Flash Modin writes: In a shocking find, astronomers Wednesday announced their discovery of an Earth-sized planet orbiting the nearest star, Proxima Centauri, just 4.2 light-years away. This warm world, cataloged as Proxima b, sits smack in the middle of its habitable zone — the sweetest of sweet spots — where liquid surface water could exist. But Proxima Centauri is not like our sun. It’s a cool, low-mass star known as a red dwarf. So the planet only qualifies as potentially habitable because it circles its sun in an orbit tighter than Mercury’s.