And Now Exo-magnetospheres

Article written: 25 Apr , 2011
Updated: 24 Dec , 2015
by

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New observations of one of the biggest and hottest known exoplanets in the galaxy, WASP 12b, suggest that it is generating a powerful magnetic field sufficient to divert much of its star’s stellar wind into a bow shock wave.

Like exoplanets themselves, the discovery of an exo-magnetosphere isn’t that much of a surprise – indeed it would be a surprise if Jovian-type gas giants didn’t have magnetic fields, since the gas giants in our own backyard have quite powerful ones. But, assuming the data for this finding remains valid on further scrutiny, it is a first – even if it is just a confirming-what-everyone-had-suspected-all-along first.

WASP-12 is a Sun-like G type yellow star about 870 light years away from Earth. The exoplanet WASP-12b orbits it at a distance of only 3.4 million km out, with an orbital period of only 26 hours. Compare this to Mercury’s orbital period of 88 days at a 46 million kilometer distance from the Sun at orbital perihelion.

So habitable zone, this ain’t – but a giant among gas giants ploughing through a dense stellar wind of charged particles sounds like an ideal set of circumstances to look for an exo-magnetosphere.

The bow shock was detected by an initial dip of the star’s ultraviolet light output ahead of the more comprehensive dip which was produced by the transiting planet itself. Given the rapid orbital speed of the planet, some bow wave effect might be expected regardless whether or not the planet generates a strong magnetic field. But apparently, the data from WASP 12-b best fits a model where the bow shock is produced by a magnetic, rather than just a dynamic physical, effect.

The finding is based on data from the SuperWASP (Wide Angle Search for Planets) project as well as Hubble Space Telescope data. Team leader Dr. Aline Vidotto of the University of St. Andrews said of the new finding. “The location of this bow shock provides us with an exciting new tool to measure the strength of planetary magnetic fields. This is something that presently cannot be done in any other way.”

Although WASP 12b’s magnetic field may be prolonging its life somewhat, by offering some protection from its star’s stellar wind – which might otherwise being blowing away its outer layers – WASP 12-b is still doomed due to the gravitational effects of the close-by WASP 12 star which has already been observed to be drawing material from the planet. Current estimates are that WASP 12-b will be completely consumed in about 10 million years.

WASP 12-b is not only one of the hottest hot Jupiters we've found, but also one of the biggest (although this may be largely a result of expansion due to heating).

There is at least one puzzle here, not really testable from such a distance. Presuming that a planet so close to its star is probably tidally-locked, it would not be spinning on its axis – which is generally thought to be a key feature of planets generating strong magnetic fields – at least the ones in our Solar System. This may need something like an OverwhelminglySuperWASP to investigate further.

Further reading: RAS National Astronomy Meeting 2011 press release.



57 Responses

  1. Member
    IVAN3MAN_AT_LARGE says

    There is at least one puzzle here, […]. Presuming that a planet so close to its star is probably tidally-locked, it would not be spinning on its axis…

    Eh? Actually, the tidally-locked planet is spinning on its axis, it just takes as long to rotate around its own axis as it does to orbit around its star.

    • Member
      IVAN3MAN_AT_LARGE says

      P.S. I realize now, after posting, that “orbit around” is tautology, but I don’t have an edit facility here to correct it. Also, I was in a rush to be the first to post a comment before that “iantresman” character turned up!

  2. Member

    I see the point you are making, but I don’t think many people would agree with the statement that (for example) the Moon spins on its axis – although it is true that the (tidally locked) Moon is rotated relative to the Sun as a result of its orbit around Earth.

    With Wasp 12-b, it is rotated relative to some arbitrary point in space – but (key point) its contents probably wouldn’t experience dynamo-like spin forces which are thought to underlie powerful planetary magnetic fields. Instead, the centrifugal force generated by its rapid orbit just stretches it out into a egg shape.

    Still – it’s pure guess work as to whether it is tidally-locked or not. I’m just saying…

    • edg says

      Steve – A tidally locked body *is* rotating around its axis – it has to be, or it wouldn’t be able to keep the same face pointing towards the object that they’re orbiting. If it wasn’t rotating, then one face would point towards the same region of distant space all the time, not towards the central body all the time. Try simulating it with something at home if you’re not convinced.

      If WASP 12-B is tidally locked to its star (which is likely for gas giants, but not actually guaranteed. Because they’re made predominantly of fluid they take longer to tidelock than a rocky planet would, so it all depends on the age of the system. I’m not sure if 1.7 billion years is long enough to tidelock a jovian this close, but I suspect it is), then its rotation period around its axis has to be the same as its orbital period around the star. A 26 hour rotation period is more than sufficient to generate a powerful magnetic field in a jovian’s interior.

