Evrim Yazgin
Cosmos science journalist
A planet-in-the-making has been discovered in a protoplanetary disk 518 lightyears away. The discovery of the small gas planet comes out of a newly developed way of detecting newborn planets.
A protoplanetary disc is a rotating “disk” of dense gas surrounding a young newly formed star.
Astronomers agree that planets are born in these protoplanetary disks. Hundreds of these rings of dust and gas have been spotted throughout the universe. However, seeing an actual planetary birth is much more difficult as the planets are often outshone by the material surrounding them.
Harvard-Smithsonian Center for Astrophysics (CFA) astronomers have used a new method to find evidence of a small Neptune – or Saturn-like planet – lurking in one of these planetary disks. Their results are published in the Astrophysical Journal Letters.
“Directly detecting young planets is very challenging and has so far only been successful in one or two cases,” says lead author Feng Long, a postdoctoral fellow at the CFA. “The planets are always too faint for us to see because they’re embedded in thick layers of gas and dust.”
Because the baby planets are too faint to observe directly, astronomers must look for indications that they are there in the protoplanetary disk.
“In the past few years, we’ve seen many structures pop up on disks that we think are caused by a planet’s presence, but it could be caused by something else, too” Long says. “We need new techniques to look at and support that a planet is there.”
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Long used new high-resolution data from the Atacama Large Millimeter/submillimeter Array (ALMA) observatory in Chile to re-examine the protoplanetary disk known as LkCa 15. The disk sits in the Taurus constellation and has previously been subject to studies indicating planet formation. But the astronomers were missing the “smoking gun.” Until now.
Two faint features from the 2019 ALMA data stood out to Long — two bright clusters of material within a dusty ring about 42 times further out from the disk’s central star than Earth is from the Sun. The material had come together into a small clump and a larger arc. Using computer models, Long found that the geometry matched that which would be expected in the presence of a baby planet.
Long recognised in the data positions in space known as “Lagrange points” where two bodies in motion – such as a star and orbiting planet – produce regions where gravity generates matter accumulation.
“We’re seeing that this material is not just floating around freely, it’s stable and has a preference where it wants to be located based on physics,” Long says.
The planet is roughly the size of Neptune or Saturn, and around one to three million years old. For a planet, that is very young indeed. By comparison, the Earth is believed to be up to be around 4.5 billion years old.
While it may not be possible to take a direct image of the planet any time soon, Long believes further observations will provide evidence to support her theory that this is a baby planet.
Long also hopes her new approach for detecting planets which focuses on Lagrange points will continue to be used by astronomers. “I do hope this method can be widely adopted in the future. The only caveat is that this requires very deep data as the signal is weak.”