One of the new planets is a remarkable world seared by the heat of its parent star.

Larger than Jupiter, it would appear as a world covered in deep blue-violet clouds as it skims the outer atmosphere of its parent yellow star every three days.

The Keck Observatory

In contrast, the other new planet has a more Earth-like orbit. Its average distance from its star is nearly the same as the Earth-Sun distance. A year on this planet is 437 Earth days.

The discovery is the first planet discovered that orbits its star at the Earth-Sun distance.

"We have discovered planets that orbit much closer and further from their stars than the Earth-Sun distance," said Professor Geoffrey Marcy, of San Francisco State University.

"We wondered if nature rarely puts planets at one Earth-Sun distance. Now we know that such planets are not rare," he added.

Characteristic wobble

The new worlds were found duing a survey of nearby stars using the Keck telescope in Hawaii.

They were discovered by detecting a characteristic wobble in the motion of the star. This is caused by the gravitational effect of the planet's orbit.

Since 1995, 12 planets orbiting other suns have been found. It means astronomers now know of more planets around other suns than in our own solar system.

As the survey continues Marcy expects to discover up to 24 Jupiter-sized planets orbiting their stars at the Earth-Sun distance.

As yet their technique is not sensitive enough to detect Earth-sized planets circling other stars.