      And if the planet was noticeably ellipsoidal then that would be due to tidal forces, not the rapidity of its orbit.

      • Member

        I think this is just semantics. You can say the Earth-Moon system is spinning about its barycenter/axis, if you like. However, as far as I am concerned I am tidally locked to the Earth and being rotated every 24 hours. I am still able to continue facing my PC screen and type without inadvertently spinning on my axis.

        I am not convinced the orbital period of a planet is significant to generating a magnetic field. The scenario you describe would not generate the internal dynamo-spin that is characteristic of planets with powerful magnetic fields – it would just create an outwardly-directed centrifugal force.

        Re: “And if the planet was noticeably ellipsoidal then that would be due to tidal forces, not the rapidity of its orbit.” Keplers third law requires that anything this close in will have to move at the velocity that it is moving at to maintain its orbit. Whatever tidal forces it is experiencing arise because it is a body moving at the velocity it is moving. If its velocity changed it would no longer be in the orbit it is currently in. But OK, I acknowledge this may be semantics 🙂

      • Member
        IVAN3MAN_AT_LARGE says

        edg:

        A 26 hour rotation period is more than sufficient to generate a powerful magnetic field in a jovian’s interior.

        Steve Nerlich:

        I am not convinced the orbital period of a planet is significant to generating a magnetic field.

        Steve, I think that you’ve misunderstood “edg”; I think that what he/she was referring to was the 26 hour rotation period of the planet, not its orbital period, being “more than sufficient to generate a powerful magnetic field in a jovian’s interior.”

        Furthermore, here is a simulation of the Synchronous Rotation of the Moon, which demonstrates that the Moon (and also any other tidally locked body) is, in fact, rotating. (N.B. Check out the crazy comment, in all CAPITAL LETTERS, from some obvious “Electric Universe” proponent!)

      • Andrew James says

        Agreed Ivan3man; Edg. has likely read or misinterpreted this figure wrongly.
        – Jupiter rotation period on its axis is about 09h 55m 30s (System III)
        – An orbital period of Wasp-12 is calculated as 1.0914222±0.0000011) days or about 26 hours.
        – We do not know the rotation period of the exoplanet st present.

        [See Extrasolar Planets Encyclopaedia @ http://exoplanet.eu/star.php?st=WASP-12 ]

      • Torbjorn Larsson OM says

        The dynamo theory of planetary magnetic fields is that “a rotating, convecting, and electrically conducting fluid acts to maintain a magnetic field.

        First, an internally (and externally) heated giant planet has plenty of convection. It likely extends to the fluid region.

        Second, the Coriolis “pseudo” force acting on a moving fluid region (see above) is equally valid for all rotating reference frames. A tidal locked planets reference frame rotates, hence large volumes of gas and liquid experience Coriolis effects.

        Whatever tidal forces it is experiencing arise because it is a body moving at the velocity it is moving.

        But tidal forces are due to divergence of a gravitational field. Ie the field differs across a body. Or in other words, velocity is neither here or there.

        Semantically we would have to say that “whatever differences in tidal forces”, say, when the tides moves around earth. Semantics, indeed. 😀

      • Torbjorn Larsson OM says

        Let me join IVAN3MAN in noting that an edit facility would be helpful at times:

        – “a rotating, convecting, and electrically conducting fluid acts to maintain a magnetic field.”

        – neither here _nor_ there.

      • edg says

        It isn’t semantics – you’re simply mistaken. It’s really easy to prove that you’re wrong. (I’ve had lengthy arguments with people about this on the bautforum, and they were wrong there too).

        Take a ping-pong ball and mark a big black X on it with a marker pen. Sit on a swivel chair, and hold that ball at arm’s length, with the X point directly at your eyes/head. Now slowly spin around while sitting in the chair, and keep the X on the ping-pong ball pointing at you. Extrapolate a line from the X on the ball out of the back of your head to the room around you, and you’ll realise that the ping-pong ball is rotating relative to the rest of the room, with a period equal to its orbital period (that extrapolated line will sweep in a circle, 360 degrees around the room)

        Now do the same thing again, but keep the ball pointing to a fixed point in space elsewhere in the room. As you rotate in the chair, the line from the X doesn’t rotate around the room, it stays fixed at that one point.

        The former scenario is what tidelocking is – one face of the satellite (the X) points towards the primary (you). The satellite MUST have the same rotation period as its orbital period, otherwise the same face can’t point towards the primary throughout the orbit.

        If the satellite wasn’t rotating at all, then you would have the second scenario where one face of the satellite (the X) points towards a fixed point in space (the point in the room). That simply doesn’t happen.

