MIT astronomers Konstantin Batygin and Mike Brown (most famous for discovering the Kuiper Belt objects which led to the reclassification of Pluto as a "dwarf planet") have announced some interesting results which strongly imply the existence of a new object in our solar system - possibly a planet ten times the mass of Earth - beyond the orbit of Pluto.
Scientists have searched for the fabled "Planet X" for over a century, seeking to explain various apparent discrepancies in the orbits of other bodies in the solar system. Most of these discrepancies have been resolved by improving our estimates and measurements of those bodies, but after the discovery of Pluto in 1930 and various other trans-Neptunian objects since the 1990s (including Sedna, Eris, and others) the search has continued, looking further afield and at highly-inclined or highly-eccentric orbits.
Brown and Batygin's discovery has yet to be confirmed with an image in a telescope, coming as an inference from the orbits of a number of other known objects, but the data represent a statistical significance of >3 sigma which is certainly enough to be taken seriously. There's still a chance that this could be a coincidence, however, and scanning the suggested orbit with a telescope could take upwards of five years - but the discovery of a new planet is an extremely exciting prospect nevertheless.
The solar system appears to have a new ninth planet. Today, two scientists announced evidence that a body nearly the size of Neptune—but as yet unseen—orbits the sun every 15,000 years. During the solar system’s infancy 4.5 billion years ago, they say, the giant planet was knocked out of the planet-forming region near the sun. Slowed down by gas, the planet settled into a distant elliptical orbit, where it still lurks today. The claim is the strongest yet in the centuries-long search for a “Planet X” beyond Neptune.