June 9, 2009 | David F. Coppedge

Planets Can’t Grow Past the Electric Fence

In the artwork, it looks so simple: dust clumps into planets that grow into nice, orbiting solar systems – like ours.  It’s not so simple when you try to nail down the real physics.  Planet-building models have to contend with a host of variables and barriers to growth (accretion).  Another barrier was discussed in Astrophysical Journal this month:1 the electric barrier.
    Satoshi Okuzumi (Kyoto University) tackled the problem of static electricity among particles trying to accrete into planets.  He specifically worked on the earliest stages, when particles are just microns to millimeters in size.  What happens when cosmic rays add static electricity to the clumps?  Astronomers have been working on planet-building for decades, so it’s surprising so little attention has been paid to this question.  “Although the importance of dust charging is well recognized in the above context, its effect on dust coagulation in protoplanetary disks has been hardly examined.
    What happens is bad news.  The static charge builds up and forms a barrier that repels particles from sticking to each other.  After a lot of math and analysis, he concluded that the particles can’t grow any further.  They “freeze out” and planetesimal growth stops.  Now what?
    Okuzumi suggested that turbulence can overcome the electrostatic barrier.  But this brings more bad news.  For one, the expected turbulence only occurs at 20AU, about the radial distance of Uranus.  So much for Earth and its lovely neighbors.  For another, the turbulence makes the collision rate over three times higher than the threshold past which destruction overtakes accretion.  That won’t work, either.  Here are his two major findings: 

  1. For a wide range of model parameters, the effective cross section for the mutual collision of aggregates is quickly suppressed as the fractal growth proceeds and finally vanishes at a certain aggregate size (Sections 3.2.1 and 3.2.2).  This is due to the strong electrostatic repulsion between aggregates charging negatively on average, and happens much before the collisional compression of aggregates becomes effective.  Both the charge fluctuation and the thermal velocity fluctuation do not help the aggregates to overcome the growth barrier.  Without strong turbulence, the quasi-monodisperse fractal growth is very likely to “freezeout” on its way to the subsequent growth stage.
  2.   Strong … turbulence will help the aggregates to overcome the above growth barrier (Section 3.2.3).  However, such turbulence is likely to occur only in MRI-active regions, i.e., at outer disk radii or high altitudes (Section 4.3).  Furthermore, it will cause another serious problem—the catastrophic disruption of collided aggregates—in later stages.  These facts suggest that the combination of electric repulsion and collisional disruption may strictly limit the collisional growth of dust aggregates in protoplanetary disks.

That’s what the abstract had warned: “These facts suggest that the combination of electric repulsion and collisional fragmentation would impose a serious limitation on dust growth in protoplanetary disks.
    One doesn’t like to end on a negative note, so he added a final section, “A possible scenario to overcome the electric growth barrier.”  But his “scenario” stretches credibility.  Realizing he had only considered a narrow size distribution of initial particles, he said, “In fact, there may exist some aggregates considerably larger than average-sized ones.”  Sure enough, if you start with the assumption that there were some big clumps to begin with, those might survive the charge barrier and the collision barrier, and continue on growing into planets.  He’s going to work on that problem next.  But wasn’t the origin of the large particles the very problem he was trying to solve? 


1.  Satoshi Okuzumi, “Electric Charging of Dust Aggregates and Its Effect on Dust Coagulation in Interplanetary Disks,” The Astrophysical Journal 698 (2009) 1122, published May 27, 2009; doi:10.1088/0004-637X/698/2/1122.

If you’re not intimidated by equations and jargon, this paper is funny.  It’s another “assume a can opener” joke.  Here he was all set out to tell us how tiny dust particles grow into planets, and in the end, he had to assume that large particles already existed.  Get a charge out of that.  Isn’t that what he does, right here?  “Let us consider a small population of irregularly large aggregates (referred to as “test aggregates”) growing with a large population of standard (D ~ 2) fractal aggregates (“field aggregates”).  Under this assumption, the kinetic energy of relative motion between test and field aggregates is written as … ” and off he goes, assuming the existence of the very thing he was trying to prove.  Some whiz-bang math later, he says it again without apology: “Therefore, if there exists an aggregate that is large and compact, it will be able to continue growing by sweeping up smaller “frozen” aggregates.”  Good grief, that is hilarious.  He followed it with a brief return back to reality: “In any case, we expect that the effect of dust charging should qualitatively modify the current scenario of dust growth in protoplanetary disks.”  That’s the evolutionist’s way; a solution exists! – in the future.  “Quick!  Can anybody else concoct a better just-so scenario?”
    Even in the most charitable description of this paper, he has created more problems than he has solved.  Now, the number of lucky dust particles hoping to become lucky mud some day has dwindled considerably.  It’s better to hear true bad news than flawed good news.
    But what about the evolutionist who argues, “We may have problems in our models, but we know planets exist, so they had to form somehow.  At least this guy was using SCIENCE to try to figure out how they came about.”  This is the error of begging the question.  It’s not a matter of science; it’s a matter of assumptions about initial conditions.  Why not assume planets were created?  The disruption of planets into smaller pieces fits the observations and the second law of thermodynamics (the best-attested law in all nature).  Why must we begin with the metaphysical assumption that things always grow from small to complex, and not assume the reverse?  Why not model things from the top down?  Why must we assume naturalism?  It’s not clear that bumping one’s head against the wall of naturalism is healthy, when more elegant solutions exist.

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Categories: Physics, Solar System

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