April 14, 2010 | David F. Coppedge

Conflicting Reports About Earthlike Planets

Are earth-like planets rare or common?  Your opinion might depend on which news stories you read.  “Polluted Old Stars Suggest Earth-like Worlds May Be Common,” reported Space.com and Science Daily.  The idea is that hydrogen in the atmospheres of white dwarfs might have come from water, which might be remains of rocky planets that got swallowed up before the old stars died.  That’s a lot of speculation riding on a series of maybes.
    PhysOrg, by contrast, had pessimistic news for earth-like planet hunters today.  A study announced at the Royal Astronomical Society turns “planetary theory upside down” by showing that a significant fraction of extrasolar planets known as hot Jupiters orbit retrograde and at high inclinations.  Theoretically, planets should orbit prograde with low inclinations from the dust disks in which they formed.  How they got into their weird orbits is a huge puzzle.  Six out of 27 studied were going backwards.  The findings “challenge conventional wisdom” about planet formation.  They also dim prospects for earth-like planets, because large gas giants in such unusual orbits would most likely perturb small rocky planets from their habitable zones:

To account for the new retrograde exoplanets an alternative migration theory suggests that the proximity of hot Jupiters to their stars is not due to interactions with the dust disc at all, but to a slower evolution process involving a gravitational tug-of-war with more distant planetary or stellar companions over hundreds of millions of years.  After these disturbances have bounced a giant exoplanet into a tilted and elongated orbit it would suffer tidal friction, losing energy every time it swung close to the star.  It would eventually become parked in a near circular, but randomly tilted, orbit close to the star.  “A dramatic side-effect of this process is that it would wipe out any other smaller Earth-like planet in these systems,” says Didier Queloz of Geneva Observatory.

Only two of the six have a nearby companion that might qualify as perturbation sources.  The addition of gravitational influences complicates an already problematic theory of planet formation.  National Geographic has diagrams of the weird orbits of the newly-discovered planets.

These reports illustrate how little scientists know about planet formation, and how much speculation rides on flimsy data.

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Categories: Physics, Solar System

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