Planet-Makers Ask Miracles to Evade Death Spiral
Remember the old artwork of planets gently forming out of dust orbiting a young star? That’s all gone. Reality has set it. Clumps of material a meter across need help – almost miraculous help – to avoid getting sucked into the star in a giant death spiral. If you don’t believe it, ask John Chambers of the Carnegie Institute why he wrote the following in Nature this week:1
The rarity of planetesimals smaller than 100 km in diameter at the end of stage 1 seems to rule out the possibility that dust aggregates somehow made it across the metre-size barrier by gradually sweeping up material from their surroundings. Instead, objects must have grown very rapidly from sub-metre-sized pebbles into 100-km-sized bodies, possibly in a single leap.
To get a handle on what he just said, he is asking people to believe that pebbles grew into planets as big as Los Angeles instantaneously. That makes the “punctuated equilibria” theory in biology look tame by comparison.
Chambers’ mostly optimistic article focused on the possibility of using current asteroid size distributions as a kind of archaeological probe into the early history of the solar system. Since most surviving asteroids appear to be at least 100 km in size (though this may be an observational selection effect), some models suggest it reflects the original size distribution after “stage 1” of planet formation, which he describes thusly: “Dust grains coalesced into planetesimals, objects of 1�1,000 km in diameter, through an unknown process.” Philosophers and logicians might enjoy a hearty debate over the difference between a miracle and an unknown process; see, for example Hugh Mclachlan’s discussion about miracles and science in New Scientist.
Chambers relied heavily on four papers he cited, so we looked them up. One, by Blum and Wurm,2 was supposed to guarantee that dust grains will accrete into boulders (see the 12/05/2007 where Wurm was less sanguine about this). That paper started with a less optimistic tone: “The formation of planetesimals, the kilometer-sized planetary precursors, is still a puzzling process.” The authors examined all the latest experiments and models, and concluded that it is possible to get pebbles up to 10cm (if charged dust particles collide below 1 m/s), but after 1 meter in diameter is reached, erosional processes dominate. In frustration they said, “Due to the experimental findings discussed in the previous sections, it seems unlikely to form planetesimals by direct collisional sticking.” They attempted some special pleading by invoking unusual conditions to make the particles more sticky, but then appealed to miracles to get around the giant sucking sound: “However done, the formation of kilometer-sized planetesimals has to happen fast, as large bodies possess a rather short lifetime owing to their effective inward drift motion.” How fast? 100 years or less. Within a century of orbit, meter-size agglomerates will meet their fate in the stellar oven. “Thus, any model explaining the growth over this meter-size barrier has to be extremely fast to prevent the radial drift of the macroscopic bodies.”
In the next few paragraphs, Blum and Wurm engaged in more special pleading, searching for solutions, only to conclude, “We now have a somehow detailed picture of how decimeter-sized dust aggregates form, but lack a self-consistent description of the further evolution of solid bodies to the planetesimal level.” Chambers borrowed his optimism from the sections that talked about the pebbles, but had to admit a big problem remains with the city-sized planetesimals: “However, the transition from pebble-sized dust aggregates to mountain-sized planetesimals is problematic and remains an unresolved issue,” he said. “This is unfortunate, because all subsequent stages of planet formation depend on it.” Then he agreed: meter-size clumps are quickly destroyed by the death spiral and collisions with neighbors. “For these reasons, it seems unlikely that objects will grow larger than about one metre as a result of the gradual accumulation of dust grains.” Bad news for planet builders.
But then, Chambers renewed his optimism by referencing two recent models (2007, 2008), that though “still in their infancy” offer hope of a solution.3,4 Both models rely in disk instability to produce large clumps almost instantaneously. Johansen et al3 recognized the problem in their abstract: “How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem: boulders are expected to stick together poorly, and to spiral into the protostar in a few hundred orbits owing to a ‘headwind’ from the slower rotating gas.” Only by special pleading, invoking local concentrations of matter, were they able to model the formation of minor-planet-size bodies by processes faster than those trying to destroy them. Cuzzi et al4 also understood the destructive processes at work, e.g., “disruption by the ram pressure of the differentially orbiting nebula gas.” Their “scenario” which relied on turbulence and local knots of material, was stated as a work in progress: “Localized radial pressure fluctuations in the nebula, as well as interactions between differentially moving dense clumps, will also play a role that must be accounted for in future studies.” Neither of these “scenarios” seem ready for the imprimatur of scientific theory. Chambers recognized this. “Ideally, one would like observational data to test their viability,” he said. Indeed. This recalls Yogi Berra’s quip that in theory, theory and practice should agree, but in practice, they often don’t.
