Pluto is very small and very far away, but it’s one of the most controversial objects in the solar system. It used to be classed as a planet; now it isn’t. Lots of people were confused, and even angry, when that changed.
So what is Pluto, really? How much do you know about this unusual object?
Most of the time Pluto can be found outside the orbit of Neptune, but its own path is highly elliptical and sometimes it’s closer to the Sun than Neptune is. Its orbit is unusual in other ways too. Pluto is small – much smaller than our Moon – and dark, so it’s impossible to see with the naked eye.
A good amateur telescope can see it sometimes, but only when it’s closest to the Sun. So if you want to know more about Pluto you’re better looking at this page than at the sky.
Mass: 1.303×1022 kg
Equatorial Diameter: 2,374km
Equatorial Circumference: 7,232km
Known Moons: 5
Notable Moons: Charon
Orbit Period: 248 years
Surface Temperature: -229°C
How far is Pluto from the Sun?
A long, long way. At its closest, Pluto is 29.656 AU from the Sun – that’s 4,436,400,000km, or 2,772,750,000 miles. It passed that point in 1989 and right now is heading for the other end of its orbit, 49.319 AU (7,378,070,000km, or 4,611,250,000 miles) out. It won’t get there until February 2114.
How hard is it to see Pluto?
It’s definitely not easy, but it’s not impossible if you have a decent telescope. A 200mm reflector can pick it up when it’s as far from the Sun as it is now. You won’t see much though. It’s so small and so distant that even through a big telescope it’s just a small faint dot.
Astronomers couldn’t even tell what size it was until the invention of space-based telescopes with adaptive optics.
The only reason Pluto is visible from Earth at all is that its surface it reflects light well. Most of the surface is nitrogen ice. There isn’t much light to reflect though, because the Sun is so distant. Midday on Pluto is about as dark as five minutes after sunset on Earth.
Who discovered Pluto?
Astronomers started searching for a ninth planet, beyond Neptune, in the late 19th century. They knew there was something out there, because its gravity was affecting the orbit of Uranus slightly, but nobody could find it. American astronomer Pervical Lowell started searching for “Planet X” in 1906.
By the time he died in 1916 he had taken two photographs of Pluto – but didn’t recognise it as a planet. Then, in 1930, Clyde Tombaugh made the discovery at Lowell’s observatory in Arizona.
The name “Pluto” was chosen because it begins with Percival Lowell’s initials.
Is there anything unique about Pluto?
Yes there is. It’s largest moon, Charon, has over 12% of the mass of Pluto itself. It’s large enough that it and Pluto both orbit a point nearly 900km above Pluto’s surface. Some astronomers think that rather than a moon Charon is also a dwarf planet, and that it and Pluto are a double system.
Pluto’s orbit is also unusual. As well as being highly elliptical it’s also tilted. Most planets and other solar system objects are on or near the ecliptic, a flat plane with the Sun at its centre. Mercury is the planet furthest from this plane, with an orbital tilt of about 7°. All the other planets orbit within 3.5° of the ecliptic. Pluto’s orbit is tilted by 17°.
How long is a year on Pluto?
One circuit of Pluto’s object is a journey of close to 40 billion kilometres. Even at an average speed of 4.67km per second it takes Pluto 248 Earth years to make the trip. For about 20 years of that time it’s closer to the Sun than Neptune is.
Pluto is an icy planet
About 70% of Pluto’s mass is rock, and the rest is ice. It’s very hard to investigate the internal structure because of the distances involved, but scientists believe radioactive decay would have heated the ice enough for the rock to settle into a dense core with an ice mantle about 300km thick on top of it. Most of the ice is water, but the surface is mainly nitrogen ice.
Does Pluto have an atmosphere?
Although it’s small and gravity is weak, Pluto does have an atmosphere – and it’s quite deep. It’s less than 0.001% as dense as Earth’s but extends more than 1,600km above the surface.
Most of it is nitrogen but there’s also some methane in there. Methane is a powerful greenhouse gas, so Pluto’s atmosphere is warmer than the surface.
More Pluto facts
- Only one spacecraft has visited Pluto. NASA’s New Horizons probe passed within 18,000km in 2015, taking the best photographs yet.
- Pluto’s orbit is unstable. Because it’s so small the gravity of other objects affects it, and over millions of years the orbit can drift unpredictably.
- The colour of Pluto’s surface varies from black, through orange to white.