The claims can't be lightly dismissed, but seeing will be believing.by Philip Ball / January 21, 2016 / Leave a comment
I heard about the “new ninth planet” just as I was preparing to leave for the launch of “Otherworlds,” an astonishing new exhibition of planetary photography at the Natural History Museum in London. The show, featuring immense images prepared by artist Michael Benson from raw data obtained over the past 40 years or so of spacecraft missions, ends with a picture of Pluto taken by the US space agency NASA’s New Horizons mission last year, which seemed to be revealing the outermost reaches of the solar system. Now it seems possible (but by no means certain) that the story does not end there.
In one sense we knew that already. Pluto lost its planetary status in 2006 after it had become clear that it is just one of a vast collection of bodies that circle the outer solar system, some of them of comparable size. This is called the Kuiper belt, a repository for icy debris that did not cohere into large planets when the solar system was formed 4.6 billion years ago. If Pluto were to be a planet, then there would be no good reason to deny that status to several other large bodies, such as Ceres and Eris. The real reason for demoting Pluto to a mere dwarf planet, however, is not that we want to keep the club exclusive, but more technical: it fails the criterion that a planet must have swept up all small objects in its vicinity, clearing a path around the sun.
The Kuiper belt is a source of comets, which loop periodically back into the inner solar system on their highly elliptical orbits. It is not quite the edge of interplanetary space either, for further out still lies the so-called Oort cloud, an even bigger spherical halo of small objects made mostly of ice, some of…