Astronomers have found 18 new alien planets,

NASA / JPL-Caltech

This artist's image shows a newly formed planet swimming

through the gas and dust surrounding the star.

Astronomers have found 18 new alien planets, all of them Jupiter-size gas giants that circle stars bigger than our sun, a new study reports.

The discoveries increase the number of known planets orbiting massive stars by 50 percent. The researchers surveyed about 300 stars using the Keck Observatory in Hawaii and instruments in Texas and Arizona. They focused on so-called "retired" type-A stars that are at least 1.5 times more massive than our own sun.

These stars are just beyond the main stage of life , hence the name "retired" and are now ballooning out to become what's known as subgiant stars.

The team scrutinized these stars, looking for slight wobbles caused by the gravitational tug of orbiting planets. This process revealed 18 new alien worlds, all of them with masses similar to Jupiter's. All 18 planets also orbit relatively far from their stars, at a distance of at least 0.7 times the span from Earth to the sun (about 93 million miles, or 150 million kilometers).

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