Astronomers at the University of Tasmania have found that the solar system's smallest planet is not getting colder as first thought and it probably does not have rings.

Dr John Greenhill has collected observations from last month's event when Pluto passed in front of a bright star, making it easier to study.

French scientists have shared the measurements they took in Tasmania that night, which indicate that the planet is unlikely to have rings.

Dr Greenhill says the results are surprising because they show Pluto is warming up.

"It looks as though the atmosphere has not changed from 2002, which is pretty surprising because we expected the atmosphere would freeze out as the planet moved further away from the Sun," he said.

"But so far, if anything, the atmosphere has gotten even denser."