Planet Nine: Are We Not That Special?

by Seth Shostak, Senior Astronomer

Is there a planet ten times the mass of Earth hanging out in the dismal and distant fringes of our solar system?

Two researchers at Caltech, Mike Brown and Konstantin Batygin, have reported phenomena that they interpret as smoking gun evidence for a world roughly 500 times farther from the Sun than our own. 

The evidence consists of a strange alignment of some so-called Kuiper Belt objects – ice-ball worlds similar to Pluto that populate the farthest realms of the solar system.  About a dozen of these KBO’s seem to have orbits that are similarly aligned – an unlikely situation, akin to throwing a handful of pencils onto a table and finding that they pretty much all point in the same direction.

A predicted consequence of Planet Nine is that a second set of confined objects should also exist. These objects are forced into positions at right angles to Planet Nine and into orbits that are perpendicular to the plane of the solar system. Five known objects (blue) fit this prediction precisely.Credit: Caltech/R. Hurt (IPAC) [Diagram was created using WorldWide Telescope.] 

What could account for this bizarre orientation?  On the basis of computer simulations, the Caltech astronomers conclude that the most likely explanation is that the KBOs are being nudged into these orbits by the gravitational interactions with a planet roughly twice the diameter of Earth.  This object would be located on the side of the solar system opposite to the lined-up Kuiper Belt objects.  

No one has actually seen this putative planet with a telescope, but you can bet that many are looking.  It will take a large instrument to bring the object into view, as sunlight so far out in the solar system would be 300 thousand times weaker than on Earth.  In addition, the exact position of this hefty planet is unknown – so the search has to cover a relatively large amount of sky.  It’s a bit like finding a floating volleyball in the ocean from 40,000 feet, when you don’t have a good fix on the volleyball’s location.  Still, Batygin estimates that the planet might be discovered within eight years or so.

And what is the significance of “Planet 9,” as it’s being called?  For those who look for biology beyond Earth, such a world would make our solar system more in keeping with those we find around other stars.  Many of the so-called exoplanets discovered by the Kepler mission and other telescopes are what are called “Super Earths” – worlds that are up to ten times the mass of our home planet.  Until now, we didn’t think that our solar system had a Super Earth.

But if the predictions are correct – if Planet 9 actually exists – then our solar system will better comport with many of those we find elsewhere.  And if our solar system is not so special, then there’s added reason to suspect that the biology it has spawned may not be so unusual either.