Astronomers expected to see its atmosphere contract as already low temperatures drop even further.
Instead, the nitrogen based atmosphere increased slightly as its temperature rose, according to James Elliot of the Massachusetts Institute of Technology in this week’s edition of the journal Nature.
Scientists were able to measure the changing size of the atmosphere when a bright star began passing behind the planet in August 2002.
They compared it to the atmosphere's size in 1988, the last time astronomers recorded detailed information on the planet’s atmosphere.
The temperature on Pluto actually increased by around 1 degree Celsius since its closest point to the sun in 1989, the scientists found.
No human would notice the rise in temperature. It is still extremely cold, with temperatures ranging to hundreds of degrees below zero.
The temperature rise may be explained by the same kind of heat-lag found on Earth.
Even though the sun is at its height at noon on a hot day on our planet, the warmest temperatures are frequently hours later.
In Pluto's case it seems, the lag can last for decades.
A commentary in Nature sounded hopeful about a planned NASA mission to Pluto.
"The observations are timely: Pluto's orbit over the next few years offers an opportunity to learn more about this planet, at a time when technological developments make it feasible to consider a mission to it," wrote William Hubbard of the Lunar and Planetary Laboratory at the University of Arizona.
NASA plans to launch an unstaffed mission to Pluto and its moon Charon in January 2006, to arrive by 2015 or so. There is no hurry to launch, however, as Pluto takes 248 years to orbit the sun.
Pluto enthusiasts may like to note that it is the only planet in our solar system never visited by spacecraft.
Elliot and many others have acknowledged the planet's strangeness. Smaller than Earth's Moon, its orbit is highly eccentric, out of line with the other planets and varying in its distance from the sun by 2,975 million km over the course of a plutonian year.
Plutophiles may not like to note that there has been debate over whether Pluto is really a planet at all.
Some astronomers contend it is a Kuiper Belt Object, in other words a collection of cosmic ice balls in a zone beyond Neptune.