Exoplanet Aurora… Light ‘Em Up!

Article written: 23 Jul , 2011
Updated: 24 Dec , 2015
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One of the most beautiful and mysterious apparitions – be it north or south – here on Earth is an auroral display. We know it’s caused by the Sun-Earth connection, so could it happen around exoplanets as well? New research shows that aurorae on distant “hot Jupiters” could be 100-1000 times brighter than Earthly aurorae, creating a show that would be… otherworldly!

“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).

As we are now aware, aurorae occur here on Earth when the Sun’s energetic particles encounter our magnetosphere and are shifted towards the poles. This in turn excites the atmosphere, ionizing the particles. Much like turning on your electric stove, this causes the “element” to glow in visible light. It happens here… and it happens on Jupiter and Saturn as well. If other suns behave like our own and other planets have similar properties to those in our solar system, then the answer is clear.

Exoplanets have aurorae, too.

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 blast. He wanted to learn the effect on the exoplanet’s atmosphere and surrounding magnetosphere. In this scenario, the solar storm is much more focused and far more concentrated when it impacts a “hot Jupiter”. In our solar system, a coronal mass ejection spreads out before it reaches us, but what would happen if it collided with a nearer planet?

“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.

Using modeling, the team took a look at the scenario. The solar blast would slice into the exoplanet’s atmosphere and weaken its magnetic shield. The auroral activity would then form a ring around the equator, 100-1000 times more energetic than seen here on Earth. It would then travel up and down the planet’s surface from pole to pole for hours, gradually weakening – yet the planet’s magnetosphere would save it from erosion. This type of study is important for understating habitable properties of Earth-like worlds.

“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.”

Original News Source: Harvard-Smithsonian Center for Astrophysics News.


10 Responses

  1. Torbjörn Larsson says

    “Even a planet with a magnetic field much weaker than Jupiter’s would stay relatively safe.”

    It is good days for astrobiology, then, if massive CMEs are short and long time survivable with magnetic fields.* From Wikipedia I get that Jupiter’s field is ~ 10 times as strong as Earth’s. Attempts to model terrestrial exoplanets show them rather robustly populated with strong magnetic fields:

    “We present estimations of dipolar magnetic moments for terrestrial exoplanets using the Olson & Christiansen (2006) scaling law and assuming their interior structure is similar to Earth. We find that the dipolar moment of fast rotating planets (where the Coriolis force dominates convection in the core), may amount up to ~ 80 times the magnetic moment of Earth, M_Earth, for at least part of the planets’ lifetime. For very slow rotating planets (where the force of inertia dominates), the dipolar magnetic moment only reaches up to ~ 1.5 M_Earth.”

    Sizable magnetic fields for terrestrials in M-dwarfs habitable zone! So what is going on here?

    First they note another (!) mitigating factor which is outside of their model:

    “Recent calculations by e.g. Correia et al. (2008), Barnes et al. (2009) and Heller et al. (2011) suggest that the planets may in fact end in stable nonsynchronous rotation states if their orbits are slightly eccentric.”

    Second, convection is now believed to play a greater part in planetary dynamos than before:

    “More recently, convection has been found to also be an important parameter for dynamo scaling laws (Olson & Christensen 2006; hereafter OC06). Compared with previous models, OC06 mispredicts by less than 15% on average the observed magnetic moments of Solar System bodies, while all other models produce mismatches up to 268 [!] %. In addition, for example, all models except OC06 fail to predict the observed absence of magnetic field in Venus.”

    The factor that oomps up fields for habitable terrestrials around M stars is then the same factor that sometimes dampen the field to make our expectations of range slightly skewed. Not having checked that, it may be an expression for the retrograde rotation of Venus.

    They make a first order optimistic model to get to possible field strengths, and then note that at least some of the planetary lifetime the larger terrestrials will have a sizable field even if rotation is locked to the orbit.

