A ‘new-born’ planet has been discovered in the protoplanetary disk surrounding a young star

Astronomers agree that planets are born in protoplanetary disks — rings of dust and gas that surround young, new-born stars. While hundreds of these disks have been spotted throughout the universe, observations of actual planetary birth and formation have proved difficult within these environments.

Now, astronomers at the Center for Astrophysics | Harvard & Smithsonian (USA) have developed a new way to detect these elusive new-born planets — and with it, ‘smoking gun’ evidence of a small Neptune or Saturn-like planet lurking in a disk.

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

“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. We need new techniques to look at and support that a planet is there.”

The presence of a planet

For her study, Long decided to re-examine a protoplanetary disk known as LkCa 15. Located 518 light years away, the disk sits in the Taurus constellation in the sky. Scientists previously reported evidence for planet formation in the disk using observations with the ALMA Observatory (Chile).

Long looked into new high-resolution ALMA data on LkCa 15, obtained primarily in 2019, and discovered two faint features that had not previously been detected. About 42 astronomical units out from the star — or 42 times the distance Earth is from the Sun — she discovered a dusty ring with two separate and bright bunches of material orbiting within it. The material took the shape of a small clump and a larger arc and were separated by 120°.

Long examined the scenario with computer models to figure out what was causing the build-up of material and learned that their size and locations matched the model for the presence of a planet.

She added: “This arc and clump are separated by about 120°. That degree of separation doesn’t just happen — it’s important mathematically.”

Long points to positions in space known as Lagrange points, where two bodies in motion — such as a star and orbiting planet — produce enhanced regions of attraction around them where matter may accumulate.

Neptune or Saturn

“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 and the objects involved,” she explained.

In this case, the arc and clump of material Long detected are located at the L4 and L5 Lagrange points. Hidden 60° between them is a small planet causing the accumulation of dust at points L4 and L5.

The results show the planet is roughly the size of Neptune or Saturn, and around one to three million years old, which is relatively young when it comes to planets.

Directly imaging the small, new-born planet may not be possible any time soon due to technology constraints, but Long believes further ALMA observations of LkCa 15 can provide additional evidence supporting her planetary discovery.

It is hoped this new approach for detecting planets — with material preferentially accumulating at Lagrange points — will be utilised in the future by astronomers, although there is the caveat that it requires very deep data as the signal is weak.

The results are described today in The Astrophysical Journal Letters.

Image: Artist’s illustration of a small Saturn-like planet discovered in the system LkCa 15. The planet resides within dense rings of dust and gas that surround a bright yellow star. Material accumulates in a clump and arc-shape, about 60 degrees away from the planet. Note: This illustration is not to scale.

Credit: M.Weiss/ Center for Astrophysics | Harvard & Smithsonian