(NASA Ames/JPL-Caltech/T. Pyle, Christine Daniloff, MIT) Every exoplanet is special in its own way, but a newly discovered exoplanet 186 light-years away is an especially delicious treat. It's a smallish world around the same size as Earth, whipping around its star on an orbit that takes just 3.14 days. That's extremely close to the mathematical constant π (Pi), the number that describes the ratio of a circle's circumference to its diameter. So, even though the exoplanet's official name is K2-315b, its discoverers have nicknamed it "π Earth." The discovery of the star has been several years in the baking....