The location and orbit of Sedna is shown in relation to asteroids and Kuiper Belt Objects, and the hypothesized Oort Cloud of distant objects orbiting the Sun.

A fabled tenth planet out beyond Neptune, often referred to as Planet X, hasn't been found despite years of searching. But astronomers involved in the hunt are beginning to speculate that something like Planet X will be discovered, along with Y and Z, SPACE.com said.
In fact, the entire alphabet may not suffice to denote the many worlds circling the Sun.
In an emerging new theoretical view of our corner of the galaxy, several worlds larger than Pluto--a few perhaps as big as Mars--lurk in the outskirts of the solar system.
For years, astronomers have been scouring the Kuiper Belt, a region past Neptune that's loaded with comet-like objects. The Kuiper Belt extends out to some 5 billion miles (8 billion kilometers) from the Sun. That's a little more than 50 times the distance between Earth and the Sun, or 50 astronomical units (AU).
Since 1992, more than 800 Kuiper Belt Objects (KBOs) have been found. "Given that our survey has covered almost the entire region of the Kuiper Belt, I'm willing to bet these days that nothing larger than Pluto will be found in the Kuiper Belt," says Caltech astronomer Mike Brown.
As hope fades, a study released earlier this month shows that some KBOs are smaller than had been assumed.
The size of a distant object is often based on an estimate of its reflectivity, a measure called albedo. For years astronomers had assumed KBOs were pretty dark, reflecting just 4 percent of the sunlight that hit them.
Last November, Mike Brown's team found a world at least half as large as Pluto. They named it Sedna, after the Inuit sea goddess. Sedna's elongated orbit is outside the Kuiper Belt, ranging from 76 to 1,000 AU.
Sedna was found only because it is currently near the innermost stretch of its travels.
Nobody expected to find an object like Sedna in the largely empty space between the Kuiper Belt and the Oort Cloud.

Some 13,000 new marine species have been discovered in the past year, according to information released by an international alliance of scientists.
The Census of Marine Life (COML) has also uncovered previously unknown migration routes used by fish such as tuna and shark.
The $1bn 10-year project, which is building a huge database, involves researchers in more than 70 countries.
The new knowledge will inform future conservation and fisheries policies "We're just skimming the surface," said Dr Ron O'Dor, Chief Census Scientist, based in Washington DC, US.
"We know something about the first 100m at this point but we know almost nothing about what lies down in the deep.
"Our analysis shows that if you catch a fish below 2,000m it is 50 times more likely to be new to science," he told the BBC News website.
Some specimens are pulled up on trawls, counted and catalogued. Other organisms are even tagged and tracked. A remarkable picture of how life operates in the deep is beginning to emerge.
"In some of the results we've had you can see a kind of doughnut of circulation which seems to concentrate life in deep water," explained Dr Fred Grassle of Rutgers University, US, who chairs the Census' International Scientific Steering Committee.
The project's Ocean Biographic Information System database now includes more than 5.2 million new and previously existing records of the location, date and depth at which a marine species was found - a rise of 1.1 million entries.
The information has allowed the COML to create a map of the distribution of 38,000 marine species, from plankton to whales. Vast areas of the world's oceans have yet to return any data at all. One survey, however, on the mid-Atlantic Ridge, recorded 80,000 specimens. It is expected to add several new fish species to the 106 marked by the census this year.

An ingredient of chocolate could put a stop to persistent coughs and lead to new, more effective cough medicines, ananova.com said.
Scientists found the key ingredient, theobromine, is nearly a third more effective in stopping persistent coughs than the leading medicine codeine.
They say it produces fewer side effects than conventional treatment - and would not leave people drowsy.
Researcher Professor Peter Barnes said: "Coughing is a medical condition which affects most people at some point in their lives, and yet no effective treatment exists.
While persistent coughing is not necessarily harmful it can have a major impact on quality of life, and this discovery could be a huge step forward in treating this problem."
The researchers gave 10 healthy volunteers theobromine, a placebo or codeine at different times. They then exposed the volunteers to capsaicin, a substance used in clinical research to cause coughing.
The concentration of capsaicin required to produce a cough in those people given theobromine was around one third higher when compared with the group receiving a placebo. When the group received codeine they needed only marginally higher levels of capsaicin to produce coughing, compared with the placebo.
Theobromine works by suppressing vagus nerve activity, which is responsible for causing coughing.
The team also discovered that unlike standard cough treatments, theobromine caused no adverse effects on either the cardiovascular or central nervous systems.