Failed Planet Formation

What is a Planet We get the term "planet" from the Greek word "Planetes" - meaning wanderer. The IAU in 2006 voted to define a planet as: 1.it must orbit the Sun. 2.it must be big enough for gravity to squash it into a round ball. 3.it must have cleared other objects out of the way in its orbital neighborhood.

Asteroids Asteroids are rocky, airless worlds that orbit our sun, but are too small to be called planets. Tens of thousands of these "minor planets" are gathered in the main asteroid belt, a vast doughnut-shaped ring between the orbits of Mars and Jupiter. Asteroids that pass close to Earth are called Near- Earth Objects (NEOs).

Asteroid Belt

Eros Vesta Ceres

Kuiper Belt The Kuiper Belt is a disc-shaped region of icy objects beyond the orbit of Neptune -- billions of kilometers from our sun. Pluto and Eris are the best known of these icy worlds. There may be hundreds more of these ice dwarfs out there. The Kuiper Belt and even more distant Oort Cloud are believed to be the home of comets that orbit our sun.

Kuiper Belt Extends from 30 to 50 AU Contains about 100,000 comets that are greater than 100 km across Comets orbit in the same plane and direction as the planets

Kuiper Belt

Plutoids

Oort Cloud A spherical cloud of predominantly icy planetesimals that may lie roughly 50,000 AU, or nearly a light- year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun.

Oort Cloud Extends out to 50,000 AU Contains trillions of comets Comets orbits have random tilts and eccentricities