Potential Ninth Planet!

Sadly no, this time we are NOT talking about Pluto. Astronomer Mike Brown from Caltech, heralded as the ‘man who killed planet Pluto’ has done some new work that might replace Pluto with a better fit for a true ninth planet, one that is ten times the mass of Earth.  Now the only problem is finding it.

This 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. Credit: Caltech/R. Hurt (IPAC)

But wait, if we haven’t seen it, how do we know it’s there? Well it certainly showcases the power of science, that an understanding of the true laws of nature can give us incredible predictive power.  It started out as a ‘that’s strange’ moment.  Chad Trujilo, a former graduate student of Brown’s, did some work showing that 13 of the outermost known Kuiper Belt objects had a strange orbital characteristic that was similar in all of them.

Brown’s interest was piqued, and so he turned to colleague Konstantin Batygin, a theoretical physicist down the hall.  Looking at the objects, they realized that orbits of the six most distant objects all pointed to the same region in space. “It’s almost like having six hands on a clock all moving at different rates, and when you happen to look up, they’re all in exactly the same place,” says Brown. “The odds of having that happen are something like 1 in 100.”  They also all had the same orbital tilt with respect to the plane of the solar system, making their orbits even less coincidental.

Their first thought was that undiscovered Kuiper Belt objects would cause the gravitational tug and place the objects in these similar orbits, but the math just didn’t work out, as they found they would need the Kuiper Belt to have over 100 times the mass it already has today.  Not likely.

So the idea became a planet, but running simulations revealed that a normal planet would not predict the exact orbital properties observed, though it was close.  Changing the simulations a little bit revealed that if a giant planet was orbiting with its perihelion (closest point to the Sun) 180 degrees away from the perihelion of the rest of the planets and solar system bodies, that the orbits worked perfectly. “Your natural response is ‘This orbital geometry can’t be right. This can’t be stable over the long term because, after all, this would cause the planet and these objects to meet and eventually collide,'” says Batygin.  “Still, I was very skeptical,” he continued. “I had never seen anything like this in celestial mechanics.”

But as they investigated other consequences of the massive planet, they found matches to other observed phenomena, like the strange orbit of Sedna slowly moving away from Neptune, and the existence of Kuiper Belt Objects that are perpendicular to the plane of the solar system.  This is the kind of thing that makes a scientist salivate.  A theory based on one observation that naturally solves other observed mysteries.  A true prediction that fits evidence.  “When the simulation aligned the distant Kuiper Belt objects and created objects like Sedna, we thought this is kind of awesome — you kill two birds with one stone,” says Batygin. “But with the existence of the planet also explaining these perpendicular orbits, not only do you kill two birds, you also take down a bird that you didn’t realize was sitting in a nearby tree.”

Now the only thing that remains is to find the planet.  It’s pretty far out, and likely only orbits the Sun once every 10,000 to 20,000 Earth years.  If it’s in the closer part of it’s orbit currently, astronomers may have already imaged it with telescopes and just didn’t notice it.  If it’s further away, we would need some of the biggest ground-based telescopes in order to see it.  Finding it has a lot of implications for how planetary systems form and how we approach the search for planets orbiting other stars.

“I would love to find it,” says Brown. “But I’d also be perfectly happy if someone else found it. That is why we’re publishing this paper. We hope that other people are going to get inspired and start searching.”

Finding this mysterious planet would make it the third planet in history to be discovered (if you don’t count Pluto’s discovery) after Uranus and Neptune.  It would also highlight the power of simulation and hypothesis in science.  It shows how we can take a proven theory and apply it to other situations in nature to make testable predictions.  Now we need the power of our technology to discover it and see if the theory is correct.


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