Giant Jupiter-like world reveals clues on how planets evolve

Astrophysicists have got a rare peek at a baby planetary system 300 light years away


Simulated image of the HD 106906 stellar debris disc, showing a ring of rocky planet-forming material. Image credit: Erika Nesvold/Carnegie Institution for Science

An enormous young planet approximately 300 light years from Earth has given astrophysicists a rare glimpse into planetary evolution.

The planet, known as HD 106906b, was discovered in 2014 by a team of scientists from the United States, the Netherlands and Italy. It is 11 times the mass of Jupiter and is extremely young by celestial standards — not more than 13 million years old, compared with our Solar System’s 4.6 billion years.

“This is such a young star – we have a snapshot of a baby star that just formed its planetary system — a rare peek at the final stage of planet formation,” says Smadar Naoz, a UCLA assistant professor of physics and astronomy.

Another of the planet’s unusual characteristics is its distance from its star. Astronomers believe that the vast majority of planets outside of our Solar System exist inside a vast dusty disc of debris relatively close to its centre. But HD 106906b is far beyond its planetary disc — so far away that it takes 1,500 years for the planet to orbit its star. HD 106906b is currently at least 650 times as far from its star as the Earth is from our Sun.

“Our current planet formation theories do not account for a planet beyond its debris disc,” Naoz says.

The study’s lead scientist is Erika Nesvold, a postdoctoral fellow at the Carnegie Institution for Science whom Naoz mentors. She wrote software called Superparticle-Method Algorithm for Collisions in Kuiper belts and debris discs, or SMACK, that allowed the researchers to create a model of the planet’s orbital path — a critical step because HD 106906b orbits so slowly that the researchers can barely see it move.

The Jupiter-like world has provided insight into the evolution of planets. Image credit: UCLA

The research suggests that the planet formed outside the disc, where it’s visible it today, as opposed to having been formed inside the debris disc and then having been thrust far beyond it. It could help explain the shape of the debris disc.
The planet’s orbit is elliptical; it gets much closer to the star on one side of its orbit than on the other side. And its gravity produces an elliptical shape in the disc as well. One side of the disc is closer to the star than the other side, and the dust on that side is warmer and glows brighter as a result.

The debris disc was photographed in 2016 by American and European astronomers. According to Naoz, it is an analog to our Solar System’s Kuiper belt — an enormous cluster of small bodies like comets and minor planets located beyond Neptune.

The researchers don’t know if there are additional planets inside the disc, but using Nesvold’s software — which also been used to study other debris discs in the universe — they were able to re-create the shape of the disc without adding another planet into the model, as some astronomers had thought would be required.

Debris discs are composed of gas, dust and ice, and they play a key role in the formation of planets. Typically planets form after a gas cloud collapses due to its own gravity, forming a disc — where planets are created — and a star. As the gas slowly evaporates, the dust and debris rotate and collide around the young star until gravity pushes them away, forming a structure like our Solar System’s Kuiper belt.

“In our Solar System, we’ve had billions of years of evolution,” says Michael Fitzgerald, UCLA associate professor of physics and astronomy. “We’re seeing this young system revealed to us before it has had a chance to dynamically mature.”

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