When a new star is born, it creates a disk full of gas and dust — the stuff of planetary formation. But it's hard to catch alien stars in the process of planetary baby-making, because the same dust that creates planets helps obscure these distant solar systems from our sight. We've found young planets and old ones alike, but none of them have actually been in the process of forming — until now.
This new study was led by University of Arizona graduate student Stephanie Sallum and Kate Follette, a former fellow graduate student who has since moved on to postdoctoral research at Stanford University. The two women were working on separate PhD projects, but had decided to focus on the same star — LkCa15, located 450 light years from Earth.
"The reason we selected this system is because it’s built around a very young star that has material left over from the star-formation process," Follette said in a statement. "It’s like a big doughnut. This system is special because it’s one of a handful of disks that has a solar-system size gap in it. And one of the ways to create that gap is to have planets forming in there."
The women and their colleagues set high-powered telescopes to look at the system and used a new technique to look for protoplanets. They searched for the light emitted by hydrogen as the gas falls toward a newly forming planet. That process is hot — roughly 17,500 degrees Fahrenheit — and it produces a signature red glow.
"The difference in brightness between a star and a young exoplanet is usually comparable to the difference between a firefly and a lighthouse," Follette said in a statement. "It's very hard to isolate the light from the planet when it is so faint and so close to the star from our point of view. But, because we could focus on a special color of light where the planet is glowing very brightly, the signal was significantly stronger than what we normally look for."
Here's a composite image made from the telescope observations:
The researchers were able to determine the orbits of two young planets, and suspect there may be a third as well. They're likely to be gas giants. The team was surprised to find that at least one of the planets has been forming for around 2 million years.
In a commentary article for Nature, Zhaohuan Zhu, an astrophysicist at Princeton University who wasn't involved in the study, wrote that such direct images of planetary formation could help answer a lot of questions. “Little is known about how microscopic dust particles can grow 14 orders of magnitude to become a giant planet," he wrote, but "the authors have demonstrated a powerful technique to find young planets in circumstellar disks, one that will discover many such planets in the future."
Correction: Due to a typo, a previous version of this post stated that the star LkCa15 was "at least two million years old". This is true, but it's actually the age of the planet that researchers found surprising.