Why isn’t the asteroid belt a planet?

There appears to be a strange gap between Mars and Jupiter filled with rocky rubble. Why didn’t the asteroid belt form into a planet like the rest of the solar system?

Beyond Mars’ orbit lies the Asteroid Belt, a vast collection of rocks and ice, remnants of the formation of the solar system. It starts at around 2 AU, ends around 4 AU. Objects in the asteroid belt range from tiny pebbles at Ceres to 950 km in diameter.

Star Wars and other science fiction have it all wrong. The objects here are hundreds of thousands of kilometers apart. There would be absolutely no danger or tactical advantage in flying your spaceship through it.

For starters, there isn’t actually that much going on in the asteroid belt. If you were to take the entire asteroid belt and form it into a single mass, that would only be about 4% of the mass of our Moon. Assuming a similar density, it would be smaller than Pluto’s moon Charon.

There is a popular idea that maybe there was a planet between Mars and Jupiter that exploded, or even collided with another planet. What if most of the debris was thrown out of the solar system and the asteroid belt was what was left?

We know this is not the case for several reasons. First, any explosion or collision would not be powerful enough to propel matter out of the solar system. So if it was an ancient planet, we would actually see more debris.

Second, if all of the asteroid belt pieces were from a single planetary body, they would all be chemically similar. The chemical makeup of Earth, Mars, Venus, etc. is unique because it formed in different regions of the solar system. Likewise, different asteroids have different chemical compositions, which means that they must have formed in different regions of the asteroid belt.

Artist’s representation of the asteroid belt between Mars and Jupiter. Credit: David Minton and Renu Malhotra

In fact, when we look at the chemical compositions of different asteroids, we find that they can be grouped into different families, each with a common origin. This gives us an idea of ​​why a planet did not form where the asteroid belt is located.

If you rank all the asteroids in order of their average distance from the Sun, you will find that they are not distributed evenly. Instead, you find a bunch, then a gap, then a more bunch, then another gap, and so on. These gaps in the asteroid belt are known as Kirkwood gaps, and they occur at distances where one orbit would resonate with Jupiter’s orbit.

Jupiter’s gravity is so strong that it makes the orbits of asteroids in the Kirkwood holes unstable. It is these shortcomings that have prevented the formation of a single planetary body in this region. So, because of Jupiter, asteroids formed into families of debris, rather than a single planetary body.

What do you think? What is your favorite object in the asteroid belt. Tell us in the comments below.

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