        And you really don’t understand tidal forces or how magnetic fields are generated either. I’d suggest reading some literature about it.

      • Member

        You are debating a straw man. I have been saying this from the beginning. Yes, it’s being rotated – but no, it’s not spinning on its axis.

        Agree that a magnetic field might still be generated by the dynamics involved – they are just not the dynamics generated by spinning on an axis.

        I have been suggesting the dominant force would be centrifugal. This is not really tidal force, though agree tidal effects are in there too. Yes, I guess some Coriolos effect as well – and OK convection too. It still sounds like a mess though – you probably need consistent rapid bulk flow in the metallic core to get a powerful magnetic field out of this.

        I will go and read some literature now 🙂

      • edg says

        Steve – no you haven’t been saying that from the beginning at all. I’m not sure if you are the same person I was arguing with a while back about this on BAUT, but you’ee saying exactly the same things, and they are *wrong*.

        The satellite has to be rotating around its own axis if it’s tidelocked to the primary. I’ve demonstrated why. If you still don’t get it then there’s not much more I can say, other than to request that you stop claiming that it’s not true.

        What I (Steve) said at the beginning was:
        I don’t think many people would agree with the statement that (for example) the Moon spins on its axis – although it is true that the (tidally locked) Moon is rotated relative to the Sun as a result of its orbit around Earth.

        With Wasp 12-b, it is rotated relative to some arbitrary point in space – but (key point) its contents probably wouldn’t experience dynamo-like spin forces which are thought to underlie powerful planetary magnetic fields. Instead, the centrifugal force generated by its rapid orbit just stretches it out into a egg shape.

        No idea who you were debating on BAUT. Look at your own analogy – you are holding the ball MOTIONLESS in your fingers while you spin in the chair. The ball is not spinning in your fingers. The system comprising you and the ball are spinning around an axis yes, but “I don’t think many people would agree with the statement that (for example) the Moon spins on its axis”.

      • Member
        IVAN3MAN_AT_LARGE says

        Steve, it is said that a picture is worth a thousand words; I provided a link, above, to a YouTube simulation of the Synchronous Rotation of the Moon that clearly illustrates what edg was trying to explain to you. Did you not bother to check it out?

      • Andrew James says

        Gee whizz, your laboured point here is so mindlessly trivial.
        We see the same face of the moon, through its orbit around the Earth. From an earthbound observer it does not appear to be rotating on its axis. It is but an illusion, because of the captured or synchronous rotation.

        To be factual, quoting Norton’s “”The moon rotates on its axis in the same time it takes to make one orbit of the centre of mass;…”

        Now if I go and live on the Moon. The earth appears to rotate around the moon. What then is the rotation of the Earth? Twenty-four hours or a lunar month? (It is a silly argument!)

        It is comparing rotational motion.to translational motion. Any motion of an extended object can be viewed as a combination of translational motion of the center of mass and rotational motion about the center of mass. It is, in the end, it is really kind of kind of obvious.
        (Q. What is the moment (or rotation) of inertia and the rotational kinetic energy of the moon? Clue: Any object has translational as well as rotational motion, then its total kinetic energy is the sum of the translational kinetic energy and the rotational kinetic energy.)

      • Andrew James says

        As for the statement; “The satellite MUST have the same rotation period as its orbital period, otherwise the same face can’t point towards the primary throughout the orbit.”

        True. Yet the orbital period is technically physically different from the rotational period. i.e. The orbital period of the Moon can be synodic, sidereal, draconic, etc. by time. Axial rotation in either by time, or say, angle/unit time. Angular momentum, for example, applies to rotation of a body only, etc. Even the moment of inertia is defined with respect to an axis of rotation.

        Note: Please stop treating everyone as if they are ignorant. Being pedantic doesn’t make you look smarter, it just makes look like a drama queen seeking attention. Believe me, you’d earn many more brownie points using honey than endless vitriol.

      • edg says

        Andrew – it’s not “mindlessly trivial” at all. People are claiming – out of ignorance or misunderstanding – that something is true when it isn’t. Since this is an educational site (even in the comments), it’s important to make sure the facts are right, and that people understand that they are wrong. I’ve presented the evidence, and that is the scientific consensus, and you’re still misinterpreting it – calling me names doesn’t change that fact.

        You admit that my statement is true and then proceed to ignore it completely. Again, the moon has to rotate to keep the same face towards Earth.

