With the many effects that Planet Nine is having on the outer solar system, we can infer many things about its properties. In practice, because the solar system is a complicated place, understanding these properties has involved massive amounts of computer simulation. We simulate a slightly larger planet, a slightly closer planet, a slightly more inclined planet, and each time we compare the results of our simulations with observations of the solar system that we know.
From these constraints we have determined that Planet Nine is about 10 times the mass of Earth, that its orbit is inclined by approximately 30 degrees to the plane of the planets, that it has an average distance of something like 600 AU from the Sun, and that when it is at its most distant point from the Sun, it lies toward the outstretched arm of the constellation Orion.
All of this relatively detailed knowledge might make it seem like we could, like Le Verrier, simply say to the world, “Go look; it will be THERE!” But we can’t. Le Verrier had the advantage of being able to analyze the full orbit of Uranus around the Sun to see its deviations. If we waited 10,000 years to fully track Sedna around its orbit, we, too, would be able to pinpoint Planet Nine.
Instead, though, we have only a snapshot of the orbits of a variety of different objects, and we must infer what should have happened in the past. In practical terms, that means that although we know the orbital path of Planet Nine through the sky, we don’t know where it is in its orbit. We no longer have to search the entire sky to find Planet Nine, but there’s still a lot of work to do.
The search will not be as hard as it might have been, however, as many sky surveys over the past few years have covered large swaths of the sky and might have detected Planet Nine had it been in their region. We know, for example, that when Planet Nine is at its perihelion, it is as bright as 18th magnitude, lying in the southern sky near the constellation Ophiuchus. Such an object would have been detected years earlier. Most likely, Planet Nine is now closer to its aphelion, where it would glow dimly, likely close to 25th magnitude.
While that is very faint, detecting such an object is well within the capabilities of the 8-meter Subaru Telescope on Mauna Kea and its impressive Hyper Suprime-Cam, a mosaic of 112 CCD cameras covering nearly two square degrees of sky with every exposure. We have already begun our search using this telescope. Other astronomers are likely to follow.
Is Planet Nine really out there? It’s always wise to be skeptical, but still, we are quite convinced that the answer is yes. Something must be responsible for all the unusual orbits that we now see in the outer solar system. Planet Nine is by far the most likely explanation.
So if it is really out there, when will we find it? The world has been alerted, and multiple teams are on the hunt. Perhaps during the next five years, someone, at some telescope somewhere, will spot a faint blip in the sky that moves to a slightly different spot the next night. When they first see it, they will gasp. Then they’ll recheck all the data and gasp again. They’ll scramble to beg and borrow a few hours on big telescopes here and there to confirm the blip’s slow march across the sky. Finally, after checking and double checking and checking 10 more times, they’ll make a dramatic announcement to a now-anticipating world: Planet Nine is found; Planet Nine is real!