Sedna (dwarf planet)

Sedna, also known by its minor-planet designation 90377 Sedna, is a dwarf planet located 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, mixed with reddish substances called tholins.

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. Either way, 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 were using a telescope at Palomar Observatory near San Diego, California, as part of a sky survey that began in 2001. On that day, they noticed an object moving across the sky, which helped them figure out it was very far from the Sun—about 90 times farther than Earth is from the Sun.

Naming

The discoverers first called Sedna “The Flying Dutchman” because it moved so slowly they almost missed it. Later, they named it after Sedna, a goddess from Inuit mythology. They chose this name because they thought it matched the object's distant location from the Sun, similar to where the goddess lived in Arctic stories. The name was shared before the object was officially recognized, which caused some disagreement, but it was eventually accepted by astronomy experts in 2004. Sedna does not have an official symbol used by astronomers, though some astrologers use a special symbol.

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, taking 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 it is farthest from the Sun, Sedna is extremely cold, with temperatures never above −240 °C. From Sedna, the Sun looks like a very bright star, about 45% as bright as the full moon looks from Earth.

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. Some long-period comets go even farther than Sedna but are too faint to see except when they come close to the Sun.

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. It might spin in 18 hours, but this seems less likely.

Physical characteristics

Sedna is a dwarf planet far from the Sun, with a diameter of about 900 km. When it was found in 2003, it was the brightest object found in our Solar System since Pluto was discovered in 1930.

Observations show that Sedna's surface is made of water ice, carbon dioxide ice, and ethane ice. It also has a reddish color because of a special mix of organic materials on its surface, called tholins. These materials change over time due to sunlight and space conditions. Scientists think Sedna might have a hidden ocean below its surface and could have some geological activity. So far, no moons have been found around Sedna.

Origin

Sedna was described as the first known object from the Oort cloud, a distant group of icy bodies that may stretch nearly a light-year from the Sun. Unlike other distant objects, Sedna’s closest approach to the Sun is too far to have been pushed by Neptune’s gravity. This suggests it belongs to an “inner Oort cloud,” located between the Kuiper belt and the main Oort cloud.

Scientists think Sedna may have formed closer to the Sun than it is now, in a flat disk of material that surrounded the young Sun. Its current stretched-out orbit might have been caused by the gravity of a passing star from the Sun’s birth cluster. Other ideas include the influence of an unseen planet far beyond the Kuiper belt or even that Sedna came from another star system and was captured by the Sun’s gravity.

Population

Main article: Sednoid

Sedna has a very stretched-out path around the Sun, making it hard to spot with our current 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. If a big planet beyond Neptune moved Sedna, all similar objects would have paths that come close to the Sun at about the same distance. If Sedna came from another solar system moving in the same direction as ours, these objects would have paths that stay close to the same flat plane. If that solar system moved the opposite way, there might be two groups of objects with different tilted paths. Passing stars could cause many different paths and distances.

Finding more objects like Sedna could help scientists figure out what really happened. One scientist said Sedna acts like a record of the early solar system, and finding more records could tell us how the Sun formed and how many stars were near it back then.

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, suggesting that a hidden distant planet might be affecting these objects. One study suggested there could be about two million objects in this far-out area, with a total weight much bigger than the asteroid belt but less than Pluto.

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

Classification

The discovery of Sedna sparked discussions 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 classified as a planet. Instead, it is called a dwarf planet because it is large 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 put 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 and the Sun's wind at its farthest point. Scientists figured out in 2011 that a spacecraft could reach Sedna in about 24.5 years if it used Jupiter's gravity to help, leaving in either May 2033 or June 2046. Sedna would be about 77 AU or 76 AU from the Sun when the spacecraft arrives near the end of 2057 or 2070. Other paths could use the gravity of Venus, Earth, Saturn, and Neptune, in addition to Jupiter. Researchers at the University of Tennessee have also looked into the chance of landing on Sedna.