Written By: Ran Levi
This article is a part of a series named ‘Astronomy Shorts’, which brings bite-sized interesting and easy to understand facts and anecdotes about our universe and solar system.
This episode will focus on a few of the lesser-known children of the Solar System neighborhood: The Oort Cloud, Kuiper’s Belt & Dwarf Planets.
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Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune are the most familiar among the Solar System members and have been researched thoroughly. But this episode will describe a few of the lesser-known children of the Solar System neighborhood.
Oort Cloud, named after the astronomer who discovered it, Jan Oort, is a collection of small planetesimals. A planetesimal is a small celestial body formed during the creation of the solar system. Oort Cloud is believed to surround the Sun at a far distance: a few hundred thousand times the distance between planet Earth and the Sun. Essentially, The Solar System is “wrapped” by a cloud which is mostly made of large frozen rocks.
Oort Cloud is a quiet area with very little activity, but every now and then something exciting happens: for example, a distant star passes by the Solar System and its gravity pushes some of those rocks towards its inner regions. As the rocks get closer to the Sun, the increasing heat melts the ice that’s accumulated on them. The water vapors are ejected from the rock, a are blown away by the ‘Solar wind’ – small particles discharged by the Sun. The result is the long and impressive tail of a comet. In fact, that is exactly how the cloud was discovered: Jan Oort assumed that if comets continuously reach us from distant areas of the Solar System, then somewhere there must be a storage of “deep frozen” comets. Observations and computerized models confirmed that Oort Cloud contains trillions of comets in different sizes.
Oort Cloud surrounds the Sun as a big ball, and its comets reach us from different directions. Yet one group of comets in particular interest astronomers – those comets whose routes are always aligned to the plane of the Solar System, the same one on which all the other planets move. This fact brought the astronomers to conclude that Oort Cloud couldn’t be the only source of comets. They assumed that there should be a belt of objects that orbits the Sun in the area between Neptune, the most distant planet, and the far Oort Cloud. The existence of this belt was confirmed by Gerard (Ky per) Kuiper and Kenneth Edgeworth in the 1940s.
In 1992, the first object in the Kuiper Belt was discovered, and since then more than 400 other objects have been observed. The area between Kuiper and Oort Cloud is called “the Circumstellar Disc,” and it contains objects that used to be a part of Kuiper’s Belt, but were thrown out due to the gravitational forces exerted by the inner planets.
When Pluto was discovered in 1930, it was considered to be the smallest planet of the Solar System, but having strange characteristics compared to other planets. For example, all other planets orbit the Sun in a circular trajectory and move on the same plane – while Pluto’s trajectory is extremely elliptic and at a high angle relative to the Sun’s path. But despite Pluto being “the black sheep,” it was classified as a real planet.
But as other small objects were discovered in the outer reaches of the solar system, like Aris whose size was more or less the size of Pluto – Astronomers encountered a dilemma. If Pluto was to be classified as a planet, then all other similar bodies should be included in the same category, too. Frankly, if Pluto were to be discovered nowadays, most likely it wouldn’t be considered a planet at all… The astronomical community debated over the question “what is a planet?” and finally, in 2006, it was decided to remove Pluto from the list of planets and define it as a “dwarf planet.” According to the official definition, a dwarf planet is a round object in direct orbit of the Sun whose gravity is not strong enough to “clean” the area around it of other small objects, as larger planets do. A bit of an insult for Pluto, but it is what it is.