Planet Formation Remains a Puzzle

Even with an rough understanding of the accretion disc, actually aggregating planets within it is difficult, when one starts only with frail dust grains of about a micron (0.001mm) in diameter, and molecular gases. We really don't know how such tiny grains survive, collide and grow, much less assemble into planets.

Scott Tremaine (Princeton) has focused more on the long term orbits of planets and comets, to determine where they come from and why they are there. He has found such interesting results as this: if you remove the Earth from the Solar System, Venus would simply fly away. This is quite surprising since the Sun completely dominates Venus's orbital dynamics. Very subtle effects of the Earth on Venus, however, keep Venus behaving. Other work has shown that many comets are probably kicked inward from the far reaches of the Solar System by the tiny gravitational kick of passing stars. Such work demonstrates the somewhat surprisingly delicate nature of planetary systems.

Although the formation of planets almost certainly occurs within accretion discs, understanding and observing such discs remain daunting tasks. Furthermore, study of the long term stability of planetary systems, such as the Solar System, remains active and surprising, although it looks as if the Earth isn't going anywhere soon.

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