The Kuiper Belt extends from just beyond the orbit of Neptune to a distance of about 50 astronomical units (4.65 billion miles) from the Sun. The Kuiper Belt is named for a Dutch-American astronomer, Gerald Kuiper, who predicted its existence in 1951, and it is the Kuiper Belt that has begun to provide the clues to answering the riddle of the solar system.
Astronomers had long theorized that many icy bodies orbit the sun beyond the planets. Astronomers predicted the existence of such objects based on a widely accepted theory that the planets in our solar system were formed from a rotating disk of gas and dust surrounding the infant sun. Theoretical models of solar system formation showed that beyond Neptune, small bodies, representing debris from the formation of the main planets, should exist. In addition, the motions of comets indicated that these small icy bodies originate far beyond the known planets.
Shortly after the beginning of the second millennium of the Christian calendar, many astronomers began to reassess the status of Pluto, the distant body that had been designated as the ninth planet. Astronomers noted that Pluto is more closely related to the Kuiper Belt objects than to the other planets. Pluto is far smaller than any other planet, and physically it is much more like a Kuiper Belt object or a large moon than a planet. Additionally, the orbit of Pluto is different from the other planets but is similar to other Kuiper Belt objects.
The reassessment of Pluto eventually led to it being "demoted" from being a planet. Indeed, other Kuiper Belt discoveries has led to the listing of Pluto with fellow Kuiper Belt objects, Haumea and Makemake, as being dwarf planets. Indeed, since the belt was discovered in 1992, the number of known Kuiper Belt objects (KBOs) has increased to over a thousand, and more than 100,000 KBOs over 100 km (62 mi) in diameter are believed to exist. The Kuiper Belt was initially thought to be the main repository for periodic comets, those with orbits lasting less than 200 years. However, studies since the mid-1990s have shown that the classical belt is dynamically stable, and that comets' true place of origin is the scattered disc, a dynamically active zone created by the outward motion of Neptune 4.5 billion years ago. Scattered disc objects such as Eris have extremely eccentric orbits that take them as far as 100 AU (9.3 billion miles) from the Sun.
Today, Pluto is the largest known member of the Kuiper belt. Originally considered a planet, Pluto's status as part of the Kuiper Belt caused it to be reclassified as a "dwarf planet" in 2006. It is compositionally similar to many other objects of the Kuiper Belt, and its orbital period is characteristic of a class of KBOs known as "plutinos".
Astronomers are fascinated by Kuiper Belt objects because they are cosmic leftovers, material that is virtually unchanged from the time when the sun and planets formed, about 4.6 billion years ago. By studying the Kuiper Belt, astronomers expect to learn much about the early solar system and how it came to be.
4.57 Billion Years B.C.T. – A supernova explosion (known as the primal supernova) seeds our galactic neighborhood with heavy elements that will be incorporated into the Earth, and results in a shock wave in a dense region of the Milky Way galaxy. The Ca-Al-rich inclusions, which formed 2 million years before the chondrules, are a key signature of a supernova explosion.
4.567 Billion Years B.C.T. – A rapid collapse of hydrogen molecular cloud h, forming a third-generation Population I star, the Sun, in a region of the Galactic Habitable Zone (GHZ), about 25,000 light years from the center of the Milky Way Galaxy.
4.566 Billion Years B.C.T. – A protoplanetary disc (from which Earth eventually forms) emerges around the young Sun, which is in its T Tauri stage.
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