So far, the models and experiments are pretty glum. That’s where Chambers turned a corner and talked archaeology. Referring to a paper by Morbidelli et al,5 he argued that present distributions of asteroids can tell us about original distributions of hopeful clumps in the early solar system. But to believe this, one has to believe that “asteroids were born big” (the title of their paper). Only by starting out with the assumption that the original clumps were 100km in diameter could they get the size distributions to match. Here is where Morbidelli et al stated the miracle in their words: “This supports the idea that planetesimals formed big, namely that the size of solids in the proto-planetary disk ‘jumped’ from sub-meter scale to multi-kilometer scale, without passing through intermediate values.” While we’re having fun with miracles, let’s pile them on: “the initial planetesimals had to have sizes ranging from 100 to several 100 km, probably even 1,000 km” they said. Now the dust particles leaped from centimeters to the size of continents. What they said next indicates that miracles must hereafter be included in planet-building scenarios: “This result sets a new constraint on planetesimal formation models and opens new perspectives for the investigation of the collisional evolution in the asteroid and Kuiper belts as well as of the accretion of the cores of the giant planets.”
Meanwhile, the giant sucking sound continues. In this month’s Astrophysical Journal, Fred Adams and Anthony Bloch6 wrote more about Type I migration – the death spiral that conveys meter-size rocks to their doom. “In many planet-forming disks, the Type I migration mechanism, driven by asymmetric torques, acts on a short timescale and compromises planet formation,” they said. Only by appeals to luck could they get some clumps to survive the “Type I migration problem” – “If the disk also supports magnetohydrodynamics instabilities, however, the corresponding turbulent fluctuations produce additional stochastic torques that modify the steady inward migration scenario.” How many survive? The results for any given set of boundary conditions is “uncertain,” they admitted, but with some “expected disk properties” they arrived at calculations of 1% to 10% might survive; however, “the fraction of surviving planets decreases exponentially with time.” They did not discuss the accretion problem. They only said that unless something happens fast, by chance or miracle, don’t expect to find any planets left.
For public consumption, JPL issued a feature story explaining all this in layman’s terms. Without blinking an eye, the story just stated that asteroids were born big, as if that is all you need to know. “Evidence is now mounting that these small space rocks quickly ‘jumped’ (or grew) in size from below one meter to multi-kilometer in size,” the article said – and that’s how they evaded the death spiral. And what is that evidence that has been mounting? It’s current asteroid distributions and computer simulations (and the realization that without starting big, they would be destroyed). The only way the simulators could keep the initial asteroids from obliterating themselves was by starting them out big – pebbles that somehow “quickly morphed into asteroids hundreds of kilometers in size.” How that happened, exactly, they couldn’t say. “Once their growth spurt was over, these massive celestial bodies began an epoch-sized game of demolition derby as they orbited the sun. Over the eons, and with each extraterrestrial pileup, came fewer and fewer large asteroids – a fragmentation process that continues to this day. Despite the modest sizes of asteroids today, the paper’s authors conclude that asteroids must have been born big.”
New Scientist followed suit, blessing the papers that prescribed instant planets with positive vibes. This “sudden leap” scenario was blessed by John Chambers as a “big step forward.” The only one calling for a little more caution was Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics. “It’s a nice story and they have a lot of evidence supporting their point of view,” he said, but he cautioned, according to New Scientist, that “it may have been difficult to complete planet formation in a reasonable time if there were no small asteroids at the outset.” Onlookers might question whether leaping over a gap is really increasing our understanding of how we got here.
