The Beautiful Violence of Exoplanet Aurorae
If you think the auroral displays we have on Earth are impressive, spare a thought for aurorae erupting in the atmospheres of massive gas giant exoplanets orbiting close to their parent stars.
Through a combination of close proximity to the star and powerful magnetic fields, aurorae on so-called “hot-Jupiters” could be 100-1000 times brighter than the displays we see on Earth. Also, as an added bonus, these aurorae are predicted to ripple across entire exoplanetary atmospheres and not just be restricted to the polar regions, according to a new study.
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“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).
Living With a Star
Space weather is of increasing importance to our planet as we send delicate electronics into space. The satellites that deliver everything from cable television to GPS coordinates are susceptible to being disrupted by the high-energy particles blasting from the sun.
But it’s not only the circuit boards of satellites that are at risk. The large-scale circuitry of entire national power grids are also prone to failure.
As energetic particles slam into our planet’s magnetosphere, the condition could arise that huge quantities of high-energy solar particles (mainly protons) are injected into the magnetic field of Earth. When this happens, aurorae will erupt at high latitudes (generating the famous Northern and Southern Lights — a.k.a. the Aurora Borealis and Aurora Australis, respectively), and powerful currents are generated through the atmosphere. Power grids can be overloaded, causing widespread blackouts.
But what if a gas giant was orbiting its star only a few million miles from its star, as opposed to Earth’s “safer” 90 million miles from the sun?
Using computer models to simulate this extreme space weather environment, Cohen and his team simulated what it would be like if a coronal mass ejection (CME) were to hit a hot-Jupiter with a compact orbit.
In a word: Fireworks.
Firstly, as the exoplanet is so close to its star, the CME will be far denser than the CMEs than plow into our little world. After CMEs have traveled nearly 100 million miles to Earth’s doorstep, their density drops as the CME cloud expands. Cohen’s hot-Jupiter on the other hand is sitting right down the throat of a stellar blowtorch.
Although the physics is similar between aurorae on Earth and extreme aurorae on this particular exoplanet, that is where the similarities end.
“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 in the CfA press release.
Slamming into the exoplanetary atmosphere with an energy 100-1000 times greater than the aurorae we experience, the extreme illumination caused by the impact of the stellar plasma with atmospheric gases will rapidly engulf the whole globe. The CfA model predicts the resulting eruption will light up equatorial regions, rippling from the north to south poles over six hours, eventually fading as the geomagnetic storm energy is dissipated.
Amazingly, even though this exoplanet has undergone a hammering, the researchers have found that the global magnetic field will be more than capable at protecting the atmosphere from erosion. This invisible “force field” that formed and shaped the ensuing storm will, like the Earth’s magnetosphere, protect lower portions of the exoplanet’s atmosphere from being blown away into space.
Besides being a fascinating study, this model may also help the search for alien life.
Red dwarf stars are an attractive target for ET-hunters as any closely-orbiting exoplanets can be easily spotted. Also, as red dwarfs pump out a lot less energy than the sun, their habitable zones are by their nature, very compact. There’s little wonder that bets are being placed on the first habitable “exoEarths” being discovered there.
But there’s a problem with this plan. Red dwarfs are nasty little balls of seething plasma, massive stellar eruptions regularly bellowing searing plasma into any unfortunate worlds orbiting them. It seems doubtful that looking for alien life inside a stellar flame-thrower would be fruitful.
However, as Cohen’s work demonstrates, gas giant exoplanets are predicted to be more than capable of defending themselves against a stellar explosion. It therefore seems logical that the team are now analyzing whether small, rocky, Earth-like worlds orbiting red dwarfs might have a similar magnetic self-defense mechanism.
If extraterrestrial life could evolve in such a violent cosmic environment, should this life become advanced enough for space travel, it would be very interesting to know whether these close-proximity solar storms hinder, or facilitate interplanetary travel. Strong stellar winds, after all, could motivate the construction of some killer solar sails, but it would be curtains for any unprotected electronics.