        Tidal forces currently push the moon away from the Earth over time, and slow down Earth’s rotation.What they also did at the beginning was slow the Moon’s (independent) rotation down so that it matched the orbital period – this is what happens with every large moon in the solar system (the tiny asteroidal moons tend to take a lot longer to tidelock because they’re so small). As the Earth’s rotation slows down, and the moon is pushed away, eventually (after a long time, and solar tides notwithstanding) they will be equal – the moon will be further away and the earth’s rotational period would be equal to the Moon’s orbital period (like the Pluto-Charon system).

        The key point in all of this is that the tidal evolution stops *once the rotation periods become equal to the orbital period*. The rotational period does *not* continue to decrease to ever slower values. The moon’s rotation period stops decreasing once it equals its orbital period around the Earth. The earth’s rotation period will stop decreasing – at least if the Earth-Moon system is isolated from the Sun, since solar tides complicate the situation – once it equals the moon’s orbital period. The process stops because there are no more forces acting to change the rotational period any further.

        Your question about the Earth is also misguided. If you stand on the moon and look at the Earth (at least, on the nearside…), you would clearly see Earth rotating around its own axis because Earth’s rotational period is 24 hours, not 28 days.

        Here’s an exercise for you – if Earth’s rotation period was instead 28 days, how many rotations would you see it complete during one lunar orbit? And if the Earth’s rotation period was infinite (i.e. it wasn’t rotating at all) what would you see as the moon orbited it?

      • Andrew James says

        I’m not interested. It is not relevant to this interesting story.
        Again. I just don’t care.

      • edg says

        I said “The moon’s rotation period stops decreasing once it equals its orbital period around the Earth.”

        Actually that’s not strictly true. The moon’s rotation period is locked to equal its orbital period around the Earth, yes – but the moon is still being pushed away from the Earth by tidal forces. This makes the orbital period longer, but at the same time those tidal forces slow the rotation period to match the longer orbital period. My point is that the rotation period is slowing in step with the orbital period – it doesn’t slow down faster (to infinite values where it’s “not rotating”).

      • Andrew James says

        I could not careless of your opinions.
        You said your piece, now move on please!

  3. andjames says

    One think that this exoplanet might be ellipsoidal and not spherical considering how close the planet is to the star. (As the artist impression shows.)

    Another thing missing in this article is the nature of WASP 12 itself. I found some information at http://exoplanet.eu/star.php?st=WASP-12 . The star is a solar-like G0, of main sequence [or only] just sub-giant (luminosity class IV)] at 1.3 or 1.4 solar masses. The star age, from the [Fe/H ratio] is 1.7 billion years. From what I see, what is remarkable is the prediction that this planet of WASP-12b will disappear in 10 million years!
    This suggest that it is remarkable that we have caught this planet on the edge of disappearing forever, It begs to ask, what was this megaplanet was like when the star as formed, and how did it survive for over a billion years?
    It leaves the question of the evaporation as ether questionable or we are missing an additional clue. (I can find little in the literature than can address this problem.)

    Lastly, considering the thermal differential between the day and night, would infer that the atmospheric rotation would be high. Also wouldn’t the source of the magnetic field be either the core of the planet or the expected high atmospheric rotation?

    • Andrew James says

      Correction : One would think that this exoplanet might be ellipsoidal and not spherical considering how close the planet is to the star. (As the artist impression shows.)

      • Member

        I said a Sun-like G type star – you’re just embellishing 🙂

        And ellipsoidal… is that kind of egg-shaped?

        Hot Jupiters almost certainly form out past the frost line of a system (like other Jupiters) but their orbit decays and they end up in a very close stellar orbit. It’s unlikely that this is a tenable situation for any gas giant and hence they probably all have very limited lifetimes.

        More than likely they are relatively rare objects, but are currently over-represented in the exoplanet database – being the easiest to detect.

        Wasp 12-b was probably smaller when it formed (it’s current size being the result of thermal expansion).

        A super-rotating upper atmosphere due to the day-night thermal gradient is likely – though the likelihood of this generating a magnetic effect is doubtful. Venus offers a analogous situation – and has no magnetic field. It’s rapidly spinning sister planet Earth has a whopper.

      • Andrew James says

        Oh. I “embellished” this only to highlight that it wasn’t a massive star and the evolution 1.4 billion years. Compared to the age of the star and that this planet is only got 10 million to go just infers that the exoplanet was bigger in the past.

        As for statement; “Wasp 12-b was probably smaller when it formed (it’s current size being the result of thermal expansion).”

        I’d somehow really doubt this as a valid conclusion. The star has been shining as is easily for one billion, based on simple stellar evolution. (It was still a G0 star a billion years ago.) At the parameters found, it has yet to leave the main sequence, and would do so in a few more billion years. (I calculate about 3.5 Gyr, actually.) Hence the stellar radius has not really changed since being a ZAMS.

