Exoplanet aurorae ‘a thousand times’ brighter than ours

Aurorae on many planets could make our own Northern and Southern lights look like a flickering candle, new research has shown.

It seems that aurorae on distant ‘hot Jupiters’ could be up to 1,000 times brighter than Earthly aurorae, rippling all the way from the equator to the poles.

“I’d love to get a reservation on a tour to see these aurorae!” says Ofer Cohen, a SHINE-NSF postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics (CfA).

Earth’s aurorae are created when energetic particles from the Sun slam into our planet’s magnetic field, which funnels them toward the poles. Here, they smash into Earth’s atmosphere, causing air molecules to glow like a neon sign.

Particularly strong aurorae result when Earth is hit by a coronal mass ejection or CME – a gigantic blast that sends billions of tons of solar plasma – electrically charged, hot gas – into the solar system.

Cohen and his colleagues used computer models to study what would happen if a gas giant in a close orbit, just a few million miles from its star, were hit by a stellar eruption.

The planet, he found, would be subjected to a much stronger and more focused blast.

“The impact to the exoplanet would be completely different than what we see in our solar system, and much more violent,” says co-author Vinay Kashyap of CfA.

In the model, a CME hits the hot Jupiter and weakens its magnetic shield before the particles reach the gas giant’s atmosphere. Its aurora lights up in a ring around the equator, 100 to 1,000 times more energetically than on Earth. Over the next six hours or so, the aurora ripples up and down toward the planet’s poles before gradually fading away.

Despite the extreme forces involved, the exoplanet’s magnetic field shields its atmosphere from erosion.

“Our calculations show how well the planet’s protective mechanism works,” says Cohen. “Even a planet with a magnetic field much weaker than Jupiter’s would stay relatively safe.”

The work has important implications for the habitability of rocky worlds orbiting distant stars. Since red dwarf stars are the most common stars in our galaxy, they’ve become the focus for the search for Earthlike worlds.

However, since a red dwarf is cooler than our sun, a planet would have to orbit very closely to be warm enough for liquid water – and, here, it would be subjected to the sort of violent stellar eruptions studied by Cohen. The team now plans to examine whether rocky worlds could shield themselves from such eruptions.