Rocket holds up a pebble from Monday's pile. "If the planets are finished, what are the asteroids?"
"Leftovers," Raven says. "Pieces that never joined a planet. What do you notice about where they are?"
Rocket checks his notes. "Mostly between Mars and Jupiter. And comets are way out past Neptune, where it is cold."
Nova hovers close. "Scientists study the leftovers to learn the recipe," she says. "Why would old pieces help?"
"Because they have not changed much," Rocket says. "They are like the first page of the story, still readable."
Raven writes it down. "The planets got cooked and mixed. The leftovers stayed raw." Nova glows. "Review time."
NASA describes asteroids as rocky, airless remnants left over from the early formation of the solar system about 4.6 billion years ago.
Most asteroids orbit the Sun between Mars and Jupiter, in the main asteroid belt.
They range in size from Vesta, the largest at about 530 kilometers across, down to bodies less than 10 meters across.
All the asteroids together have less mass than Earth's Moon.
Comets are cosmic snowballs of frozen gases, rock and dust. When frozen, a comet is about the size of a small town.
When a comet's orbit brings it near the Sun, it heats up and sends out dust and gas. That forms a tail that stretches away from the Sun.
NASA says asteroids in the belt are bits and pieces of the early solar system that could never quite form a planet.
Comets, far out, have not changed much at all since the solar system formed.
OpenStax Astronomy explains where they ended up. Stable orbits in two regions let leftover planetesimals avoid being swept up or thrown out.
Those regions are the asteroid belt between Mars and Jupiter and the Kuiper belt beyond Neptune.
Studying asteroids and comets is what lets scientists piece together the whole long story of how the solar system formed.
| Object | Made of | Where most of them are | Role in the story |
|---|---|---|---|
| Asteroid | Rock, no air | Between Mars and Jupiter | Pieces that never formed a planet |
| Comet | Frozen gases, rock and dust | Far out, beyond Neptune | Little changed since the beginning |
| Planet | Rock, or gas and ice | Eight orbits in one flat plane | Leftover bits that clumped into spheres |
| Week review | True or false? |
|---|---|
| The solar system appears to have formed from a disk of dust and gas drawn together by gravity. | ? |
| The shrinking cloud spun slower as it collapsed. | ? |
| The planets orbit the Sun in the same direction and nearly the same plane. | ? |
| Rocky planets formed close to the Sun; gas and ice giants formed farther out. | ? |
| All the asteroids together have more mass than the Moon. | ? |
Strong week. Tomorrow is Sky Night: a Moon check and a spinning family demonstration.