Light fantastic

In this animation, stunning aurorae ripple around a “hot Jupiter”  (hot Jupiters are exosolar planets, or planets outside our solar system). When a stellar eruption — known as a coronal mass ejection —  hit the planet, it triggered these aurorae, which are the planetary equivalent of Earth’s northern and southern lights. However, this exoplanet’s aurorae shine up to a thousand times brighter than Earth’s, and extend from the equator to the poles. Animation created by Hyperspective Studios. Credit: CfA

Earth’s aurorae, or northern and southern lights, provide a dazzling light show in the polar regions. Shimmering curtains of green and red undulate across the sky. New research shows that aurorae on distant “hot Jupiters” could be 100 to 1,000 times brighter than Earth’s aurorae. Hot Jupiters are exosolar planets, or planets outside our solar system, that are large and gaseous like Jupiter but much hotter and in very close orbit with a star. Due to the hot Jupiters’ proximity to stellar eruptions, the planets’ aurorae would also ripple from equator to poles, treating the entire planet to a dramatic spectacle.

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

Aurorae are created when energetic particles from the sun slam into a planet’s magnetic field. The field guides solar particles toward the poles, where they smash into the planet’s atmosphere, causing air molecules to glow like a neon sign.

Particularly strong aurorae result when a planet is hit by a coronal mass ejection (CME) — a gigantic blast that sends billions of tons of solar plasma, or electrically charged, hot gas, into the solar system. A CME can disrupt a planet’s magnetosphere — the bubble of space protected by its magnetic field — causing a geomagnetic storm. In 1989, a CME hit Earth with such force that the resulting geomagnetic storm caused power failures in huge regions of Quebec.

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. (Earth is more than 93 million miles from the sun.) He wanted to learn the effect on the exoplanet’s atmosphere and surrounding magnetosphere.

The alien gas giant would be subjected to extreme forces. In our solar system, a CME spreads out as it travels through space, so it’s more diffuse once it reaches us. A hot Jupiter would feel a 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,” said co-author Vinay Kashyap of CfA.

In the model, a CME hits the hot Jupiter and weakens its magnetic shield.

Then CME particles reach the gas giant’s atmosphere. Its aurora, 100 to 1,000 times stronger than Earth’s aurorae, lights up in a ring around the equator. Over the course of about six hours, the aurora then ripples up and down toward the planet’s north and south 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,” explained Cohen. “Even a planet with a magnetic field much weaker than

Jupiter’s would stay relatively safe.”

This 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, astronomers have suggested focusing on them in the search for Earth-like worlds.

However, since a red dwarf is cooler than our sun, a rocky planet would have to orbit very close to the star to be warm enough for liquid water. There, it would be subjected to the sort of violent stellar eruptions Cohen and his colleagues studied. Their future work will examine whether rocky worlds could shield themselves from such eruptions.

This research has been accepted for publication in The Astrophysical Journal, and is available online.