    ————————–
    I would add two – three other mitigating factors to all this. Larger terrestrials could and would on average have denser atmospheres which protects more and last longer. And M stars hyperactive CME period are in most cases a shorter period, with some of that used up for the rather slow terrestrial planet formation.

    In fact, those two should be able to combine to a “goldilocks” atmosphere thickness, where the remaining is “just so” thick for habitability. Rarer, but possibly happening.

    • Anonymous says

      Convection is thought to be a major aspect of planetary magnetic field. I am not sure if theory for how magnetic fields are generated has been worked out satisfactorily. I seem to remember there were controversies over this.

      LC

      • WaxyMary says

        An old friend has the idea that a superconductor iron/hydrogen alloy might be the source of the dynamo effect. He is now retired and has a lot of time on his hands… his thoughts are below. It seems very hard to test any of them currently.

        “Tests in diamond anvils indicate that hydrogen enters the iron lattice at super high pressures, and the ratio is close to unity. The resulting crystal structure, designated FeH, matches the seismic velocity and average density of the Earth’s core, and its layered pattern makes it a good candidate for a superconductor.”

        “The ponderous circulation of liquid iron in the outer cores of differentiated planets serves only to cause slowly varying regional distortions of the pure dipole field originating within the solid core and provides a buffer allowing the solid core to rotate independently of the mantle. The magnetic field in the solid core is `pumped up’ by impulses of charged particles from the Sun spiraling into the poles.”

        Mary

      • Torbjörn Larsson says

        “Core strength to the rescue!”

        Though I assume of C et al are correct, such theories of inherent core strength and next to no role for convection fails.

      • WaxyMary says

        An old friend has the idea that a superconductor iron/hydrogen alloy might be the source of the dynamo effect. He is now retired and has a lot of time on his hands… his thoughts are below. It seems very hard to test any of them currently.

        “Tests in diamond anvils indicate that hydrogen enters the iron lattice at super high pressures, and the ratio is close to unity. The resulting crystal structure, designated FeH, matches the seismic velocity and average density of the Earth’s core, and its layered pattern makes it a good candidate for a superconductor.”

        “The ponderous circulation of liquid iron in the outer cores of differentiated planets serves only to cause slowly varying regional distortions of the pure dipole field originating within the solid core and provides a buffer allowing the solid core to rotate independently of the mantle. The magnetic field in the solid core is `pumped up’ by impulses of charged particles from the Sun spiraling into the poles.”

        Mary

      • Torbjörn Larsson says

        More phenomenology, I am afraid. But it should be sound, as there are a few planets to check with.

        Actually there may be more than a few, since exoplanet magnetic fields can be detected.

        Speaking of which, I also googled that Christensen et al has proposed more mundane detection based on their results:

        “Now, a group of German theorists claims that this divide is too simplistic in the case of small stars and very large planets. They suggest that some planets and stars, less than a third as massive as the Sun, generate strong dipole fields like the Earth’s. If true, this would lead to very large emissions of synchrotron radiation which could be detected on earth. […]

        Perhaps the most promising aspect is the radio waves stemming from strong magnetic fields. This year — the International Year of Astronomy — a new central European radio telescope array known as LOFAR will begin scanning the skies for low frequency radio waves. Christensen said, “I hope projects like LOFAR will take notice of our research and this may lead astronomers to detecting new stars and possibly extrasolar planets.””

        You can also find some of the controversy in that last link.

      • Anonymous says

        However, a tidally locked planet would be rotating rather slowly. If I understand right, the magnetic field intensity is proportional to the angular velocity of the planet.

        LC

      • Anonymous says

        However, a tidally locked planet would be rotating rather slowly. If I understand right, the magnetic field intensity is proportional to the angular velocity of the planet.

        LC

    • Anonymous says

      Convection is thought to be a major aspect of planetary magnetic field. I am not sure if theory for how magnetic fields are generated has been worked out satisfactorily. I seem to remember there were controversies over this.

      LC

  2. Anonymous says

    Bonito articulo,queremos saber mas.

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