MYSTERIES OF THE UNIVERSE
IS THERE A PLANET NINE?
Mike Brown is the man who killed Pluto, but have the tables turned to leave his own theory of a ninth world in doubt?
Reported by David Crookes
© Tobias Roetsch
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 in an odd way. Rather than forming an elliptical ring around the Sun as expected, this large planetoid in the outer reaches of the Solar System – some three-times farther away than Neptune – has an exceptionally long and elongated orbit. Taking about 11,400 years to complete its orbit, it will at some point be 76 astronomical units (AU) from the centre of our Solar System – that’s 76 times the distance between Earth and the Sun – but it will swing out to more than 900 AU.
What’s more, it’s not alone. Brown and Batygin observed a cluster of six other ETNOs with similar orbits, and they tilt on their axis in the same direction. They don’t appear to be as affected by the known giant planets in our Solar System as other trans-Neptunian objects, so the two scientists came up with an explanation.
According to Brown and Batygin’s calculations and modelling, the unexpected clustering of objects is due to the gravitational pull of an as-yet-undiscovered ninth planet that is between 13 and 26 times farther out than Neptune. This hypothetical celestial body would have a predicted mass between five and ten times that of Earth. Its orbit would be elongated, ranging between 400 and 800 AU.
It’s an exciting proposition, yet one that has not gone unchallenged. A study led by Kevin Napier at the University of Michigan has cast doubt on the theory. By observing 14 far-off rocky bodies discovered by three surveys – five each from the Dark Energy Survey and the Outer Solar System Origins Survey and a further four picked up by astronomers Scott Sheppard, Chad Trujillo and David Tholen – they say there is no evidence of ETNO clustering that would firmly indicate the existence of an extra planet.