May 13, 2005 | David F. Coppedge

How Privileged Is Our Planet?

Several recent news stories touch on the uniqueness of Earth.

  • Eccentric neighbors:  A story in Science Daily draws attention to the highly eccentric orbits of most extrasolar planets found so far.  The press release from Northwestern University begins, “Except for the fact that we call it home, for centuries astronomers didn’t have any particular reason to believe that our solar system was anything special in the universe.  But, beginning with the discovery 10 years ago of the first planet outside our solar system, evidence suggests that, as far as planetary systems go, the solar system might be special indeed.”  The point is that, because gravitational interactions between planets often slingshot them into extremely elliptical orbits or even eject the smaller ones away, having a solar system with planets mostly in circular orbits is a rare thing.  One scientist in the article stated, “This is what makes the system so peculiar.  Ordinarily, the gravitational coupling between two elliptic orbits would never make one go back to a nearly perfect circle.  A circle is very special.”  The long-term stability of our system is also “rather peculiar,” he said.  A similar story is found on  A related article on reviews what is known about extrasolar planets after ten years of cataloging them.
  • Ionic Breeze:  A press release from Purdue University proposes that our atmosphere may be more effective at removing pollutants than previously thought.  Some naturally-occurring chemicals can react with sunlight to produce ions like OH radicals that can scrub the atmosphere of smog.  Unlike Titan, the Earth may possess a self-cleaning atmosphere.
  • Heaven or Hell?  Two articles seem to contradict each other about the early Earth.  Bjorn Carey on MSNBC News suggests that the “Early earth [was] not so hellish” as previously thought.  But then Robert Roy Britt on surmises that “The Sun might have had an incredibly violent youth in which tremendous X-ray flares battered the Earth into being, a new survey suggests” – yet somehow these favored the formation of rocky planets.  A short piece on Science Daily agrees that super-flares from the early sun might have helped form our rocky planet.  Based on Chandra X-ray Observatory data, scientists propose that “The bigger flares would prevent Earth-like planets from plummeting into the star, while such a planet would likely be drawn into a star with smaller flares and disintegrated.”

For comparison, we can now look at Titan.  This week in Science,1 the first detailed research papers from Cassini relate findings that, compared to our planet, look both familiar and bizarre.  No life has been found there, of course.

1Brooks Hanson, “First of Many Returns,” (introduction to seven research papers on Titan), Science, Vol 308, Issue 5724, 968 , 13 May 2005, [DOI: 10.1126/science.308.5724.968].

Nothing in any of the observations suggest life is common on other planets around other stars, but rather the opposite.  In the Q&A of the film The Privileged Planet, Guillermo Gonzalez asked how his hypothesis could be tested.  If the observations continue to support the connection between habitability and discoverability, he said, it would gain strength, and if not, it would lose credibility.  So far it’s looking pretty good.

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Categories: Solar System

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