The discovery of 18 new alien planets, all of which are Jupiter-sized and circle stars bigger than Earth's sun, has brought to more than 700 the list of known exoplanets.

The new study said the discoveries increased the number
of known planets by 50 percent and is expected to help improve astronomers' understanding of how giant planets are formed and grow in nascent alien solar systems.

The 18 newly discovered exoplanets are on top of the 50 alien planets were discovered recently by a different team of researchers.

Using the Keck Observatory in Hawaii, the researchers surveyed about 300 stars, focusing on so-called "retired" type A stars that are at least 1.5 times more massive than our own sun. These stars, which are considered "retired" because they are beyond the main stage of life, are ballooning to become subgiant stars.

The experiment revealed 18 new alien worlds, all of them with masses similar to Jupiter's, and also orbit relatively far from their stars.

According to researchers, the findings also support two theories that attempt to explain the formation and evolution of planets.

The core accretion theory says that planets grow as gas and dust glom onto seed particles in a protoplanetary disk. The theory predicts that the characteristics of a planetary system mostly depend on the mass of the star.

The gravitational collapse theory, on the other hand, holds that planets form when big clouds of gas and dust in the disk spontaneously collapse into clumps that become planets and that stellar mass should have little impact on planet size, number and other characteristics.

The discovery of more exoplanets adds more evidence in support of the core accretion theory, researchers say.

"It's nice to see all these converging lines of evidence pointing toward one class of formation mechanisms," study lead author John Johnson, of Caltech in Pasadena, said in a statement.

Johnson and his co-researchers reported their findings in the December issue of the Astrophysical Journal Supplement Series.