Planet X might still lie somewhere in the Kuiper Belt

This is Sedna, not the theoretical planet.

I was thinking about this subject just the other day when explaining to a friend of mine just how many dwarf planets there are in the Kuiper Belt. Given that we've grown up learning the standard nine planets in the Solar System and remembering anything above that is a chore, it's no surprise that most are still unaware just how many large bodies there are out there past Pluto. And lo and behold, Space.com has an article on the subject today. It turns out that a body with 30 to 70 percent of the mass of the Earth in between 100 and 200 AU could explain the weird orbits of a lot of the other bodies we've found:

An icy, unknown world might lurk in the distant reaches of our solar system beyond the orbit of Pluto, according to a new computer model.

The hidden world -- thought to be much bigger than Pluto based on the model -- could explain unusual features of the Kuiper Belt, a region of space beyond Neptune littered with icy and rocky bodies. Its existence would satisfy the long-held hopes and hypotheses for a "Planet X" envisioned by scientists and sci-fi buffs alike.

and:

The Kuiper Belt contains many peculiar features that can't be explained by standard solar system models. One is the highly irregular orbits of some of the belt's members.

The most famous is Sedna, a rocky object located three times farther from the sun than Pluto. Sedna takes 12,000 years to travel once around the Sun, and its orbit ranges from 80 to 100 astronomical units (AU). One AU is equal to the distance between the Earth and the Sun.

According to the model, Sedna and other Kuiper Belt oddities could be explained by a world 30 to 70 percent as massive as Earth orbiting between 100 AU and 200 AU from the sun.

At that distance, any water on the world's surface would be completely frozen. However, it might support a subsurface ocean like those suspected to exist on the moons Titan and Enceladus, said Mark Sykes, director of the Planetary Science Institute in Arizona.

Yes, a subsurface ocean! Just like this other post from before on rogue planets and life.

As for the others, luckily we have Wikipedia to group them together under a single image to show just how many there are:



A short description of each:

• Eris is the largest dwarf planet in the Solar System with a diameter of about 2,500 km, so larger than Pluto. It is 96.7 AU away and orbits once every 557 years.
• 2005 FY9 is the third largest with a diameter of about 1,500 km. Orbits once every 310 years (more than Pluto) and needs a name.
• 2003 EL61 also needs a name. It's one-third the mass of Pluto, has a very rapid rotation and two moons. About the same diameter of Pluto along its long side.
• Sedna is the farthest away, moving as far as 975 AU away from the Sun during its orbit. It's especially close now at 89 AU or so. Diameter estimated at 1,180 to 1,800 km. It's especially well-known because when it was discovered it was the largest we knew after Pluto. Now it's fifth-largest of them all.
• Orcus: Um...we now know that this one has a satellite. Seems to be a typical trans-Neptunian object. Diameter almost 1000 km and has a high albedo.
• Quaoar: Diameter about 1,260 km, the first trans-Neptunian object to be measured directly from Hubble Space Telescope images. A bit farther out than Pluto.
• Varuna: Just under 1,000 km, also a bit farther out than Pluto.
  

Those are a lot of new planets! But this Planet X will prove to be the most interesting if/when it is discovered. New studies like this are important because it's hard to find something when you don't know what you're looking for. In this case if a planet of this size and mass is a way to explain phenomena we already know, then it narrows down the area to search that much more.

On the space.com article there are already over two dozen comments.