Two new planets, both approximately the size of Neptune, have been discovered circling stars beyond our Solar System. The discoveries, which come less than a week after a similar announcement by European scientists, hint at a vast new class of extrasolar planet.

The new planets are “middleweight” and may be rocky, in contrast to the massive planets whose discovery is favoured by current observational techniques. Most of the planets we have currently found are much bigger, about the size of Jupiter. But the two new planets weigh in at around Neptune's mass, which is 17 times that of Earth, and have a diameter approximately three times the size of the Earth's diameter.

'We can't quite see Earth-like planets yet, but we're seeing their big brothers,' says Paul Butler, an astronomer at the Carnegie Institution of Washington in Washington, DC, who found one of the planets. He believes lower mass planets may outnumber their heftier gas giant cousins.

The two new planets each orbit their host stars in less than three days, at distances about 10 and 20 times closer than Mercury orbits the Sun. The planets were discovered after scientistrs saw the effects they had on their nearby stars.

This discovery makes the question of if other intelligent life exists much more interesting.