The evolution of a faraway star system

A young planet located 150 light-years away has given astrophysicists the rare chance to study a planetary system in the making.

The findings, published in The Astronomical Journal, suggest the planet dubbed DS Tuc Ab – which orbits a star in a binary system – formed without being heavily impacted by the gravitational pull of the second star.

“We expected the pull from the second star to tilt the rotating disc of gas and dust that once surrounded the main star – a process that would skew the orbit of the planet,” says lead author Benjamin Montet, from the University of New South Wales, Australia. 

“Surprisingly, we found no evidence the planet’s orbit was impacted. We also found the planet formed through relatively calm processes – which means it could be possible for Earth-like planets to survive in binary systems like this.”

At 40 million years old, the gas giant DS Tuc Ab is considered a ‘pre-teen’ in planetary years; fewer than ten known planets are this young. 

Its age is a unique chance for astrophysicists to study a system in development before external influences interfere.

“To find out how long planetary systems last, we need systems that are too young to go through dynamical interactions, but old enough to have formed planets. The DS Tuc system is exactly in that niche,” Montet says.

A team of international researchers used the Planet Finder Spectrograph to measure the Rossiter-McLaughlin effect, which is the relative angle between the orbit of the planet and the spin of its star.

They discovered DS Tuc Ab orbits its star in a relatively flat plane, at approximately 12 degrees incline from the star’s rotational axis. 

This low incline – called obliquity – suggests that the pull from the companion star did not significantly tilt the orbit of the protoplanetary disc where DS Tuc Ab formed.

While planets in the solar system all have a low obliquity, it’s unusual for planets like DS Tuc Ab.

“Most similar planets orbit their star at random angles, sometimes reaching up to 90 degrees above the axis of their star,” Montet says.

“The DS Tuc system is the first piece of evidence that higher orbital angles don’t get defined early on in a star’s life – they are an effect that happens only later on.”