StarChild Question of the Month for December 2001

Question:

Where do comets come from?

shuttle divider

Answer:

Comets are believed to have two sources. Long-period comets (those which take more than 200 years to complete an orbit around the Sun) originate from the Oort Cloud. Short-period comets (those which take less than 200 years to complete an orbit around the Sun) originate from the Kuiper Belt.

Danish astronomer Jan Oort proposed that comets reside in a huge cloud at the outer reaches of the solar system, far beyond the orbit of Pluto. This has come to be known as the Oort Cloud. Statistics imply that it may contain as many as a trillion comets and may account for a significant fraction of the mass of the solar system. However, since the individual comets are so small and so far away, we have no direct evidence about the actual existence of the Oort Cloud.

diagram showing 
relatively small size of orbits of Neptune and Pluto compared to the 
surrounding Oort Cloud

The Kuiper Belt is a disk-shaped region past the orbit of Neptune roughly 30 to 100 AU from the Sun. The Belt contains many icy bodies which can become comets. Occasionally the orbit of a Kuiper Belt object will be disturbed by gravitational interactions with the giant planets in such a way as to cause the object to take up an orbit that crosses into the inner solar system.

Although the Oort Cloud is much farther away from the Sun than the Kuiper Belt, it appears that the Oort Cloud objects were formed closer to the Sun than the Kuiper Belt objects. Small objects formed near the giant planets would have been ejected from the solar system by gravitational encounters. Those that didn't escape entirely formed the distant Oort Cloud. Small objects that formed farther out had no such interactions, and remained as the Kuiper Belt objects.

shuttle divider

The StarChild site is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/ GSFC.

StarChild Authors: The StarChild Team
StarChild Graphics & Music: Acknowledgments
StarChild Project Leader: Dr. Laura A. Whitlock
Curator: J.D. Myers
Responsible NASA Official: Phil Newman
If you have comments or questions about the StarChild site, please send them to us.