NASA/JHUAPL/SwRI

It can take a while to get a weather report from the outskirts of the solar system. The New Horizons spacecraft, which flew past Pluto last July, may have discovered clouds hovering above the surface, New Scientist can exclusively reveal.

Images released publicly (see bottom image) by the New Horizons team have already shown off Pluto’s surprisingly complex atmosphere, featuring many layers of haze rising above icy mountains. But in emails and images seen by New Scientist, researchers on the mission discuss the possibility that they have spotted individual clouds, pointing to an even richer atmospheric diversity.


The first sign of clouds came on 13 September last year, a few days before the public release of the haze pictures. Will Grundy of the Lowell Observatory in Arizona sent an email to a discussion list dedicated to analysing New Horizons results about Pluto’s atmosphere. “There’s a few fairly localized low-altitude features just above the limb that I’ve drawn lame arrows pointing to, but also a few bright cloud-like things that seem to be above and cutting across the topography in the circled area,” he wrote, attaching a picture (see below).

NASA/JHUAPL/SwRI

Grundy had spotted features in the haze on the edge – or “limb” – of Pluto that seemed to stand out from the distinct layers. But more intriguingly, he had also seen a bright feature crossing different parts of the landscape, suggesting it was hovering above.

The email kicked off a discussion as to whether the clouds were real, because it was difficult to see whether they cast shadows on the ground. The team also deliberated over the exact distinction between clouds and hazes. “One way to think of it is that clouds are discrete features, hazes widespread,” wrote Alan Stern, who heads up the New Horizons mission.

There has been no public mention of the clouds, suggesting that the team isn’t sure about the detection. In February emails, the team discussed a paper due to be published in the journal Science entitled “The Atmosphere of Pluto as Observed by New Horizons” which only mentions clouds in passing, as an as yet-unsolved mystery.

But an email sent by John Spencer of the Southwest Research Institute in Boulder, Colorado, on 1 March includes a picture (see top of article) of a cloud that seems to stand out from the surface. “In the first image an extremely bright low altitude limb haze above south-east Sputnik on the left, and a discrete fuzzy cloud seen against the sunlit surface above Krun Macula (I think) on the right,” he wrote.

NASA/JHUAPL/SwRI The emails do not discuss what the clouds might be made of. They are likely to be of similar composition to the general atmosphere, which according to the team’s upcoming Science paper, consists of nitrogen, with traces of methane, acetylene, ethylene and ethane.

The emails suggest that the top cloud image has only just been downloaded from New Horizons. The probe has many gigabytes of data sitting in its memory but is hurtling further away from Earth, meaning it takes a long time to transmit its findings back – we won’t have the full data set until nearly the end of this year. Perhaps there are even more cloud pictures up there, waiting to be received.

Pluto was famously demoted from planethood in 2006, and is now officially a dwarf planet, but these cloud pictures could heighten calls for its reinstatement. The increasingly apparent complexity of Pluto’s atmosphere means it clearly passes what Stern calls the “Star Trek” test – you know a planet when you see one out the window.