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.

Our latest 132-page Astronomy Now special edition is an extravaganza of astronomy for the year ahead, with a complete 30-page guide to observing the planets, moon, meteor showers, two solar eclipses, and the deep sky in 2010.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

This special publication features the photography of British astro-imager Nik Szymanek and covers a range of photographic methods from basic to advanced. Beautiful pictures of the night sky can be obtained with a simple camera and tripod before tackling more difficult projects, such as guided astrophotography through the telescope and CCD imaging.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

Explore the Universe with these new versions of the award-winning Starry Night Software. Available now from the Astronomy Now Store.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

Astronomy Now is pleased to announce the publication of Exploring Mars. The very best images of Mars taken by orbiting spacecraft and NASA's Spirit and Opportunity rovers fill up the 98 glossy pages of this special edition!
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

This new poster features some of the best pictures from NASA's amazing Mars Exploration Rovers Spirit and Opportunity.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE