January 4, 2008 | David F. Coppedge

New Planet, or Dusty Brown Dwarf?

A planet has been found associated with a dusty disk, reported National Geographic News and Astrobiology.com.  This is “one of the most exciting discoveries in the study of extrasolar planets,” a Max Planck Institute researcher said, because they “have directly proven” that planets form from dust disks.  Moreover, they must form rapidly, because the planet cannot be older than its parent star, which is much younger – 8 to 10 million years – than other extrasolar planets found so far, they said.
    Previous estimates called for a hundred million of years or more for a planet to form.  Earth is thought to be 4.5 billion years old, with multicellular life appearing only in the last 0.5 billion.  This star, TW Hydrae, is 1/500th the age of the sun according to the scientists.  Dust disks are thought to evaporate within 30 million years, so it was reassuring for them to find a planet forming within the time limit.
    The NG News article mentioned some cautions.  For one thing, the giant planet is very close to the star and revolves around it every 3.5 Earth-days, with a mysterious disruption in the motion every nine days.  Also, the mass of the planet is in doubt.  Jack Lissauer (NASA-Ames) appreciated the discovery.  “However,” he said, “I do think that the authors have substantially underestimated the uncertainties in the mass of the object.”  If it is as big as a brown dwarf, that makes the combination a binary star system – a very common occurrence among stars.

The astronomers have merely associated a dust disk with an orbiting object that is not observed but only inferred via wobbles in the parent star (circumstantial evidence).  Unless they explain how dust particles accrete into large bodies (a major problem in planetary physics; see 12/05/2007), they have not proved that this object emerged from the disk.  Perhaps it did, but the observations do not create an open-and-shut case.
    Astronomers do not know how stars form; they do not know how planets form (see 07/15/2005 entry and its embedded links).  They have not, therefore “directly proven” that planets form from dust disks.  Distinctions are important in science.  Reporters often charge into conclusions without proper warrant.  The astronomers’ claims may be plausible.  They might even be true.  Hypothesis, though, is not confirmation.  Let him who puts his armor on not boast like the one who takes it off.
    Alert readers must constantly beware of claims that go far beyond the evidence: the star “is” 8-10 million years old (no human observers watched it for that long).  The planet “is” about ten times as massive as Jupiter (it could be far smaller or far bigger).  “Its host star is still surrounded by the disk of gas and dust from which it was only recently born” (theories of planet birth are full of problems).  “This discovery allows scientists to draw important conclusions about the timing of planet formation”  (the conclusions are based on assumptions about the timing – circular reasoning).  “Finally, perhaps in the future we will be able to answer the question: Are we alone in the Universe?”  (How did they get there from a blip on a graph?)

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