Ageing stars seem to destroy their closest planets, new Warwick research reveals

A new study by astronomers at the University of Warwick and University College London (UCL) suggests that ageing stars destroy the planets orbiting closest to them.

Published in the Monthly Notices of the Royal Astronomical Society, the study consisted of observing nearly half a million nearby stars. The researchers aimed to better understand how common it is for a planet to survive their host star becoming a red giant.

This destruction is presumably caused by “the gravitational tug-of-war between the planet and the star, called tidal interaction”. This interaction strengthens as the star expands, causing the planet to slow down and either spiral into the star, or break apart in the process

The researchers found that planets were much less likely to be orbiting close to red giant stars, suggesting that the planets which were originally in close orbit had been destroyed when the star evolved. Only 130 planets and planet candidates were found to be closely orbiting these ageing stars.

Lead author Dr Edward Bryant, an Astrophysics Prize Fellow at the University of Warwick, suggests that evolving stars “can quickly cause planets to spiral into them and be destroyed”.

This destruction is presumably caused by “the gravitational tug-of-war between the planet and the star, called tidal interaction”. This interaction strengthens as the star expands, causing the planet to slow down and either spiral into the star, or break apart in the process.

The sun is predicted to become a red giant in around five billion years – but whether Earth will survive its evolution is up for debate

A red giant is defined as a “dying star in the final stages of stellar evolution”.  Stars undergo nuclear fusion throughout their lives to exert the pressure needed to counteract the inward force of gravity.  As this process continues, stars fuse progressively heavier elements together. Eventually, a star is exhausted of hydrogen and is unable to counter its own gravity, causing its helium core to collapse as the surrounding hydrogen shells reignites.

Consequently, the star rapidly inflates at extremely high temperatures; as the star’s outer envelope cools, it reddens – hence the name ‘red giant’. The sun is predicted to become a red giant in around five billion years – but whether Earth will survive its evolution is up for debate.

Co-author Dr Vincent Van Eylen, at Mullard Space Science Laboratory at UCL, said that, unlike the giant planets in their study which were much closer to their own star: “Earth itself might survive the Sun’s red giant phase. But life on Earth would probably not.”

The researchers are yet to conclude why some planets, but not all, are destroyed by ageing stars. Bryant concluded that using the masses of such planets would help to deepen their understanding of this phenomenon and better predict the survivability of planets orbiting closest to their host stars.