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When planets collide: Galway astronomer detects huge impact that stripped away atmosphere

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Luca Matra

Luca Matra

The Moon itself is believed to have formed from a planetary impact when the Earth was young

The Moon itself is believed to have formed from a planetary impact when the Earth was young

Dr Luca Matrà, Lecturer in the Centre for Astronomy, School of Physics at NUI Galway

Dr Luca Matrà, Lecturer in the Centre for Astronomy, School of Physics at NUI Galway

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Luca Matra

A massive collision be- tween Earth-sized planets in a nearby star system that stripped their atmospheres has been detected by a Galway-based astronomer.

Astronomers interested in understanding how planets form have theorised that young planets grow by smashing into each other in huge impacts, which can reduce a planet’s atmosphere to dust and gas. This is the first concrete evidence to confirm this theory.

“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,” said Dr Luca Matra, a lecturer in the Centre for Astronomy, School of Physics at NUI Galway, who contributed to the findings today in the journal Nature.

Dr Matra, along with colleagues at the Massachusetts Institute of Technology (MIT), observed dust and gas left over after the collision of two planets orbiting a star called HD 172555.

They did this using a collection of 66 radio telescopes, working together, at the ALMA high-altitude observatory in the Atacama Desert, Chile.

“This is the first time we’ve detected this phenomenon – of a stripped protoplanetary atmosphere in a giant impact,” said Tajana Schneiderman, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Science, who led the research effort.

“Everyone is interested in observing a giant impact because we expect them to be common, but we don’t have evidence in a lot of systems for it. Now we have additional insight into these dynamics.”

The HD 172555 star system is a hot young star at just 23 million years old where planets are being formed, unlike our solar system, which is 4.57 billion years old and where planets formed long ago.

It is an interstellar neighbour too at 95 light years from Earth and it can be viewed with the naked eye in the southern hemisphere.

Astronomers are intrigued with HD 172555 because it is somewhere nearby where they can watch as planets are being formed.

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Dr Matra and his colleagues were specifically interested in discovering why HD 172555 had so much dust orbiting where its planets should reside, something that had been previously observed by other astronomers.

“Our ALMA observations unexpectedly detected a ring of carbon monoxide gas co-located with the dust in this system,” Dr Matra said. “This indicates, for the first time, that impacts can release large amounts of gas as well as dust and that this gas can survive long enough to be detected.”

The astronomers estimate that the giant impact that created all the dust and gas happened at least 200,000 years ago.

The collision had to have occurred at least that recently because otherwise the star’s UV radiation would have destroyed the gas.

“It provides evidence that young terrestrial planets, like Earth would have been in its youth, can have their atmospheres removed, or stripped, by an impact,” Dr Matra said. “The moon itself is believed to have formed from a planetary impact when the Earth was young.”

This research can also help search for extra-terrestrial life as even small amounts of certain gases can be found that represent signs of life on other planets.


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