Back in August, we published our detailed analysis which gave predictions of where we should expect to find Planet Nine and how big and bright it might be when we do find it. One of the very interesting parts of the analysis was the result that Planet Nine should be on the closer and brighter end of our initial range of expectations. Planet Nine might be close enough and bright enough, that it might even already have been imaged in one of the many ongoing sky surveys taking place on moderately small telescopes all around the world.
In many ways, the idea that Planet Nine might have already been imaged should not be a surprise. All of the planets discovered had been seen before they were recognized. Galileo saw Neptune -- without realizing it was a planet -- in 1612, more than 200 years before Neptune was found. Flamsteed cataloged Uranus as a star in 1690 -- a century before its discovery -- and it is even possible that Uranus was observed and cataloged by eye in the second century BCE. Eris, which I first spotted in 2005, showed up on photographic plates from the 1950s (from the same Palomar 48-inch Schmidt telescope used to discover it in 2005!). Images of Pluto have been found from 16 years before its discovery.
The problem, of course, is that just getting an image of an object in the solar system is not enough. You need to recognize it as something interesting. In all of the historical cases, the solar system body was simply cataloged as a star. The only way to know that it is not a star is to see it move.
Can't be too hard, right? Herschel saw Uranus one night and came back the next and saw that it had moved. We found Eris by taking three images of the right place in the sky over three hours and noticing one little bright star slowly sliding east.
Sadly, Planet Nine will be a little harder, but only because the data are not designed for searching for Planet Nine. Most of the big surveys out there cover large fractions of the sky, but only ever couple of days or weeks or months. If you look one night and see something where nothing has ever been before (a "transient" in astronomical language) it could be an asteroid, a supernova, a variable star, a satellite glint, a defect in the camera. Who knows? And when you see the same area of sky a month later there might be dozens of other transients nearby. Is one of them the same object, just moved? Who knows?
In our new paper out today, we search for Plane Nine by combing through the transients from the Zwicky Transient Facility (ZTF; which, once again, uses the Palomar 48-inch Schmidt!), of which there are ~13 million over a three year period. Finding one Planet Nine in that big of a haystack is daunting.
The good news is we figured out a cool trick to make the searching moderately efficient. It relies on a simple yet delightfully clever insight from Matt Holman at the Harvard-Smithsonian Center for Astrophysics. While it is true that, when viewed from the Earth, asteroids and planets have complicated looping paths across the sky that are difficult to link, if viewed from the sun they simply travel on great circles. Since we're unlikely to move all of our telescopes to the sun, we have to fake it, by transforming everything we see in the sky into what it would look like if we were on the sun. It's more complicated than this, as we don't know how far away solar system things that we haven't discovered are, so we have to search over a range of potential distances and do more stuff too, but in the end the algorithm is able to smoothly link single detections of Planet Nine over many years. Check out the paper for the gory details.
Given the 13 million transients in the sky, sometimes they will accidentally line up and look like something real. We find that requiring 7 detections over 3 years allows us to avoid all of the coincidental detections and only find the real Planet Nine.... which we don't. It's not in the ZTF data. Insert sad face emoji.
Searching for and not finding Planet Nine is only useful if we can somehow figure out what it means, and here is the second part of this paper about which I am excited. We take the results from our paper two months ago and make what we call the Planet Nine Reference Population -- a collection of 100,000 statistically sampled realizations of our Planet Nine prediction, we inject these into the ZTF data, and we see which we would detect. Astonishingly, this one survey would detect 56% of the reference population, or, said differently, this survey covers 56% of the Planet Nine parameter space.
Knowing this 56% is great, of course, but even better is that we know where (and how bright) the remaining 44% of the reference population is. Basically we took the treasure map from the last paper and we've removed 56% of the search area, giving us an updated treasure map (the updated treasure map is noisier looking than the original because it is a little more sparsely sampled with only 100,000 members of the reference population, but you can see the important points there).
In this map you can see not only where Planet Nine is predicted to be, but how far away and how bright at all of the points across across the sky. And you can see what the ZTF analysis has done. ZTF would have found most Planet Nines brighter than ~20.75th magnitude, as long as they weren't too far south or in the southern galactic plane.
My final favorite part of this paper is that now the reference population and treasure map are easily accessible for anyone who either wants to look for Planet Nine, wants to think about how to look for Planet Nine, or wants to ponder whether or not they have a telescope/survey/whatever that could possibly detect Planet Nine. Just go to the permanent archive of the reference population , download the reference population, and start making plots like the above. Software to help is even included, conveniently written in IDL for those born before 1970 (ahem).
The reference population includes a flag of everything that would have been detected in ZTF, so you know where not to look. We're happy to include similar flags for any other survey that would have detected any part of the reference population. We've got a couple of new surveys that we are working on including, but get in touch if you'd like yours there.
Planet Nine is out there. Except in the 56% part of parameter space we have now ruled out.