Pluto Facts: Size, Moons, and More

Pluto is a dwarf planet in the Kuiper Belt, and the most surprising facts about it are that it is smaller than Earth’s Moon, has a heart-shaped glacier, and takes 248 Earth years to orbit the Sun. Once the ninth planet, Pluto was reclassified in 2006 after astronomers found similar icy worlds nearby. Here is what makes this distant world so fascinating.

Formula

The calculator above works out Pluto’s average orbital speed using the circumference of its orbit divided by the time it takes to complete one orbit. The formula is:

Speed = (2 × π × r) / T

Where:

• r = average distance from the Sun in astronomical units (AU), converted to kilometers (1 AU = 149,597,870 km)
• T = orbital period in Earth years, converted to seconds (1 year = 365.25 days × 86400 seconds)
• Speed = average orbital speed in kilometers per second (km/s)

Worked Example

Using the defaults: Pluto orbits at 39.5 AU and takes 248 Earth years.

1. Convert distance to km: 39.5 × 149,597,870 = 5,909,115,865 km
2. Circumference = 2 × π × 5,909,115,865 ≈ 37,128,000,000 km
3. Convert time to seconds: 248 × 365.25 × 86400 = 7,823,000,000 s
4. Speed = 37,128,000,000 / 7,823,000,000 ≈ 4.75 km/s

The result shows about 4.75 km/s, which matches the known average orbital speed of Pluto (about 4.7 km/s).

Why Pluto Is a Dwarf Planet

In 2006, the International Astronomical Union (IAU) defined a planet as a body that orbits the Sun, has enough gravity to be round, and has cleared its orbital neighborhood of other debris. Pluto fails the third test because its orbit is crowded with other Kuiper Belt objects. So it was reclassified as a dwarf planet, along with Eris, Haumea, Makemake, and Ceres.

Pluto’s orbit is also unusual: it is highly elliptical and tilted 17 degrees above the plane of the solar system. For about 20 years of its 248-year orbit, Pluto is closer to the Sun than Neptune is — the last time was from 1979 to 1999. Yet the two never collide because their orbits are in a 3:2 resonance: Neptune orbits three times for every two Pluto orbits.

Pluto’s Size and Composition

Pluto’s diameter is about 2,377 kilometers (1,477 miles), which is roughly two-thirds the size of Earth’s Moon. Its surface is mostly nitrogen ice, with methane and carbon monoxide frost. The heart-shaped region, called Tombaugh Regio, is a large plain of nitrogen ice that is constantly resurfacing itself through convection — like a slow-boiling pot.

Beneath the ice, Pluto likely has a rocky core and an ocean of liquid water that may still exist today. The heat from radioactive decay in the core keeps the ocean from freezing solid, even though the surface temperature is about -230°C (-382°F).

Pluto’s Five Moons

Pluto has five known moons. The largest, Charon, is so big that Pluto and Charon orbit a point between them — effectively a double dwarf planet system. Charon is about half the diameter of Pluto, and its gravity keeps Pluto tidally locked, meaning the same side of Pluto always faces Charon.

The other four moons are much smaller: Styx, Nix, Kerberos, and Hydra. They are irregularly shaped and probably formed from a giant impact that also created Charon. The entire system is a miniature version of the early solar system, where collisions shaped the worlds we see today.

What New Horizons Revealed

NASA’s New Horizons spacecraft flew past Pluto in July 2015, giving humanity the first close-up look. It found:

• Mountains of water ice as tall as the Rockies, floating in a sea of nitrogen ice
• Glaciers that flow like those on Earth, but made of nitrogen
• A thin atmosphere of nitrogen, methane, and carbon monoxide that extends far into space
• Evidence of an active surface — no impact craters in some areas, meaning the surface is young

Common Misconceptions

One myth is that Pluto is dark and dead. In reality, its surface reflects about 50-70% of sunlight — brighter than Earth’s Moon. Another myth is that Pluto is the only dwarf planet; there are at least five in our solar system, and hundreds more likely in the Kuiper Belt.

Also, Pluto is not the coldest place in the solar system. That record goes to Neptune’s moon Triton, which has a similar surface but is slightly colder. Pluto’s temperature varies as it orbits, warming slightly when closer to the Sun.

How Pluto Compares to Other Dwarf Planets

Pluto is the largest known dwarf planet, but Eris is slightly more massive. Eris is about 27% more massive than Pluto, even though it is smaller in volume. Haumea is elongated and spins so fast that it is shaped like an egg. Makemake is similar to Pluto but has no known moons. Ceres is in the asteroid belt and is the only dwarf planet that is not in the Kuiper Belt.

Frequently Asked Questions

Why was Pluto reclassified?

Because it has not cleared its orbit of other objects. The Kuiper Belt is full of icy bodies, and Pluto shares its orbit with thousands of them. The IAU created the dwarf planet category for round objects that orbit the Sun but have not cleared their neighborhood.

How long is a day on Pluto?

Pluto rotates once every 6.4 Earth days. Its axis is tilted 122 degrees, so it rotates on its side, similar to Uranus. This means for part of its year, one pole faces the Sun continuously for decades.

Can you see Pluto with a telescope?

Yes, but you need a large telescope (at least 200 mm aperture) and dark skies. Pluto appears as a tiny point of light, not a disk. It is about 100 times fainter than the faintest star visible to the naked eye.

Does Pluto have rings?

No confirmed rings. Some scientists hypothesized that the small moons could create a faint ring system, but New Horizons found none. The spacecraft did detect a dust halo around the system, but it is not a ring.

What would it be like to stand on Pluto?

You would weigh about 6% of your Earth weight, so a 50 kg person would weigh 3 kg. The sky would be dark even at noon, and the Sun would appear as a bright star. The surface is covered in soft nitrogen ice, and you would need a spacesuit because the atmosphere is extremely thin.

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in t | Orbital period | Earth years | 248 | 1 | 10000
in d | Distance from Sun | AU | 39.5 | 10 | 100
out speed | Average orbital speed | km/s | 2 * pi * d * 149597870 / (t * 365.25 * 86400) | 2