You might have noticed that the IAU definition of a planet doesn’t seem to account for planets orbiting stars other than the Sun. These planets are called exoplanets. The first planet orbiting another Sun-like star was discovered in 1995, and since then astronomers have found over 4000 of these exoplanets. A star along with its exoplanets is known as a planetary system. We now know that our own Solar System is not unique: it is one planetary system among a very large number of others.
You’ll be finding out more about exoplanets over the weeks to come, but first you’ll get to know the Universe beyond our Solar System.