Analysis: we know for the first time that planetary impacts can release large amounts of gas as well as dust
Gas released in the aftermath of a massive impact between planets is the first evidence that planetary atmospheres can be stripped away by such collisions. The impact occurred in a nearby young planetary system, just 95 light years from Earth.
Terrestrial planets, like our own Earth, form when a star is relatively young, as in the first 10 to 100 million years of its life (by comparison, our own Sun is roughly 4.6 billion years old). Theory predicts that when a star is 10-100 million years old, a sea of so-called planetary embryos and protoplanets, the precursors of the terrestrial planets, populates the inner Solar System and other planetary systems.
The gravitational interactions in this crowded environment cause total chaos, and commonly produce giant impacts between (proto)planets. Eventually, some protoplanets end up eaten up by the central star, some are flung out of the planetary system, and some lucky ones grow to produce terrestrial planets like our Solar System's Earth and Venus, for example. As well as larger planets, these catastrophic collisions can often lead to the formation of satellites like the Moon.
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From Brigthside, what happens if two planets collide?
But such gigantic collisions also produce debris. While it would be very unlikely to directly witness an impact itself, we can use telescopes on Earth to observe the wreckage. However, to date, our telescopes have provided little evidence of giant impacts around other young stars in our 'neighbourhood’.
A rare glimpse of an impact comes from a nearby star called HD172555, which is visible with the naked eye from dark sites in the Southern Hemisphere. This star is young (23 million years old), and 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 indicates that it most likely represents post-impact debris from the aftermath of a planetary impact - akin to the one that led to the formation of the Moon in our Solar System.
A team of researchers, led by the Massachusetts Institute of Technology (MIT) and NUI Galway, discovered gas, particularly carbon monoxide, around this young HD172555 planetary system. The gas was found orbiting in large amounts in a region analogous to the outer terrestrial planet region of the Solar System.
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From Deep Space TV, the birth of the ALMA observatory in Chile
The observations were made using the ALMA (Atacama Large Millimeter/submillimeter Array) observatory in Chile, made of 66 radio telescopes working together, which Ireland gained access to after joining the European Southern Observatory in 2018. The ALMA observations revealed a ring of carbon monoxide gas in orbit around the star, in roughly the same place as where the dust was known to be.
But what does the presence of gas tell us? The group tested various scenarios to explain the abundance of gas in the terrestrial planet region, concluding that the same planetary impact that produced the dust most likely released the observed gas as well.
This indicates for the first time that planetary impacts can release large amounts of gas as well as dust, and that this gas can survive long enough to its signal to be picked up by telescope on Earth. 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.
The amount of gas and debris indicate that the impact was likely massive, involving two young planets of size likely comparable to the Earth
The data favours a scenario where carbon monoxide is produced through the stripping of the atmosphere(s) of the colliding planets. In other words, the planetary collision was sufficiently energetic to remove at least some of the atmosphere on either (or both) of the planets. In fact, the amount of gas discovered is tiny in astronomical terms - only 10-20% of the mass of Venus' atmosphere, which goes to showcase the ability of the ALMA telescope to pick up incredibly faint signals. Moreover, this discovery implies that gas observations are a promising way to find more terrestrial planet-forming collisions around other stars, and to study the composition of atmospheres stripped from young planets.
Modelling of the observed gas and dust wreckage shows that two protoplanets likely slammed into one another at high velocity at least 200,000 years ago, releasing some of their planetary atmospheres into space. Over time, this debris from the collision spread out into the planet’s orbit creating a ring of gas and dust around the star, which is what is now observed. The amount of gas and width of the observed ring of debris indicate that the impact was likely massive, involving two young (proto)planets of size likely comparable to the Earth.
The full research article is published in Nature journal.
The views expressed here are those of the author and do not represent or reflect the views of RTÉ
