Moons of Pluto

Pluto has five moons.

One moon is very big. Its name is Charon.

Charon is so large it stays close to Pluto. They always show the same side to each other.

Four other moons are much smaller. They are named Styx, Nix, Kerberos, and Hydra.

These small moons are not round. They look like bumpy rocks.

It is fun to think about these tiny worlds.

Pluto has five moons.

The largest moon is Charon. It was found in 1978. Charon is very big. It is nearly one eighth the mass of Pluto. Because it is so large, the two bodies orbit a center point between them. This center is called a barycenter.

Pluto and Charon are tidally locked. This means they always show the same face to each other. They are both shaped like round spheres.

Four other moons orbit Pluto. These are Styx, Nix, Kerberos, and Hydra. They are much smaller than Charon. These moons are not round. They have irregular shapes.

Scientists think these moons formed from a crash. A large object may have hit Pluto long ago. This crash could have made a ring of dust. That dust then clumped together to make the moons. This would explain why the moons are grey. They are mostly made of water ice.

These moons stay in a tight group. They orbit in a special way called resonance. This means their paths follow a steady pattern.

Pluto is a dwarf planet with a very busy family of moons. There are five known moons orbiting this distant world.

The largest moon is named Charon. It is so big that it is nearly one eighth the mass of Pluto. Because they are so heavy, Pluto and Charon orbit a center point between them. This point is called the barycenter. The barycenter actually sits above the surface of Pluto. Some people even call them a double dwarf planet.

Charon and Pluto work together in a special way. They are tidally locked to each other. This means they always show the same face to one another. As they move, they never turn away. Charon is a perfect sphere, which means it is shaped like a round ball.

The other four moons are much smaller. They are Styx, Nix, Kerberos, and Hydra. These moons do not look like balls. They have irregular shapes instead.

They orbit Pluto at different distances from the barycenter.

Finding these moons took many years of searching. James Christy discovered Charon on 22 June 1978. This discovery changed how we thought about Pluto's size. Later, astronomers used the Hubble Space Telescope to find more. They imaged Nix and Hydra on 15 May 2005. The International Astronomical Union gave them these names in 2006.

Kerberos was announced on 20 July 2011. Scientists found it while looking for rings. Finally, Styx was announced on 7 July 2012. This happened while looking for hazards for the New Horizons mission.

These small moons follow a very tidy pattern. This pattern is called an orbital resonance.

Styx, Nix, and Hydra share a 3-body Laplace resonance. This means their orbital periods follow a specific ratio of 18:22:33. Nix and Hydra also share a 2:3 resonance. The moons are also very close to each other. They are packed tightly in a small region. Styx is 42,700 kilometres from the barycenter. Hydra is the furthest at 64,800 kilometres.

Scientists wonder how this whole system began. They think a huge collision happened a long time ago. A large object may have crashed into Pluto. This crash could have sent dust and rocks flying into space. This debris might have clumped together to form the moons.

This would explain why the moons are grey. They are mostly made of water ice. This is different from Pluto, which is very red. The moons might have moved outward from Pluto after they formed. This would explain why they are in their current orbits.

The Pluto system is a complex group of celestial bodies. It consists of the dwarf planet Pluto and its five known moons. These moons are Charon, Styx, Nix, Kerberos, and Hydra.

The system is unique because of the massive size of its largest moon. Charon is nearly one-eighth the mass of Pluto. Because of this high mass, the two bodies orbit a common center of mass. This center is called the barycenter. In this system, the barycenter actually lies above the surface of Pluto. This relationship is so strong that some consider Pluto and Charon a binary dwarf planet.

Charon and Pluto are tidally locked to one another. This means they always show the same face to each other as they orbit. Charon is shaped like a perfect sphere within measurement uncertainty. The other four moons are much smaller and have irregular shapes.

These four moons are circumbinary, meaning they orbit the center of mass of both Pluto and Charon. They follow nearly circular, prograde orbits in the same plane. Their orbits are very flat, with inclinations differing by less than 0.4 degrees. The distances from the barycenter range from 42,700 kilometers for Styx to 64,800 kilometers for Hydra.

Finding these moons required decades of astronomical work. James Christy discovered Charon on 22 June 1978. This discovery forced scientists to revise their estimates of Pluto's size. Previously, they thought Pluto was much larger because they attributed all the system's mass to it. On 15 May 2005, the Pluto Companion Search Team used the Hubble Space Telescope to image two more moons. These were later named Nix and Hydra by the International Astronomical Union in 2006.

Kerberos was announced on 20 July 2011 after a search for rings. Styx was discovered on 7 July 2012 while looking for hazards for the New Horizons mission.

The small moons exhibit a fascinating mathematical pattern called orbital resonance. Styx, Nix, and Hydra are thought to be in a 3-body Laplace resonance. Their orbital periods follow a specific ratio of 18:22:33. This 3-body relationship includes a 2:3 resonance between Hydra and Nix.

Additionally, Styx and Nix share a 9:11 resonance. Styx and Hydra share a 6:11 resonance. These resonances mean the moons pass each other in predictable patterns. For every two conjunctions of Styx and Nix, there are five Styx–Hydra conjunctions and three Nix–Hydra conjunctions. This mathematical harmony suggests the moons have interacted gravitationally for a long time.

Scientists have different theories about how these moons formed. One theory suggests a massive collision occurred in the distant past. A large Kuiper Belt object may have struck Pluto. This impact would have ejected debris into space. This debris could then aggregate to form the moons.

The grey color of the moons supports this idea. While Pluto is very red, the moons are likely dominated by water ice. This happens if volatiles are lost during a high-speed impact. Another theory suggests a slower collision at 2,000 miles per hour. In this scenario, Pluto and Charon remained attached for ten hours before separating. This separation would have been caused by centrifugal force from Pluto's rapid rotation.

The system is also notable for its compact nature. The four small moons are packed very tightly together. They occupy only the inner 3% of the region where prograde orbits are stable. The stable region for prograde moons extends to 53% of the Hill radius, which is 6 million km. Despite the potential for more moons, New Horizons confirmed no moons larger than 4.5 km exist out to 180,000 km. While some thought the debris from a collision might form rings, data from Hubble and New Horizons suggest no ring system is present. The moons themselves also show interesting rotation. Instead of tumbling chaotically, they have high obliquity. This means their axes are significantly tilted.