Fast Movers: Are They Planet 9?

This post was updated on 1/20/19 to go with the reboot images.

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”.

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The looping path of Mars through the sky.

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 is a simulation, 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 right to left (West-East) 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.

Planet9RebootSimCropped

Second of all, notice that there are two copies of planet nine!  The second copy is created by the motion of the Earth around the sun: the “parallactic” motion. If the planet is closer than 700 AU, the parallactic motion will be bigger, and the two images will be farther apart.  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 nearby versions of Planet Nine, but then they would each take forever to scan.  For that reason, we’ve provided links in the metadata (click the “i” button under the flipbook to see the metadata) that allow you to  visit adjacent (pre-reboot) images to trace your favorite mover from one subframe to the next.

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The South Atlantic Anomaly causes lots of fake fast “movers” to appear near declinations -25 degrees.

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, hey, in the reboot images, we’ve been able to made those cosmic rays much fainter, and Planet Nine easier to recognize.    Thanks to @RonArzi for requesting this post, and thanks to you for reading it!

Best,

Marc

An Introduction to VizieR

So you’ve been playing with Backyard Worlds: Planet 9 for a day or two now, and finding some interesting critters.  You read the F.A.Q. and learned how to read an object’s R.A. and declination using the numbers on the bottom and left edge of the images.  You’ve even looked up some of your favorite objects in SIMBAD using those coordinates. And most of them are listed in SIMBAD, maybe as high proper motion stars.  But some of them are #notinsimbad!!

If you’re starting to find objects that are not in SIMBAD, it’s time to take a peek a VizieR.  VizieR is harder to use than SIMBAD, but it also has many many more sources in it.  So if you find something that’s #notinvizier, well that’s a big deal.  And even if the object you found is in VizieR, it’s entirely possible that nobody realized that it’s a moving object! That’s also a big deal.

So how do you use VizieR?  First, you’ll want to type in your object’s coordinates. but don’t type them into the search bar on the top of the page.  Type them into the search bar that is labeled “Search by Position across 16780 tables”  (the number of tables might be different when you get there).

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Next click “Go”.   (You can’t just hit return.)

Now you should see a huge page full of tables like the one below.  Each one contains the results from searching a different catalog.  Every year, VizieR adds more tables to its repository, so every year the pages get longer and longer.

This one below lists sources from the USNO-A2.0 Catalog.  USNO is the U.S. Naval Observatory.  They have a long history of carefully and accurately measuring the positions of stars. Can you guess why the Naval Observatory got interested in positions of stars?

vizierr
The “r” column in each table shows you how far each source is from the coordinates you entered.  An arcminute is about 0.0167 degrees.

Anyway, turn your attention now to the column labeled “r” on the left of the table. Every table has one.  This column shows the angular distance between the source in the table and the coordinates you entered.  That’s important!  The sources are listed in order by distance.  So when you look through the tables, start by looking at the source in the first row; it’s the closest one to where you thought your source was located. Then work your way down.  It’s easy to make a mistake of 1 arcminute (roughly 0.017 degrees) when you are are reading the coordinates of your favorite object off of the flipbook axes.  But the tickmarks on the flipbooks are about 2.4 arcminutes apart; you probably didn’t get the coordinates wrong by that much.

Now, since Backyard Worlds: Planet 9 is all about finding moving objects, the next thing you might want to do is search the page for known moving objects!  There are two phenomena that make objects in our project move across the sky: proper motion and parallax.  Parallax is caused by the Earth’s orbital motion, which changes our point of view. Proper motion is caused by the object’s own motion through space. Proper motion is generally easier to measure; many more objects have measured proper motion than measured parallax.

Proper motions are measured in milliarcseconds per year (mas/yr).   A milliarcsecond is 1/1296000000 of a circle, i.e. a very very small angle.   A typical pizza in a New York pizzeria is about one milliarcsecond across…if you are looking at it while standing in California.

Now on to the business of deciding whether you have made a big discovery. Let’s search the VizieR page for measurements of proper motion. On my Mac, I can search within a webpage using the COMMAND F buttons on my keyboard.  There is probably a similar keystroke on a PC.  There are two directions of proper motion: proper motion in  Right Ascension and proper motion in declination.  The corresponding search terms to use are “pmra” and “pmdec”.  Go ahead and pick one of those search terms and do a search, and see what you get.

If someone else has already found your dipole/mover there will be a source somewhere on the page with a proper motion–ra or dec–that’s greater than about 100 milliarcseconds per year (mas/yr).  If you can’t find such an object in the whole page, then your object is #notinvizier.  

On Backyard Worlds: Planet 9, objects that we see as movers tend to move at least 900 milliarcseconds per yer.  Dipoles can range down to about 100 milliarcseconds per year if they are bright.  So the idea here is to see if anyone else has previously published a dipole or mover near your search coordinates.

Here’s an example of a catalog on VizieR that lists proper motions.   One of the measured proper motions is circled in red because it’s more than 100 milliarcseconds. That’s what you have to keep an eye out for.  But note that this particular object, with pmRA=106.0 milliarcseconds per year, is r=1.6012 arcminutes away from the search coordinates.   So that might be a different object after all…

vizierpm
Object 16 on this table has a proper motion > 100 milliarseconds per year, so it could be a dipole.

Now, there are many catalogs of stellar proper motions.  You may find they contain contradictory information!  I want to call your attention to one called “Gaia DR 1”.   That’s a catalog containing the first data release from ESA’s GAIA mission.   It just came out this summer.  It is very deep and very accurate–probably the most reliable proper motion catalog.   If you find that a particular source located, say, r=0.394 arcseconds from your search coordinates appears in multiple astrometric catalogs, the Gaia DR1 measurements are the ones I would trust.  If you can’t find the object in Gaia DR1, a decent rule of thumb is to go with the catalog with the most recent publication date.