        Also the thermal expansion by the heat on such a nearby star is trivial if this star has been shining for that long. The energy of the atmosphere would be raised in a very short period of time compared to the age of the star.

        As for the ellipsoidal shape (prolate ellipsoid), yes I did meant-egg shaped. Its proximity to the star would distort the planet, much like a component in an EB type eclipsing binary. Forces pulls on the facing towards the star, and also the back of the star. As the distance of the near side of the body is significantly different from the back of the body. This pulls it into an egg shape. If WASP 12b is locked into a synchronised orbit, that makes the egg shape quite likely the case.

        I’d agree it is possible the orbit could have decayed over a billion years, but with the mass loss of the exoplanet you would likely counter that. Angular momentum loss by planets (let alone exoplanets) is fairly contentious.
        In the solar system, the planetary orbits and mean distances (‘a’) change over time due to perturbations by other bodies. We don’t know if other bodies exist in the Wasp 12 system, so it quite difficult to draw this assumption. (From what I’ve read on this system I don’t think we can conclude as yet to the stability of the orbit.) Also the eccentricity of the orbit is given as 0.0 (a circular orbit), meaning there is no variability in the orbital distance.

        As for the hot atmosphere to generate a field it is possible. Theoretical fields can be generated that depend on the rotation rate of the object. (We also suspect that such fields are all in contact binaries, where they are also distorted by the ellipsoidal shape of the stars.)

        Its atmosphere would be quite different from the Earth or Venus, as some of the atmosphere is easily ionised by the incoming radiation and probably made worst by being well inside the star’s corona in which the exoplanet is certainly immersed. (It lies 3.43 million kilometres from the heat of the star. The radius of the star at 1.4 solar masses for the spectral type is about 2.5 million kilometres!) Needles to say, an electric field could be easily generated in the atmosphere, and the observed magnetic field is then enhanced by it.

      • Andrew James says

        Just to clarify,

        (It lies 3.43 million kilometres from the heart of the star. The radius of the star at 1.4 solar masses for the spectral type is about 2.5 million kilometres! [A diameter of 5.0 million kilometres])
        So Wasp 12b lies just 900,000 kilometres (3.43-2.5 m. km.) above the photosphere!

      • Torbjorn Larsson OM says

        I’d agree it is possible the orbit could have decayed over a billion years, but with the mass loss of the exoplanet you would likely counter that. Angular momentum loss by planets (let alone exoplanets) is fairly contentious.

        In the solar system, the planetary orbits and mean distances (‘a’) change over time due to perturbations by other bodies. We don’t know if other bodies exist in the Wasp 12 system, so it quite difficult to draw this assumption.

        As I understand the field, scientists were already seeing extensive migration in models of planetary systems dynamics (such as in the Nice model). But observations now points to even larger effects (hot Jupiters; massive eccentricities; et cetera)!

        I don’t think the idea is considered controversial, but the mechanisms. They have to start research (all over) now. I have seen suggestions such as extensive amount of remaining planetesimals, asteroid belts, et cetera, which interaction with planets can bleed of momenta at various times.

      • Hon. Salacious B. Crumb says

        Agreed.
        – Yet three small points. Wasp-12b has an eccentricity of 0.0 (circular). It is better than the Earth’s e=0.2!)
        – Effects of migration are based on perturbations of other planets or bodies. I.e. The Nice model works in the solar system, of say, planetesimals. (See YouTube video on Venus’s Orbit Eccentricity Precession, Last 10 Myr. [http://www.youtube.com/watch?v=p4rOctyc8xU])
        As for Wasp-12b, it is the only other body known in the system. Hence, we cannot say if migration is occurring. (The only way to tell is to look for changes in the nodes of the orbit, which is lost as the orbit is mostly circular.) As the exoplanet website says;

        “Eccentric orbits may occur relatively commonly in extrasolar planetary systems. The second law of thermodynamics suggests that orbits, once scrambled, will remain so. While an eccentric giant planet would certainly induce dynamical dominoes for terrestrial planets, the supposed demise of life may be a circular argument.”

        One planet orbiting a star is unlikely to be “scrambled”, especially when it is a close as Wasp-12b.

        – Also the probability of variances in eccentricity ‘e’ decreases as the semi-major axis ‘a’ decreases. I.e. In the majority eclipsing binaries and spectroscopic binaries have near perfect circular orbits. It is caused by the so-called low-entropy state.

