Planet Nine has been making the rounds again, largely because the Division of Planetary Science had their annual meeting last week, and if you’ve had some new thoughts on planets in the past year, this is the time to announce them. So in the past week, articles have popped up informing us that Planet Nine might be responsible for why the Sun's rotation appears slightly tilted relative to the orbits of the rest of our planets, that we’ve found a few more distant objects which have odd orbits best explained by another large planet, and the very long orbit of a new distant world was also claimed for Planet 9 (though the scientists who discovered it prefer a non-Planet 9 explanation).
All of these victories of Planet Nine are based on simulations of what our solar system would look like, if Planet Nine were there, given our current best guesses for what Planet Nine should look like. At the moment, Planet Nine has to be at least several times the size of Earth, and several more times the mass of Earth. Planet Nine happens to be at the farthest point in its orbit from the Sun, as it slowly putters around the Sun in a very elliptical orbit, once every ten to twenty thousand years, at 700 times the distance of the Earth to the Sun.
This distance from the Sun means that the hypothetical Planet Nine is much fainter than it would be if it were at its closest approach to the Sun. That faintness, in turn, means that your best chance for spotting it lies with the biggest telescopes we can turn towards the sky. However, these massive telescopes are in high demand, with scientists all over the world competing for the use of them for a few nights. To get to use them, your proposed science must beat out the proposed science of many other scientists, who also have good ideas for what to do with the telescope. If you go to the telescope and say 'I need to use a lot of the telescope time to survey a huge area of the sky, to find a single object that I hope is there', you are not going to get that telescope time. That’s much too risky a way of spending telescope time – what have you learned if it isn’t there?
A quick note on NASA’s role in all this - while NASA is involved in the USA’s space based telescopes, they’re mostly involved in the construction, launch, and management of the data that is taken with them - major tasks. The science that is done with those telescopes is done by teams of people - scientists - who have made a case for why they should be entrusted with the instruments for a few hours, to point at their favorite patches of sky. These are not necessarily NASA employees, and many of them aren't. For Planet Nine purposes, NASA’s space telescopes are not the ideal facilities to do a major planetary hunt. That honor goes instead to the Subaru telescope on Mauna Kea, which is operated by the National Astronomical Observatory of Japan.
There are a number of groups of scientists already hunting for Planet Nine. Critically, the hunts are not all being done in exactly the same way. Some scientists are trying to narrow down the area of sky which needs to be surveyed. Some are looking for other, smaller objects in the outer solar system, which might help to rule out some possible orbits of Planet Nine. (These authors prefer an orbit of 17,000 years specifically.) Some are running more simulations to see if the data we already have is enough to put constraints on whether or not the planet could be there (not so far, is the answer). Some have dug out all the old Pluto observations that we have, to see if Pluto's orbit has been jostled by Planet 9. And some are hunting for Planet Nine itself, while also looking for other moving objects in the solar system - that way, even if they don’t find it, they’ve found a new piece of the outer solar system to fit into the jigsaw puzzle.
This hunt has already turned up some new objects. These particular new discoveries are among the most distant from the sun on their closest visits to the inner solar system. They may not travel the furthest away from our Sun, but they’re also never coming in. The more information scientists can get about what’s happening at the edge of the solar system, the more we will understand how much an extra giant planet, wandering frigidly and slowly, would change those happenings.
If Planet Nine is found, credit will go not just to the group of scientists which finally got photons from the distant world into their telescope’s camera, but to all those who led the way to get there - other scientists who have laid the foundations of understanding of how our solar system works very much included. To be able to notice oddities in the very outskirts of our solar system is an achievement already. We will either find Planet Nine, or a reason that it cannot exist.
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