Is there another major planet orbiting the Sun, far out past Neptune?

It’s possible. I haven’t written about this much for the newsletter, though I did a few times on The Old Blog™ and at Scientific American. The idea is that there are objects in the distant solar system called Trans-Neptunian Objects, or TNOs, and some of them are acting funny.

Most are on elliptical orbits, and we expect them to be fairly randomly oriented. So some will have their long axes pointed one way, others in another, every which-way. Their orbital inclinations (how much their orbits are tipped relative to Earth’s) will be all over the place as well.

However, as more of these objects were discovered, a suspiciously large number of them seemed to be oriented the same way. This could happen if there were a massive body — a planet — that was tugging on them over the eons, aligning them. They’re too far out from Neptune for its gravity to affect them, so this hints there could be another planet out there. Two astronomers championing this idea are Konstantin Batygin and Mike Brown (full disclosure: Mike’s a friend of mine), and they’ve done some theoretical work to figure out where in the sky such a planet might be given the way it appears to be affecting the TNOs. No survey has yet turned it up though.

Another team of astronomers, though, tried something different. This object will be incredibly faint, since it could be more than 40 billion kilometers from the Sun, over 20 times Neptune’s distance. However, it will still be warm enough to emit infrared light, so the scientists turned to a couple of infrared surveys of the sky, one from the 1980s called IRAS and another one called AKARI. The surveys were taken about 23 years apart, and during that time the orbital motion of the purported planet would make it move in the sky, revealing its presence.

This is a difficult thing to do, though. The two satellites had very different ways of observing the sky and saving the data, and we’re talking about something pretty faint. Still, it wasn’t completely hopeless; they could look for something that was in one spot in the IRAS survey but not AKARI, and vice-versa, and separated by a reasonable amount given the range of possible distances for the planet. Also they knew roughly how bright it would be, so they could eliminate pairs that were vastly different in brightness, as well as other methods for whittling down candidates.

In the end, they detected a single potential finding [link to journal paper]. It ticks a lot of the right boxes… except it doesn’t match the orbital orientation calculated by Batygin and Brown. They expect Planet Nine to have a tilt of 15 – 20°, but this object would have one more like 120°! That’s a huge difference. They also find it would be between 75 and 100 billion kilometers out, farther than expected.

If this pans out it would be a different planet than predicted, and would actually preclude the existence of Planet Nine as predicted by Batygin and Brown! If there were two planets like that out there, they’d eventually affect each other gravitationally and change their orbits, so only one of them can be real. That’s a bit ironic, since it was Batygin and Brown’s work that inspired this research.

So is it real? Wellllll, the certainty is pretty low. They only have two observations of it, once in each survey (the details are complicated, but it boils down to this). The two observations might be real, but they could be stars that change brightness over time, so one appears in one survey and the other star in the other survey. Or it might be a fluke, a random blob of noise in one or both surveys. Or or or.

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