Beyond the gas giant Neptune lies a region of space filled with icy bodies. Known as the Kuiper Belt, this chilly expanse holds trillions of objects, remnants of the early solar system. Dutch astronomer Jan Oort first proposed in 1950 that some comets might come from the the solar system’s far suburbs. That reservoir later became known as the Oort cloud. Earlier, in 1943, astronomer Kenneth Edgeworth had suggested comets and larger bodies might exist beyond Neptune. In 1951, astronomer Gerard Kuiper predicted the existence of a belt of icy objects that now bears his name. Some astronomers refer to it as the Edgeworth-Kuiper Belt.
Astronomers are now hunting for a planet in the Kuiper Belt, a true ninth planet, after evidence of its existence was unveiled on Jan. 20, 2016. The so-called "Planet Nine," as scientists are calling it, is about 10 times the mass of Earth and 5,000 times the mass of Pluto.
Let's take a closer look at this distant section of the solar system and the small worlds most commonly known as Kupier Belt Objects (KBOs) and, in recent years, dwarf planets.
Kuiper Belt facts
The Kuiper Belt is an elliptical plane in space spanning from 30 to 50 times Earth's distance from the sun, or 2.5 to 4.5 billion miles (4.5 to 7.4 billion kilometers). The belt is similar to the asteroid belt found between Mars and Jupiter, although the objects in the Kuiper Belt tend more to be icy rather than rocky.
Scientists estimate that thousands of bodies more than 62 miles (100 km) in diameter travel around the sun within this belt, along with trillions of smaller objects, many of which are short-period comets. The region also contains several dwarf planets, round worlds too large to be considered asteroids and yet not qualifying as planets because they’re too small, on an odd orbit, and don’t clear out the space around them the way the 8 planets do.
Kuiper Belt formation
When the solar system formed, much of the gas, dust and rocks pulled together to form the sun and planets. The planets then swept most of the remaining debris into the sun or out of the solar system. But bodies farther out remained safe from gravitational tugs of planets like Jupiter, and so managed to stay safe as they slowly orbited the sun. The Kuiper Belt and its compatriot, the more distant and spherical Oort Cloud, contain the leftover remnants from the beginning of the solar system and can provide valuable insights into its birth.
The classical Kuiper Belt — the most crowded section — lies between 42 and 48 times Earth's distance from the sun. The orbit of objects in this region remain stable for the most part, although some objects occasionally have their course changed slightly when they drift too close to Neptune.
Kuiper Belt Objects
Pluto was the first true Kuiper Belt Object to be seen, although scientists at the time didn't recognize it as such. The existence of the belt wasn't realized until scientists discovered a slow moving, small world in the outer solar system in 1992 (David Jewitt and Jane Luu found the KBO, 1992QB1.). Other objects soon followed, and astronomers quickly saw that the region beyond Neptune teemed with icy rocks and tiny worlds.
Sedna (sed’nah), about three-fourths the size of Pluto, was discovered in 2004. It is so far out from the sun it takes about 10,500 years to make a single orbit. Sedna is about 1,100 miles (1,770 km) wide and circles the sun on an eccentric orbit that ranges between 8 billion miles (12.9 billion km) and 84 billion miles (135 billion km).
In July 2005, astronomers announced the discovery of an object in the Kuiper Belt thought to be larger than Pluto, though subsequent observations revealed it was slightly smaller. Known as Eris, it orbits the sun approximately once every 580 years, traveling almost one hundred times farther from the sun than Earth does. Eris' discovery revealed to some astronomers the problem of terming Pluto a full-scale planet, and in 2006, Pluto, Eris, and the largest asteroid Ceres were reclassified as dwarf planets. Two more dwarf planets, Haumea and Makemake, were discovered in the Kuiper Belt in 2008.
Planet Nine orbits the sun at a distance that is 20 times farther out than the orbit of Neptune. (The orbit of Neptune is 2.7 billion miles from the sun at its closest point.) The strange world's orbit is about 600 times farther from the sun than the Earth's orbit is from the star.
Scientists have not actually seen Planet Nine directly. Its existence was inferred by its gravitational effects on other objects in the Kuiper Belt.
Scientists Mike Brown and Konstantin Batygin at the California Institute of Technology in Pasadena described the evidence for Planet Nine in a study published in the Astronomical Journal. The research is based on mathematical models and computer simulations using observations of six other smaller Kuiper Belt Objects with orbits that aligned in a similar matter.
Because of their small size and distant location, Kuiper Belt Objects are a challenge to spot from Earth. Infrared measurements from NASA's space-based telescope, Spitzer, have helped to nail down sizes for the largest objects.
In order to catch a better glimpse of these remote leftovers from the birth of the solar system, NASA launched the New Horizons mission. The spacecraft reached Pluto in 2015 and continued on with an aim to examine multiple KBOs.
See more about Discoveries and Discoverers of KBOs & Dwarf Planets.