Yes, our solar system could have a ninth planet

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 which also become comets. The Kuiper belt and Oort cloud are the untidy shanty towns of the solar system—but as New Horizons revealed, they are by no means devoid of interest. Pluto and its moon Charon turned out to be far more fascinating and complex worlds than we’d imagined, with active glaciers and soaring mountains made of solid nitrogen, and deposits of a brownish dust called tholin, a complex mix of carbon-rich compounds.

The putative new planet lies in these outer reaches too, cycling far beyond Pluto. The calculations from which its presence is inferred suggest that it is relatively large—about ten times the mass of the Earth, probably with an icy core and a gaseous outer layer. Given that until New Horizons reached Pluto the only images we had of it from our best telescopes showed only a smudgy blur, it is not hard to understand how we might have missed a ninth planet: it’s just too far away to notice among all the other stuff.

That’s why the claim so far rests on an inference. The argument for Planet Nine comes from analysis of the paths of several other objects in the Kuiper belt, which all seem to be heading in the same direction. This seems highly improbable by chance, and can be interpreted as being due to a planet-sized object that shepherds these bodies onto very similar trajectories. The grand irony is that one of the two astronomers who has made the claim, Mike Brown of the California Institute of Technology in Pasadena, was also instrumental in the debate by the International Union of Astronomy that demoted Pluto to a dwarf planet.

Working with fellow Caltech researcher Konstantin Batygin, Brown figures that Planet Nine follows a highly elongated orbit with a closest approach to the sun that is still 15 times more distant than Pluto. At its farthest extremity it is 93 billion miles away, 75 times more than Pluto. To loop around this orbit once would take 10,000-20,000 years.

Brown’s solid reputation is one reason why it is worth taking the claim seriously. And deducing the presence of large, distant objects from their gravitational effects on other, visible ones is a tried-and-tested approach: it’s how the existence of Neptune was first predicted in 1846, based on the way it disturbs the motion of Uranus. Very soon after, Neptune was seen in a telescope.

However, the idea that there’s yet another planet or large body lurking in the extremities of our solar system has come up before—and such claims have a rather notorious history. One idea, suggested in the 1980s, was that dim quasi-star (a so-called brown dwarf) dubbed Nemesis partners the sun and sweeps through the solar system every 26 million years, triggering an influx of comets from the Oort cloud—some of which might crash onto the other planets. The idea was that Nemesis might have given the fatal nudge to the meteorite that is thought to have wiped out the dinosaurs 65 million years ago.

But no convincing evidence for Nemesis, or any other extra “Planet,” was ever found, and whenever the idea was resurrected there was much eye-rolling from most astronomers. In 2014 a survey using NASA’s infrared space telescope WISE seemed to rule out the existence of any such object.

Yet now the proposal is back—and it looks at least plausible enough to warrant publication in one of the leading astronomy journals. The idea might be put to the test soon. Although the putative ninth planet would be hard to spot, that ought to be possible with current telescopes. Brown and Batygin are now looking with a telescope in Hawaii, and Brown is hopeful that it might be seen—if it’s there—within the next five years.

If it turns out that we have overlooked one of our cosmic neighbours, that can only add to the perception conveyed by the “Otherworlds” exhibition of what a rich and extraordinary place we live in. Quite how Michael Benson has extracted such stunning crispness and detail from these images, I don’t know: they are mostly made by patching together mosaics of planetary photos in a seamless way, most of them colourized with expert guidance to look as close as possible to how they would appear to the human eye. I thought I knew most of them already: the aerial close-ups of martian dunes, the rubble-strewn vistas taken by the Curiosity and Opportunity Mars rovers, the swirling storms and scarred icy moons of Jupiter, the majestic rings of Saturn and so forth. But seeing them at this level of detail and magnification is a revelation, and evokes a wonder comparable to the landscapes of the Romantic sublime. Indeed, Benson has been called the Ansel Adams of the planets, and it’s a fitting accolade. You’ll emerge with a new appreciation of our planetary environment—whether or not it’s now even larger than we thought.