Pluto

The beloved planet that controversially got reclassified as a dwarf planet. The largest and best studied object in the Kuiper Belt, learn more about Pluto here!

Formation

The Kuiper Belt has millions of objects in orbit around the Sun beyond the orbit of Neptune. Pluto is expected to have formed closer to the Sun in a more circular orbit compared to its present position and eccentric orbit. Rather than collecting the debris in the area and becoming a planet, Pluto stayed as a planetesimal. Some models suggest that when Neptune migrated outwards towards its present position, this caused the Kuiper Belt objects to experience drastic repositioning with Pluto migrating outwards. This is suggested to have stunted Pluto’s growth where it never gained enough mass to become a planet, orbiting the vast empty space that dominates the Kuiper Belt.

Composition

The surface of Pluto is mostly composed of nitrogen ice, with faint traces of less than one percent each of carbon monoxide ice and methane ice. Mountains on Pluto are made of primarily water ice. The side of Pluto that is tidally locked with Charon has more carbon dioxide, while the opposite side has more methane.

Structure

The structure of Pluto is under debate. The core of Pluto is thought to be dense, mostly composed of rocky materials that migrated to the center of the dwarf planet. The rocky core dominates Pluto, taking up 70% of the volume. Surrounding the core is a water ice mantle, with some models even suggesting that there is a liquid water mantle due to the heating mechanism from radioactive decay within Pluto.

Atmosphere

The atmosphere of Pluto is tenuous, with it being mostly composed of methane. Back when the atmosphere for Pluto was first discovered, scientists believed that the atmosphere existed because at the time Pluto was at the closest point to the Sun in its orbit. When New Horizons visited Pluto, the atmosphere was discovered to not be losing gas as the Planet moves away from the Sun at the rate expected. Since methane is a powerful greenhouse gas, this actually warms the atmosphere and causes ices on the surface to sublimate into Pluto, refueling any gas lost. And thanks to methane in the atmosphere, the height of the atmosphere is not large enough to have Solar radiation strip the top of the atmosphere off easily.

Satellites

Pluto has been visited by New Horizons, the only satellite to perform a flyby of the dwarf planet. Taking images of around 80% of its surface, detailed views into the geography and composition of Pluto were obtained. New Horizons was also able to measure the atmosphere of Pluto, refuting the assumption that gases were being lost at a rate to deplete the atmosphere since its discovery. Pluto’s largest moon, Charon, was also imaged by New Horizons as the spacecraft passed the system. Much about Pluto remains to be seen, from the rest of the surface to the other smaller moons, and as such an orbiter would be needed to truly explore this distant world.