How big is Sedna?

Sedna is a small, cold world that orbits the Sun far beyond Neptune, on a long, stretched-out path. It is about 1,000 kilometers (620 miles) across, an estimate worked out from the heat it gives off. Some astronomers call Sedna a dwarf planet, but it is not one of the five that the International Astronomical Union (IAU) recognizes as of 2026.

Size

Sedna is so far away that telescopes see it only as a point of light, so its width cannot be measured directly. Its brightness is not enough either: a small, shiny object and a large, dark one can reflect the same amount of sunlight. Instead, astronomers measure the heat it gives off. A large object at a given temperature gives off much more heat than a small one.

A 2012 study used heat measurements from the Herschel Space Observatory, a European space telescope. It found Sedna to be 995 kilometers across, give or take 80 kilometers. Sedna turned out smaller and more reflective than expected, which suggests that more of its surface is covered in ice than scientists had assumed. Earlier, the Spitzer Space Telescope had failed to detect Sedna's heat at all, which showed only that it could be no more than about 1,670 kilometers across.

Size comparisons

If the Sun were shrunk to the size of a basketball, Sedna would be a speck 0.17 millimeters across, about twice the width of a human hair.
At that scale, Earth would orbit 26 meters (85 feet) from the basketball, while Sedna would come no closer than about 2 kilometers (1.2 miles) and would swing out to about 24 kilometers (15 miles).
Quaoar, one of the next larger objects in the explorer and another icy world beyond Neptune, is about 1,090 kilometers across, a little larger than Sedna.
Pluto is about 2.4 times as wide as Sedna.

A long, cold orbit

Distances in the outer solar system are measured in astronomical units (AU), where one AU is the average distance from Earth to the Sun, about 150 million kilometers. Sedna never comes closer to the Sun than about 76 AU, or 11 billion kilometers. At its farthest it is about 937 AU away, more than 12 times as far. One trip around the Sun takes more than 10,000 years.

Sedna will next be at its closest to the Sun around 2075. Even then, the temperature on its surface is expected to be only about −243 degrees Celsius. Near its farthest point it falls to about −264 degrees Celsius, less than 10 degrees above absolute zero, the coldest possible temperature. At such low temperatures, many kinds of ice can last on the surface.

The James Webb Space Telescope has found frozen ethane, a compound of carbon and hydrogen, on Sedna's surface, along with other hydrocarbons and water ice. Its surface is also strongly red, a color that probably comes from complex carbon compounds.

Discovery and a puzzle

Mike Brown, Chad Trujillo and David Rabinowitz found Sedna on November 14, 2003, using the Samuel Oschin Telescope at Palomar Observatory in California. It was then about 90 AU from the Sun, the most distant object ever seen in the solar system at the time. Because it is so cold, the team named it after Sedna, the Inuit goddess of the sea, from whom all sea creatures were created.

Sedna's orbit is hard to explain. Its closest approach to the Sun is too far out for any known planet to have flung it there. Its discoverers suggested three possible causes: a planet not yet discovered, a star that once passed unusually close to the Sun, or the Sun's birth in a cluster of other stars.