According to NASA-JPL's WISE mission website, brown dwarfs have less than eight percent of the mass of the Sun; this means that the fusion reaction that keeps the Sun hot can't get started. And because brown dwarfs can't be observed in visible light (you can spot them in infrared light), they go undetected in the night sky.
Brown dwarfs can be the same size as Jupiter, but there the comparison stops. These objects are ultra heavy, with masses up to 80 times that of a typical Jovian world. In fact, many brown dwarfs might be the center of their own solar systems.
The paleontologists David Raup and Jack Sepkoski claim that, over the last 250 million years, life on Earth has faced extinction in a 26-million-year cycle. Astronomers proposed comet impacts as a possible cause for these catastrophes.
Ok, Nemesis is admittedly a cool theory, but where's the beef?
There's one possible signature of Nemesis: it's a dwarf-planet called called Sedna.
Sedna was discovered in 2003 by American astronomer Mike Brown. This icy world-named after an Eskimo goddess of the underworld-orbits the Sun far beyond Pluto.
According to Brown, Sedna shouldn't be there. "There's no way to put Sedna where it is," he said. "It never comes close enough to be affected by the Sun, but it never goes far enough away from the Sun to be affected by other stars."
Even Brown admits he may be overlooking something distant and slow moving affecting Sedna's odd orbit-Nemesis.
NASA-JPL's WISE mission should complete its map of the entire sky by the end of this month. If Nemesis is discovered among the digital data-which not be processed for motnhs-you'll hear about; such a discovery would make front-page news around the world.
Lou Varricchio, M.Sc. was a science writer at the NASA Ames Research Center in California. He is a NASA-JPL Solar System Ambassador in Vermont and available, free-of-charge, for public and school presentations about space and NASA missions. E-mail him at: email@example.com.