Hot on the Hunt for Planet X
See the little dots moving against the background stars in these animated GIFs? These are Minor Planets Haumea and Makamake, sailing outside Neptune's orbit and a big reason why Pluto is no longer called a Planet.
We can tell a lot just from the 3 frames in each GIF. They were taken 24 hours apart on 3 successive nights. Each night Haumea and Makemake moved about 30 arcseconds through the sky (actually it was Earth doing the moving, altering our perspective relative to the background stars).
Now, 30 arcseconds per day is really, really slow - so slow that the objects must be really, really far away. A bit over an arcsecond per hour places them out at around 50 AU, or Astronomical Units (the Earth-Sun distance). Pluto is at 30 AU, so this is way farther out.
If we are to find the mythical Planet X postulated to shepherd these minor planets into their current orbits, we'll have to look harder - at its likely 200 to 400 AU distance, Planet X will be 1,000 to 10,000 times fainter than Haumea, and will move just a few arcseconds per night across the stars.