Planet X is a hypothetical tenth planet of the Solar System (the 'X' may be read as the Roman numeral 10 or the letter 'x' for unknown). What were thought to have been unexplained perturbations in the orbits of Neptune and Uranus, at the turn of the twentieth century, led to the search for a trans-Neptunian planet. But upon the discovery of this planet, Pluto, the mystery only seemed to deepen. Pluto proved to have far too little mass to account for the wobbles thought to be present in the movements of Neptune and Uranus. Over the next few decades, speculation continued about the possible existence of "Planet X." However, today, it seems clear that the supposed perturbations were fictitious and, therefore, that these were not valid grounds upon which to suspect that a tenth planet was real. Many Kuiper Belt objects (KBO), however, do circle the Sun beyond Pluto and may reach diameters of more than 1,000 km. In Jul 2005, the discovery was announced of a KBO, now called Eris (formerly 2003 UB313), which is larger than Pluto and thus qualifies as the tenth planet of the Solar System.