The Scientific Truth About Planet Nine, So Far

May 12, 2017

This artistic rendering shows the distant view from Planet Nine back towards the sun. The planet is thought to be gaseous, but smaller than Uranus and Neptune. Hypothetical lightning lights up the night side.

Last year, two astronomers were looking at the most distant objects orbiting our Sun ever discovered, when they noticed something funny. These ultra-distant Kuiper belt objects, instead of having their orbits oriented at random, were both swept off in one particular direction and tilted in the same direction.

If you only had one or two objects doing this, you might chalk it up to random chance, but we had six; the odds that this would be random was around 0.0001%. Instead, astronomers Konstantin Batygin and Mike Brown proposed a radical new theory: that there was an ultra-distant ninth planet — more massive than Earth but smaller than Uranus/Neptune — knocking these objects into their new orbits. 16 months later, here’s the scientific truth, so far.

First off: the idea is brilliant. Anytime you can take a slew of observations that don’t seem to make sense on their own and explain what caused them with a single new object, it’s very compelling. But like many brilliant ideas, it’s also possible that it’s simply wrong. Seeing six ultra-distant objects doing something slightly unusual doesn’t mean there aren’t also six million ultra-distant objects doing something perfectly normal, but those aren’t the ones we’ve seen yet.

Astronomers call this bias: in any data set, you’re only looking at the objects that are easiest to see/find/measure, and those objects tend to be outliers in many regards by their nature. If you were looking above the tall grass and only saw giant elephants, you might conclude that there was no such thing as an elephant calf; that’s your bias. But there’s a way to overcome this bias: ask what will happen if you collect new, better, and additional data? What specific predictions can you make to confirm or invalidate your hypothesis? In the case of the Planet Nine idea, there are five.

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