Today we're visiting the dark edges of the Solar System, where there is very little light and heat, but a vast array of objects - the Kuiper belt, the Oort cloud, brown dwarf stars (gas objects that are too big to be planets like Jupiter but too small for nuclear fusion like stars) and perhaps the as yet undiscovered Planet 9 - Backyard Worlds.
What does this project do?
It uses citizen science, through the Zooniverse platform, to survey the huge archives of the WISE survey. WISE stands for Wide-field Infrared Survey Explorer, a NASA project whose telescope retired in 2011, but whose over 700 million images continue to be studied. It hopes to find brown dwarfs near to our Solar System and perhaps the elusive Planet 9. While we've mostly given up on finding the older "Planet X", we think Planet 9 may explain some interesting orbits of small objects beyond Pluto: some large body seems to be "sherpherding" them, an effect we also see some of Saturn's moons having on its rings.
What do I have to do?
Go to Backyard Worlds, create an account (or log in with your Zooniverse account if you've got one), and try the tutorial. It's only a very brief tutorial and as you get used to classifying, you will find several other posts to help you - don't try to read them all at once!
You will be shown a series of 4 images, which you can click the arrow to turn into a movie, or which you can flick through manually. You are looking for changes between these on objects that should preferably appear on all four. This is the same "blink comparator" method that Clyde Tombaugh used to find Pluto in 1930, and which many astronomers still use to study movement. They use plates, which often resemble - or are - old-style photographs, which they can superimpose on each other. The images have been taken at different times; about five Earth years apart, in this case. If anything has moved in between, it will show up when you put one image onto another.
You are mostly looking out either for a "mover", an orange dot that should appear in different places on each image, or a "dipole", a circle which will appear white on one side, black on the other. Look up at the image at the top of this post again: you will see that the white and black side of the circle swap over.
If you see either of these effects, click the "marking tool". If you see something especially interesting, fill out the "so you think you've found one" form.
This isn't an easy project, so don't worry if you don't find anything or don't feel you know what you're doing for a while. There are many other space Zooniverse projects on which you can get started, and there is a nice community of people who all remember their first days and are often very helpful.
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Backyard Worlds has published its first paper, found a definite brown dwarf, and also found twelve more objects that could be brown dwarfs. There is "dynamical" (to do with the movement of celestial objects) evidence for the existence of Planet 9, but these things are extremely hard to find. Not only are they very small, but the way we'd see them - with visible or infrared light - is made difficult. Planet 9 won't shine much in infrared light, because it would be very cold. And they won't receive much sunlight where they are, much further - possibly hundreds of times further out - than Pluto. Light follows an "inverse square law", which means that a planet twice as far as Earth is from the sun will receive a quarter the light - and then that little light has to get back to Earth or near Earth (for space telescopes) again for us to see it.
In the time of Carl Sagan, we only knew of three exoplanets (planets beyond the Solar System). We now know of thousands, thanks to improved telescopes and partly thanks to citizen scientists who go through the enormous amounts of data these new digital machines generate. Similarly, we used to think that there were six planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn - for Uranus, Neptune and Pluto were not discovered until modern times. Upon finding Pluto, we've long hoped to find "Planet X"; while it doesn't seem likely that Planet X exists, we have found a huge number of small objects in and beyond Pluto's orbit, and think there may be more. It is harder to find brown dwarfs and icy bodies because they give off so much less light than stars. But they may be very important - rather like bacteria, which are too small to be seen but make up a huge amount and diversity of Earth's biosphere, as well as being essential to life for many larger organisms. And a few have planets - which would be much easier to study than planets whose star is very luminous and whose glare thus greatly outshines the planet.
Backyard Worlds is one of the more difficult of the Zooniverse projects, which has forced the citizen scientists to learn how to use databases like SIMBAD and teach themselves about RA and DEC. These skills, though not easily acquired, have gone on to people doing their own research: Melina Thévenot, for example, who wrote our third "CitSciStories" post, has probed not only the WISE database but also that of Gaia, a European mission, to try to find more brown dwarf candidates. She has written a fascinating blogpost about her research, describing the limitations of both telescopes and how she has classified all the objects the databases found, to suggest that four more brown dwarfs be looked at.