The Oort Cloud is a huge, distant reservoir of icy objects surrounding the Solar System. It is often called a comet nursery because many long-period comets are thought to begin there. These objects are leftovers from the early formation of the Sun and planets.
Studying the Oort Cloud helps astronomers understand both the history of the Solar System and the origin of comets that visit the inner planets.
Unlike the asteroid belt or Kuiper Belt, the Oort Cloud is believed to form a roughly spherical shell around the Sun. Passing stars, giant molecular clouds, or the gravity of the Milky Way can disturb icy bodies and send some inward. As one falls toward the Sun, sunlight heats its ice and dust, creating a glowing coma and tail.
Because the Oort Cloud is so far away, its existence is inferred mainly from the orbits of long-period comets rather than direct images.
Key Facts
- The Oort Cloud is a proposed spherical shell of icy bodies surrounding the Solar System.
- Estimated distance from the Sun: about 2,000 AU to 100,000 AU, where 1 AU is the average Earth-Sun distance.
- 1 AU = 1.496 x 10^8 km.
- Long-period comets often have orbital periods greater than 200 years.
- Kepler's third law for objects orbiting the Sun: T^2 = a^3 when T is in years and a is in AU.
- A comet develops a coma and tail when solar heating causes ices to sublimate near the Sun.
Vocabulary
- Oort Cloud
- A distant, roughly spherical region of icy objects thought to surround the Solar System far beyond the planets.
- Long-period comet
- A comet with an orbit around the Sun that takes more than 200 years to complete.
- Astronomical unit
- A unit of distance equal to the average distance from Earth to the Sun, about 149.6 million kilometers.
- Sublimation
- The process in which a solid changes directly into a gas without becoming a liquid first.
- Coma
- The fuzzy cloud of gas and dust that forms around a comet nucleus as it is heated by the Sun.
Common Mistakes to Avoid
- Thinking the Oort Cloud has been directly photographed, which is wrong because its objects are too small, dark, and distant to observe clearly with current telescopes.
- Placing the Oort Cloud just beyond Neptune, which is wrong because that region is the Kuiper Belt and the Oort Cloud begins much farther away.
- Assuming comet tails point behind the comet along its path, which is wrong because tails are pushed away from the Sun by solar radiation and the solar wind.
- Treating the Oort Cloud as a flat disk, which is wrong because it is expected to be a roughly spherical shell surrounding the Solar System.
Practice Questions
- 1 The inner edge of the Oort Cloud is about 2,000 AU from the Sun. Using 1 AU = 1.496 x 10^8 km, calculate this distance in kilometers.
- 2 A comet has a semi-major axis of 10,000 AU. Using T^2 = a^3, estimate its orbital period T in years.
- 3 Explain why a passing star could send an Oort Cloud object toward the inner Solar System and why that object might become visible as a comet near the Sun.