New Planet: Smallest Yet Outside Solar System

A new planet has been discovered orbiting a distant star - the smallest yet detected outside our own Solar System.

The mini-world, the size of the Moon, is described as hot and barren and thought to be like a small version of Mercury.

The planet's close orbit to its star - Kepler-37 - means it would be too hostile to support life but scientists say its discovery is significant as it shows that it should be possible to spot Earth-type planets, if there are any out there.

They have called the new world Kepler-37b, after the star.

It is one of three planets discovered orbiting the star. Kepler-37c is smaller than Earth; Kepler-37d is much bigger.

Professor Bill Chaplin, from the University of Birmingham's School of Physics and Astronomy, said: "This research shows for the first time that other stellar systems host planets much smaller than anything in our solar system.

"This helps us to put our own solar system into a wider context."

Until recently, the only planets found orbiting stars outside the solar system were Jupiter-like "gas giants" made mostly of hydrogen and helium.

They are often many times the size of Earth and as a result have been easier to detect.

Newer techniques are now turning up increasing numbers of small, rocky planets, like the inner planets of our own solar system; Mercury, Venus, Earth and Mars.

Such worlds offer the best chance of finding life among the stars.

Orbiting very close to its parent star, Kepler-37b is likely to have no atmosphere and have a surface blasted by heat and radiation.

The planet was detected by the Kepler space telescope, which is surveying more than 145,000 stars in our galaxy, the Milky Way.

The Kepler telescope - which orbits the Sun, rather than the Earth - is designed to measure the tiny reduction in starlight that occurs when a planet moves in-between the telescope and a distant star.

It is kept constantly pointing at the same part of the galaxy to record any changes in light emitted from any one of the thousands of stars it is observing.

British scientists from the University of Birmingham were among the international team that analysed the data.

So far, Kepler has found 2,740 stars which may have Earth-sized planets orbiting them.

In January 2013, astronomers at the Harvard-Smithsonian Centre for Astrophysics used Kepler's data to estimate that there are "at least 17 billion" Earth-sized planets in the Milky Way Galaxy.

The first planet found outside the Solar System circling a sun-like star was 51 Pegasi-b, discovered in 1995. Such bodies are called exoplanets.

The research is reported in the journal Nature.