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Early Solar System Catastrophism

The two moons of Mars have always presented planetary scientists with something of a mystery. These tiny moons, Phobos and Deimos, whizz around Mars at no great height at all: Phobos whips around the red planet in less than 8 hours, at a height of only 3,700 miles – the closest of any moon to its parent planet. I say ‘parent’ advisedly because a new theory of the origin of these peculiar little moons suggests that they emerged from a major impact between mars and a dwarf planet. It has generally been assumed that they were captured asteroids, but the relative circularity of their orbits argued against such a capture. Work on the possibility of a catastrophic origin was carried out last year by two separate teams of researchers, after decades of battling intense scepticism within the scientific community (1). An important finding of the modelling at that time was that the resultant debris would circulate around the red planet at a relatively low altitude, which is in keeping with the orbits of the two extant moons.
More recently, further computer modelling of various impact scenarios carried out by one of those teams has narrowed down the range of masses of an impactor to about the size of Pluto. The resultant debris field was initially far more extensive than the two moons left today:
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Massive Planet X Now Urgently Sought by Top Planet-Hunters

We seem to be getting very close now to a discovery of a massive Planet X in the outer solar system.  I heard this report on the evening BBC news, a slot which indicates the seriousness with which this subject is now being taken by the scientific community:

“American astronomers say they have strong evidence that there is a ninth planet in our Solar System orbiting far beyond even the dwarf world Pluto.  The team, from the California Institute of Technology (Caltech), has no direct observations to confirm its presence just yet.  Rather, the scientists make the claim based on the way other far-flung objects are seen to move.  But if correct, the putative planet would have 10 times the mass of Earth.

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