Planet-Making a Lost Art
Exclusive Solar system theorists are trying to reverse engineer the planets without the recipe. Planets exist, but they can’t get from a rotating disk of dust and gas to a solar system from their models. They are at a loss to explain Jupiter, Saturn, Uranus, Neptune and a host of Jupiter-class planets around other stars.
A press release from Astronomy & Astrophysics explains some of the problems. Two British astronomers found a show-stopper in their models: any hopeful clumps tend to march in lockstep to their deaths in the center, like lumps of oatmeal washing down the drain before they can solidify. This is called “Type I migration” – the viscosity of the stellar disk carries material inward like a spiral conveyor belt. The migration is so rapid (a few thousand years), there is simply not time for a gas giant to form by core accretion. (If the planet is able to open a gap in the disk, a more benign “Type II” migration still keeps it moving inward, but more slowly.)
Dr. Alan Boss (Carnegie Institute of Washington) shared some of his “heretical” views at a presentation March 21 to scientists and engineers at the Jet Propulsion Laboratory. He listed many cons outweighing the pros of the core accretion theory. Core accretion was the leading model dating back to Laplace’s original Nebular Hypothesis, until in the 1990s the problem of migration came to light. The problem was exacerbated by the discovery of “hot Jupiters” around other stars – gas giants closer to their parent stars than Mercury is to the sun. Earlier theory prohibited gas giants from forming so close. Also, many red dwarf stars have been found to have Jupiter-size planets, contrary to predictions. Gas giants seem to form regardless of the metallicity of the star (i.e., the proportion of elements heavier than hydrogen and helium). Furthermore, our Saturn appears to have a much larger core than Jupiter, when the reverse should be true.
While core accretion is a bottom-up hypothesis, there is an alternative: a top-down approach. Dr. Boss presented his newer “disk instability” model (the heretical one), not so much to pit it against core accretion (the conventional one), but to pit both models against the observations. Both leave many problems unsolved. For instance, while disk instability overcomes some of core accretion’s defects, it adds new problems. In the model, eddies in the rotating stellar disk collapse quickly into clumps. It is not clear, however, that a clump will survive and continue to shrink into a planet. Also, the gas giants need to form closer in than expected, then get kicked outward, to account for Jupiter and Saturn’s radial distances. This means larger clumps must form to take the low road into the star while ejecting the others to the high road. This process, however, would spell death for any incipient rocky planets like Mercury, Venus, Earth and Mars.
Disk instability was invented primarily to try to save the timescale, he said. No one realized how quickly migration would carry a planet in: an upper limit is 10,000 years, when core accretion is assumed to require millions of years. Modeling planet formation under these constraints is tricky. Models vary in one parameter by five orders of magnitude. Boss cited a model that simply ignored Type I migration, and another that artificially set the viscosity very high. That one got a Jupiter, but no Saturn. Also, most modelers ignore the situation in most gas nebulas like Orion and Eta Carina, where photoevaporation from high-mass stars blows away the gas in stellar disks quickly. Boss’s model gets a Jupiter in about 245 years, but that’s only in the computer. Eventually the models need to account for the highly diverse and anomalous extrasolar planets – currently 150 and counting – being found around other stars, to say nothing of those in our own solar neighborhood. “Eventually, observers will tell us what the answer is,” he ended.
Footnote: Dr. Boss mentioned several times that core accretion is only a problem with gas giants; he claimed it worked well with rocky terrestrial planets like Earth. In the Q&A session, however, he did admit that there is a gap in our understanding of how the initial particles begin to accrete. Bodies need to reach at least 10 meters before gravitation can pull in more material. He referred to studies performed in space demonstrate that dust grains moving with slow relative velocities in a vacuum will clump into filaments and irregular clumps he called “dust bunnies,” but after they get to a certain size, they begin to impact one another too fast for further accretion to occur. At that stage, more material is lost than accreted. So he confessed there is a question mark between the dust-bunny stage and the 10-meter stage. Also, he said there are problems in the outer disk. While accounting for Kuiper Belt objects was theoretically not too difficult, he asked, “Can you really explain the Oort Cloud?”
Alan Boss was fairly frank about the problems and difficulties, but his thinking is enslaved to a larger molecules-to-man world view that assumes everything from the big bang to man can be explained with references to natural causes alone. The possibility that planets were designed and created is utterly alien to their thinking. The film and book The Privileged Planet should challenge these modelers with stringent reality checks on the ability of natural processes alone to account for Earth, for the solar system that protects life on our planet, and for the galactic and cosmic systems of which our planet is a part. Creationists, on the other hand, need to do serious thinking also in the light of the discoveries of extrasolar planets. Are all planets equally designed, even for stellar systems devoid of life? If not, did natural processes form them? If so, how does one differentiate the need for design in our solar system? Intelligent design theorists argue that it is not necessary to claim everything is designed to make the case against materialism. To show that some things cannot be explained with reference to natural causes, and that it is possible to discriminate design from chance and natural law, is sufficient to establish ID. Such questions may forever remain outside the purview of scientific investigation and remain debate topics for philosophers and theologians. As shown by today’s story, with more anomalies than successes, materialists are in no position to claim the upper hand. In the meantime, all players can benefit from more observations.