Boffins spot baby planet forming around young star – and that's rather cool

In dust, we trust

Astronomers have found a baby planet hidden in clouds of gas and dust swirling within a young solar system, by studying the accumulation of material around Lagrange points.

That's according to research published this week in The Astrophysical Journal Letters. 

Studying these protoplanets is difficult. Their stellar nurseries are shrouded in thick, hot clumps of mostly hydrogen gas, preventing astronomers from clearly observing the birth of stars and planets.

"Directly detecting young planets is very challenging and has so far only been successful in one or two cases," Feng Long, first author of the study and a postdoctoral fellow at the Center for Astrophysics at Harvard, said. "The planets are always too faint for us to see because they're embedded in thick layers of gas and dust."

To overcome this hurdle, Long and her colleagues developed a method to detect baby worlds, and used it to discover what appears to be a young planet forming around LkCa 15, a juvenile star located 518 light-years from Earth.

Here's how the team said they did it. They used observational data gathered from the ALMA telescope, which revealed a clump of mass and an arc-shaped feature, both telltale signs that something else is forming within the dense protoplanetary disk of matter surrounding the young star. 

These images did not, however, provide hard evidence of a planet forming around that sun. But another measurement connecting the pair of features convinced the team they had found an alien world in the making. "This arc and clump are separated by about 120 degrees," Long said. "That degree of separation doesn't just happen — it's important mathematically."

The separation showed these two features lie at Lagrange points, points in space around which objects can orbit stably thanks to the gravitational pull of two nearby large objects – for example, a star and a planet. NASA's James Webb Space Telescope is in the second Lagrange point or L2 in the Solar System with the Earth between it and the Sun.

That means the Webb telescope effectively circles the L2 point due to the Sun and Earth's combined pull on it. It stays put, relatively speaking, as a result of those two bodies. As NASA explains: "At Lagrange points, the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them."

The data from LkCa 15 showed the arc is located at the L4 point and the clump is at L5. These are so placed because another object – a hidden planet – is orbiting between them; the Lagrange points are the result of the gravitational pull by the young star and its forming world, just as the Sun and Earth form Lagrange points, one of them being home for the Webb probe.

Long and her colleagues used the data to simulate the growth of a planet with similar properties to the one they thought they had found, and compared their model's results with the telescope's images.

Strong similarities between the simulations and observational data showed a planet is likely forming around LkCa 15. The mystery object is estimated to be about the size of Neptune or Saturn, and orbits around the star at quite a distance – 42 times the distance between the Sun and Earth, and is thought to be between one to three million years old.

"[We] put a planet into a disk full of gas parcels and dust particles, and see how they interact and evolve under known physics," Long said. "In the end, usually an image will be made to present the system architecture at certain time steps. This model image will show what the millimeter wavelength emission would look like, [so we can] make a direct comparison with our observations."

In order for the simulations to be effective, the astronomers had to gather as much information as they could on new planets to inform their modeling.

"There are one or two studies in the past that tried to predict planets associated with similar features, but in these examples, they only found one feature, so it's not that convincing," Long told The Register. "We found two, the clump and the arc."

"We are the first to detect mass accumulation around both Lagrange points, so the location of the planet can be more precisely predicted. We look forward to applying this method to other systems," she added.

The main thing in hunting for growing planets is to look for empty gaps in the protoplanetary disk, she explained.

"Systems with known gaps are the prime search targets," Long told us. "This technique in finding young planets actually has many unknown dependencies on other system properties that we are still exploring. So [finding more] samples with those features would be super helpful in defining future search directions." ®

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