The inner Solar System is a spectacular show of planets and moons dancing around the Sun on the notes played by Gravity. But there is something else lurking in the shadows of the outer Solar System, a cold home to millions of floating icy bodies, asteroids and comets. It is also the home of former planet Pluto!

The outer Solar System is that region of Space beginning right after Neptune’s orbit at around 30 Astronomical Units (AUs) from the Sun, where 1 AU is the average distance between Earth and the Sun (roughly 150 million km or 93 million miles). It is an extremely cold place, with temperatures close to the absolute zero (approx. -270 C); here even hydrogen freezes.

The outer Solar System is composed of two main areas: the Kuiper Belt and the Oort Cloud. The two form a relatively vast area that extends to a distance between 50,00 and 200,000 AUs, according to some estimations. Here, the gravitational field of the Sun progressively loses its strength, and interplanetary space begins.

The Kuiper Belt

Also referred to as the “third zone”, the Kuiper Belt is a donut-shaped region that comprises millions of tiny, icy objects that are thought to be remnants of the early formation of the Solar System.

As these trans-Neptunian objects (TNOs) often collide with each other, they produce even smaller objects, in particular comets and dust.

Comets are wandering bodies of frozen gases, rock and dust; as they orbit the Sun, they gradually melt leaving behind a visible tail. Today, astronomers count a trillion or more short-period comets originating in the Kuiper Belt. These are comets that take less than 200 years to complete an orbit around the Sun.

The Kuiper Belt is also home to relatively larger bodies, the most famous being Pluto. Discovered in 1930, Pluto was initially classified as a planet before being downgraded to “dwarf planet” by the International Astronomical Union in 2006, having failed to clear the area around its orbit.

Sometimes called “The King of the Kuiper Belt”, Pluto is a very peculiar dwarf planet. It rotates counter-clockwise (like Uranus), has its own atmosphere, which is mainly made of nitrogen and methane, and has a surface temperature that varies between about -235 C and -210 C. It also has five moons: Charon, Hydra, Nix, Kerberos and Styx.

Other dwarf planets in the Kuiper Belt include Eris (one of the largest in the Solar System), Haumea and Makemake.

The Oort Cloud

The Oort Cloud is a large region of space made of millions of icy bodies circling the entire Solar System, marking its frontiers. Toward the end of the cloud the Sun’s gravitational pull is less significant and the influence of nearby stars begins to appear.

From this area originate a large number of long-period comets, which have their orbit around the Sun completed in up to thousands of years. The extreme distance from Earth makes it difficult for astronomers to identify the tiny bodies that compose the Oort Cloud and so far only 5 objects that are believed to belong to this area were detected.

Among these there is Sedna, a dwarf planet about three-fourth the size of Pluto that wanders happily in the Oort Cloud at a distance of approximately 900 AU from the Sun, completing an orbit around it in around 10,500 years. Sedna’s most distinctive feature, however, is its reddish colour. It is the second-reddest object in the Solar System, after Mars.