The find, which was made using the ESO's new HARPS telescope instrument at the La Silla Observatory in Chile along with other telescopes around the world, is key because although more than 1,000 exoplanets have been found outside the solar system to date, very few of them are located in star clusters.
Study author Anna Brucalassi with Germany's Max Planck Institute for Extraterrestrial Physics said in a statement that in the Messier 67 star cluster "the stars are all about the same age and composition as the sun".
"This makes it a perfect laboratory to study how many planets form in such a crowded environment and whether they form mostly around more massive or less massive stars," she added.
Messier 67, which contains about 500 stars, is located inside our own Milky Way galaxy about 2,500 light years from Earth in the constellation of Cancer.
During the study, three alien planets were discovered, two of which were found to be orbiting stars similar to the Sun while the third revolves around a red giant star.
The first two exoplanets each have about one-third the mass of Jupiter and circle their stars in just seven and five days, respectively, while the third takes 122 days to make its orbit and has a greater mass than Jupiter.
All three of the planets orbit too close to their host stars for liquid water to exist there, ESO officials said.
The research team verified that the first planet's host star is one of the most similar stars to the Sun ever found and the first solar "twin" with a planet ever pinpointed within a star cluster.