1. John Chambers, “Planetary science: Archaeology of the asteroid belt,” Nature 460, 963-964 (20 August 2009) | doi:10.1038/460963a.
2. Blum and Wurm, “The Growth Mechanisms of Macroscopic Bodies in Protoplanetary Disks,” Annual Review of Astronomy and Astrophysics, Vol. 46: 21-56 (Volume publication date September 2008) (doi:10.1146/annurev.astro.46.060407.145152).
3. Johansen et al, “Rapid planetesimal formation in turbulent circumstellar disks,” Nature448, 1022-1025 (30 August 2007), doi:10.1038/nature06086; Received 19 December 2006; Accepted 5 July 2007.
4. Cuzzi, Hogan and Sharif, “Toward Planetesimals: Dense Chondrule Clumps in the Protoplanetary Nebula,” The Astrophysical Journal, 2008 ApJ 687 1432-1447, doi: 10.1086/591239.
5. Morbidelli, Bottky, Nesvorny and Levison, “Asteroids Were Born Big,” Icarus (July 2009), doi:10.1016/j.icarus.2009.07.011.
6. Adams and Bloch, “General Analysis of Type I Migration with Stochastic Perturbations,” The Astrophysical Journal, 701 (August 2009) 1381, doi:10.1088/0004-637X/701/2/1381.
This has to be one of the most egregious lapses of scientific integrity in modern times. It is so bad, so full of special pleading and ad hoc speculation and storytelling contrary to the evidence, it is almost as bad as Darwinism – and you know what that means. Elsewhere we have complained that appeals to the Stuff Happens Law are not scientific.
In chapter 8 of his recent book Signature in the Cell, Stephen Meyer elaborated on the “chance hypothesis” and described when it is legitimate in scientific explanation and when it is not. Something that is “one of the normal possible outcomes of a regular underlying process” can be explained as an outcome of chance. But when a one-time, highly-improbable outcome that exhibits specified complexity is explained by chance (the Stuff Happens Law), it amounts to an admission of ignorance. It’s a fancy way of saying, “We don’t know what happened” or “We can’t explain it” (p. 176).
So when astronomers attribute the existence of planets to the luck of the draw, when our planet exhibits numerous cosmic “coincidences” that make life possible, they are using chance as an escape hatch to avoid clear evidence of design. Should we bless it with the honor of science?
Some astronomers reading this may object to our use of the word “miracle” in their explanations. Planets, after all, do exist. They must have gotten here somehow. And the authors of these distinguished papers are scientists, not priests. They know calculus. They work at universities. They have degrees. It follows that anything they say and do must be respected as authoritative, even when they say that clumps went from pebble size to continent size without passing through the intermediate stages. Come on; scientists put on their pants one leg at a time like other mortals do. If we are to treat them like gods, then they are like Janus – one face weeping over the insurmountable problems with the physics, and the other smiling blissfully over the visions of what they can achieve in their models with a few miracles sprinkled in.
You know what would get these false gods out of their troubles? A little exercise in thinking outside the box. They have forced themselves into the impossible situation of believing that they have to explain humans from the bottom up. Once upon a time, a bang happened; this led to some subatomic particles, then some atoms, then some clumps, some stars, some galaxies, some planets, some life, and some people. This sequence requires invocations of the Stuff Happens Law and miracles at every turn. Ask yourself: why must scientific explanation start from the bottom up? Why not a top-down approach?
The answer is, of course, the necessity of a designing intelligence when explanation starts from the top down. But look at the benefits: it fits the scientific evidence (for example, see this JPL press release). The two greatest laws of physics (the first and second laws of thermodynamics) no longer have to be violated. Planets can have an explanation for their origin. Instead of dust disks building themselves up into systems, they collide and decay – just as we observe them doing. The order and design we observe has a sufficient cause. Things have meaning and purpose – including our desire to understand the world through science.
Janus is a bad scientist. He needs a head transplant. We suggest the head of Kepler, who accepted his priesthood as a scientist and believed in a designing intelligence. He said, “Since we astronomers are priests of the highest God in regard to the book of nature, it befits us to be thoughtful, not of the glory of our minds, but rather, above all else, of the glory of God.”