Is there a Planet Nine in our solar system? After Pluto got kicked out of the planet club most of us woke up to the reality that there were only eight planets. There was no reason to believe that some mysterious Planet Nine out there that met the requirements of the club. We would have seen by now, correct?

Planet Nine and other orbits looking from above the solar ecliptic

Nothing New In Planet Discovery

All the planets except Uranus and Neptune were discovered by Babylonian astronomers, although the six innermost planets were most likely noted by humans before writing was invented. Uranus was discovered in 1781, and Neptune was found in 1846. Despite all the advances in telescopes and space exploration no other ‘planet’ has been discovered in our solar system. So, why would anyone think another planet might exist?

Odd Ducks Out There

If the Sun and the planets were formed from a disk of debris, then most objects would be aligned in that disk or solar ecliptic plane, or the plane that the major planets are on as they orbit the Sun. Collisions between asteroids can send them off in unusual orbits that don’t align with the solar ecliptic.

However, there are a group of objects beyond Neptune that have unusual orbits. These objects are called trans-Neptunian objects. These are on a different plane and defy easy explanation. In addition, the Sun itself is a bit odd in its rotation. The Sun seems to rotate slightly off the ecliptic plane that all the planets follow.

Doing the Math

Two astronomers, Chad Trujillo and Scott S. Sheppard, tackled the odd orbits of the trans-Neptunian objects and discovered that some of the orbits of the objects could be explained if there were a large planet farther out in the solar system. That didn’t sit well with other astronomers so they decided to prove them wrong.

Planet Nine’s likely orbit and the orbits of trans-Neptunian objects

Konstantin Batygin and Michael E. Brown from CalTech decided to re-do the calculations of Trujillo and Shepard. They eliminated some of the objects studied because they might be influenced by Neptune’s gravity. That left six objects to study. To their surprise, they discovered that a planet ten times the size of Earth in an off-plane orbit much farther out than Neptune explained the orbits of the six objects to a 99% degree of certainty.

Is There a Ninth Planet?

Despite the mathematical support, there is no Planet Nine…yet. An analogy would be that if someone picked a date ten years in the future, say 3 March 2028, and was asked if that day would fall on a weekday. Without a calendar to look at it would be hard to say ‘yes’ or ‘no.’ It is very likely, but it is not certain until there is proof.

We currently have no proof of another planet, and spotting Planet Nine will be difficult at best. It would be a relatively small target with almost no sunlight reflecting off of it. It couldn’t be seen with today’s telescopes in either the visible or infrared spectrums. Even if we could determine where it is in its orbit, it would take a probe as many as twenty years to get into the Planet Nine neighborhood.

So the answer remains ‘no.’ There is no Planet Nine…but stay tuned.