August 31, 2010 | David F. Coppedge

Exoplanet Hunters Fail Predictions

Before the first extrasolar planets were discovered, astronomers had high confidence that other solar systems would resemble ours.  We have rocky planets close to the sun, and gas giants farther out.  Planetary scientists were pretty sure the pattern would hold up around other stars.  Now that we have hundreds of examples to compare, the reality has been far different from expectations.  The number of surprises in real exoplanet systems underscores the potential flaws in building models based on a sample size of one.
    In Caltech’s latest Engineering and Science magazine,1 John Johnson was interviewed about the state of extrasolar planet hunting.  Johnson has been involved with leading planet-hunting pioneers.  A recurring theme in the interview is the surprise that planetary systems were found to be radically different from predictions.

What are some of the current big questions that you guys are trying to tackle?
We’re interested in how the solar system formed.  We’re interested in our immediate environment and describing its origins.  And beyond that, we’re interested in general in how planetary systems formed.  There are some very specific questions that arise at every turn.  There are so many surprises in this field—almost nothing is turning out as we expected.  There are Jupiter-mass planets in three-day orbits.  There are planets with masses that are between those of the terrestrial planets in our solar system and the gas giants in the outer part of our solar system.  There are Jupiter-mass planets with hugely inflated radii—at densities far lower than what we thought were possible for a gas-giant planet.  There are giant planets with gigantic solid cores that defy models of planet formation, which say there shouldn’t be enough solids available in a protoplanetary disk to form a planet that dense.  There are planets with tilted orbits.  There are planets that orbit the poles of their stars, in so-called circumpolar orbits.  There are planets that orbit retrograde—that is, they orbit in the opposite direction of their star’s rotation.  There are systems of planets that are in configurations that are hard to describe given our understanding of planet formation.  For instance, some planets are much too close to one another.
But a lot of those surprises have to do with the fact that we have only one example of a planetary system—our solar system—to base everything on, right?
What’s interesting is that we’ve found very little that resembles our example.

Johnson went on to say that the leading theory of planetary migration to explain how the so-called hot Jupiters get so close to their star has “gone into the dustbin” now that so many inclined and retrograde examples have been found.  “We’re scrambling to find a new way of describing how these gas giants can move in that also causes their orbits to be tilted,” he added.
    Although Johnson reaffirmed the old Laplace nebular hypothesis with a “2.0” upgrade, the number of “wacky” things his team has discovered belies any attestation of confidence.  “We’re going out into the solar neighborhood, where there are things that we thought were just familiar, things that we thought we understood,” he said.  “But just the wackiest stuff comes up—and it’s sure keeping me busy.”  He compared it to going on safari and discovering a blue lion.  “That might be the level of wackiness I would attach to it.


1.  Marcus Y. Woo, “Discovering New Worlds,” Engineering & Science, Volume LXXIII, Number 3, 2010, pp. 18-23.

He didn’t really find a blue lion.  He found a natural lion, but the funny glasses he was wearing made it look blue.
    Johnson went on to describe how Stephen Hawking’s book A Brief History of Time had made a profound influence on him.  He also affirmed at the end that he thought humans would figure out that their place in the universe is insignificant, following the theme of the positivists and Carl Sagan: “We are coming out of the darkness from a couple hundred years ago and we’re rubbing our eyes today, realizing that we are on a really small planet around a really average star in an unspectacular part of the galaxy, and we’re learning our place in this whole universe,” he said.  “Once we find more planets like our own, it’ll further define our place and give us a better universal context for what it means to be human.”
    This kind of bluffing means Johnson has been a good apprentice.  His mentor Hawking was similarly prone to wild speculation without evidence, pontificating as he did in his book about how close humans were to finding a “theory of everything” when in fact he could point to little more hard evidence than mathematical speculations whirring about in his nimble imagination.  If anyone in economics or sportscasting had this bad a track record of predictions, though, they would be out of a job.  Cosmology is one of the many evolutionary sciences where you can brag about how wrong you have been, and people will still think you are wonderful because you are busy stamp collecting.

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