Friday, March 24, 2006

The locked migration of giant protoplanets

The locked migration of giant protoplanets, March 21, 2006 . In an article to be published in Astronomy & Astrophysics, two British astronomers present new numerical simulations of how planetary systems form. They find that, in the early stages of planetary formation, giant protoplanets migrate inward in lockstep into the central star. Gravitational interaction between the gaseous protoplanetary disc and the massive planetary cores causes them to move rapidly inward over about 100,000 years in what we call the "migration" of the planet in the disc. The prediction of this rapid inward migration of giant protoplanets is a major problem, since this timescale is much shorter than the time needed for gas to accrete onto the forming giant planet. Theories predict that the giant protoplanets will merge into the central star before planets have time to form. This makes it very difficult to understand how they can form at all. For the first time, Paul Cresswell and Richard Nelson examined what happens to a cluster of forming planets embedded in a gaseous protoplanetary disc. Previous numerical models have included only one or two planets in a disc. But our own solar system, and over 10% of the known extrasolar planetary systems, are multiple-planet systems. ... Cresswell and Nelson's work is the first time numerical simulations have included such a large number of protoplanets, thus taking into account the gravitational interaction between the protoplanets and the disc, and among the protoplanets themselves. The primary motivation for their work is to examine the orbits of protoplanets and whether some planets could survive in the disc for extended periods of time. Their simulations show that, in very few cases (about 2%), a lone protoplanet is ejected far from the central star, thus lengthening its lifetime. But in most cases (98%), many of the protoplanets are trapped into a series of orbital resonances and migrate inward in lockstep, sometimes even merging with the central star. ... [If this holds up it will mean that Earth-like planets will be much rarer than previously thought. If this theoretical model proves robust and predicts there should be a lot less exoplanets, then maybe some (if not most) of these claimed exoplanets are actually dwarf stars in binary star-systems? See this article of last year which suggested that is possible.]

Stephen E. Jones, BSc (Biol).
"Problems of Evolution"

"It may be also objected, that the power of reaching upwards, acquired by the lengthening of the [giraffe's] neck and legs, must have necessitated a considerable increase in the entire size and mass of the body (larger bones requiring stronger and more voluminous muscles and tendons, and these again necessitating larger nerves, more capacious blood-vessels, &c.), and it is very problematical whether the disadvantages thence arising would not, in times of scarcity, more than counterbalance the advantages. For a considerable increase in the supply of food would be requisite on account of this increase in size and mass, while at the same time there would be a certain decrease in strength; since, as Mr. Herbert Spencer says: ["Principles of Biology," vol. i. p.122] 'It is demonstrable that the excess of absorbed over expended nutriment must, other things equal, become less as the size of an animal becomes greater. In similarly-shaped bodies, the masses vary as the cubes of the dimensions; whereas the strengths vary as the squares of the dimensions. ... Supposing a creature which a year ago was one foot high, has now become two feet high, while it is unchanged in proportions and structure-what are the necessary concomitant changes that have taken place in it? It is eight times as heavy; that is to say, it has to resist eight times the strain which gravitation puts on its structure; and in producing, as well as in arresting, every one of its movements, it has to overcome eight times the inertia. Meanwhile, the muscles and bones have severally increased their contractile and resisting powers, in proportion to the areas of their transverse sections ; and hence are severally but four times as strong as they were. Thus, 'while the creature has doubled in height, and while its ability to overcome forces has quadrupled, the forces it has to overcome have grown eight times as great. Hence, to raise its body through a given space, its muscles have to be contracted with twice the intensity, at a double cost of matter expended.' Again, as to the cost at which nutriment is distributed through the body, and effete matter removed from it, `Each increment of growth being added at the periphery of an organism, the force expended in the transfer of matter must increase in a rapid progression - progression more rapid than that of the mass.'" (Mivart St.G. J., "On the Genesis of Species," Macmillan & Co: London, Second edition, 1871, pp.31-32)

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