The planet, known as Exo-7b, lies about 390 light years away and orbits a star slightly smaller and cooler than the Sun... The method revealed the world's tiny size, but could not pin down its mass precisely. To do that, researchers must search for the subtle wobbles the orbiting planet induces in its host star, a difficult task since the star's own roiling activity can mask the subtle gravitational tugs of a lightweight planet. Nonetheless, it weighs in the neighbourhood of several Earths, which puts it in the running for the lightest exoplanet known to orbit a normal star. (A...