Sedna (dwarf planet)

Sedna, also known by its minor-planet designation 90377 Sedna, is a dwarf planet in the farthest parts of our Solar System. It orbits the Sun much farther out than even Neptune, taking about 11,400 years to complete one journey. Discovered in 2003, Sedna is about 1,000 kilometers wide and has a surface made of frozen water, carbon dioxide, and other ices.

Unlike most objects in its area, Sedna does not have any moons, so scientists do not know its exact mass or density. Its path around the Sun is one of the most stretched-out orbits known, reaching a farthest point about 19 times farther from the Sun than Pluto. As of 2026, Sedna is about 83 astronomical units from the Sun—more than two and a half times farther than Neptune.

Some experts think Sedna belongs to a group called the scattered disc, while others believe it is part of the inner Oort cloud. Sedna is the main example of a new class of objects called sednoids, which have very long, stretched-out orbits. Its unusual path might help us learn more about how our Solar System formed long ago, perhaps due to the influence of a hidden planet sometimes called Planet Nine.

History

Discovery

Sedna was discovered on November 14, 2003, by scientists Michael Brown, Chad Trujillo, and David Rabinowitz. They used a telescope at Palomar Observatory near San Diego, California. They were looking at the sky as part of a project that started in 2001. That day, they saw something moving in the sky. This helped them learn it was very far from the Sun—about 90 times farther than Earth is from the Sun.

Naming

At first, the discoverers called Sedna “The Flying Dutchman” because it moved so slowly. Later, they named it after Sedna, a goddess from Inuit mythology. They picked this name because the object was very far from the Sun, like where the goddess lived in stories. The name was shared before it was officially approved. This caused some disagreement, but experts accepted the name in 2004. Sedna does not have an official symbol that astronomers use, though some astrologers have a special symbol for it.

Orbit and rotation

See also: List of Solar System objects most distant from the Sun

Sedna has the longest orbit of any known object in the Solar System of its size. It takes around 11,400 years to go around the Sun. Its path is very stretched out, going as far as 937 times the distance from the Earth to the Sun and as close as 76 times that distance.

When Sedna was found in 2003, it was 89.6 times the distance from the Earth to the Sun away and was the most distant object known. It was later passed by Eris, found by the same team near its farthest point at 97 times the Earth-Sun distance. As of 2024, both Eris and Gonggong are farther from the Sun than Sedna, even though their average distances are shorter.

At first, scientists thought Sedna turned very slowly, maybe taking 20 to 50 days for one spin. They wondered if a large companion, like Pluto’s moon Charon, was slowing it down. But the Hubble Space Telescope found no such companion. Later observations showed Sedna actually spins much faster, about 10 hours, which is more normal for an object its size.

Physical characteristics

Sedna is a dwarf planet far from the Sun. It has a diameter of about 900 km. It was found in 2003 and was the brightest object found in our Solar System since the discovery of Pluto in 1930.

Sedna's surface is made of water ice, carbon dioxide ice, and ethane ice. It has a reddish color because of special materials called tholins. These materials change over time because of sunlight and space conditions. Scientists think Sedna might have a hidden ocean below its surface. No moons have been found around Sedna.

Origin

Sedna was the first object found in the Oort cloud, a very distant group of icy objects. Sedna’s path around the Sun is too far for Neptune’s gravity to affect it. This means it may be in an “inner Oort cloud” between the Kuiper belt and the main Oort cloud.

Scientists believe Sedna may have formed closer to the Sun and was moved to its current position by the gravity of a star that passed near the Sun when it was young. Other ideas include the pull of an unseen planet beyond the Kuiper belt or that Sedna came from another star system and was caught by the Sun’s gravity.

Population

Main article: Sednoid

Sedna has a very long path around the Sun, making it hard to see with our tools. Because of this, scientists think there might be about 40 to 120 objects about the same size as Sedna far out in space.

Astronomers have studied how different events could have shaped Sedna's path.

In 2014, astronomers found an object about half the size of Sedna with a path very similar to Sedna's. In 2018, they found another object with an even longer path that seems to line up with Sedna's path.

In 2020, Sedna was found again in data from a space telescope, as part of early work to search for unknown objects beyond Neptune.

Classification

The discovery of Sedna made people talk about what should be called a planet. In 2006, astronomers decided that a planet must clear its area around its orbit. Sedna has not done this, so it is not a planet. Instead, it is called a dwarf planet because it is big enough to be round but shares its space with other objects.

Sedna is also grouped based on its orbit. It travels far beyond the planet Neptune and is called a trans-Neptunian object. Some astronomers think objects like Sedna should be in a new group because of how they move in space.

Exploration

Sedna will come closest to the Sun around July 2076. This will be a great chance to study it because it won't happen again for over 11,000 years. Since Sedna spends most of its time far beyond where the Sun's wind stops, looking at it can teach us about space outside our solar system.

Scientists figured out in 2011 that a spacecraft could reach Sedna in about 24.5 years if it used Jupiter's gravity to help. Researchers at the University of Tennessee have also looked into the chance of landing on Sedna.