There are days when it seems that our outer solar system has the singular goal of frustrating astronomers. In a way, we have no one to blame but Newton. After all, his equations of motion allowed researchers to realize that Uranus’ motion required something to be pushing and pulling on its orbit.
William Herschel found Uranus in March 1781, and for the next several years, he and other observers carefully watched its slow motion through the stars. By 1845, it had completed an orbit that could only be explained by the presence of another large planet in the outer solar system. Mathematicians Urbain Jean-Joseph Le Verrier in Paris and John Couch Adams in Cambridge, England, made their calculations, and in September 1846, astronomer Johann Gottfried Galle found Neptune within a degree of where Le Verrier predicted.
Neptune’s also somewhat wonky orbit also led folks to go searching for another planet, and ultimately, Clyde Tombaugh found Pluto – an object much much smaller than what anyone expected. We now know that Pluto is not responsible for Neptune’s out-of-round orbit, and in fact, no large world is required to explain its elliptical orbit around the Sun.
And in 2006, Astronomer Michael Brown killed off Pluto’s planetary status with the discovery of other, similarly sized icy objects out in orbits near Neptune and beyond.
Pluto, by the way, was not the first planet to get demoted. That title goes to Ceres, the largest of the asteroids. Like Pluto, it was the first object discovered in what would turn out to be a belt of objects. Today, we call Ceres the largest object in the asteroid belt, and Pluto one of the largest objects in the Kuiper belt, and we argue over the definition of planet.
As I said, the outer solar system has a goal of frustrating astronomers.
As we’ve found more and more objects in the Kuiper Belt, it was expected that they’d map out a random set of orbits, much like the asteroid belt, with things oriented every which way as they spiral around the Sun.
Weirdly, that isn’t the case, and trying to understand why there is a freakishly large number of orbits weirdly oriented in one direction has become what sometimes feels like a combat sport.
The opening round of shots fired came from Michael Brown and Konstantin Batygin who called for a new search for a Planet 9 capable of gravitationally tweaking minor planet after minor planet into vaguely aligned orbits. They made predictions for its orbit, and then they started searching.
And… it hasn’t been found.
It’s entirely possible that it is out there doing its best to reflect as little light back at us as possible. A low albedo – not very reflective – object could easily be hiding from all our existing data sets. We are, after all, looking for something that is significantly farther out than Neptune, like potentially 20 times further out than Neptune, and at that distance, it would both receive a lot less Sunlight and its light would have to travel a whole lot farther to reach us. The combined effect is an object 100s to 1000s of times fainter than Neptune.
But what is a factor of 1000 among friends?
It is understood that Rubin Observatory will be able to see Planet 9 if it is out there. Within a couple of years, we will either have found it or we will know something else is weird about our solar system.
But no one wants to wait, and astronomers have started getting super creative in how they are using archival data to look for potential new worlds.
In general, planets give off most of their light in the infrared as they shine warmly. This means our best bet for finding planet 9 is to look for moving dots in infrared sky surveys.
Not many such surveys exist, and none of them lasted long enough to be able to readily see the kind of super slow motions we’d expect from an object far beyond Neptune. That said, two surveys were similar enough and spaced out enough in time to compare their data. These surveys are the Infrared Astronomical Satellite’s survey or the IRAS survey, which did science from February to November 1983, and the Japanese AKARI infrared satellite, which surveyed the sky from February 2006 to Aug 2007 in far and mid-infrared
The 26-year gap between these two surveys means that together they should be able to catch the motion neither can see alone. Over the course of 1 generation, we expect a distant plant to move not quite a degree across the sky. Put another way, such a world would travel less than twice the moon’s diameter.
Using software, researchers led by Terry Long Phan looked for objects that appeared in one survey but not the other, and that appeared within a degree of an object that only appeared in the opposite survey, and that appeared about the same brightness in both surveys. This is a much harder software problem than it sounds like – the two surveys are very different, and a lot of calculations have to be done to figure out what it means for two points to have the same brightness.
But… It is still just math, and computers can do this for us.
A whole lot of software writing and waiting later, the team found exactly one pair that fit the bill… and that one pair – if it is real – is nowhere near where Planet 9 is supposed to be. In fact, if this is a real planet, then the Planet 9 Brown and Batygin predicted probably isn’t out there, because its orbit would interfere with this new possible object’s orbit.
Back to Slide 12
But here is the thing… All they have are two points of light. In the AKARI data, which looked at the same spot 6 months apart, the point only appears in 1 image… which means it either is an object moving as expected… or it is something that changed in brightness.
All sorts of stars vary in brightness for different reasons. It is entirely possible that two stars just happened to vary in brightness between the two surveys. It’s also possible one of these points is a false detection, or … well… a lot of things are possible.
But… the 2 points they observed are enough to predict, somewhat vaguely, where on the sky that object just might be today, and with a night or two of telescope time, she should be able to go looking for this object using existing ground-based telescopes. If they can get a good enough position with something like the Dark Energy Camera, they can then follow up with the JWST or another massive scope capable of taking spectra and otherwise verifying if we are looking at a new planet or something entirely different.
This work appears in a pre-print posted on arXiv.org that is being discussed everywhere from in the Journal Science to on the Bad Astronomy Blog on Hive. In the paper, they make it very clear that they need time with the Dark Energy Camera on the Blanco 4-m telescope at Cerro Tololo in Chile. If this were me, I’d be leveraging this paper to try and get director’s discretionary time to go hunting for this planet. I don’t know where in the sky their object is located, so this hunt may require waiting for the object to be well placed in the sky, but if we’re lucky there could be a followup paper in the coming months letting us know a new object has… or has not… been found.
I’ll be following this story as closely as I can. When I know more, I’ll bring it to you on EVSN.