May 24, 2011 | David F. Coppedge

Earth Still Privileged Planet

Astronomers have found over a thousand extrasolar planets now.  How does our solar system compare?  Thanks to the Kepler spacecraft, we now have a catalog of 1,235 alien planet candidates after just four months of operation.  Of the 408 that have been found in multiple-planet systems, 170 of these containing two to six planets have been pictured in a “Kepler Orrery” posted by the Harvard-Smithsonian Center for Astrophysics.  The press release says, “most of those look very different than our solar system” (see also 05/21/2011, bullet 2).
    The poster is accompanied by an animated version that shows the 170 systems revolving like gears (see also PhysOrg).  Due to selection effects of the transiting method, Kepler has tended to find systems with low inclinations.  These have planets smaller than Neptune, because large gas giants can perturb the orbits of member planets into higher inclinations.  The Kepler team was surprised to find so many multiple-planet systems in their quarry: over 100, when only two or three were expected.  It is still too early, though, to detect earth-mass planets within their stars’ habitable zones.
    How do planets form?  The astronomical community has undergone a paradigm revolution in the last decade about planet formation (05/07/2001).  Ever since Laplace, astronomers have assumed that disks of dust and gas will slowly condense into planets (the nebular hypothesis).  The discovery of “hot Jupiters” (gas giants orbiting extremely close to their parent stars) was shocking.  It indicated that planets migrate inward and will quickly be destroyed unless they can form faster than the core accretion model permits.
    This was a factor contributing to a newer “disk instability” model that posits clumps within the disk condensing rapidly into planets – a “heretical” view when first proposed (06/03/2003, 03/21/2006).  All such models have problems of their own, however (09/22/2003, 08/27/2004, 08/06/2004, 07/15/2005).  In either case, an upper limit typically given for planet formation has been ten million years or less to avoid the death spiral.
    Ten million years now appears too long.  Observations of IC 348, a cluster of stars thought to be two or three million years old, shows that the dust is rapidly depleting in nine disks detected.  Universe Today interpreted what this means: “If planets are forming in IC 348 at the same frequency in which they form in systems astronomers have observed elsewhere, this would seem to suggest that the gravitational collapse model is more likely to be correct since it doesn’t leave a large window in which forming planets could accrete.  If the core accretion model is correct, then planetary formation must have begun very quickly.”  Join Voisey’s headline for the story was, “Want to Make Planets?  Better Hurry.”  The 05/07/2001, 06/03/2003 and 05/21/2009 entries show this has been known to be a problem for at least a decade.
    New Scientist commented on the Kepler results.  “Exoplanet systems around other stars are surprisingly flat compared with our own,” reporter Jeff Hecht wrote.  “The discovery means that the solar system must have had a far more colourful history than many of its counterparts and is forcing astronomers to rethink their ideas about the way planetary systems form.”
    Another factor essential to life on earth is its magnetic field.  Space.com posted an image of how earth’s magnetic field would look from space.  “This view is conceptual, but based on real science observations that have been made since the beginning of the Space Age,” the article said.  Without a magnetic field, life would be bombarded from “blasts and harmful radiation from the sun.”
    In combination with earth’s atmosphere and ozone layer, the transparent atmosphere lets in the benevolent wavelengths for photosynthesis and vision.  See the film The Privileged Planet on Illustra Media’s YouTube channel for numerous other factors that combine on earth to make it not only habitable but an ideal platform for scientific discovery.

Take the time to re-read the commentaries from our earlier entries linked above (if only time for one, try 05/21/2009).  There’s not much more to say that’s new, other than that astronomers are no nearer a solution to their problems than they were in 2001, 2003, 2005, 2006, and 2009, even with the latest Kepler Spacecraft observations.  What does this say about their epistemic status?  Maybe they should go back and reconsider what made Johannes Kepler so delighted with his discoveries.

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