In July 2020, observers in North America were greeted by a sight not seen since the 1990s - a bright comet visible in the sky. This comet was one of the brightest seen in the northern hemisphere since Comet Hale-Bopp in 1997. It was discovered on March 27, 2020, by the NEOWISE space telescope. At the time of its discovery, the comet was very faint, approximately 18th magnitude, but brightened as it began to reach the Sun. This was Comet NEOWISE, an icy dustball that was reaching the apex of its nearly 5,000-year journey. Comet NEWOISE is highly likely to have originated from the Oort Cloud.
But what exactly is the Oort Cloud? Astronomers believe it is likely that the Oort Cloud is a vast area of icy bodies ranging from 1 to 10 kilometers in size that surrounds the Sun at distances greater than 10,000 times that of the Earth and the Sun. At these distances, the Sun’s gravitational force is very weak, and it takes many thousands of years for objects to complete one orbit around the Sun. As such, these bodies spend most of their time at great distances from the Sun, where temperatures are close to absolute zero and reflect almost no light. Because of this, the Oort Cloud has never been directly imaged, but its presence can be inferred based on the number of long-period comets that enter the solar system. These comets are nudged out of their orbits by passing stars and fall towards the Sun in highly elliptical orbits. As they approach the Sun, they heat up and form a great tail made of ionized gas that we can see from Earth.
Named after astronomer Jan Oort, it’s thought the Oort cloud formed after the formation of the planets, some 4.5 billion years ago. Initially, it was thought that the objects in the Oort Cloud formed much closer to the Sun, but gravitational interactions with the young giant planets in our solar system, especially Jupiter, ejected these objects into highly elliptical orbits around the Sun, and many more were ejected outright from the Solar System. Some of these objects could be captured objects captured from the interstellar medium, originating from other solar systems. The early solar system was a very chaotic time. In fact, most craters in the solar system appear to originate during what was called the Late Heavy Bombardment. One possible explanation is that the gas giants migrated outward to their current positions, with Uranus and Neptune even potentially switching places. This chaotic period in the early solar system may have ejected many icy bodies into highly elliptical orbits, forming the Oort Cloud.
Comets act as time capsules from the solar system’s infancy, as their icy compositions preserve materials otherwise lost or altered in the inner solar system over billions of years. Each new comet offers astronomers a rare chance to study the primitive building blocks of the solar system and test our models of solar system formation.
References
NASA Jet Propulsion Laboratory. (2020). Comet C/2020 F3 (NEOWISE). National Aeronautics and Space Administration.
https://science.nasa.gov/solar-system/comets/c-2020-f3-neowise/
NASA Parker Solar Probe Team. (2020, July 15). NASA’s Parker Solar Probe spies newly discovered comet NEOWISE. National Aeronautics and Space Administration.
https://www.nasa.gov/science-research/heliophysics/nasas-parker-solar-probe-spies-newly-discovered-comet-neowise/
NASA Science. (n.d.). Oort Cloud: Facts. National Aeronautics and Space Administration.
https://science.nasa.gov/solar-system/oort-cloud/facts/
NASA Science. (2020). Hubble observes comet NEOWISE. National Aeronautics and Space Administration.
https://science.nasa.gov/asset/hubble/comet-neowise/
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