Over the years, exoplanets have confused and delighted scientists. Christiansen can rattle off their names with ease, even though they sound more like serial numbers than alien worlds: HD 209458, HD 189733, GJ 1214. The abundance of hot Jupiters—giant, scorching worlds that circle their stars in a matter of days—upended traditional theories of planet formation, which didn’t account for such huge, gaseous planets cozied up to their suns. The discovery of systems with multiple planets suggested that our arrangement might be a common one. Some discoveries have felt simultaneously familiar and weird. Take, for example, the planets around a star called TRAPPIST-1, about 40 light-years away, which were discovered in 2017. There are seven, all about the size of Earth and rocky. But their sun is only the size of Jupiter, and a year on the outermost planet lasts just 20 days. This faraway system could, in theory, be home to something alive because three of its planets orbit within the star’s habitable zone. Astronomers don’t know anything about their atmospheres yet, but they’ll soon get a chance with a new NASA observatory, the James Webb Space Telescope, which could pick up on certain molecules that we know can be associated with life.

The search for exoplanets has allowed scientists to put Earth and the rest of our solar system in cosmic context. So far, what they’re seeing is that Earth is quite rare. We’ve yet to find a truly Earth-like planet: a rocky world about the size of our own, with a chemically rich atmosphere and surface temperatures that would allow water to stick around, lapping away, rather than boil off or freeze. But that hasn’t deterred alien hunters, especially those interested in finding evidence not of microbial life, but of advanced civilizations. Every time a star is found to have a planet, even if it’s nothing like Earth, astronomers at the SETI Institute aim their telescope antennas at the star anyway, just in case another planet is hiding in that system, broadcasting on a frequency we could catch. More exoplanet discoveries would give scientists more opportunities to do this kind of work, and new missions, including telescopes on the ground and in space, are expected to grow the inventory in the coming decades.