Pluto
What is Pluto?
Dwarf planets· Diameter 2,377 km. The best-known dwarf planet of the Kuiper belt. It was reclassified from planet to dwarf planet in 2006, and in 2015 New Horizons photographed its heart-shaped plain of ice.
Orbit and size
| Diameter | 2,377 km |
|---|---|
| Distance | 29.7~49.3 AU |
| Year | 248 yr |
| Heart | Sputnik Planitia |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Moons of Pluto
Related events
How Pluto's thin atmosphere rises and falls
Pluto has a very thin atmosphere, with a surface pressure about one hundred-thousandth of Earth's. It is made mostly of nitrogen. When sunlight warms the surface a little, nitrogen ice turns into gas and joins the atmosphere; when it gets colder, the gas freezes back onto the ground. Pluto passed its closest point to the Sun in 1989 and is now slowly moving away.
Astronomers measure the atmosphere when Pluto passes in front of a distant star. If there is air, the starlight fades gradually instead of switching off at once. These measurements show that the pressure roughly tripled between 1988 and 2015, even as Pluto moved away from the Sun, probably because heat stored in the surface ice is released slowly. Since about 2019 the pressure has been falling a little.
It was once predicted that Pluto's atmosphere would freeze out almost completely as it moved away from the Sun, and that worry was one reason New Horizons was sent in a hurry. Recent calculations, however, suggest temperatures will drop by only a few degrees, so the atmosphere should not disappear. New Horizons photographed layers of blue haze reaching more than 200 km above the surface.
The dwarf planet in depth
Pluto was called the ninth planet of the solar system for 76 years after its discovery in 1930. At 2,377 km across it is about two-thirds the width of our Moon. It takes 248 years to go around the Sun, so it has not yet finished a single lap since it was found. Its orbit is so stretched that from 1979 to 1999 Pluto was closer to the Sun than Neptune was. That will not happen again until the 23rd century.
In 2005 the discovery of Eris, a body about as large as Pluto, raised a hard question: should Eris be called a planet too? In 2006 the International Astronomical Union set new conditions for a planet. One was that it must have cleared the neighbourhood around its orbit of other bodies. Pluto shares its region with countless icy bodies beyond Neptune, so it failed this test and became a dwarf planet.
In 2015 the New Horizons spacecraft became the first to fly past Pluto. Scientists expected a cold, unchanging ball of ice. Instead the pictures showed nitrogen ice flowing like glaciers and a wide heart-shaped plain called Sputnik Planitia. The plain has no craters at all, so its surface is thought to be less than 10 million years old. Pluto turned out to be a world whose surface is still changing.
Its moon Charon is about half as wide as Pluto. It is so large for a moon that the two circle together around a point that lies outside Pluto. Zoom in within the orbit view and you can see Pluto itself tracing a small circle around that point. Charon keeps the same face toward Pluto and goes around once every 6.4 days.
Frequently asked
- How big is Pluto?
- The diameter of Pluto is 2,377 km.
- How far is Pluto from Earth?
- The distance of Pluto is 29.7~49.3 AU.
- What are the main moons of Pluto?
- The main moons of Pluto are Charon.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.