Making Earths the Natural Way

Creating a solar system is as easy as spinning a dust cloud around a star.  Before long, rocky orbs will emerge from the dust as platforms on which life can evolve.  Is it that simple?  We know now that planets surround a number of other stars – perhaps most of them.  Textbooks and artwork make the process seem as natural as add dust and stir, but real world planetary scientists have some challenges to work out.

1. Light shields:  EurekAlert reported earlier this month problems with oxygen.  Ratios of oxygen isotopes in a meteorite are very different from those in all other solar system bodies, including the Earth, moon and Mars.  A leading theory that UV photoshielding would yield the anomalous ratios was tested and found to be wrong.  Did a nearby supernova seed the early nebula with the isotopes?  That’s too unlikely and ad hoc an explanation for most scientists.  One other theory is being tested, but the article was titled, “Theory of the sun’s role in formation of the solar system questioned.”
       Science Daily provided more detail.  One researcher for the Genesis mission, that collected samples of the solar wind, commented, “You can see the ratios of the isotopes brought back by Genesis, but that doesn’t tell you how they came about.  The isotope ratios themselves don’t tell you why they were different in the early universe than they are today, so there’s lots more science to do in the laboratory.”
2. Comet upsets:  Astrobiology Magazine reported on the surprise discovery that comets are not the pristine objects from the fringes of the solar system as was long thought (12/27/2007, 01/25/2008).  “Observations from this sample are changing our previous thinking and expectations about how the solar system formed,” a Stardust mission researcher said.  Models now have to worry about how material can migrate radially across the disk.  “This really complicates our simple view of the early solar system,” said another.  The apparent mixing of material near and far from the sun is “causing a revision of theories of the history of the solar system.”
       Science Daily agreed.  “Chemical clues from a comet’s halo are challenging common views about the history and evolution of the solar system and showing it may be more mixed-up than previously thought,” the subtitle read.  A Stardust team member explained, “They were originally hoping to find the raw material that pre-dated the solar system.  However, we found many crystalline objects that resemble flash-heated particles found in meteorites from asteroids.”  Such heating was supposed to be impossible beyond the “frost line.” a theoretical radius beyond which volatiles in the early solar system would have frozen into comets, never to heat up again till tugged toward the sun long after their formation.
3. Demolition derby:  Watching planets form would take a long time, but watching them get destroyed is quick and easy.  Space.com and Science Daily reported the collision of two Earth-like planets around a sunlike binary star 300 light-years away (an apocalyptic ending for any life there).  Benjamin Zuckerman of UCLA said, “Astronomers have never seen anything like this before.  Apparently, major catastrophic collisions can take place in a fully mature planetary system.”  Astronomy Picture of the Day posted the artwork of the proposed collision.  (Actually, the collision was inferred from dust, not witnessed.)
       OK, so planets destroy each other, but does this observation provide any evidence for how they form?  Not exactly; the article mentioned theories that our moon formed from a collision, and that the dinosaurs went extinct because of a collision.  They surmised that the planets that they think collided were in the final stages of its dust disk’s evolution.  But there seemed to be more surprise than confirmation of theory.  For one, they were surprised planets could form at all around a binary star.  For another, they were surprised to see a collision in such a mature system: “How do planetary orbits become destabilized in such an old, mature system, and could such a collision happen in our own solar system?” asked one.  The observations here seem to relate more directly to planetary destruction models, not planetary formation models.
4. Shooting gallery:  Last month, Science Daily reported on computer simulations at Northwestern University.  Results showed that our solar system is “pretty special” to have ended up with nice, stable rocky planets in nearly circular orbits inside the habitable zone.  Even assuming that planets can coalesce from a dust disk, most of the time wild things happen: the star eats up the planets, the large bodies fling the small bodies out of the system, and the remaining ones end up with elongated orbits that would prohibit life.  Out of a hundred runs on “very powerful computers,” none of the systems ended up like ours except under Goldilocks conditions – ones that were “just right.”  The senior author of the report commented on what they learned: “We … know that the solar system is special and understand at some level what makes it special.”

To add a little optimism, Space.com published a report that makes planet formation sound simple.  “Solar systems under construction” was the cheerful title of an article about observations of three stars where planets might be forming.  Trouble is, “The researchers did not actually see any planets.”  They inferred their presence from properties of the dust disks.  Gaps in the dust where planets might exist were determined indirectly, and “they used planet-formation models to project the presence of alien worlds.”  Unfortunately this begs the question whether the planets, if there are any, actually formed from the dust, because the model was made to simulate planet formation, not planet destruction.

What we see in the above examples are destructive processes, not creative ones.  The creative ones exist only in the minds of the scientists and in their contrived models.  It may make someone feel good to imagine worlds forming naturally.  It may conform with their world view: an evolving universe, full of evolving stars, with evolving planets, on which life emerges and evolves.  Everything happens naturally (whatever that ambiguous word means).  If it feels good, call it a belief.  Science, however, demands we stick with observations that are measurable and repeatable.  Thomas Kuhn would look at the number of anomalies in current models and predict a paradigm shift.  Sidney Harris would look at the number of miracles inserted into the models and draw a cartoon.
    There are numerous problems with solar system formation theories we have addressed before (e.g., 03/21/2006, 08/06/2004, 02/03/2004, 09/22/2003, 11/20/2002).  This time, notice primarily that there is no way to distinguish between whether we are seeing planets forming out of dust disks, or pre-existing planets grinding down into dust disks.  The latter would seem to comport better with well-known processes of thermodynamics and probability.  Dust particles lack the gravitational mass to accrete and grow.  It is much more plausible that disks erode into dust rather than self-assemble into planets.
    Simplistic models need not apply.  Chondrules, comets, and coincidences that make life possible must be explained with observation, measurement and repeatable experiments.  A model is not evidence.  One cannot assume what needs to be proved.  Let the scientific empiricist show his evidence; until then, we must classify planet formation theory what it is thus far: a belief, not a science.