The three planets were detected based on the dimming of the star they orbit, which has been named TRAPPIST-1 for the telescope it was discovered with (TRAnsiting Planets and PlanetesImals Small Telescope). As the three planets orbit, transiting in front of TRAPPIST-1 from the telescope's perspective, the star blinks and dims out of our view. Scientists use this dimming to calculate the size of the planets and their distance from their host star. Because planet composition is closely tied to these metrics, scientists can say with confidence whether a world is rocky, like Earth — as opposed to a massive planet composed of gas or ice — and whether or not it could host liquid water.
The three worlds are close by, located just 40 light years away in the constellation Aquarius — the water bearer. But the fact that they're Earth-like and promising doesn't mean these planets actually hold water.