Dwarf Planets and Minor Bodies

Learning Objectives

Define a dwarf planet and list the criteria for this classification.

Describe the properties and locations of major dwarf planets like Pluto and Ceres.

Identify the composition and origin of comets and asteroids.

Distinguish between the Asteroid Belt, the Kuiper Belt, and the Oort Cloud.

Leftovers of the Solar System

While the eight major planets dominate the solar system, they are accompanied by a vast number of smaller objects that hold clues to our system's formation.

Dwarf Planets

In 2006, the International Astronomical Union (IAU) established a new classification for celestial bodies. A dwarf planet is a celestial body that:

1.Is in orbit around the Sun.

2.Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape.

3.Has not cleared the neighborhood around its orbit.

4.Is not a satellite (a moon).

This third criterion is what distinguishes a dwarf planet from a major planet.

Pluto: The most famous dwarf planet. It resides in the Kuiper Belt and has a complex system of five moons. Its surface is composed of nitrogen ice, methane, and carbon monoxide.

Ceres: The largest object in the Asteroid Belt, large enough to be spherical. It is the only dwarf planet in the inner solar system.

Eris, Makemake, Haumea: Other major dwarf planets located in the Kuiper Belt or beyond.

Asteroids

Asteroids are small, rocky bodies that orbit the Sun. Most are found in the Main Asteroid Belt, a region between the orbits of Mars and Jupiter. They are remnants from the early solar system that failed to coalesce into a planet due to Jupiter's immense gravitational influence. They are rich in metals and silicate rock.

Comets

Comets are often called "dirty snowballs" and are composed of ice (water, CO₂, ammonia), rock, and dust. They originate from the outer reaches of the solar system.

The Kuiper Belt: A doughnut-shaped ring of icy bodies extending just beyond the orbit of Neptune. It is the source of short-period comets.

The Oort Cloud: A theoretical, immense spherical cloud of icy planetesimals surrounding the Sun at a great distance. It is the source of long-period comets.

When a comet's orbit brings it into the inner solar system, the Sun's heat causes the ices to sublimate (turn directly to gas), creating a glowing coma (atmosphere) and often two tails: a plasma tail and a dust tail.

Key Terms

Dwarf Planet: A celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of other objects.
Asteroid Belt: The circumstellar disc in the Solar System located roughly between the orbits of the planets Mars and Jupiter, occupied by numerous irregularly shaped bodies called asteroids.
Kuiper Belt: A region of the Solar System beyond the orbit of Neptune, believed to contain many comets, asteroids, and other small bodies made largely of ice.
Oort Cloud: A theoretical thick-walled bubble of icy debris that surrounds our solar system, thought to be the origin of most long-period comets.
Sublimation: The transition of a substance directly from the solid to the gas state, without passing through the liquid state.