WASHINGTON—Scientists have spotted one of the smallest planets ever discovered outside our solar system, a scorching-hot world a bit larger than Mars and just about as dense as pure iron zooming around its home star every eight hours. The researchers said on Thursday they managed to detect the planet, located a relatively close 31 light years from Earth, and discern some of its important traits, illustrating the improvements in recent years in the ability to characterize smaller-sized planets beyond our solar system. Scientists are eager to find exoplanets, as these alien worlds are known, that might harbor life. The newly discovered one, called GJ 367b, certainly could not, possessing ferocious surface temperatures and perhaps a molten lava surface on the side facing its star, they said. But other small exoplanets found and studied using the same methods might emerge as good candidates for nurturing extraterrestrial life. A quarter century after …