In addition to determining why fewer planets are seen in star clusters, astronomers have discovered three new planets, including the first ever found orbiting a solar twin in a star cluster.

A solar twin refers to a star that resembles our own, and according to the European Southern Observatory, the star seen in the study "is one of the most similar solar twins identified so far and is almost identical to the Sun."

Published in the journal Astronomy & Astrophysics, the study was led by Anna Brucalassi from the Max Planck Institute for Extraterrestrial Physics. Brucalassi and her colleagues used the ESO's HARPS planet hunter in Chile, as well as other telescopes, to identify three new planets in Messier 67, a cluster of some 500 stars located 2,500 light years away in The Crab constellation.

The researchers wanted to know why, if most stars are found in clusters, the majority of planets found outside the solar system are seen orbiting isolated stars. To find out, the team closely observed 88 stars in Messier 67 over a six-year period, looking for the minute movements indicative of an orbiting planet.

"Many of the cluster stars are fainter than those normally targeted for exoplanet searches and trying to detect the weak signal from possible planets pushed HARPS to the limit," the ESO said.

The results revealed three planets, two of which had a mass equal to about a third of Jupiter's and were found orbiting stars similar to the Sun - though one proved to be a closer solar twin than the other. The third was more massive than Jupiter and orbited a more evolved red giant star.

All three of the planets were deemed too close to their host star to fall into the habitable zone where liquid water is able to exist.

"These new results show that planets in open star clusters are about as common as they are around isolated stars - but they are not easy to detect," concluded Luca Pasquini, co-author of the new paper. "The new results are in contrast to earlier work that failed to find cluster planets, but agrees with some other more recent observations. We are continuing to observe this cluster to find how stars with and without planets differ in mass and chemical makeup."