July 29, 2004 | David F. Coppedge

Solar Systems Defy Theories

Stuart Ross Taylor (Australian National University, Canberra) feels left behind.  The astronomers have their nice, neat H-R diagrams to explain stars, but no such diagram exists for planetary scientists.  Our hodgepodge collection of planets, moons and small bodies defies classification, to say nothing of the extrasolar planets that have been discovered so far, mostly in wild elliptical orbits or close-in to the parent stars.  Writing in the July 29 issue of Nature,1 he compares stellar and planetary astronomy:

By contrast, planets are individuals that show few systematic relationships and have resisted attempts at classification or even definition, as witnessed in the furore over the status of Pluto, which is an eccentric dwarf when placed among the planets, but is better suited to be the king of the many icy bodies in the Kuiper belt.  So far, there is no planetary equivalent of the Hertzsprung-Russell diagram.  Even if we arrive at a satisfactory explanation for the formation and evolution of our planetary system, there is no guarantee that this will apply elsewhere.  Perhaps this is the reason, as Stephen Brush has commented, that the origin of the Solar System represents one of the oldest unsolved problems in science.

Taylor says this in spite of mentioning in the first paragraph that Laplace in 1796 had explained the planets as condensing from a solar nebula.  As an example of the difficulties in explaining the origin of planets, he points to the long history of trying to explain Earth, from Hutton (1788) to plate tectonics.  But even today’s theory of plate tectonics may be too specialized to apply anywhere else:

But this process is unique to the Earth among the planets of the Solar System and was only made possible by the late stochastic addition of a water content of a few hundred parts per million.  Many of the difficulties in trying to understand the evolution of the Moon arose from the uncritical attempts to apply our hard-won experience with wetter terrestrial rocks to those from our bone-dry satellite.

Another example of the difficulty is comparing Venus and Earth.  They should be twins, but “the Earth resembles Venus much as Dr Jekyll resembled Mr Hyde.”  What causes the difference between these twins?  “The short answer is water,” he gives as if a rote answer out of the textbook, but “As we search for terrestrial-like planets elsewhere, we need to find out the reasons for these differences and the conditions that allow these diverse bodies to form at all.”  That can only come from a new interdisciplinary approach, a distinct new mindset “somewhere between the approaches of astronomers – who want to treat planets mathematically like stars – and geologists, who want to generalize from their detective-like experience with the Earth.”


1Stuart Ross Taylor, “Why can’t planets be like stars?” Nature 430, 509 (29 July 2004); doi:10.1038/430509a.

The fact that Taylor raises these questions means that the typical rote answers given simplistically in textbooks are wrong.  He sounds like astronomers are at square one explaining the planets, despite 208 years since Laplace famously remarked, when asked where God fit into his model, “I have no need of that hypothesis.”  Well, put up or shut up.  We’re still waiting.  We know a lot of things now that Laplace did not, and the trend of recent discoveries has been contrary to the expectations of nebular and planetesimal theorists, so much so that Hal Levison called his own theory a fairy tale, and others crazy (see 05/30/2002 headline).  His subsequent suggestion was the only wise thing he said: “We have to start thinking of alternatives.  Probably there’s a method for their formation that no one has even thought of yet.”  Well, some have, but their views are categorically disallowed by the reigning naturalistic paradigm.
    We hate to break it to Taylor, but the stellar astronomers don’t have everything so neatly tied up as he supposes, either (see 07/08/2004 headline).  In fact, does any naturalist have anything neatly tied up? (See next two headlines.)

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Categories: Astronomy, Solar System

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