An artist’s impression of the dust disc and a forming giant icy planet, similar to Neptune, around TW Hydrae. NAOJ

A gap in the dusty ring whirling around nearby star TW Hydrae could be a sign that a giant icy planet, around the size of Neptune, is forming.

Researchers led by Takashi Tsukagoshi at Ibaraki University, Japan spied on the star with the Atacama Large Millimetre/submillimetre Array and saw a distinct ring devoid of big dust particles – perhaps kicked out by a developing planet.

The paper, accessible on Arxiv and to be published in The Astrophysical Journal, could help planetary scientists better understand how planets form.

TW Hydrae is one of the closest young stars to us. It’s only around 10 million years old and 176 light-years away.

Luckily for us, we have a fantastic view of the star’s disc – astronomers can see the dusty disc from above.

Earlier this year, gaps in the disc were spotted. But astronomers couldn’t tell any information about the size of the dust particles in those gaps – information that could tell them if a planet was likely forming there (or not)

So Tsukagoshi and colleagues looked at the solar system with two radio frequencies, allowing them to estimate the size of dust grains.

In the most prominent gap, which is 22 astronomical units from the star (one astronomical unit is the distance from the Earth to the sun), smaller, micrometre-sized, dust particles dominate and larger dust particles are absent.

ALMA image of the disc around the young star TW Hydrae. Researchers found that the size of the dust particles in the inner gap is smaller than in the other bright regions, and guess that a planet similar to Neptune is in this gap. ALMA / ESO / NAOJ / NRAO / Tsukagoshi et al.

Theoretical studies have predicted that a gap in the disk is created by a massive planet and that gravitational interaction and friction between gas and dust particles push the larger dust out from the gap, while the smaller particles remain in the gap.

These ALMA observations match theoretical predictions.

“Combined with the orbit size and the brightness of TW Hydrae, the planet would be an icy giant planet like Neptune,” Tsukagoshi says.