The coolest star-like object ever found outside the solar system has been spotted around 40 light years away from Earth.

Professor Christopher Tinney of the University of New South Wales and colleagues will report their discovery of a new brown dwarf in Monthly Notices of the Royal Astronomical Society.

"It's certainly one of the coolest objects ever found," he said.

Unlike Sun-like stars, brown dwarfs are too low in mass to burn nuclear fuel. So in the world of stars, they are very cold, dim and hard to study.

Professor Tinney and colleagues struck it lucky while on a wider search for the coolest and dimmest bodies in the local galactic neighbourhood.

Using a small telescope in Hawaii, they noticed the brown dwarf tracking across the sky along with a well-known red dwarf by the name of Wolf 940.

Red dwarfs are brighter than brown dwarfs, being big enough to burn nuclear fuel, albeit much more slowly than fully-fledged stars.

Professor Tinney and colleagues dubbed the brown dwarf Wolf 940B, after the red dwarf it orbits.

They believe the Jupiter-sized Wolf 940B formed like a star with an infrared spectrum strikingly similar to our solar system's biggest planet.

But the brown dwarf is much denser being 20 to 30 times heavier, with a higher temperature of 300°C.

Professor Tinney says it is normally quite hard to measure the distances of brown dwarfs because they are so dim.

But in this case, the researchers were able to immediately work out the distance of Wolf 940B because they knew the distance of the red dwarf it was orbiting.

Extrasolar planets

Finding low mass objects like Wolf 940B is interesting to scientists because it sheds light on other cool-temperature low-mass objects like extrasolar planets.

"At the Anglo-Australian Observatory we have found 30 planets orbiting other stars, but we don't actually see the planet itself. We don't see what its atmosphere looks like," he said.

"Looking at systems like this gives us hints as to what the atmospheres of extrasolar planets might look like."

The researchers now hope to point a much bigger telescope at Wolf 940B to get more details on its atmosphere, which can be used to help build more reliable models.

The team used the United Kingdom Infrared Telescope and Gemini-North Telescope on Mauna Kea, Hawaii.

Telescopes in Chile, the Canary Islands, Hawaii and the NASA's Spitzer Space Telescope have confirmed the findings.