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Posted: 27 July 2011

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Scientists at the Harvard-Smithsonian Center for Astrophysics have looked into what would happen if an alien gas giant, which tightly orbits its star at just a few million miles, were struck by a stellar eruption.

Ofer Cohen, a postdoctoral fellow at Harvard, and his colleagues used computer models to uncover what effects a stellar blast would have on an exoplanet's atmosphere and surrounding magnetosphere. The new research illustrates that aurorae on distant hot jupiters can be up to 100–1000 times more energetic than the aurorae we see on Earth. 

An Alaskan aurora from a geomagnetic storm. Image: IBTimes/NASA.

Aurorae can be seen on the Earth as the Northern and Southern Lights, and can observed at the polar regions of our planet. The curtains of green and red are created when energetic particles from the Sun slam into our planet's magnetic field. “The topology of the aurora is determined by the location of the 'open' magnetic field lines, where particles along these lines can penetrate and hit the planetary atmosphere,” says Cohen, who is lead author of the research paper accepted for publication in The Astrophysical Journal . “There particles hit the atmospheric neutral atoms so that photons are emitted via the interaction to generate the light that we see.” It is this process that can occur on distant worlds as they dance around their parent stars. “The exact location of the auroral region depends on how strong the coronal mass ejection driver is,” he adds. “The stronger it is, the lower the latitude the aurora will be.”

A coronal mass ejection, or CME, is a gigantic blast from the Sun which throws billions of tons of electrically charged hot gas into our Solar System – on some occasions we have been on the receiving end of the Sun's fiery temper. A CME can disrupt the Earth's protective magnetic shield – the magnetosphere – causing a geomagnetic storm. “During the Halloween storm event in 2003, the aurora could be seen in Florida and Texas due to its massive strength,” says Cohen. Two other famous solar storms are the 13 March 1989 event when one a storm knocked out power across the entire Canadian province of Quebec for more than nine hours, and the Carrington Event of 1859, whose severity is the greatest to date, shocking telegraph operators and setting some areas on fire.

In the case of an alien gas giant close to its star, it would be subject to these extreme forces creating much stronger and more focused blasts in comparison to our Solar System, where the CME spreads out as it travels through space, somewhat softening the blow. “The main difference is that in our Solar System and on Earth, the CME is not affected by the planet, since it is much larger,” says Cohen. “Think about a large wave in the sea that hits a small stone. The stone is affected by the wave, but the wave does not 'feel' the stone. Now think about a very large stone, an island; in this case, the wave will break as it hits the island. This is what happens in our simulation – the CME and the planet are comparable in size, so that the CME is highly affected by the interaction.”

An artist's conception showing a "hot Jupiter" and its two hypothetical moons with a Sun-like star in the background. The planet is cloaked in brilliant aurorae triggered by the impact of a coronal mass ejection. Theoretical calculations suggest that those aurorae could be 100-1000 times brighter than Earth's. Image: David A. Aguilar (CfA).

In their simulation, a CME blasts the hot jupiter, with a strength analagous to being within a mile of an active volcano, weakening its magnetic shield. The CME particles then reach the the gas giant’s atmosphere, creating a display of auroral lights around the equator. Over the course of 6 hours, the aurora ripples up and down towards the planet's north and south poles before gradually fading away. “In the simulation, the aurora starts near the equator and migrates towards higher latitudes,” says Cohen. “This is due to the fact that the CME opens up some of the originally closed field lines of the planet near the equator. The main point here is that the aurora on such a planet is not only much more intense, but it is also all over the place.” 

However, despite the extremity of the forces involved, the exoplanet's magnetic field shield still manages to protect its atmosphere from erosion. “Our calculations show how well the protective mechanism works,” explains Cohen. “Even a planet with a magnetic field much weaker than Jupiter's would stay relatively safe.”

The investigation is also important in our understanding of potentially habitable rocky worlds orbiting distant stars. However, since there are more red dwarf stars in our Galaxy, scientists have suggested that we should be focussing on them in our search for Earth-like planets, rather than hotter Sun-like stars. Since red dwarfs are cooler, a planet must orbit very closely to be warm enough to accommodate liquid water and it is at this distance that it would be subject to violent stellar eruptions. 

“In this simulation, the detailed physics of the planetary upper atmosphere and the magnetosphere could not be studied in detail, due to the computational limitations,” says Cohen of his research. “We plan to extend our study to models for the upper atmosphere of the planet, so that the detailed impact, as well as the parametric study, could be done in a more precise manner.”

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