Sculpture of the goddess Sedna, made for the Canadian Parliament in Ottawa by Inuit master carver Bart Hanna.

In November 2003, Mike Brown, Chad Trujillo and David Rabinowitz were observing at Palomar Observatory in southern California. They discovered something that was farther away than any other known Solar System object except for long period comets. We now know it as Sedna, named for the Inuit sea goddess, and it's probably a dwarf planet.

Where is Sedna?
At the time of discovery, Sedna was over 90 AU from the Sun, but it's been getting slowly closer. (AU=astronomical unit, the Earth-Sun distance) In 2076 it will be at its closest to the Sun – this is called perihelion. At that time Sedna will be 76 AU away, which is still two and a half times farther away than Neptune. At its most distant – aphelion – Sedna is some 30 times farther from the Sun than Neptune.

Sedna is well beyond the Kuiper Belt which lies at roughly 30-55 AU. The Minor Planets Center lists Sedna as an SDO (Scattered Disc Object). The scattered disc has some overlap with the Kuiper Belt, since SDOs may come into that region at perihelion. But at aphelion they can be at over 100 AU.

The planets and the Kuiper Belt are on the orbital plane. [Diagram: Tim Gunther, National Geographic] However, scattered disc objects are bodies kicked out of the Kuiper Belt under Neptune's gravitational influence. Both they and long period comets have highly tilted elongated orbits. It takes Sedna over 10,000 years to orbit the Sun once.

Mike Brown, among others, maintains that since Sedna isn't close enough to Neptune to be influenced by its gravity, it's not in the scattered disc. He considers it to be the first known inner Oort cloud object. The Oort cloud is where long period comets originate. It's a distant region of icy objects surrounding the Solar System with an outer edge thought to be 100,000 AU or more.

The view from Sedna
If you were on Sedna in 2020, you'd see the Sun as no more than a tiny bright circle of light. Yet there would still be enough sunlight to see colors, to read by, and to cast shadows. The shadows would be very dark with razor sharp edges. Despite the Sun's tiny size, you'd need eye protection to look at it. At aphelion, it would all be quite different. The Sun would look like a bright star, and visibility on Sedna would be very limited.

What we know about Sedna
We don't know a lot about Sedna. It's too far away to see any surface features, so we rely on spectroscopy, our understanding of other distant objects, and educated guesses. We do know that there are methane, water and nitrogen ices. Also that the surface is a uniform red color, nearly as bright as Mars. On Pluto and Neptune's moon Triton, the reddish material – tholins – are an organic material created by the action of solar radiation. The uniform coloring suggests that Sedna gets few meteor strikes. Considering its location in the back of beyond, there's probably not much around to collide with it.

Obviously, Sedna is very very cold. It's around -240°C (-400°F). Triton has pink snow on the surface, but Sedna is too cold for snow to form.

A dwarf planet?
In 2006 the International Astronomical Union (IAU) created the dwarf planet classification, and placed Pluto and four other Solar System bodies in it. No others have officially been added, but Sedna is often referred to as a dwarf planet.

According to the IAU definitions, planets and dwarf planets orbit the Sun. They must also be massive enough to be rounded into spheroids by their own gravity. (Small bodies have irregular shapes.) However, unlike a planet, a dwarf planet isn't massive enough to clear its neighboring region of small irregular bodies.

We can find the size and shape of a body using measurements from a telescope, and can find its mass if it has a moon. So far, no moon has yet been discovered for Sedna. Herschel Space Observatory observations suggested that Sedna was about 1000 km in diameter, somewhat smaller than Pluto's moon Charon. Yet it seems to be twice the size of Uranus's moon Miranda, and Miranda is thought to be spherical.

Naming
Mike Brown chose the name Sedna, saying

Our newly discovered object is the coldest, most distant place known in the Solar System, so we feel it is appropriate to name it in honor of Sedna, the Inuit goddess of the sea, who is thought to live at the bottom of the frigid Arctic Ocean.