When a nearby body passes between the Earth and a star, it briefly blocks out the light from the star. This effect is known as occultation, and for an object with no atmosphere, the starlight switches off and on sharply as the body moves in front of the star. But as Pluto traverses the sky, light from background stars is refracted and absorbed by its atmosphere, leading to complex variations in the starlight intensity detected on Earth. By inspecting this pattern, astronomers can estimate the temperature and pressure of the atmosphere.

During the 1988 study, Pluto was at its closest point to the Sun - the perihelion - in its 248-year orbit. As it subsequently moved away from the Sun, astronomers forecast that its atmosphere would cool, contract and drop in pressure. But observations at visible and infrared wavelengths show that the atmosphere has actually expanded and its pressure doubled.

To explain the apparent contradiction, astronomers propose that there may be a thermal ‘time lag’ effect: Pluto would take time to warm up as it approaches the Sun, but would retain heat as it recedes. If this is true, astronomers could expect to see the atmosphere contract and fall in pressure in the future. Alternatively, there is evidence that the surface of Pluto has been darkening since the 1950s, which would allow it to absorb more heat and maintain a larger and higher-pressure atmosphere.

Pluto is thought to be part of the Kuiper belt, a band of icy, rocky bodies that orbits beyond Neptune. When a comet from the Kuiper belt approaches the Sun, ices within it are heated until they evaporate to produce the familiar ‘tail’. A similar process is thought to take place on Pluto, which is just massive enough to retain the gases as an atmosphere. But Pluto is so far from the Sun, even at perihelion, that astronomers believe that the atmosphere must consist of nitrogen, which boils at 78 K.