Von Weiszacher’s efforts were generally accepted as making the nebular hypothesis scientifically acceptable. But was this really the case? The answer is No, because as we will see, observational evidence for it is lacking. And as with accretion theory, the nebular hypothesis has become more complex with time because the simpler failed. The nebular hypothesis now includes (1) an accretion stage; (2) a planetesimal formation stage; (3) a planetary core (planetary embryos) stage; and (4) a planetary migration stage.36 The planetary migration stage is necessary because, according to theory, once planetary cores have formed, they are in the wrong places to resemble a planetary system, so must be made to ‘migrate’ to their proper location. We have seen that observational evidence for the accretion stage is absent, but so are data confirming the other stages.
No evidence of nebula collapse
If the nebular hypothesis were true, astronomers should observe clouds of debris elsewhere in the galaxy collapsing, as the solar nebula did. Each of these giant molecular clouds is supposed to be like the solar nebula was billions of years ago, before it collapsed. A giant molecular cloud is a nebula (plural nebulae), or is considered part of a larger nebula. “No astronomer has ever observed the process of cloud collapse,”44 and “no one has caught a molecular cloud in the act of collapsing.”45 For clumps in clouds that have been observed, Blitz says, “None of the clumps in the clouds … observed are gravitationally bound [collapsing]. … Because the clumps are so far from being gravitationally bound … the clumps must be expanding.”46 For Blitz, “This conclusion is difficult to accept.”46 So GMCs exist, but their non-collapse says that the solar system could not have formed from nebular collapse.
No evidence of stars forming
If the nebular hypothesis were true, astronomers should see stars forming from debris contracting inward, as the sun supposedly did. But “no one has unambiguously observed material falling onto an embryonic star, which should be happening if the star is truly still forming.”45 Accordingly, theorists have concluded that, “Giant molecular clouds are not collapsing dynamically and have, in fact, generally a very low efficiency for stellar genesis.”47 Thus, GMCs cannot be expected to collapse into stars, despite the widespread belief that they are. Gravitational collapse cannot happen in a diffuse, rarified gas cloud to form a star; it is not dense enough. “The only way for a … cool interstellar cloud to contract from nebular to stellar dimensions is to be dense enough so that the gravitational attraction of its particles for each other is strong enough to start it contracting”.48 Thus theorists recognize that a GMC cannot begin collapsing on its own. There must be an external force to bring the GMC to a density high enough to trigger collapse.
Nebular theory must suppose that another physical body provides this force, such as other clouds already in collapse or unstable stars sending shock waves (density waves) into the surrounding space. Thus the theory presumes the pre-existence of a successfully-collapsing cloud or an already-formed star, which is what the theory seeks to explain in the first place. As theorists have said, “Star formation can also be induced [or] triggered by a mechanism external to the clump. … Shocks, which can be due to supernovae [unstable stars] or to cloud-cloud collisions, have been invoked frequently as a mechanism for inducing star formation.”49 In other words, “The general model requires some mechanism to trigger a cloud’s collapse: a supernova explosion, a shock wave from the galaxy’s spiral arms, cloud collisions, or stellar winds. Why clouds don’t collapse on their own … is still a ‘great mystery’.”50 Another theorist wrote, “Since the 1960s, in numerical models of protostellar collapse, thermonuclear ignition temperatures are not attained solely by the in fall of matter; an additional shock wave-induced sudden flare-up is assumed.”51 In the nebular hypothesis, it takes a star to make a star. The nebular hypothesis has not explained how stars first formed.
Since the nebular hypothesis has been fashionable off and on for over two centuries, but has not explained the origins of stars or planetary systems, why don’t its advocates abandon it? One reason is that the only viable alternative is biblical creation, or at least unknown mechanisms. Another reason is that the nebular hypothesis is a model, a way of visualizing the cosmic past. As a model, science alone cannot disprove it, because any necessary ad hoc assumptions can be generated as needed. Supposing the pre-existence of collapsing clouds and functioning stars to trigger the collapse of new clouds is an ad hoc assumption. Indeed, “If stars did not exist, it would be easy to prove that this is what we expect”, Geoffrey R. Burbidge once quipped.52 Burbidge apparently understood that a model can incorporate any observations and can never be disproved by any of them. The impossibility of model disproof is also why advocates of the nebular hypothesis can continue to say, “There is strong evidence that star formation is going on at the present time.”53 It is easy to confuse theoretical or popular assertions of the truth of the nebular hypothesis with observational evidence, which is absent.