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The Question

(Submitted August 10, 1998)

I noticed that the orbits between Mars and Jupiter (where the asteroid belt is) looks as though there ought to be a planet there instead of the asteroid belt. Has it ever been considered that the asteroid belt was maybe a destroyed planet?

The Answer

Thank you for your question! The fact that the asteroid belt has such a well-defined, high concentration of asteroids suggests two things. One, that they are fragments of a planet that broke-up long ago, or two, that they are rocks that never managed to accumulate into a genuine planet. Currently, scientists tend to favor the latter explanation. According to Eric Chaisson and Steve McMillan, the authors of the text book "Astronomy Today", 1993 edition, "There is far too little mass in the belt to constitute a planet, and the marked chemical differences between individual asteroids strongly suggest that the asteroids could not all have originated in a single planet. Instead, astronomers believe that the strong gravitational field of Jupiter continuously disturbs the motions of these chunks of primitive matter, nudging and pulling at them, thereby prohibiting them from aggregating into a planet. The existence and composition of the asteroid belt joins the general properties of the planets and their moons on our list of features that any theory of solar system formation must explain."

Here is a good web page about the asteroids: http://nineplanets.org/asteroids.html

Hope this helps!

Maggie Masetti & Koji Mukai
for Ask an Astrophysicist

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