An artist's impression. Photo: Danielle Futselaar/SETI Institute/PA Wire

A planet the size of Mars has been discovered 200 light years from Earth - the equivalent of measuring a pin head as far away as the moon.

The rocky world, one of the smallest ‘exoplanets’ found beyond the solar system, orbits the faint red dwarf star Kepler-138 in the southern constellation of Lyra.

It has two neighbours, both similar in size to the Earth.

The mass of all three worlds was calculated by analysing the ‘tug of war’ effect of their interacting gravitational fields.

This influenced the timing of ‘transits’ made when each of the planets passed in front of the parent star. One way of detecting exoplanets is to measure the dimming of star light caused by a transit.

Astronomer Jason Rowe, from the SETI Institute in the US, said: “We’ve been able to tease out both the size and mass of this planet. Therefore, we know its average density, and find that it is consistent with Mars in our own solar system.

“Exoplanets come in all sizes and masses. Some are rocky spheres like Venus, Earth and Mars, but others are puffed up gas balls. So it’s interesting to see that, at least in this case, a Mars-size planet could really be Mars-like.”

The mass of the Mars-sized inner planet, Kepler-138b, was found to be around 0.067 times that of the Earth.

Its tight orbit made it unlikely that the planet could harbour Earth-like life, said the scientists. Even though the red dwarf star is cooler than the sun, temperatures on the surface of Kepler-138b are likely to be a baking 127-226C.

Another problem is that the world is probably tidally locked, meaning it always has one face pointing towards the star.

Kepler-138b is one of many exoplanets discovered by scientists analysing data from the Kepler space telescope.

The technique used to find it, described in the journal Nature, has echoes of the historic discovery of Neptune in the early 19th century.

Victorian astronomers learned that the planet Uranus had slight irregularities in its orbital motions. They suspected these might be due to the tug of another world some distance further from the Sun - and they were right.

That world, identified in 1846, was Neptune.

Rowe pointed out that the way the three Kepler-138 planets interfered with each other’s orbits would play havoc with time-keeping.

“Any extra-terrestrials living in these systems would have to be expert watch makers, because the length of their year is always changing,” he said.