Sedna
What is Sedna?
Dwarf planets· Diameter ~1,000 km. One of the most distant orbiting bodies known — never closer than twice Neptune's distance, reaching 937 AU at its farthest on an 11,400-year orbit. A red world named for the Inuit sea goddess, it nears perihelion around 2076. See its extreme ellipse in the orbit view.
The orbit of Sedna at this moment: https://nowsky.org/solar/orbit?fw=sedna&sp=1000 — replayed live, no login.
Orbit and size
| Diameter | ~1,000 km |
|---|---|
| Distance | 76~937 AU |
| Year | ~11,400 yr |
| Next perihelion | ~2076 |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Frequently asked
- How big is Sedna?
- ~1,000 km.
- How far is Sedna from Earth?
- 76~937 AU.
- When does Sedna return?
- ~2076.
The Sun from Sedna, a pinprick of light
At aphelion (937 AU) the Sun as seen from Sedna is a millionth as bright as from Earth — still the brightest star in the sky, but a point a fingertip could cover. On that light the surface hovers near −260 °C, twelve degrees above absolute zero. Sedna is now inbound at about 84 AU, where the Sun is a hundred times brighter than at aphelion, yet still 7,000 times fainter than in Earth's sky.
In this darkness Sedna's surface has become among the reddest in the solar system: over billions of years cosmic rays and faint ultraviolet light have altered its methane and nitrogen ices into tholins, and even near perihelion the Sun is too weak to sublimate the ice and renew the surface. After perihelion around 2076 it recedes again for more than ten thousand years, so these decades are the best chance to observe it.
The dwarf planet in depth
Sedna was discovered in 2003 by Michael Brown's team and named for the Inuit goddess who lives at the bottom of the frozen Arctic sea. It was 90 AU away at discovery, and its computed orbit turned out to be extreme: a perihelion of 76 AU and an aphelion of about 937 AU. One lap takes roughly 11,400 years — its last perihelion passage came as humans were inventing agriculture.
That orbit is a headache for theories of the solar system's formation. Sedna never comes close enough for Neptune's gravity to have shaped its path, so today's planets cannot explain it. Proposed answers include a passing star in the cluster where the Sun was born, or an undiscovered ninth planet. Its surface is redder even than Pluto's, likely coated in tholins with methane and nitrogen ice; it spins every 10 hours and no moon has been found.
Track Sedna in the orbit view and zoom all the way out — every other body collapses into a dot at the center beside its enormous ellipse. With perihelion approaching around 2076, this is the observing window of several millennia, and proposals for a spacecraft visit are already circulating.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.