The primordial process that turns enormous clouds of cosmic dust into newborn planets over millions of years has been observed directly for the first time.
Astronomers caught sight of a planet in the making around a young star in the neighbourhood of Taurus, 450 light years from Earth.
The discovery is a boon for scientists who have never before had a real star system against which they can check theories of how the universe came to be dotted with different worlds.
“This is our first chance to watch the planet formation process happening,” said Stephanie Sallum, a graduate student at the University of Arizona. “We can go and look at this and do more detailed studies now, to try to understand how planets are built.”
Although nearly 1,900 alien worlds have been spotted beyond our solar system, none are still forming. And with no growing planets to gaze at, scientists can only compare their models for how planets are born with the end results, such as fully mature rocky worlds and gas giants.
That leaves an enormous gap in astronomers’ understanding. The latest ideas on planetary formation put broad margins on the time the process takes, ranging from 1m to 10m years. What happens between the start and finish is hazy. As Zhaohuan Zhu, an astrophysicist at Princeton University, puts it: “Little is known about how microscopic dust particles can grow 14 orders of magnitude to become a giant planet.”
What is known is that particles left over from the dusty disc that surrounds a newborn star coalesce and coalesce until eons later, a nascent planet takes shape. It grows as material from its own dusty disc rains down on the surface, forming a huge sphere under its own gravity.
Sallum and her colleagues commandeered two different telescopes in their search for a planet in the making. They used the large binocular telescope in Arizona to look at infrared light coming from the vicinity of LkCa 15, a 2m-year-old star around which astronomers had spotted a candidate protoplanet, LkCa 15b, in 2012. Infrared light received by the telescope pointed to two, perhaps three, young planets in orbit around the star.
The team then turned to another telescope, the Magellan adaptive optics system in Chile. This time, they looked for light that is known to be released by hydrogen atoms when very hot material falls on to a growing planet. They picked up the signature emissions from the closest body to the star, the suspected protoplanet LkCa 15b. It was strong evidence yet that the planet was in the midst of forming.
From past observations, the team went on to reconstruct the movement of the new planets and found that their orbits looked circular.
“We can’t say much about their size, but you could have stable orbits for millions of years if these planets were somewhere between half as massive as Jupiter and three times as massive as Jupiter,” said Sallum. The planets are likely to be gas giants that orbit between 12 and 24 times further from their star than Earth is from the sun.
Astronomers will now watch the young planets to see if they grow at different rates. “If we observe again in the future and see an accretion signature coming from one of the other planets in there, that would be a sign that the process is variable,” said Sallum, whose study appears in the journal Nature. More detailed observations could potentially pick out how material compacts on to the planet’s surface.
Zhu believes scores more young planets will now be found with the procedure, allowing scientists to work out how young and old planets are distributed through our cosmic neighbourhood.
“Such an understanding of the young planet population will shed light on the decades-old problem of planet formation and reveal how young planetary systems can evolve into older ones such as our solar system, billions of years after they were born,” he said.
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