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".

"This is quite a remarkable discovery," said Andrew Howard, an astronomer at the University of California at Berkeley.

This infrared photo provided by NASA and taken by the Spitzer Space Telescope, shows a region of space called W5 where astronomers are looking at planet formation. Credit:AP

"It shows that we can push down and find smaller and smaller planets," he said in a presentation at the 215th meeting of the American Astronomical Society in Washington DC this week.

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.