The Dwarf Planet
Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun. After 1992, its status as a planet was questioned following the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, a dwarf planet in the scattered disc which is 27% more massive than Pluto, was discovered. This led the International Astronomical Union (IAU) to define the term “planet” formally in 2006, during their 26th General Assembly. That definition excluded Pluto and reclassified it as a dwarf planet.
Pluto 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. 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 (39.5 AU).
Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), 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.
Layers of Pluto
Because the decay of radioactive elements would eventually heat the ices enough for the rock to separate from them, scientists expect that Pluto’s internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of water ice.
The diameter of the core is hypothesized to be approximately 1700 km, 70% of Pluto’s diameter. It is possible that such heating continues today, creating a subsurface ocean of liquid water 100 to 180 km thick at the core–mantle boundary. In September 2016, scientists at Brown University simulated the impact thought to have formed Sputnik Planitia, and showed that it might have been the result of liquid water upwelling from below after the collision, implying the existence of a subsurface ocean at least 100 km deep. Pluto has no magnetic field.
Pluto has a tenuous atmosphere consisting of nitrogen (N2), methane (CH4), and carbon monoxide (CO), which are in equilibrium with their ices on Pluto’s surface. According to the measurements by New Horizons, the surface pressure is about 1 Pa (10 μbar), roughly one million to 100,000 times less than Earth’s atmospheric pressure. It was initially thought that, as Pluto moves away from the Sun, its atmosphere should gradually freeze onto the surface; studies of New Horizons data and ground-based occultations show that Pluto’s atmospheric density increases, and that it likely remains gaseous throughout Pluto’s orbit. New Horizons observations showed that atmospheric escape of nitrogen to be 10,000 times less than expected.
Alan Stern has contended that even a small increase in Pluto’s surface temperature can lead to exponential increases in Pluto’s atmospheric density; from 18 hPa to as much as 280 hPa (three times that of Mars to a quarter that of the Earth). At such densities, nitrogen could flow across the surface as liquid. Just like sweat cools the body as it evaporates from the skin, the sublimation of Pluto’s atmosphere cools its surface. The presence of atmospheric gases was traced up to 1670 kilometers high; the atmosphere does not have a sharp upper boundary.
- Pluto is named after the Greek god of the underworld. This is a later name for the more well known Hades and was proposed by Venetia Burney an eleven year old schoolgirl from Oxford, England.
- Pluto was reclassified from a planet to a dwarf planet in 2006. This is when the IAU formalised the definition of a planet as “A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.”
- Pluto was discovered on February 18th, 1930 by the Lowell Observatory. For the 76 years between Pluto being discovered and the time it was reclassified as a dwarf planet it completed under a third of its orbit around the Sun.
- Pluto has five known moons. The moons are Charon (discovered in 1978,), Hydra and Nix (both discovered in 2005), Kerberos originally P4 (discovered 2011) and Styx originally P5 (discovered 2012) official designations S/2011 (134340) 1 and S/2012 (134340) 1.
- Pluto is the largest dwarf planet. At one point it was thought this could be Eris. Currently the most accurate measurements give Eris an average diameter of 2,326km with a margin of error of 12km, while Pluto’s diameter is 2,372km with a 2km margin of error.
- Pluto is one third water. This is in the form of water ice which is more than 3 times as much water as in all the Earth’s oceans, the remaining two thirds are rock. Pluto’s surface is covered with ices, and has several mountain ranges, light and dark regions, and a scattering of craters.
- Pluto is smaller than a number of moons. These are Ganymede, Titan, Callisto, Io, Europa, Triton, and the Earth’s moon. Pluto has 66% of the diameter of the Earth’s moon and 18% of its mass. While it is now confirmed that Pluto is the largest dwarf planet for around 10 years it was thought that this was Eris.
- Pluto has a eccentric and inclined orbit. This takes it between 4.4 and 7.3 billion km from the Sun meaning Pluto is periodically closer to the Sun than Neptune.