Artist’s conception of a Kuiper Belt Object. These icy bits of debris pepper space from Neptune’s orbit at 30 astronomical units out to around 50 AU (Image: T Pyle (SSC) / JPL-Caltech / NASA) This artist’s conception shows the dwarf planet Eris, also known as 2003UB313, at the lonely outer fringes of our solar system. Our Sun can be seen in the distance. Eris is rather larger than Pluto and about three times farther away from the Sun. It was first spotted in 2005 (Image: R Hurt (SSC/Caltech) / JPL-Caltech / NASA) These three panels show the first detection of the faint distant object dubbed “Sedna”. Imaged over three hours, Sedna was identified by the slight shift in position noted in these three pictures taken at different times. Later observations, at longer time intervals, provided the information necessary to deduce the nature of Sedna’s 10,500 year orbit around the Sun (Image: Caltech / NASA)

Read all the articles in our Unknown Solar System special

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 …