Every Planet Surprises Secular Astronomers, III: Formation Theories
“Surprise” or “puzzling” are the most common words in news reports about bodies in the solar system. Here are recent examples that discuss the origin of planets.
Lucky breaks: In a Perspective piece in Science Magazine, “Forming Terrestrial Planets,” John Chambers describes the mess between competing theories of the rocky planets. He only treats two theories, both secular, to explain why Mars is puny compared to Earth (1/10 Earth’s mass). One theory involves motions in and out by Jupiter (the “Grand Tack” model) that results in a depletion of accretionary resources at the distance of Mars (3 AU). A recent competing theory by Izodoro posits that gas was depleted in that region as it migrated toward the sun. Both models require Mars to form near the sun and get pushed into the depleted zone. As Chambers admits, whatever happened, it’s all a matter of luck:
Some caveats are in order. Like all current theories for planet formation, the models described here are incomplete. In particular, we lack a clear understanding of how and where planetesimals formed in the solar nebula, which obviously has a bearing on subsequent events. The final stage of growth is also notoriously chaotic—a tiny change in initial conditions can completely change the final outcome. So, luck played a big role in shaping the solar system. Conventional planet-formation simulations generate respectable Mars analogs in a few percent of cases without requiring any special measures. This leaves the slim possibility that Mars represents a bizarre statistical outlier, and its small size contains no deeper truths about our solar system.
In other words, the situation is like Sidney Harris’s famous cartoon, in which a student, describing a process on the blackboard, inserts “Then a miracle occurs.” His teacher demands he be a little more explicit on that point. An “incomplete” theory is little more than a suggestion if a lack of “clear understanding” about a major point—the origin of planetesimals—remains. Also, explanations that rely on sheer dumb luck are not explanations at all, strictly speaking, in science, even if the theorist (John Chambers) is an esteemed scientist at the Carnegie Institution for Science. (See the “Stuff Happens Law” in the Darwin Dictionary.)
Late stage evolution: Eric Ford (Penn State), in a review article for PNAS, starts after “the miracle occurs” and tries to make sense of the rest of the post-planetesimal story. His article is mostly a promissory note: “astronomers can look forward to a much better understanding of planet formation in the coming decade,” he says. Whether anyone will be able to hold him accountable to that remains to be seen. Meanwhile, he describes the hubbub created by the discovery of “hot Jupiters” (gas giants orbiting very close to the star). The median orbital period for these anomalous bodies has increased to a year, but their location still presents problems to theorists:
The large masses of hot Jupiters imply a substantial gaseous component and therefore rapid formation, before the protoplanetary disk is dispersed. In situ formation of the rocky cores of hot Jupiters is problematic due to the high temperature and low surface density of the disk so close to their host star. Therefore, theorists explain hot Jupiters starting with the formation of a rocky core at larger separations from the host star, followed by accretion of a gaseous envelope and migration to their current location. The mechanism for migration is less clear.
By “less clear,” he means that both competing “broad classes of models” suffer seemingly falsifying problems. It’s unlikely a rocky core would just wander in through the disk on its own; how would it stop? “It is unclear how the planets would avoid migrating all of the way into the host star or why the migration would be halted to leave planets with orbital periods of ∼2–5 days” like some hot Jupiters seen closer to their stars than Mercury is to our sun. On the other hand, if the gas giant formed with initial high eccentricity (perhaps due to gravitational encounters with neighbors) and then tidal interactions circularized the orbit close to the star, the notion relies on sheer dumb luck. Ford remains optimistic about what no astronomer has witnessed: “In practice, planetary systems that form two giant planets may well form additional massive planets, leading to a series of planet–planet scattering events and substantially increasing the probability for one to achieve a pericenter of just a few stellar radii.” Perhaps it was a combination plate, he imagines: maybe migration brought the gas giant to about 1 AU where scattering could be more efficient. This is qualitative guesswork. Migration was not even in the cards for planetary theory till the discovery of hot Jupiters forced the idea.
Needless to say, stars with hot Jupiters are unlikely to have Earth-like worlds. “While the future hot Jupiter is circularizing, it cleans out the inner solar system by scattering any rocky planets in the inner planetary system into the star or the outer regions of the planetary system.” Thank goodness our solar system did not meet this fate. When Ford is not talking about actual observed planets, he is using a lot of escape words like perhaps, maybe, and could, (see perhapsimaybecouldness index) or is pushing the explanation into the future. Theorists of the future, though, will have their work cut out for them. Jupiter is hard enough to explain, but “Unfortunately, characterizing the architectures of outer planetary systems is likely to prove significantly more difficult than planets with orbital periods of a few years or less.” Same for inner rocky planets, many of which the Kepler spacecraft found are “short-period, tightly-packed inner planetary systems” (STIPS). Maybe they should work on the accretion problem (the “bouncing barrier discussed in Part II) before tackling migration. Ford didn’t even get into that; he just assumed that planets accreted (somehow).
Eliminate the negative, accentuate the positive: Meanwhile, news sources routinely put a positive spin on planet formation. Assuming planets must have formed by natural causes somehow (since they exist), astrobiologists are ready to look for them in the wild west. For instance, contrary to earlier speculations, Science Daily holds out hope for “tilt-a-worlds” (planets with high obliquity) being habitable (see 1/12/12). “Don’t forget F-type stars” (hot white stars more massive than the sun) as possible planetary homes, PhysOrg wrote; one astronomer says they “are not hopeless” in spite of their heat and radiation.
Hope springs eternal in astrobiology. Since it’s a science without a subject, that’s all they have. There’s no -biology in astrobiology, and the astro- part faces serious challenges when it comes to explaining planet formation.
Here at Creation-Evolution Headlines, we are very proud of the space program’s achievements in planetary exploration, including the discoveries of exoplanets. We would never disparage the highly gifted and intelligent astronomers who have brought us all these incredible observations. Nevertheless, when it comes to explaining the origin of what they observe, are we getting anything better than the ideas of witch doctors? Look at the astrobiologists’ appeals to magic and miracles. Look at the forward passes to future astronomers. Look at their beliefs in spite of the evidence. At least witch doctors dress up as shamans, not pretending to be scientists. Which is the more deceptive?