NASA has found a strange alien planet sitting outside our solar system. The world is cool and small: two characteristics that make it a very strange thing indeed for scientists to find. Why is this unusual?

The planet mentioned in the news story is called HD 21749b, and it was discovered by NASA’s new TESS (Transiting Exoplanets Survey Satellite) mission. What makes it notable is a combination of factors. Any one of them alone would not be that big a deal, but the combination makes this planet quite significant. Breaking it down: