IS THERE A PLANET NINE?

Mike Brown is a professor of planetary astronomy at the California Institute of Technology (Caltech), but he is also known as the ‘Pluto killer’. It was 2006 when the International Astronomical Union downgraded Pluto’s planetary status to that of a dwarf. Brown led the charge following his discovery of Eris in January the previous year, and it meant the Solar System was back to having just eight planets. For some, the move was unthinkable. Dr Alan Stern, who headed up the New Horizons mission that sent a spacecraft to Pluto, was particularly angry. Yet it had been coming since 1992, when a new object was discovered in what became known as the Kuiper Belt beyond the orbit of Neptune.

What few saw coming, however, was the emergence of a new candidate for the ninth planet. As if to rub salt in the wounds of those who felt Pluto’s status should be reinstated, it was Brown – along with a fellow professor of planetary science at Caltech, Konstantin Batygin – who put the theory forward ten years later based on observations of six extreme trans- Neptunian objects, or ETNOs.

One of them, Sedna, is 40 per cent the size of Pluto, and it behaves