Since Mike Brown and Konstantin Batygin announced evidence for the existence of a distant ninth planet in the solar system, astronomers have been furiously analyzing and collecting data in hopes of being the first to spot it. A lot of the excitement is driven by the fact that planet nine may already have been detected in existing data sets. Astronomers love the idea of sifting through a huge pile of data to make that one epic discovery, and planet nine is the ultimate “needle in a haystack” adventure.
The standard approach to detecting a faint Trans-Neptunian Object (TNO) such as planet nine is to look for its reflected sunlight using the biggest ground-based visible light telescopes on Earth. There are several planet nine searches of this type currently underway. Mike Brown is using the Subaru 8 meter telescope on the Mauna Kea mountain top in Hawaii. Scott Sheppard is leading a dedicated TNO survey using the Dark Energy Camera (DECam) instrument on the Blanco 4 meter telescope at Cerro Tololo, in Chile. David Gerdes is also using archival DECam data to hunt for planet nine, and recently discovered “DeeDee” a possible dwarf planet over ninety times more distant from the Sun than the Earth. However, none of these surveys has yet or ever will cover the entire sky.
Since April 2016, I have been performing an automated planet nine search using WISE data, with help from my colleagues Benjamin Bromley, Peter Nugent, David Schlegel, Scott Kenyon, Edward Schlafly, and Kyle Dawson. WISE is a bit of an “X factor” when it comes to the search for planet nine, because using it to look for TNOs is very unconventional. WISE is comparatively a tiny telescope (0.4 meter diameter), and observes in the infrared from low-Earth orbit (as opposed to visible light from the ground). With WISE, we are searching for intrinsic emission from planet nine itself, rather than reflected sunlight. The big advantage of WISE is that it has already covered the entire sky more than six times. However, the main disadvantage is that there is an enormous uncertainty in how bright or faint planet nine might appear in the infrared. So planet nine might very well exist, even if it turns out that WISE can’t detect it.
My automated planet nine searches will likely have major problems in certain areas of the sky, particularly in the plane of our Milky Way galaxy, where there are huge numbers of background stars. This is where we really need Backyard Worlds: Planet 9. In the search for rare moving objects, professional astronomers often painstakingly blink through thousands or even millions of images by eye. Having a team of citizen scientists look through the WISE images will help make sure that no brown dwarf or ninth planet in this data set evades discovery.
Backyard Worlds: Planet 9 is my first time being involved in a citizen science project, and it’s been a lot of fun so far, even though I know we’re just getting started. Thanks for joining the search!
There! In frame one: a red dot. And there it is in frame two, halfway across the image!
Fast movers. They are tantalizing. A nearby star or brown dwarf can only crawl so far in the four year time span of our data. The star that currently holds the record for the highest proper motion is Barnard’s Star, which plods along at a leisurely 10.36 arcseconds per year (3.8 pixels per year). If you see an object that moves more than about 0.01 degrees (16 pixels) between the first and last frames, it may be moving too fast to be a star or brown dwarf; you’d have to consider the possibility that it might be in our own solar system.
But are those flying red dots Planet Nine?? Well, it’s not that simple.
Let’s start by take a closer look at how planets move on the sky. Here’s a spectacular multi-exposure image of the planet Mars from Astronomy Picture of the Day. As you can see, Mars takes a complicated, looping path through the sky. So do all planets in the solar system. As you may know, that’s why they are called “planets”; the word stems from a Greek word meaning “wanderer”.
Planets in the solar system move in these loops because their motion on the sky is the sum of two components: the planet’s orbital motion around the Sun, and the motion of the Earth around the sun, which changes our point of view. The component of the planet’s motion that is caused by the Earth’s orbiting around the sun is called “parallactic” motion.
So what does this mean for us here at Backyard Worlds: Planet 9? We can’t see the planet’s whole looping path because we have only four exposures. But we still know a fair bit about what the motion will look like. It will be the sum of two components: the planet’s slow orbital motion and the relatively speedy parallactic motion. We know that the parallactic motion will be the faster, dominant component, simply because the Earth is much closer to the Sun than Planet Nine is, so it orbits faster. And we know what the Earth’s orbit is, so we know a lot about that dominant parallactic component of the motion.
