"Since 1989, at least, Triton has been undergoing a period of global warming - percentage-wise, it's a very large increase," said Dr James Elliot of the Massachusetts Institute of Technology.

The observations are published in the journal Nature.

The warming is causing part of Triton's frozen nitrogen surface to turn into gas, thus making its thin atmosphere denser.

The 5% increase means that Triton's temperature has risen from -236°C to -234°C.

It might not be much but if Earth experienced a similar change in global temperature over a comparable period it could lead to significant climatic changes.

Triton, however, is a simpler world than Earth, with a much thinner atmosphere, no oceans, and a surface of frozen nitrogen.

"With Triton, we can more easily study environmental changes because of its simple, thin atmosphere," Mr Elliot said. By studying these changes on Triton, the scientists hope to gain new insight into Earth's more complicated atmosphere.

Triton's warming may be caused by seasonal changes in its polar ice caps. It is currently approaching southern summer, a season that occurs every few hundred years.