The Search for the Ninth (or 10th) Planet

a possible ninth planet
his artistic rendering shows the distant view from Planet Nine back towards the sun. The planet is thought to be gaseous, similar to Uranus and Neptune. Hypothetical lightning lights up the night side. Caltech/Robert Hurt (IPAC)

There may be a giant planet in the distant reaches of the solar system! How do astronomers know this? There's a clue in the orbits of smaller worlds "out there".

When astronomers look out to the Kuiper Belt in the outer regions of our solar system and observe the motions of the known objects such as Pluto or Eris or Sedna, they chart their orbits precisely. They do this with all the objects they observe. Sometimes, things don't look quite right with a world's orbit, and that's when astronomers get to work trying to figure out why.

In the case of more than half a dozen Kuiper Belt Objects discovered in the past decade, their orbits seem to have some unusual characteristics. For example, they don't orbit in the plane of the solar system and they all "point" the same direction. That implies there's something else "out there massive enough to have an effect on the orbits of those tiny worlds. The big question is: what is it? 

Discovering another World "Out There"

Astronomers at CalTech (California Institute of Technology) may have found something to explain the anomalies in those orbits. They took the orbital data and did some computer modeling to figure out what might be perturbing the orbits of the recently found Kuiper Belt Objects. At first, they assumed that a collection of objects out in the distant reaches of the Kuiper Belt would have enough  mass to mess with the orbits. However, it turned out that whatever is affecting those orbits would need a lot more mass that is available among the scattered KBOs.

So, they plugged in the mass of a giant planet and tried that in the simulation. To their surprise, it worked. The computer sim suggested that a world ten times more massive than Earth and orbiting 20 time farther from the Sun than Neptune's orbit would be the culprit. This giant world, which the Caltech astronomers nicknamed "Planet Nine" in a scientific paper, would have to orbit around the Sun once every 10,000 to 20,000 years. 

What Would it Be Like?

No one has seen this world. It hasn't been observed.Whatever it is, it's very distant — at the outermost edge of the Kuiper Belt. Astronomers will no doubt begin tooling up to use giant telescopes here on Earth and in space to find this place. When they do, they may find themselves looking at something as massive as a gas giant, perhaps a Neptune-like world. If so, it would have a rocky core smothered by layers of gas and liquid hydrogen or helium. That's the general makeup of gas giants closer in toward the Sun.

Where Did it Come From?

The next big question to answer is where this world came from. Its orbit is not in the plane of the solar system, as the orbits of the other planets are. It's perpendicular. So, that means it was likely "kicked out" from the inner third of the solar system early in its history. One theory suggests that the cores of the giant planets formed closer to the Sun. As the infant solar system grew up, those cores were jostled and ejected away from their birth regions. Four of them settled out to become Jupiter, Saturn, Uranus, and Neptune — and spent their infancy gathering gases to themselves. The fifth one may have been ejected WAY out into the Kuiper Belt, becoming the mystery planet the CalTech scientists think is perturbing the orbits of smaller KBOs today. 

What's Next?

The orbit of "Planet Nine" is roughly known, but hasn't been completely charted yet. That will take more observations.  Observatories such as the Keck telescopes can begin the search for this missing world. Once it's found, then Hubble Space Telescope and other observatories can zero in on this object and give us a dim, but distinct view of it. That will take some time — perhaps several years and hundreds of telescope sessions.