How did the Asteroid Belt Form?

Discovering the asteroid belt started an ongoing debate.  It continues today albeit with somewhat less controversy.  This debate centers on the formation, dynamics, and stability of planetary systems.

Well, discussing that would require a series of articles.  I’ll stick to how the asteroid belt came to be.

One hundred and twenty years ago Giuseppe Piazzi discovered an object between the orbits of Mars and Jupiter.  He thought it a planet and named it Ceres.  That discovery satisfied the then recently penned Titus-Bode law – each planet is approximately twice the distance from the Sun as the next closer planet.  That was great, a nice tidy solution.

It all got messed up about fifty years later as many objects had then been observed in the space between Mars and Jupiter.  That gap seemed to be full of small objects.  Ceres got demoted to dwarf planet status, the astronomy community was in turmoil.  What to do?

Do what scientists do…study it and figure it out.

It wasn’t long before astronomers came up with a new hypothesis.  The objects in this area between Mars and Jupiter must be the remnants of a destroyed planet or one that failed to form.  Jupiter and/ or Saturn might have disrupted the process (accretion). They even gave the non-planet-mess a name, Phaeton.  Seemed an elegant solution.  Over time, as the asteroid belt gave up more of its objects the Phaeton hypothesis started to lose favor.  The problem is, even using extreme extrapolation, there isn’t enough mass in the asteroid belt to make our Moon, much less a planet.  Not so elegant. Keep trying.

Thanks to missions to the asteroid belt a confounding issue has evolved. We find much diversity in asteroid composition. A fair number, due to their composition, appear to have origins in the outer solar system. Now things are become interesting, at least theoretically.

More recent hypotheses, using computer modeling to try and sort out our early solar system’s progress propose a curious phenomenon.  The outer planets Jupiter, Saturn, Uranus, and Neptune might have spent time trading orbits early in the formation of our solar system.  And there might even have been another, fifth gas giant involved. As these massive planets jockeyed around, small objects were kept scattered. Some objects from the outer solar system were flung inward, past Jupiter, and the hypothetical fifth gas giant was ejected from the solar system.  Jupiter’s gravitational influence eventually shepherded the objects into a ring or belt shaped configuration between Mars and Jupiter. Whew!

All this disruptiveness by big planets would also help explain why Mars is so small. Planetary formation theory says it should be at least as big as Earth.  Due to the constant stirring up of small objects Mars was deprived of building materials.  What if…?

What’s in the Sky?

March 9&10; 30 minutes before sunrise; southeast:  A waning crescent Moon shares the low pre-dawn sky with Mercury, Jupiter, and Saturn.