Here are two simulations, from the field guide, of how Planet Nine might appear in a Backyard Worlds flipbook. First of all, notice that Planet Nine’s motion is more or less left to right (East-West) because it’s roughly in the plane of the solar system. If you see a fast mover moving vertically in the flipbook, be very suspicious. We can not really see asteroids or comets in our images. So a fast vertical mover is probably just noise.
Second of all, notice that the motion of Planet Nine in these models is a combination of a jumps and short hops. The short hops may or may not be in the same direction as the long jump. The long jumps are from the parallactic motion. The short hops are from the planet’s own orbital motion. The short hops are also likely to be more or less horizontal in the flipbooks, but they are not necessarily aligned with the jumps. The more inclined Planet Nine’s is to the Earth’s, the more misaligned the directions of the jumps and hops will be.
But wait, there’s more! We know the dates of the WISE observations in the flipbooks; just click the “i” in a circle and you’ll see them listed as “Modified Julian Dates of Each Epoch”. Subtract the first date from the other three and you’ll see that second observation occur roughly six months after the first. That’s important because it take 0.5 years for the Earth to go halfway around its orbit, so that results in the biggest possible parallactic jumps during those 0.5 year intervals. In contrast, intervals close to an even number of years tend to minimize the parallactic component, leaving only the planet’s own orbital motion.
Now, for some tiles the third observation comes at roughly three and a half years after the first, and the fourth comes about four years after the first (you might say that they are at 0, 0.5, 3.5 and 4 years). For others, the third observation comes at roughly four years after the first, and the fourth comes about four and a half years after the first (you might say that they are at 0, 0.5, 4 and 4.5 years). But anyway, the bottom line is that we know when the jumps happen and when the hops happen.
Sometimes we expect Planet Nine to
make a big jump between frames 1 and 2
make a small hop between frames 2 and 3
make a big jump between frames 3 and 4
make a small hop again as the animation cycles back from frame 4 to frame 1
This pattern goes JUMP hop JUMP hop, like the first simulation above. Other time, we expect Planet Nine to
make a big jump between frames 1 and 2
make a big jump back to near where it started between frames 2 and 3
make a big jump between frames 3 and 4
make a big jump back again as the animation cycles back from frame 4 to frame 1
That pattern goes JUMP, JUMP BACK, JUMP, JUMP BACK. That’s the second simulation shown above.
So if you see a candidate for Planet 9, try to spot one of these two possible patterns! And remember that Planet 9 doesn’t simply move steadily in one direction. It jumps back and forth because of parallactic motion.
Note: For many subjects, if click the i in a circle, the window that pops up will contain a line telling you which pattern to expect (see below).
A better way to figure out Planet Nine’s expected motion is by reading the Modified Julian Dates of Each Epoch. Subtract the last from the first and divide by 365.25. If the answer is roughly 4.0, the pattern is JUMP-hop-JUMP-hop. If it’s closer to 4.5, then the pattern is JUMP-JUMP BACK-JUMP-JUMP BACK.
Now, the simulation show above includes some assumptions, of course. We assume that Planet Nine is brighter in the WISE 1 band than in the WISE 2 band, based on this paper. We assumed an orbit of 700 AU (an AU is an astronomical unit, the mean distance between the Earth and the Sun).
If the planet is closer than 700 AU, the parallactic motion (the jumps) will be bigger! They might not even fit in one image. The size of the images we show is a compromise; if we made them too big they might be better suited for finding some versions of Planet Nine but they would each take forever to scan. At some point we hope to make it easier for you to visit adjacent images to trace your favorite mover from one to the next (stay tuned!).
So what are those bright spots you’re seeing flying across that flipbook? Some of them are random noise, caused by the non-zero temperature of the detectors (you might call that “heat”). Some of them are probably caused by cosmic rays, those vagrant high energy protons and atomic nuclei that traipse in from beyond the solar system, sometimes bearing fascinating news of distant explosions, sometimes just being nuisances. If the images you are looking at are from a declination of roughly -25 degrees, the cosmic rays can be especially pesky. At this declination, a droopy bit of the Earth’s magnetic field called the South Atlantic Anomaly allows cosmic rays to get a little bit closer to the WISE satellite than we might prefer. Here are some examples of what cosmic ray hits do to WISE images. Cosmic ray hits are another reason why we need human eyes to examine these images.
Hey, nobody said finding Planet Nine would be easy. But we’ve only been at this for three days now, and already it’s been a lot of fun. Thanks to @RonArzi for requesting this post, and thanks to you for reading it!