      • Hon. Salacious B. Crumb says

        The earlier quote is from “Circular Orbit: The Planetary Norm?” @ http://exoplanets.org/ecc.html

      • Hon. Salacious B. Crumb says

        @ Torbjorn Larsson
        Discussion on orbit eccentricities see; Peter Goldreich, Re’em Sari; “Eccentricity Evolution for Planets in Gaseous Disks”; http://arxiv.org/abs/astro-ph/0202462
        Whilst not all written here is relevant, the basic principles of low or non-existent eccentricities is well highlighted here.
        Bottom line, I’d expect that the Wasp-12b orbit is likely very stable.

      • Hon. Salacious B. Crumb says

        @ Torbjorn Larsson
        Ah! Now I remember. On the last point…
        The eccentricity versus distance ‘a’ can be seen in the following graph “Eccentricity vs. Semimajor axis for extrasolar planets. The”; http://exoplanets.org/ecc_vs_a_col.html

        This quite clearly shows the orbit is more circular with closer distances.

    • Torbjorn Larsson OM says

      I’m fairly certain that they had to figure in the planet shape in the analysis of the transit. However the differential timing was very extensive (IIRC 4 planetary radius on the UV blockage) so not an atmospheric effect if that is your concern.

  4. Member

    Check – the planet’s body shape is stretched into eggyness, not the orbit. I am only saying this because exoplanet researchers have said it – and it seems plausible.

    The super rotating atmosphere causing magnetic field effect is an interesting idea, really. I only raise Venus as an obvious counter argument – not to debunk this interesting idea. The few planetary magnetic fields we know about tend to arise from spinning dense metallic cores.

    Otherwise… I was saying WASP 12-b – i.e. the exoplanet – expanded when it got closer to the star and hence got hotter. I am only saying this because exoplanet researchers have said it – and it seems plausible.

    A hot Jupiter just should not be able to form this close to a star, since there would not be enough free hydrogen available within the radius of its current orbit. So, regardless of the mechanism (which we can only guess at), it must have migrated in from further out.

    • Andrew James says

      “The few planetary magnetic fields we know about tend to arise from spinning dense metallic cores.”

      This is not likely possible, as the calculate mean density of Wasp 12b is around 0.3. (Unlike the Earth which is 5.52 g.cm^-3.) It is even less dense than Saturn at 0.69 g.cm^-3, and that too has a meagre magnetic field.
      Wasp 12b’s core, if the 0.3 density were true, would be similar to Saturn’s core. The field strength to divert the particles streaming from the star Wasp 12, would not be powerful enough. Logically, something else is likely generating the powerful field. (Hence, why I say it might be the atmosphere interaction.)

      Cheers.

      • gopher65 says

        My guess would be that it’s neither the upper atmosphere nor the core that’s generating the field (if it turns out to be there at all, of course), but rather the “liquid” layers in between. They can be exceptionally dense, and the various layers might not spin at exactly the same rate. I don’t know how it would work, but it seems to me that there is no other possible explanation for where the field is coming from. It *can’t* be the core. At least not by itself.

      • gopher65 says

        Unless of course the magnetic field is being created by the torn off material itself.

      • Andrew James says

        Possible, but quite unlikely. Anything passing outside the Lagrangian point would be just blown away by the stellar wind. The field is more probably being generated by the planet itself.

      • Andrew James says

        There can be no “liquid” layers in between”, as the hydrogen, like the cores of Jupiter and Saturn are already solid metallic hydrogen or as gas. The electrons are free to roam in the dense core at quarter-of-a-million atmospheres, and it is the electron free motion through the metal which generates the field. Also, unlike water, there is no phase transition here under these pressures, so the hydrogen goes from gas to solid.
        We know of few objects that have the combinations solid core, liquid mantles and gaseous atmospheres. (except perhaps Enceladus and Europa, and they are thought to be caused by tidal interactions transferring energy to melt the water into liquid. In Jupiter’s and Saturn’s case, there is only gaseous atmosphere and core — and nothing in between!
        In Wasp-12b’s case, the tear-shaped (egg shaped disk), would mean the hot ‘mantle’ would be subject to similar tidal distortions. If it were true, the energy would be generated within the atmosphere, and that has to be released somehow; most probably by MHD. If it were really “just a dynamic physical effect” (like in Jupiter or Saturn), we would see quite different phenomena. [However, Dr. Aline Vidotto has rejected this, saying it is likely a “magnetic field.”]

        An interesting problem.

        Note: So while the core may or may not generate the field, we still cannot rule it out altogether as not being the true cause. Future theory and observations may tell us more in the future.

      • Torbjorn Larsson OM says

        “Jupiter is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen.” [Wikipedia.]

        I’m not saying that is correct; I’m inferring that there are several models and the likely most dominant _is_ suggesting a (conducting) liquid layer. Or Wikipedia needs your help.

