What is the Kuiper belt?



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    Kuiper Belt 1

    It is no mystery what planets belong to our solar system. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune make up the celestial version of college basketball’s “Elite Eight”. The furthest planet, Neptune, lies a whopping 30.1 astronomical units (AU) from the Sun. That’s over 2,700 million miles away from the Sun – truly, an astronomical distance!
    What exactly lies beyond Neptune – that is, what scientists have deemed the “trans-Neptunian region” – is significantly more mysterious. In this article, I’ll be looking to unpack a specific section of this region of the galaxy. I’ll be telling you about the Kuiper belt, which extends twenty-five AU past Neptune.  
    “Belt”? Hey, I know one Belt…
    Like the Asteroid belt, the Kuiper belt is largely made up of “small bodies” – asteroids, minor planets, space debris, and the like. However, the Kuiper belt is much larger than the Asteroid belt. It is twenty times wider than the Asteroid belt, and has over twenty times as much mass. 
    History and Naming

    The Kuiper belt was formally discovered in 1992 by two astronomers – one from the University of Hawaii, and another from Cal-Berkeley. In their subsequent Nature article, they referred to the Kuiper belt as the “hypothesized population of objects beyond Neptune”. 
    It is referred to as the “Kuiper” belt out of respect for astronomer Gerard Kuiper. In the 1950s, Kuiper wrote a paper speculating on the existence of objects beyond Pluto (which was then the ninth planet in the solar system). 
    There has been some debate over whether “Kuiper belt” is the correct and/or definitive name for this entity. Here, two individuals that also deserve mention are astronomers Kenneth Edgeworth and Fred Leonard. Each of these men pursued the notion of “trans-Neptunian” objects before Kuiper. Edgeworth published two papers on the subject, one in 1943 and the second in 1949. With Leonard, the year was 1930. 

    The Kuiper belt is primarily composed of ice. As of now, over 1,000 “Kuiper body objects” (KBOs) have been found, and there may be as many as 70,000 out there. 
    Kuiper belt 2
    I will now move to discussing the seven largest known KBOs: Pluto, Makemake, Haumea, Charon, Orcus, Quaoar and Ixion. You can compare the sizes of five of these KBOs by looking at the above diagram. (Note Planet Earth on the bottom edge.)

    Pluto was originally classified as a planet, but has since been categorized as a dwarf planet and the largest object within the Kuiper belt. Only Eris outranks it among our solar system’s dwarf planets. It is known for its icy and rocky composition, as well as its three moons: the large Charon, and the comparatively tiny Nix and Hydra. 
    Pluto was discovered by American astronomer Clyde W. Tombaugh on February 18th, 1930.
    Diameter: 2,390 kilometers
    Mass: 1.31 x 10^22 kilograms 


    Makemake is a plutoid – a dwarf planet with an orbit outside of Neptune’s. It is known for its reddish color and its extremely low temperature – hundreds of degrees below zero Celsius! This low temperature has established an icy surface composition – primarily ices of the gases of methane and ethane. 
    Makemake was discovered by a trio of American astronomers – Michael E. Brown, Chad Trujillo and David Rabinowitz – on March 31st, 2005.
    Diameter: ~1,500 (not precisely known) kilometers
    Mass: Not precisely known


    Haumea is another plutoid. It is known for its “extreme elongation” – Haumea is much closer to an ellipsoid than a sphere. This elongation is thought to be the result of two conditions: Haumea’s high rate of rotation, and its accompanying low gravity. Scientists believe that the rotation has essentially stretched Haumea into an ellipsoid.
    There is a controversy over who has officially discovered Haumea. Two teams claim credit – an American team led by Michael E. Brown, and a Spanish team led by Jose Luis Ortiz Moreno. Both teams claim to have discovered Haumea in the early-to-mid 2000s.
    Diameter: Not precisely known
    Mass (of Haumea and its moons): 4.2 × 10 ^ 21 kilograms


    Charon is Pluto’s largest moon. Since Pluto and Charon actually revolve about each other, a suggestion has been made that Charon is not Pluto’s moon; instead, the two are actually twin dwarf planets. 
    Charon was discovered by American astronomer James W. Christy on June 22nd, 1978. Interestingly, Charon was first seen as a “bulge” jutting out from Pluto on one of Christy’s photographic plates.
    Diameter: 1,207 kilometers
    Mass: 1.52 (+/- 0.06) x 10 ^ 21 kilograms


    Orcus has been categorized as a “plutino” due to how its orbit resonates with that of Neptune. It is also connected to Pluto – and is known as the “anti-Pluto”. This is so because Orcus’ and Pluto’s orbits ensure that they will always be on opposite sides of the Sun.
    Orcus was discovered by the Brown – Trujillo – Rabinowitz trio on February 17th, 2004.
    Diameter: ~950 kilometers
    Mass: 6.32 (+/- 0.05) x 10 ^ 20 kilograms


    Quaoar has the potential to be a dwarf planet. One scientists has suggested that Quaoar used to be larger than it is now. It then collided with another dwarf planet; this collision left behind the Quaoar that we now know. 
    Quaoar was discovered by Brown and Trujillo on June 5th, 2002. 
    Diameter (2004 estimate): 1260 +/- 190 kilometers
    Mass: ~2.5 x 10 ^ 21 kilograms


    Like Quaoar, Ixion has the potential to be categorized as a dwarf planet. It is the fifth largest plutino. 
    Ixion was discovered by the Deep Elliptic Survey on May 22nd, 2001.
    Diameter: ~650 kilometers
    Mass: 2.8 x 10 ^ 20 kilograms 
    Stephen C. Tegler (2007). “Kuiper Belt Objects: Physical Studies”. in Lucy-Ann McFadden et. al.. Encyclopedia of the Solar System. pp. 605–620.
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