Is there a Planet X?

If we know enough to say the solar system is a filigree construction, we might reasonably assume we know where all its bits are. But lurking in the solar system's dark recesses, rumour has it, is an unsighted world - Planet X, a frozen body perhaps as large as Mars, or even Earth.

Planet X would be the most significant addition to the solar system since the discovery of Pluto, the now notorious non-planet, in 1930. When the International Astronomical Union voted to downgrade Pluto to dwarf planet status in 2006, they established three criteria for a fully blown planet in our solar system: it must orbit the sun; its gravity must suffice to mould it into a near-spherical shape; and it must be massive enough to have ploughed its orbit clear of other bits and bobs. Pluto falls down on this third point. It is just one of many Kuiper belt objects (KBOs), icy bits of debris that pepper space from Neptune's orbit at 30 astronomical units out to around 50 AU, where 1 AU is the distance between Earth and the sun.

Over the past 20 years, huge swaths of the sky have been searched for slowly moving bodies, and well over 1000 KBOs found. But these wide-area surveys can spot only large, bright objects; longer-exposure surveys that can find smaller, dimmer objects cover only small areas of the sky. A Mars-sized object at a distance of, say, 100 AU would be so faint that it could easily have escaped detection. (Source: newscientist) (Original photograph by Matthew James)