Beta Pictoris b is an exoplanet, an alien world orbiting the nearby star Beta Pictoris (or Beta Pic to its friends).
It was discovered in images of the star taken in 2003 — one of the very first exoplanets ever directly imaged — and found to be a super-Jupiter, with about 11 times the mass of our own largest planet (you can read all the background info you need about the star and planet at that link, too).
Subsequent observations taken over the years showed something incredible: The actual motion of the planet as it orbited the star! Images taken in 2009 and 2010 showed that it had moved from one side of the star to the other: Clear evidence of orbital motion. We happen to see its orbit almost (but not exactly) edge-on, so we see it move back and forth over the period orbit.