When the icy red world called Sedna edged into the solar system from the shadows of deep space, astronomers marveled at its unexpected arrival even as they wondered at its origins. Where did it come from? And why was it there?
A year after its public debut, Sedna remains an enigma in search of an explanation.
It is the most distant object in the solar system ever identified -- traveling around the sun every 10,500 years in a highly elliptical orbit that keeps it 7 billion to 93 billion miles from Earth. Nothing else that far out has ever been seen.
All the planets in the solar system orbit the sun in a circle. Not Sedna. All the planets orbit in the same plane. Sedna's orbit is canted 12 degrees. All the planetoids and comets that orbit in deep space just beyond Pluto were probably hurled there by Neptune's gravity. Sedna is too far away for that.
Unlocking Sedna's secrets has important implications for scientists' understanding of Earth's origins, for whatever happened to Sedna must have happened 4.5 billion years ago as the infant sun's "dust disk" created the solar system. Sedna is a visitor from the beginning of time.
Last month, Alan Stern, based in Boulder, Colo., for the Southwest Research Institute, reported in the Astronomical Journal that computer models showed Sedna could have formed from the dust disk much like the planets -- as a circular-orbiting body.
He said in a telephone interview, however, that for Sedna to be at its current distance, the disk had to have extended at least 7 billion miles into deep space, with particles traveling at slow enough relative speeds to "accrete" -- gathering together to form planets, rather than bouncing off one another like balls on a billiard table.
"These are considerable ifs," he said, because no evidence exists of anything substantial besides Sedna beyond 4.7 billion miles from the sun, even though other stars have dust disks that extend for 100 billion miles or farther.
There are other, more exotic, possibilities. One team has suggested that Sedna formed inside the planetary system, traveled in a scattered disk kicked outward by Neptune's gravity, then somehow flew even farther into space before the gravity of a passing star stretched its orbit into an ellipse and yanked it out of the solar plane.
Others have suggested there is a 10 percent chance that the passing star may have resulted in an exchange of material between the two solar systems. The sun may have lost a big piece of its dust disk to this interloper and picked up Sedna in exchange. Sedna, literally, may be an arrival from outer space.
Astronomers led by Michael Brown of the California Institute of Technology announced the discovery of Sedna last March. It is a small, spherical, body 800 to 1,100 miles in diameter, about one-seventh the size of Earth, and colored bright red -- redder than anything in the solar system except Mars. The team named the discovery Sedna, after the Inuit goddess of the sea.
The first question about Sedna was whether it is a planet or a smaller orbiting body known as a planetoid. This is also quite likely to be the last question as astronomers cannot agree on what a planet is. Exhibit one is Pluto, which after 75 years of debate remains in semantic limbo.
Far more provocative are efforts to fit Sedna into what astronomers know about the origins of the solar system, when the sun and its spinning disk of dust and gas emerged from a star cluster to spiral into space like a gigantic pinwheel.
Over time, masses of material accreted to form the sun and the planets, including Earth. Beyond Neptune this fan created an icy girdle of asteroids known as the Kuiper Belt. Pluto is a Kuiper Belt object, as are many comets that migrate to the inner solar system.