Astronomers may have uncovered how the most common types of planets in our Galaxy grow, a new study has revealed. The discovery confirms astronomers’ theories that these planets start as 'bloated' babies but quickly lose much of their thick atmospheres.
The study, published in Nature, looked at four infant exoplanets in the V1298 Tau system. Their star is just 20 million years old, making it the cosmic equivalent of a 5-month-old baby.
Currently, the planets are five to 10 times the radius of Earth but have just five to 15 times its mass – giving them a similar density to polystyrene foam.
This means the planets are ‘puffed up’, as the heat and light of their young star is causing their atmospheres to expand enormously. The result is that the planets lose huge amounts of gas into space, meaning they will eventually contract and end up somewhere between Earth and Neptune in size.
Though planets this size – also known as super-Earths and sub-Neptunes – have been found around many other stars, and are the most common variety we currently know of, one place you won’t find them is in our own Solar System.
Without an example of one of these intermediate worlds to study up close, they have been something of a ‘missing link’ in our understanding of how planets grow and form.
“V1298 Tau is a critical link between the star- and planet-forming nebulae we see all over the sky, and the mature planetary systems that we have now discovered by the thousands,” said Prof Erik Petigura from the University of California, who took part in the study.
Astronomers had previously theorised that infant planets grew in this way, as they had noticed many young planets were quite large, but this is the first time they have managed to catch the act as it was happening.
“These planets have already undergone a dramatic transformation, rapidly losing much of their original atmospheres and cooled faster than what we’d expect from standard models,” said Prof James Owen, a co-author from Imperial College London.
“But they’re still evolving. Over the next few billion years, they will continue to lose their atmosphere and shrink significantly, transforming into the compact systems of super-Earths and sub-Neptunes we see throughout the galaxy.”
A hole in one
Like so many astronomical discoveries, the find relied as much on good luck as hard work and perseverance.
The team were only able to examine the planets by looking at their transits – the brief dips in their star’s brightness caused when the planet passed in front. The size of the dip can reveal information about a planet’s radius, while the timing exposes the details of its orbit.
While the scientists knew the planets were on the larger side, some of the transits for the outer two planets had been missed, leaving a large question mark over their orbits.
“There were hundreds of possibilities which we whittled down by running computer models and making educated guesses,” said Petigura.
They guessed well, it seemed, as when they went to look for the planet again with ground-based telescopes, they found it on the first try.
“I couldn’t believe it,” said Petigura. “The timing was so uncertain that I thought we would have to try half a dozen times at least. It was like getting a hole-in-one in golf.”
With all four orbits pinned down, the team could then do a much closer analysis to work out the planet’s masses.
As the planets pass each other, their gravity pushes and pulls their orbits, subtly changing the timing of their transits. The bigger the mass of the planet, the bigger the pull, meaning the team were able to pick apart the timings of the transits to reveal the masses of the planets.
“The unusually large radii of young planets led to the hypothesis that they have very low densities, but this had never been measured,” said Trevor David from the Flatiron Institute, who led the initial discovery of the system in 2019.
“By weighing these planets for the first time, we have provided the first observational proof. They are indeed exceptionally ‘puffy,’ which gives us a crucial, long-awaited benchmark for theories of planet evolution.”
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