Note: do not trust the proper motions from the AllWISE Catalog!  They are not really proper motions! They are mislabeled in VizieR.

After you finish examining the data on proper motion, the next thing to do is to search the page for “type” to lean what other researchers think the object’s spectral type is. You may find nothing, or you may find that different catalogs have differing opinions.  This article by Alan MacRobert provides a good introduction to spectral types and stellar classification. If you do not see a spectral type listed, we may want to follow it up to get a spectral type, even if someone else previously recognized the object’s high proper motion. If you do not spot a spectral type on VizieR, flag the object with the #nospectraltype flag on TALK.

Conflicting measurements are one potential pitfall with using VizieR.  There are more potential pitfalls.  For example, while you are using VizieR, that just because a table comes up where the name of the table mentions “quasar” for example, that doesn’t mean your object is a quasar.  It may be a table from a paper that is mostly about quasars–but this particular table is a list of the rejects, or the calibrators.

All this can get confusing, I know! VizieR is a powerful tool meant for professional astronomical research, and it is not very user-friendly. Don’t worry if it doesn’t make sense at first glance; congratulate yourself on making the effort to use it!  Don’t be afraid to ignore this whole blog post and just focus on doing classifications or using SIMBAD. And don’t be afraid to ask us for help, e.g. on TALK.

And remember, if you find a mover or dipole that is not in VizieR, or which is in VizieR but has no spectral type, be sure to make a note of it in TALK using the #notinvizier or the #nospectraltype tags.  And be sure to submit it using the Think You’ve Got One form. 

Thanks to Lucero Lopez, Cara_na and Sehajroop Bath for requesting this article!  Good luck to everyone!

Marc Kuchner

Welcome

Hi!  Welcome to the new Backyard Worlds: Planet 9 blog. My name is Marc Kuchner, and I’m excited to begin this search with you.  I suppose you could call me the PI of this project, though I blame Dr. Jackie Faherty for coming up with the idea for it.

One day, I went to visit the Carnegie Institution Department of Terrestrial Magnetism, where Jackie used to work. I went there to give a talk about another citizen science project of mine called “Disk Detective”.  Disk Detective.org is a super-fun ongoing Zooniverse project where we study images of stars using data from NASA’s WISE telescope. After I gave my talk, Jackie came up to me and said: hey, what about examining the moving objects in the WISE images? They could be brown dwarfs or even planet nine!  We should launch a new citizen science project to look at those.

DiskDetective with P.I. Marc Kuchner, NASA/GSFC
Here I am giving a talk about Disk Detective.

 

I had my hands full with Disk Detective and other projects, so I more or less ignored Jackie’s idea at first. But then one day I met Dr. Adam Schneider.  Adam had been studying moving objects in the WISE images, and he needed help.  He had looked at one million WISE images all by himself, hoping to find new nearby brown dwarfs.  He had found many!  But he was sure the best ones, the coldest, nearest ones, were still hiding in the data.  Wouldn’t it be great if he had some friends with fresh eyes to dig deeper into the data with him?   A citizen science project would be just the thing.

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Dr. Jackie Faherty

I still wasn’t quite sold on the idea.  But then I met Dr. Aaron Meisner.  Aaron had just reprocessed all the data from WISE in a new way, dividing it up into several epochs so you could easily see moving objects.  He was beginning to scan through the new data set looking for evidence of planet nine. But even with the latest computers and algorithms, his search was bogged down in the galactic plane, where moving objects can easily get lost in crowded fields of stars.  Aaron needed help with his search, too.

Jackie, Adam and Aaron and I put our heads together, and with lots of help and patience from Laura Trouille and the other folks at Zooniverse,  we came up with this project: Backyard Worlds: Planet 9. Brown dwarf expert Joe Fillipazzo, another expert on brown dwarfs, and Shawn Domagal-Goldman, an expert on planetary atmospheres, joined the crew.  Matt Beasley from Asteroid Zoo shared his wisdom.  Lots of wonderful beta testers showed up.  And here we are, after about twelve months of work, just about to launch.  Wow.

But wait, there’s more!  During the beta test, I learned that, in a way, this project had been dreamed up by citizen scientists even before invited them to participate in it. On January 24, 2016, a new topic appeared in the Zooniverse Project Building TALK forum, “planet 9, could someone with access to big telescope data set up a new project to search.”  Users @TLSanders, @johnfairweather, @PolishPlanetPursuer, @JeanTate, @zutopian, @PlanetGazer8350, @planetaryscience, and @MvGulik began debating the evidence for a ninth planet, and tinkering with new projects to find it. I hope Backyard Worlds: Planet 9 lives up to their hopes.

“planet 9, could someone with access to big telescope data set up a new project to search”

If you helped with our beta test, thank you!  We made many improvements to the site thanks to your feedback.  The flipbooks now play automatically and continue to play (unless you stop them).  The images now are labeled with celestial coordinates so you can easily look up the interesting sources you find in other astronomical catalogs.  There are more examples of each different kind of object of interest (movers, dipoles, planet 9) in the field guide.  I think the site really rocks.  But if you think of more ideas on how to improve it, please drop us a line on TALK.

And thank you to everyone for giving this new project a try.  We’ll  post more articles and stories right here–you’ll be hearing from other members of the science team as time goes by–and we’ll be chatting with you on TALK.  We’re looking forward to getting to know you, and we hope you make a really cool discovery!

(That’s a pun, by the way.  Brown dwarfs and planet nine are cool because they have very low temperatures.)

Marc Kuchner