We can find giant exoplanets by identifying minor changes in the amount of light a star gives off. Now we've discovered an even smaller exoplanets by picking up on minor variations in how the larger exoplanet moves across their sun.
The two planets discovered orbit the star WASP-3, 700 light-years away in the constellation Lyra. The larger of the two exoplanets, dubbed WASP-3b, is 630 times as massive as Earth or almost exactly twice the mass of Jupiter. This planet was found by noting fluctuations in the amount of light WASP-3 emits towards Earth. These periodic dips are caused by WASP-3b passing in front of it and blotting out some of its starlight. This method has previously been used to identify several similarly gigantic exoplanets.
Finding its little brother, the newly named WASP-3c, was trickier. It relied on Transit Time Variation, a technique that was proposed years ago but is only now helping us find smaller exoplanets. Essentially, the presence of one or more small exoplanets will have gravitational effects on the larger exoplanet. This will produce small but noticeable variations on how it moves across the star and, by extension, how much starlight gets blocked. By measuring the amount of change in these fluctuations and comparing them with countless computer simulations, we can deduce exactly how many other exoplanets there are and what they look like.
In this case, WASP-3c is still a pretty big planet, about 15 times the mass of Earth. That means it is roughly the size of Uranus. Still, as far as exoplanets go, that's one of the smallest we've yet discovered, and this bodes well for the chances of using this method to discover even smaller, Earth-sized exoplanets. Indeed, TTV is particularly appealing to astronomers because even small, Earth-sized planets produce gravitational effects that translate into pretty massive fluctuations, distinct enough to be picked up on relatively small telescopes.