Sedna
A dwarf-planet candidate about 995 km across on one of the most distant orbits known, never closer to the Sun than 76 au and about 12,700 years around.
At a glance
- Status
- Candidate: likely large enough to be round, not formally classified[4]Not among the five dwarf planets the International Astronomical Union recognizes
- Size
- About 995 km across[3]995 km, give or take 80, from the Herschel Space Observatory; its surface reflects about 32 per cent of sunlight, more than expected
- Orbit
- 76 au from the Sun at the closest to about 1,200 au at the farthest[2]A detached object: it never comes near enough to Neptune to feel its pull
- Orbit period
- About 12,700 years, tilted 11.9° to the plane of the planets[5]Semi-major axis 544 au, eccentricity 0.86; a trans-Neptunian object in JPL's catalogue
- Day
- About 10.3 hours[5]From its light curve
- Moons
- None known[6]The one large body of its class without a known companion, as of the 2017 discovery of Gonggong's moon
- Color
- Very red[1]After Mars, the second-reddest object in the solar system
Timeline
- November 14, 2003Discovered[1]Mike Brown, Chad Trujillo and David Rabinowitz with the Samuel Oschin Telescope at the Palomar Observatory, California
- March 15, 2004Announced[1]As the most distant object then known in the solar system, about 13 billion km away
- 2012Size measured[3]The Herschel Space Observatory's infrared measurements give 995 km, smaller and brighter than expected
Sedna is a dwarf-planet candidate on one of the most distant orbits known for any body in the solar system.[1] NASA describes it as a detached object: the closest it ever comes to the Sun is 76 astronomical units, far beyond the main Kuiper belt, and at its farthest it travels out to about 1,200.[2] It is about 995 km across, large enough that it is probably round, but the International Astronomical Union has not classified it.[3][4]
How it was found
Mike Brown of the California Institute of Technology, Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University found Sedna on 14 November 2003 with the 48-inch Samuel Oschin Telescope at Palomar Observatory, and announced it on 15 March 2004 as the most distant object yet seen in the solar system, about 13 billion km away.[1] They named it for the Inuit goddess of the sea and mother of all sea creatures, who rewards the people of the land with food from the ocean; without her blessing hunts fail.[1][5] The name suited a world where, as the discoverers put it, the temperature never rises above -240 °C.[1]
Orbit
Sedna's orbit is unlike any Kuiper belt object's. JPL's catalogue gives a semi-major axis of 544 astronomical units and an eccentricity of 0.86: it swings from 76 astronomical units out to about 1,010, tilted 11.9 degrees to the plane of the planets, and one trip around the Sun takes about 12,700 years.[5] The discovery team's first estimate was a 10,500-year orbit reaching 130 billion km, about 900 times Earth's distance from the Sun.[1] Because it never comes close enough to Neptune to have been thrown onto such a path, astronomers suspect some other force shaped it: an undiscovered distant planet, the gravity of passing stars or disturbances while the Kuiper belt was forming.[2] Even so, Sedna is about ten times closer than the Oort cloud is predicted to begin.[1]
Size and surface
The discoverers estimated Sedna at roughly three quarters the size of Pluto, less than 1,700 km across.[1] The Herschel Space Observatory's measurement of its heat, published in 2012, gave a diameter of 995 km, give or take 80, and a surface reflecting 32 per cent of the sunlight that falls on it, more than expected, which suggests a larger fraction of its surface is covered in ice.[3] After Mars it is the second-reddest object in the solar system.[1] It rotates once every 10.3 hours or so.[5]
Moons
None is known. When Hubble found a moon around Gonggong in 2017, NASA noted that all the known large dwarf planets except Sedna now had satellites.[6]
What is not yet known
No spacecraft has visited Sedna, and at its distance none could soon. What put it on its orbit is the central question, because the answer bears on whether an unseen planet lies far beyond Neptune.[2] Its composition, interior and the absence of a moon are open as well.
Sources
- [1]NASA/JPL, Most Distant Object in Solar System Discovered — jpl.nasa.gov, retrieved November 5, 2026
- [2]NASA, Kuiper Belt: Facts — science.nasa.gov, retrieved November 5, 2026
- [3]arXiv (Astronomy & Astrophysics Letters, 2012), "TNOs are Cool": A survey of the trans-Neptunian region. VII. Size and surface characteristics of (90377) Sedna and 2010 EK139 — doi.org, retrieved November 5, 2026
- [4]NASA, Pluto & Dwarf Planets — science.nasa.gov, retrieved November 5, 2026
- [5]NASA/JPL Small-Body Database, 90377 Sedna (2003 VB12) — ssd.jpl.nasa.gov, retrieved November 5, 2026
- [6]NASA, Hubble Spots Moon Around Third Largest Dwarf Planet — science.nasa.gov, retrieved November 5, 2026
Related
- What is a dwarf planet?The definition that made Pluto a dwarf planet in 2006, the five the IAU recognises, why the list grows so slowly, and the candidates waiting to join it.
- The Kuiper belt and beyondThe ring of icy worlds past Neptune, the scattered disk and the Oort cloud beyond it, how many objects are known, and how new ones are found.
- ErisThe dwarf planet about the size of Pluto whose discovery in 2005 led to Pluto's reclassification; it orbits about 68 au from the Sun once every 557 years.
- GonggongA dwarf-planet candidate about 1,230 km across on a tilted, elongated orbit far beyond Pluto, with a small moon, Xiangliu, found in Hubble images.
- QuaoarA dwarf-planet candidate about 1,110 km across on a nearly circular orbit beyond Pluto, with a moon, Weywot, and two rings where none were expected.
- OrcusA dwarf-planet candidate about 917 km across on an orbit much like Pluto's, in the same resonance with Neptune, with a large moon, Vanth, about half its size.
- 2026 ED138A dwarf-planet candidate found by the Rubin Observatory beyond Neptune and announced on 27 October 2026, an icy world that may be 600 to 1,500 km across.