the case of HD172555, its nascent planet’s atmosphere contained Carbon Monoxide gas”.

The observations were made using the ALMA (Atacama Large Millimeter/submillimeter Array) observatory in Chile, made of 66 radio telescopes working together at 16,000 ft elevation in the Atacama Desert. ALMA is the only telescope capable of detecting such a small amount of gas, which is equivalent to 10-20% of the mass of Venus’ atmosphere. 

Dr. Sebastian Marino, co-author in the study and Research fellow at Jesus College and IoA, said “We knew this star was particularly interesting as it was surrounded by a dust ring, but we didn’t expect to detect gas. Extracting this detection from the data was a serendipitous result and only possible thanks to ALMA's public archive. The data had been briefly inspected before by the team that originally designed and led these observations, but the carbon monoxide emission was missed. The data nevertheless stayed publicly accessible in the archive and after a careful second look by our team, we were able to discover the presence of gas.”

Based on the article by the National University of Ireland Galway and MIT.

Read the full study in Nature here:


For more about the study contact Dr Sebastian Marino or Prof Mark Wyatt, Institute of Astronomy, University of Cambridge, at or   

Page last updated: 20 October 2021 at 18:27