Astronomers get rare glimpse into planetary formation

New evidence of how planets grow by colliding with each other in "giant impacts" has been discovered by an international team of researchers including an astrophysicist from NUI Galway.

These massive collisions produce planets, satellites like the moon and vast amounts of debris, while the resulting dust can be observed by telescopes on Earth – even if they occurred hundreds of thousands of years ago. In our own solar system, the Earth and moon are thought to be products of this type of collision.

Astronomers believed they should be commonplace in “early systems” where planets are formed, but they have been difficult to observe around other stars.

The team detected evidence of a giant impact in the form of dust and gas left over after the collision of two planets orbiting a star. It occurred in a nearby star system, just 95 light years from Earth – “the first discovery of a planetary atmosphere being vaporised by a giant impact”.

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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”, their study published in Nature on Wednesday found.

The research was carried out by NUIG; Massachusetts Institute of Technology (MIT) and Cambridge University. The rare glimpse into planetary formation came from observing the young star known as HD172555, which relatively near our solar system and visible with the naked eye from dark sites in the southern hemisphere.

‘Post-impact debris’

“This star has been an object of intrigue among astronomers because of the large amount of dust orbiting within its terrestrial planet region,” explained Dr Luca Matrà of NUIG school of physics, an adviser for the study. “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.”

The astronomers made the observations using the ALMA observatory in Chile, which is made up of 66 radio telescopes working together. Ireland gained access to it after joining the international European Southern Observatory in 2018.

“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,” Dr Matrà said.

The HD172555 planetary system is about 23 million years old, compared to our solar system at about 4.6 billion years. The observations found carbon monoxide orbiting in large amounts in a region similar to the outer terrestrial planet region of the solar system.

Detection method

Dr Matrà added: “The amount of gas discovered is 10-20 per cent of the mass of Venus’s 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 of MIT 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.

Dr Christine Chen, an astronomer at the Space Telescope Science Institute in Baltimore, US who was not involved in the 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."