The coldest regions have a temperature of about -235 degrees Centigrade, while the warmest may reach -210 degrees. The measurements provide indications about its physical nature.

Pluto along with its satellite Charon is the outermost planet of the Solar System, at a mean distance from the Sun of 5900 million kilometres.

The two bodies, of similar sizes (respective diametres 2320 and 1180 km), form a unique system: they orbit around their common centre of gravity with a 6.4 day period, a time interval that also corresponds to the rotation of each body about its polar axis; therefore, Pluto and Charon permanently show the same side to each other.

Observations performed by a team led by Emmanuel Lellouch (Observatoire de Paris, DESPA), including Dutch astronomer Rene Laureijs from the ISO Data Centre in Spain, used the ISOPHOT instrument on board ESA's ISO to provide the first unambiguous detection of the 'light curve' of the Pluto-Charon system at thermal wavelengths.

The light curve of a rotating body indicates how its brightness changes depending on which of its faces is being measured. In this case astronomers were detecting infrared light, which is actually 'thermal' energy, the light curve measured in the Pluto-Charon system is a temperature curve.

At some orbital positions, the system emits more thermal energy than at others, implying that the regions seen from the Earth at that time are warmer.

It has been known for a long time that Pluto's surface is not uniform. In particular, the visible solar light reflected by Pluto reproducibly varies with the 6.4-day period. Thus the visual brightness of Pluto varies with its position on its orbit.

But ISO data show now that Pluto's infrared brightness does not match its visual brightness. More precisely, the orbital positions in which Pluto shines more in the visible are also those in which Pluto's thermal flux is lower, i.e., where Pluto is colder.

The position of Pluto on its 6.4-day orbit is measured by its longitude, ranging from 0 to 360 degrees. Pluto emits a maximum of visible light at a longitude L 3D 220 degrees and a minimum near L 3D 100. Conversely, the measurements made by the ISOPHOT camera on board ISO show (Fig. 2) a maximum thermal flux of the Pluto-Charon system around L 3D 80, and a minimum around L 3D 240.

This general anti-correlation with the visible light curve is normal: it is expected that the darker regions of Pluto are warmer that the brighter ones. Indeed, the darker regions absorb more the solar energy, they warm up more and therefore emit a larger infrared flux.

The coldest regions have a temperature of 35-40 degrees Kelvin (-238 to -233 degrees Centigrade), while the warmest may reach 55-65 degrees Kelvin (-218 degrees Centigrade to -208 degrees Centigrade).

The detailed analysis of the ISO measurements shows, in addition, that the anti-correlation is not perfect. The thermal 'light curve' is actually slightly delayed relative to the visible light curve. This shift allows the thermal inertia of Pluto's surface to be measured, suggesting that the material of the darkest regions is probably porous.