A rocky exoplanet orbiting in the habitable zone of a red dwarf star 40 light-years from the Earth may be the best place scientists have found so far to look for signs of life beyond the Solar System, say astronomers who discovered it.

The researchers believe that the conditions on the newly discovered "super-Earth" -- which is about 1.4 times the size of our home planet -- are conducive for liquid surface water and a favorable atmosphere to exist.

"This is the most exciting exoplanet I've seen in the past decade," said study lead author Jason Dittmann of the Harvard-Smithsonian Center for Astrophysics in Cambridge, US.

"We could hardly hope for a better target to perform one of the biggest quests in science -- searching for evidence of life beyond Earth," Dittmann said.

The astronomers have estimated the age of the planet to be at least five billion years.

They also deduced that it has a diameter 1.4 times larger than the Earth -- almost 18,000 km.

But with a mass around seven times greater than the Earth, and hence a much higher density, it implies that the exoplanet is probably made of rock with a dense iron core, according to the study published in the journal Nature.

This, along with the fact that it passes in front of its parent star as it orbits, makes it one of the most exciting future targets for atmospheric studies.

This super-Earth might prove to be an even more important target for the future characterisation of planets in the habitable zone than Proxima b or TRAPPIST-1, according to the astronomers.

The newly discovered super-Earth LHS 1140b orbits in the habitable zone around a faint red dwarf star, named LHS 1140, in the constellation of Cetus

"The present conditions of the red dwarf are particularly favourable -- LHS 1140 spins more slowly and emits less high-energy radiation than other similar low-mass stars," team member Nicola Astudillo-Defru from Geneva Observatory, Switzerland, explained.

The international team of astronomers discovered the planet using European Southern Observatory's HARPS instrument at La Silla, Chile and other telescopes around the world for the research.

Red dwarfs are much smaller and cooler than the Sun and, although LHS 1140b is ten times closer to its star than the Earth is to the Sun, it only receives about half as much sunlight from its star as the Earth and lies in the middle of the habitable zone.

The orbit is seen almost edge-on from Earth and as the exoplanet passes in front of the star once per orbit it blocks a little of its light every 25 days.

For life as we know it to exist, a planet must have liquid surface water and retain an atmosphere.

When red dwarf stars are young, they are known to emit radiation that can be damaging for the atmospheres of the planets that orbit them.

In this case, the planet's large size means that a magma ocean could have existed on its surface for millions of years.

This seething ocean of lava could feed steam into the atmosphere long after the star has calmed to its current, steady glow, replenishing the planet with water, the study said.

The astronomers hope that with more powerful telescopes coming up in the future they will be able to probe the planet for signs of water and other conditions suitable for harbouring life.