Astronomers have spotted a planet in the act of formation for the first time ever, by searching for the special red light the process emits.
Despite all that has been discovered about the basics of planet formation, there is still much that scientists don’t know about how planets like those in our Solar System were first made.
Finding a so-called protoplanet, a world in the process of being born, has been a goal of astronomers for some time, but the faint light given out by a growing planet has made them difficult to spot.
The alien world LkCa 15b, which orbits a star 450 light years away, is on its way to becoming a planet similar to our system’s Jupiter and is the first world formation ever seen in action.
“I was pretty excited as soon as I processed the data, but I wanted to be cautious,” said Kate Follette, a postdoctoral researcher at Stanford and co-lead author.
“I was pretty sure I had found something interesting, but in this field we’re always chasing objects that are just at the edge of what we can detect. The really cool thing is that it survived all of our tests to make sure it was real.”
Follette and her colleagues were able to produce a digital picture of this alien world growing in the light of ultra-hot hydrogen gas by chasing the light of a particular signature predicted by planet formation theory.
Astronomers theorise that planets form in a transition disc – a ring of dust and rocky debris orbiting a sun. This debris is swept together to form first the core and then the rest of the planet, leaving central clearings in the disc. But finding these gaps was no easy task.
Follette’s team decided to chase the characteristic wavelength of light created by hydrogen gas falling into the core of the protoplanet and being heated up until it glows, a wavelength of visible light called H-alpha.
Using the’s Magellan Telescope in Chile, the team were able to find this shade of red light emanating from LkCa 15b as it formed, even though it’s being born close to the much brighter light of its sun.
“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. “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 colour of light where the planet is glowing very brightly, the signal was significantly stronger than what we normally look for.”
The process was possible because of Magellan’s adaptive optics system, which corrects for the bending of light as it passes through Earth’s atmosphere and has a visible light camera capable of imaging at H-alpha. The same technique allowed Follette’s adviser at Stanford, Professor Bruce Macintosh, to find the slightly older planet 51 Erdani b.
“51 Eri b is an adolescent – about 20 million years old, already full-grown and still cooling off from the energy released during its formation,” he said. “Kate’s planet is a baby, still heating up and growing.”
At the same time as Follette’s team were at work, an independent study was going on at the University of Arizona using the Large Binocular Telescope’s adaptive optics system and a new imaging technique to coincidentally image the same system.
Monitoring LkCa 15b as it forms should help scientists to better understand the process of planet formation and potentially find other markers to look out for when searching for worlds being born. It also gets astronomers one step closer to finding Earth-like planets in the Universe.
“One of the fundamental human questions is whether we’re alone or unique,” Follette said.
“It’s cool to look at Jupiter-like exoplanets like LkCa 15 b, but ultimately we’re trying to push the technology to be able to detect Earth-like exoplanets. I’ve always been inspired by the famous ‘pale blue dot’ image of Earth taken by Voyager as it passed Saturn. We’d really like to do that some day for a planet around another star, and this sort of work is moving us in that direction.”