July 21, 2007 | David F. Coppedge

We Live in a Rare Solar System

Surveys of extrasolar planets are making our solar system look unusual.  Most stars that host a family of planets have the gas giants close in, an article on Space.com states.  The “hot Jupiters” seen around many stars would most likely eject any rocky planets from the habitable zone.  “Of the nearly 250 planets discovered so far outside our solar system, most are gas giants that orbit extremely close to their stars.”
    The observations may be a selection effect.  It’s easier to detect hot Jupiters than distant ones.  A team from University of Arizona looked for gas giants at the 10 AU range, assuming that “young” gas giants would be brighter.  They found none around 54 nearby stars.  This could mean that gas giants at large radial distances are too faint to detect, or that they are rare.
    Current theory also predicts that gas giants would be less common the farther away from the star.  “The two leading theories about how planets form—core accretion and disk instability—have problems making gas giants out at distances beyond 20 AU.”  In our solar system, Jupiter is at about 5 AU, Saturn at 10, Uranus at 20, and Neptune at 30.  This arrangement allows a suite of rocky planets (Mercury, Venus, Earth, and Mars) to occupy stable orbits closer in to the sun.  The Earth-Moon system occupies the narrow “sweet spot” called the continuously habitable zone.
    A more speculative article on Space.com claimed that signatures of heavy elements on some stars might indicate that planets had fallen in.  The article claims that this might support the “disk instability” theory which suggests planets form rapidly from knots in the debris disk.  These inferences were only made indirectly from spectra, however, not from actual observations of planets impacting their host stars.  But if validated, it would indicate another hazard in making solar systems: keeping a planetary system in orbit safely out of reach of the planet-eating monster.

How many AU is the Earth from the sun?  Exactly one!  That’s a silly question, because the astronomical unit (AU) is defined in terms of the sun-Earth distance.  Impress your gullible friends with this coincidence.
    Though insufficient as a standalone piece of evidence, this article’s claim adds to the growing realization among astronomers that our solar system is special.  When you add in all the other independent evidences as described in The Privileged Planet, the cumulative case becomes convincing.

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Categories: Solar System

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