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Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune.
It was the first Kuiper belt object (KPO) to be discovered. It is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun.
It is the largest known trans-Neptunian object by volume but is less massive than Eris, a dwarf planet in the scattered disc. Like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small: about one-sixth the mass of the Moon and one-third its volume.
It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU (4.4–7.4 billion km) from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding. Light from the Sun takes about 5.5 hours to reach Pluto at its average distance.
Pluto was discovered by astronomer (and UFO witness) Clyde Tombaugh in 1930, and was originally considered the ninth planet from the Sun. After 1992, following the discovery of several objects of similar size in the Kuiper belt, it was considered a dwarf planet.
In 2005, Eris, which is 27% more massive than Pluto, was discovered, which led the International Astronomical Union (IAU) to define the term "planet" formally for the first time the following year. This definition excluded Pluto and reclassified it as a member of the new "dwarf planet" category.
Pluto has five known moons: Charon is the largest, with a diameter just over half that of Pluto, and Styx, Nix, Kerberos, and Hydra.
Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body.
In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be ingredients for the emergence of life, and produced from methane, nitrogen and related gases.
On July 14, 2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto. During its brief flyby, New Horizons made detailed measurements and observations of Pluto and its moons.
NASA's New Horizons spacecraft flew by Pluto in 2016, and evidence appeared that the dwarf planet may have a liquid ocean buried under its icy shell.
By modeling the impact dynamics that created a massive crater on Pluto's surface, a team of researchers made a new estimate of how thick that liquid layer might be.
The study, led by Brown University geologist Brandon Johnson and published in Geophysical Research Letters, found a high likelihood that there is more than 100 kilometers of liquid water beneath Pluto's surface!
The research also suggests that it likely has a salt content similar to that of the Dead Sea.
Brandon Johnson, an assistant professor in Brown's Department of Earth, Environmental and Planetary Sciences, said:
"Thermal models of Pluto's interior and tectonic evidence found on the surface suggest that an ocean may exist, but it's not easy to infer its size or anything else about it. We've been able to put some constraints on its thickness and get some clues about composition."
The research focused on Sputnik Planum, a basin 900 kilometers across that makes up the western lobe the famous heart-shaped feature revealed during the New Horizons flyby. The basin appears to have been created by an impact, likely by an object 200 kilometers across or larger.
How the basin relates to Pluto's suspected ocean starts with its position on the planet relative to Pluto's largest moon, Charon. Pluto and Charon are tidally locked, they always show each other the same face as they rotate. Sputnik Planum is directly on the tidal axis linking the two worlds.
The position suggests that the basin has what is called a positive mass anomaly: it has more mass than average for Pluto's icy crust. As Charon's gravity pulls on Pluto, it would pull proportionally more on areas of higher mass, which would tilt the planet until Sputnik Planum became aligned with the tidal axis.
Johnson said:
"An impact crater is basically a hole in the ground. You're taking a bunch of material and blasting it out, so you expect it to have negative mass anomaly, but that's not what we see with Sputnik Planum. That got people thinking about how you could get this positive mass anomaly."
Part of the answer is that, after it formed, the basin has been partially filled in by nitrogen ice. That ice layer adds some mass to the basin, but it is not thick enough on its own to make Sputnik Planum have positive mass, the study shows. The rest of that mass may be generated by a liquid lurking beneath the surface.
Models simulated the impact of an object large enough to create a basin of Sputnik Planum's size hitting Pluto at a speed expected for that part in the solar system. The simulation assumed various thicknesses of the water layer beneath the crust, from no water at all to a layer 200 kilometers thick.
The scenario that best reconstructed Sputnik Planum's observed size depth, while also producing a crater with compensated mass, was one in which Pluto has an ocean layer more than 100 kilometers thick, with a salinity of around 30 percent.
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