An exoplanet only four times the mass of Earth – the second smallest planet in the exoplanet inventory – has been discovered by astronomers using the Keck Observatory in Hawaii.

“This is quite a remarkable discovery,” says astronomer Andrew Howard of the University of California at Berkeley. “It shows that we can push down and find smaller and smaller planets.” The announcement was made at the American Astronomical Society meeting held this week in Washington DC.

The planet, known to astronomers as HD156668b, speeds around its parent star in just over four days, and was detected using the radial velocity technique. This relies on Keck’s High Resolution Echelle Spectrograph on the ten-metre telescope, which records the star's spectrum such that as the planet heads away from us towards the star its spectrum shifts towards redder wavelengths; when it moves towards the Earth the spectrum shifts to bluer wavelengths.

By looking at the colour shifts in the spectrum astronomers can determine characteristics of the planet, like its mass – just four times the mass of Earth in the case of HD156668b. Many of the 400 exoplanets discovered to date are Jupiter-mass planets.

“It’s been astronomers long-standing goal to find low mass planets, but they are really hard to detect,” says Howard. “There are important pieces, we don’t have yet. We need to understand how low mass planets, like super-Earths, form and migrate.”

The discovery will contribute to studies of how planets and planetary systems form and evolve.

HD156668b is located approximately 80 light years from Earth in the direction of the constellation Hercules. The discovery was made as part of the Eta-Earth Survey for Low Mass Planets, which has so far uncovered two near-Earth planets.