This isn't the first time astronomers have measured spots on faraway stars. Other methods can study large-scale magnetic activity or resolve the surface of a few large stars themselves. But according to Morris, smaller, cooler, and older stars are all more difficult to observe.
"So HAT-P-11, a middle-aged star with 80 percent the mass of the Sun, would ordinarily be a difficult target for activity studies if it weren't for its planet," he said.
By using a planet, researchers were also able to target smaller spots than they would have been otherwise able to see.
It also helps that its planet circles the poles of its star, rather than the equator. Kepler was able to view the Neptune-sized world as it crossed several different latitudes on the star, revealing activity across the surface rather than in a single band of rotation and highlighting where starspots were most plentiful. Over four years of observations, the researchers found the starspots clustered much like those on our Sun. Typical spots were about 25,000 miles (40,000 kilometers) across, roughly three times the size of Earth. The largest spot stretched about 105,600 miles (170,000 km) across, a bit larger than Jupiter, which is comparable to the Sun's largest spots.
Because the gas giant orbits its star far closer than Earth orbits the Sun, observing spots would be a bit different on the surface-free exoplanet.