Planets orbiting near their stars may not last for very long (Illustration: Mark Garlick/HELAS)

Exoplanets that venture near their host stars are doomed to premature deaths – even before they get close enough to be ripped apart by the stars’ gravity, two new studies suggest.

A star’s gravity can put a nearby planet on a ‘fast track’ to spiralling into the star and may also cause the planet to lose much of its atmosphere, the studies say. The research may help explain why few exoplanets have been found right next to their host stars.

More than 300 exoplanets have been catalogued to date. Many are situated close to their host stars, where it is thought to be too hot for gas and dust to collapse into planets in the first place. That implies that the planets came from farther away and migrated inwards.

But strangely, the closest-in ones are commonly found some 0.05 astronomical units (AU) from their host stars (1 AU is the distance from the Earth to the sun). This distance, which corresponds to a three-day orbit around a star as heavy as the sun, is sometimes called the “three-day pile-up”.


No one is sure why the planets seem to pile up there. Very close to a star, at a boundary called the Roche limit, planets are dismembered by the star’s gravity. But the migration of planets seems to stop well outside this limit.

Dragged inwards

So why do planets seem to stop there? Some models suggest gas and dust in the disc around a star could drag the planets inward. If the star managed to clear away the debris close to it, that could stop planet migration.

But Brian Jackson of the University of Arizona in Tucson and colleagues offer an alternative explanation. There may be planets that orbit closer in, but they will not do so for very long before they get dragged inwards by their host star’s gravity.

The tugging is caused by tidal forces between the planet and its star – differences in the pull of gravity on the objects’ near and far sides.

Fast planets

Counter-intuitively, the same force is causing the moon to slowly widen its orbit around Earth. But in that case, the moon orbits the Earth more slowly than the Earth spins, and that causes the moon’s distance to increase.

Close-in planets, on the other hand, seem to orbit their stars faster than the stars themselves rotate, so this tidal friction will have the opposite effect. It causes the stars to deform – their gaseous atmospheres are stretched towards the close-in planets – and causes the planets to migrate inwards.

Planets may only last in close-in orbits for perhaps tens of millions to a few billion years before spiralling into their stars. “Once a planet gets that close, the tide raised on the star by the planet causes the planet to migrate in so quickly they’re hard to catch,” Jackson told New Scientist.

Moribund world

The study suggests the closest-in planet known, Corot-7b, may not have that much longer to live. Tidal forces may cause the planet, which orbits just 0.017 AU from its host star, to be dragged to the star’s deadly Roche limit in as few as 25 million years.

Future studies could detect evidence of this type of violence by looking for the chemical signatures of gobbled-up planets in starlight. Stars that are spinning abnormally fast for their age could also be a sign that they have absorbed a planet and “spun up” as a result, Jackson told New Scientist.

Stellar outbursts

Intense tidal forces are not the only factor that can take a toll on vagabond planets. Outbursts of activity early on in a star’s life can also strip a planet of much of its mass, according to a study led by Helmut Lammer of the Austrian Academy of Sciences in Graz.

As ultraviolet radiation from a star heats a nearby planet’s upper atmosphere, the atmosphere expands out to a distance where it feels a stronger gravitational tug from the host star than the planet and is pulled off the planet.

The star-hugging planet WASP-12b, which orbits its host star just once a day, seems to have been a particularly dramatic victim of stellar activity. The planet seems to have lost roughly 24% of its mass over the course of its estimated 2-billion-year lifetime, the new work suggests. It now weighs about 1.4 times the mass of Jupiter.

Journal reference: Astrophysical Journal (forthcoming)