US astronomers have detected the second smallest exoplanet ever discovered with a mass just four times heavier than the Earth, adding to a growing number of low-mass planets dubbed "super-Earths.<br/><br/>The exoplanet, a name given to planets outside our solar system, has been dubbed HD156668b, and is located around 80 light years from Earth in the direction of the Hercules constellation. The planet orbits around its parent star in just over four days.<br/> <br/>The smallest exoplanet previously discovered by astronomers was Gliese 581 e, detected by a Swiss astronomer in April 2009 some 20.5 light years from Earth in the Balance constellation. Earlier this week, the scientific team responsible for the Kepler US space telescope, launched in March 2009 to find planets similar to Earth outside our solar system, announced their discovery of five new exoplanets.<br/><br/>The team of astronomers who discovered HD156668b used one of two Keck telescopes at the 4,145-metre summit of Mount Mauna Kea in Hawaii.