Kuiper belt

The Kuiper belt is a big area filled with icy objects far beyond the planet Neptune in our Solar System. It stretches from about 30 to 50 times the distance between the Earth and the Sun. This region is much larger and heavier than the asteroid belt between Mars and Jupiter. Instead of rocks and metals, Kuiper belt objects are made mostly of frozen gases like methane, ammonia, and water.

The Kuiper belt is named after the astronomer Gerard Kuiper, who suggested its existence in 1951. The first object in this belt, besides Pluto, was discovered in 1992, and since then, thousands more have been found. Many experts think there could be over 100,000 objects in the Kuiper belt that are bigger than 100 kilometers across.

Several dwarf planets, including Pluto, Haumea, Quaoar, Makemake, and Orcus, live in the Kuiper belt. These objects help us learn about how our Solar System formed and how planets grow. The Kuiper belt marks one edge of our Solar System, along with more distant areas like the Oort cloud.

History

After Pluto was found in 1930, people started wondering if it was alone. The area now called the Kuiper belt was guessed about for many years. In 1992, the first real proof that it exists was discovered.

Many astronomers thought there might be many small objects beyond Neptune. After careful searching, these objects were found, showing that the Kuiper belt is real.

Structure

The Kuiper belt is a big area beyond the planet Neptune. It stretches from about 30 to 55 astronomical units (AU) from the Sun. It is much larger than the asteroid belt, about 20 times wider and even more massive.

Instead of being flat, the Kuiper belt is shaped like a doughnut. Objects are spread out above and below where most planets orbit.

Neptune's gravity affects the Kuiper belt. It creates gaps where objects cannot stay in stable orbits. One part, called the classical Kuiper belt, lies between two of these gaps and holds most known objects. These objects are sometimes called cubewanos. Another group, called plutinos, orbit in sync with Neptune and include the dwarf planet Pluto.

Why there are fewer objects beyond about 50 AU, known as the Kuiper cliff, is still a mystery.

Origin

The Kuiper belt is made of small pieces left over from when the Solar System was formed. These pieces are called planetesimals. They did not join together to make planets. Instead, they stayed as smaller objects.

Scientists use computer simulations to study how the Kuiper belt formed. These simulations show that big planets like Jupiter and Neptune changed the belt a lot. As these planets moved to where they are now, they pushed many small objects around. This shaped the Kuiper belt we see today.

Composition

Kuiper belt objects are made of rock and ices like water, methane, and ammonia. Because they are far from the Sun, these objects have stayed mostly the same since the Solar System formed. This helps us learn about the early days of our Solar System. Scientists use a tool called spectroscopy to study these objects. Spectroscopy looks at the light from an object to find out what it is made of.

Some of the largest Kuiper belt objects, like Pluto, have surfaces with methane, nitrogen, and carbon monoxide. These molecules can turn into gas and then fall back as snow. Smaller objects can be gray or red, which might be because of hydrogen sulfide on their surfaces. Water ice has been found on many of these objects.

Mass and size distribution

The Kuiper belt does not have a lot of material. Scientists think the moving part has about 1% of Earth’s mass. The calmer part has only about 0.03% of Earth’s mass. This small amount makes it hard to explain how bigger objects in the Kuiper belt formed.

Researchers study how many objects of different sizes are in the Kuiper belt. They find that there are fewer objects as they get bigger, and this follows certain patterns. These patterns help scientists learn about how the Kuiper belt developed and how objects there may have formed.

Scattered objects

Main articles: Scattered disc and Centaur (minor planet)

The scattered disc is a region near the Kuiper belt. It stretches far out, up to 100 times the distance from the Sun. Objects here have stretched and tilted orbits because of past pulls from the planet Neptune. These objects may be the source of some comets that visit the inner Solar System.

According to the Minor Planet Center, objects inside the Kuiper belt are called Kuiper belt objects. Those outside are called scattered objects. Sometimes, scientists call scattered objects scattered Kuiper belt objects, even if they now orbit far from the Kuiper belt. For example, Eris, which is bigger than Pluto, is a scattered object but is often called a Kuiper belt object.

Triton

Main article: Triton (moon)

Neptune may have captured a large Kuiper belt object called Triton as it moved outward. Triton has a special orbit that goes against Neptune's spin. This suggests it was not born around Neptune but was caught from space. Triton’s surface is made of materials like methane and carbon monoxide, similar to Pluto. This supports the idea that it was once a Kuiper belt object.

Largest KBOs

See also: List of the brightest Kuiper belt objects

Since 2000, many big objects in the Kuiper belt have been found. These objects are from about 500 to 1,500 kilometers across—more than half the size of Pluto. Some of these include Quaoar, Makemake, and Haumea, which are especially large.

Pluto used to be called a planet. But after finding other objects of similar size, scientists decided Pluto was more like its neighbors. Pluto shares many features with these large Kuiper belt objects, such as having moons and similar compositions. This led to Pluto being called a dwarf planet instead of a regular planet.

Exploration

See also: New Horizons

The first spacecraft to explore the Kuiper belt was New Horizons. It was launched on January 19, 2006. New Horizons flew by the dwarf planet Pluto on July 14, 2015. After that, it explored other objects in the Kuiper belt.

In 2014, the Hubble Space Telescope found three possible targets for New Horizons to visit. The spacecraft later visited an object named Arrokoth (previously called 2014 MU₆₉) on January 1, 2019. This object looks like two pieces stuck together and is about 32 kilometers long. Scientists are still learning from the data New Horizons sent back. There are no plans to send New Horizons on more missions, but scientists are thinking about new missions to visit other objects in the Kuiper belt, like the dwarf planets Makemake and Haumea.

Extrasolar Kuiper belts

Main article: Debris disc

By 2006, scientists found dust discs around nine stars that might be like our Kuiper belt. These discs come in two types: wide belts that stretch far out and narrow belts that are tighter. Many stars also show signs of having big, Kuiper-belt-like structures. The Hubble Space Telescope shows these discs in stable shapes, even though they are very old.