170 years ago, French mathematician Urbain le Verrier discovered the planet Neptune. Irregularities in the orbit of Uranus, though slight, must be caused by another planet. His calculations said where the new planet should be, astronomers looked – and there it was. Neptune, the eighth known planet in our solar system.

The Planet Nine Show At The Adler PlanetariumNow a Caltech scientist has raised the likelihood of another planet, a gas giant a little smaller than Neptune but much, much farther out – so far, in fact, that it will be undetectable by telescope because so little light falls on it. The Adler Planetarium in Chicago has put on a new show setting out the case for believing that Planet Nine, as it is known, exists.

The show illustrates two centuries of exploring the outer Solar System, including the discovery of Neptune and several “dwarf planets” (including Pluto) too small to be termed planets. When that story has been told, the audience is led through the reasoning that leads to believe in Planet Nine’s existence.

Caltech astronomers believe they will discover Planet Nine by 2020, but that is not, in fact, what the Adler show is about. It’s about all the objects in the Kuiper Belt, how gravity between bodies works even when they are huge distances apart, and the logical steps that indicate that Planet Nine must be there.

What the Adler and Caltech scientists point to is the behavior of Sedna, a dwarf planet. Sedna has a very strange orbit, which is not the orbit followed by similar objects in other parts of the Kuiper Belt; the orbit is eccentric and takes Sedna way into the very outermost reaches of the solar system. “Not the orbit followed by similar objects in other parts of the Kuiper Belt” – but it IS the orbit followed by seven close neighbors (or what count for close neighbors in this rarefied part of space). The chance that the similarity is coincidence is so tiny it can be discounted. There’s a planet out there drawing these dwarves – and the Adler Planetarium is showing the whole story.