        Also, since it gives the simplest prediction for observed fields and without concomitant problems of outer dynamos (as in MHD effects?), it is nicely tested by the current observation.

      • edg says

        There are a few ways that WASP-12B could have a magnetic field.

        If pressures are high enough internally, it could have liquid metallic hydrogen as a conductor (like Jupiter and Saturn). Convection currents caused by the planet’s rotation and or tidal-flexing (if any) could drive the field.

        If not, it could have (salty?) water in there as a conductor (Uranus and Neptune generate their fields in this layer). Again, convection currents caused by the planet’s rotation and or tidal-flexing (if any) could drive the field.

        Or it could have salty water in there but the field is induced by the planet’s motion through the star’s field (like Europa – though this probably won’t result in a very strong planetary field).

      • Andrew James says

        None of the above would work to generate such a strong field as suggested for Wasp-12b.

        Tidal effects are weak on such field, as the star is not spherical but ellipsoidal. It is more likely atmospheric turbulence or internal mixing is a good possibility, especially at the pointy-end (towards the star of the ellipsoid.

        Water (liquid or ice) in the interior would be unlikely, and would only suggest a field one hundredth the strength of metallic hydrogen.

        Alternatives, not discussed here, is that the chemical composition of these close exoplanets are more like the stars than planets. (Some recent work says the opposite.) Some speculate these could be mergers of W UMa stars, whose result is a close orbiting planetary body. I.e. Martin and Spruit; “Inflated Hot Jupiters from Merger Events” http://arxiv.org/pdf/1102.3336

      • Andrew James says

        @ edg
        All of these scenarios are not really sufficient to generate such fields.
        Convection currents are highly possible as the flow of the atmosphere is more turbulent is the distorted shape of the star
        Some other possibilities, not mentioned here; See “Inflated Hot Jupiters From Merger Events” (arXiv,. 7th March, 2011) http://arxiv.org/pdf/1102.3336
        This says; “However, in these models the higher mass planets that form in the protoplanetary disk have abundances closer to that of the central star.” Others contradict this and say the opposite!

        Clearly more observations of such systems are need to prove or disprove what is happening here.

  5. Lawrence B. Crowell says

    I second Andrew James’ observation. It seem rather fortuitous that we caught this planet within 1/100-th of its end of duration..

    It is likely there is a complex magnetohydrodynamic system of the sort written about with Saturn and Enceladus. This may serve to prevent the stellar wind from battering the planet, but the extreme heat and the tidal bulging probably bleeds off material, As a result this magnetosphere is similar to an accretion disk.

    LC

    • Andrew James says

      Nice idea of the magnetosphere and accretion disk, but its proximity would be greatly torn apart from the corona and strong gravitational field. It would be unlikely to survive. Accretion disks only works between a range of stellar radii. Too close it is killed by the forces of the stellar component, too far way and it has nothing to feed it. Also the Roach lobe being exceeded means that atmosphere is force out by the exoplanet at its Lagrangian point. [I’ll have to read up on some of my Kopal eclipsing binary books for the lower limit, but I’m sure Wasp 12b well exceeds it.]

      I still do suspect something odd is going on in the ‘cool’ (relative to hot stars) atmosphere here that we haven’t experienced before. (it might even be some new science.)

      This system I think will be studied for years to come.

  6. Andrew James says

    @ Steve;
    “Instead, the centrifugal force generated by its rapid orbit just stretches it out into a egg shape.”
    Do you meant the exoplanet orbit or the exoplanet body shape?
    The former cannot be true, as the eccentricity of the orbit is 0.0. I.e. Circular.

  7. Andrew James says

    I just did some Roche Lobe potential calculations on the exoplanet Wasp 12b.
    At the mean diameter of 230,900 km., the ellipsoid is shaped with the polar diameter of 215,492 km. and the equatorial diameter of about 315,140 km.
    The calculated egg shape of Wasp 12b based on the data from Extra Solar Planets Encyclopaedia; Wasp 12b measures from “near side” (facing the star) to centre and “back or far side” to centre is, 172,140 and 143,000 km. and respectively. (A difference of 29,140 km. or about 13%) [According to Errmann below, “Li et al. (2010) predict that the planet may be losing mass at a rate of ?10^?7 MJ yr^?1 by exceeding its Roche lobe” This article then asserts the ellipsoidal nature of the planet. They also say that the oblateness ‘f’ is (f=1-(r1/r2)) is 0.083, equal to about Saturn; based on the stellar mass of Wasp 12 as 1.35±0.14 solar masses (2009) They do not, however, discuss the nature of the ellipsoid, which can be easily interpreted from our knowledge of eclipsing binaries.]
    We can conclude, Wasp 12 is certainly grossly distorted by the proximity to the star, Wasp 12. (The stellar diameter is not significantly influenced, being 1050 times more massive.)

