October Astronomers detect gas released in a giant planetary collision

Astronomers detect gas released in a giant planetary collision

Gas discovery in space is first observational evidence of an atmosphere stripped away by a planetary impact

 An astronomer at NUI Galway is part of an international team which detected evidence of a giant impact that occurred in a nearby star system, just 95 light years from Earth.

It is the first discovery of a planetary atmosphere being vaporised by a giant impact.

Based on the amount of gas present, the impact was likely massive and took place about 200,000 years ago involving two young planets, likely to have been similar in size to Earth.

The findings, based on the research by NUI Galway, Massachusetts Institute of Technology (MIT) and Cambridge University have been published today in the leading international journal Nature.

The rare glimpse into planetary formation comes from the young star HD172555, relatively near our solar system and 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 unusual composition of dust indicated that it most likely represents post-impact debris from the aftermath of a planetary impact – similar to the one that led to the formation of the moon.

Dr Luca Matrà, advisor for the study and Lecturer in the Centre for Astronomy, School of Physics, NUI Galway, said: “Our ALMA observations unexpectedly detected a ring of carbon monoxide gas co-located with the dust in this system, which for the first time indicates that impacts can release large amounts of gas as well as dust, and that this gas can survive long enough to be detected.

“This has the potential to revolutionise our understanding and observability of giant impacts.”

The HD172555 planetary system is about 23 million years old, compared to our solar system at about 4.6 billion years.

The astronomers made the observations using the Atacama Large Millimeter/submillimeter Array observatory in Chile, known as ALMA, which is made up of 66 radio telescopes working together, and which Ireland gained access to after joining the international European Southern Observatory in 2018.

The observations found carbon monoxide orbiting in large amounts in a region analogous to the outer terrestrial planet region of the solar system.

Dr Matrà added: “The amount of gas discovered is 10-20% of the mass of Venus’ atmosphere, which goes to show the incredible sensitivity of the observations. This puts forward gas observations as a viable detection method of terrestrial planet-forming collisions, and as a window to the composition of young planets.”

Lead author, Tajana Schneiderman, a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, said: “This is the first time we’ve detected this phenomenon, of a stripped protoplanetary atmosphere in a giant impact. 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 formation of terrestrial planets like Earth takes place over the first tens of millions of years of a star’s lifetime. Scientists have theorised that in the latest formation stages, planets grow by colliding with each other in giant impacts.

The massive collisions produce planets, satellites like the moon and debris, where the latter can be observed by telescopes on Earth.

The observations by the team from NUI Galway, MIT and Cambridge University confirm the predictions of planet formation models, as the HD172555 planetary system is precisely in the age range where terrestrial planet formation through giant impacts is expected to happen. The data favours a scenario where carbon monoxide is produced through the stripping of the atmosphere of the colliding planets.

The presence of the gas tells us that the same planetary impact that produced the dust most likely released the observed gas as well.

Dr Christine Chen, Associate Astronomer at the Space Telescope Science Institute in Baltimore, USA, and expert in the field external to this study, said: “The modelling convincingly demonstrates that two protoplanets slammed into one another at high velocity at least 200,000 years ago, launching their planetary atmospheres into space. Over time, the debris from the collision spread out into the planet’s orbit creating a ring of gas and dust we can now observe.”

Read the full study in Nature here: https://www.nature.com/articles/s41586-021-03872-x.

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