    Also according to Errmann, R., et.al., High-precision photometry of WASP-12 b transits
    “The central star is found to be a late-F type dwarf whose age is estimated to be 2±1 Gyr.”
    This is twice my earlier estimate, so it is possible the star Wasp 12 has just begun to move for the main sequence. Hence, the star will swell and consume Wasp 12b fairly quickly. (This may explain the recent behaviour of mass loss rather than a decaying orbit.)
    This same paper say the surface temperature is 2516±36K, making this as hot as the surface temperature of a late M-type supergiant, with Wasp 12’s temperature being 6300K.

    Hope this additional information helps.

  8. Member

    One more shot at the tidally locked – no spin issue. The good Dr Plait has a nicely balanced view on the issue:
    http://www.badastronomy.com/bad/misc/moon_spin.html

    So, sure you can find a frame of reference where the Moon rotates on its axis, but the statement has no useful meaning unless you define the point of reference you are observing from. You are equally correct to say that from a point of reference on the Earth’s surface, the Moon does not rotate on its axis.

    There is a need for plain English terminology which can differentially define an object with intrinsic spin versus a tidally locked object – without inciting a nerd duel 🙂

    • Andrew James says

      God I dislike this edg dude…
      Why is it, when you try to have an intelligent discussion about an interesting story that has some real puzzeling issues, then some less intelligent nitwit feels it is perfectly OK to sidetrack the whole story just to drag everyone to his own dumb level?
      Meanwhile you are supposed to “be nice”, don’t present personal theories, stick to the topic, ignore all the trolling ratbags that you really want to strangle; then have to pretend that everything is A OK while they openly slap you around with little diversionary tactics to waste yours and everyone else time!

      In the end it is not semantics nor “plain English terminology” at all, it is just playing all of us as utter fools. Please Steve, now and in the future, just start deleting all the really crazy comments and keep the relevant ones. Thankx.

      • Member

        Deletion is a last resort. I learnt a bit from this anyway. It’s all good.

      • edg says

        Andrew – If you want to keep a discussion civilised, you can stop calling people names. I’ve not insulted anyone here at all, and yet in two posts you’ve insulted me and made unwarranted personal attacks against me because you don’t agree with what I said when I’m trying to educate people about how tidal locking really works?

        And people wonder why science education is in such a bad state. If people who try to educate others are called names for correcting people’s mistakes, or insulted for their efforts then who can blame them for giving up?

      • edg says

        And yes, everything I say here does agree with Phil’s article.

        This all arose because Steve claimed that “I don’t think many people would agree with the statement that (for example) the Moon spins on its axis”. All I did was point out why that statement was incorrect (as demonstrated by myself earlier and in Phil’s article) – it doesn’t *appear* to spin on its axis when viewed from Earth, but it actually must be in order to keep the same face towards Earth.

      • Andrew James says

        Enough. No one cares.
        Make your point, then go away. Please!!!!!!!!!!!!!!

      • edg says

        I’ll tell you what, Andrew. If you’re ever appointed Moderator of the comments section, then I’ll do what you say. Otherwise… no. I’ll make my comments as necessary, and if you don’t like them, then you can always choose not to read them.

        The discussion between Steve and myself was pretty rational, and caused largely by miscommunication (now resolved). As far as I can see, the only person shrieking about anything and “not caring” and insulting anyone here is *you*. I would suggest that you step back and calm down, beceause you’re really not doing yourself any favours with this behaviour.

        I’m sure you’ll throw another insult my way instead though.

      • Andrew James says

        Two choices. Talk about Wasp 12 or Wasp 12b or just move on.
        Argue with yourself, if you must, but really, it is just not my problem.

    • Member
      IVAN3MAN_AT_LARGE says

      O.K., Steve, no hard feelings, then? 🙂

  9. Andrew James says

    To summarise. All of these scenarios of field generation are not really sufficient to generate such magnetic fields and bow shocks.
    Convection currents are highly possible as the flow of the atmosphere is more turbulent is the distorted shape of the star.
    Some other possibilities, not mentioned here; See “Inflated Hot Jupiters From Merger Events” (arXiv,. 7th March, 2011) http://arxiv.org/pdf/1102.3336
    This says; “However, in these models the higher mass planets that form in the protoplanetary disk have abundances closer to that of the central star.” Others contradict this and say the opposite!
    Another theory is these amazing hot exoplanets are a consequence of stellar mergers of W UMa eclipsing variables.
    Clearly more observations of such systems are need to prove or disprove what is really happening here.

Comments are closed.