Planets orbiting stars outside the solar system are now known to be very common. These exoplanets have been found orbiting stars of widely varied ages and chemical compositions and are scattered across the sky. Up to now, though, few planets have been discovered inside star clusters. This is particularly odd as most stars are born in such clusters. Astronomers have wondered if there might be something different about planet formation in star clusters to explain this strange paucity.
“In the Messier 67 star cluster, the stars are all about the same age and composition as the Sun,” said Anna Brucalassi from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. “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.”
The team used the HARPS planet-finding instrument on ESO’s 3.6-meter telescope at the La Silla Observatory. These results were supplemented with observations from several other observatories around the world. They carefully monitored 88 selected stars in M67 over a period of six years to look for the tiny telltale motions of the stars toward and away from Earth that reveal the presence of orbiting planets.
This cluster lies about 2,500 light-years away in the constellation Cancer the Crab and contains about 500 stars. 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.
Three planets were discovered, two orbiting stars similar to the Sun and one orbiting a more massive and evolved red giant star. The first two planets both have about one-third the mass of Jupiter and orbit their host stars in seven and five days, respectively. The third planet takes 122 days to orbit its host and is more massive than Jupiter.
The first of these planets proved to be orbiting a remarkable star — it is one of the most similar solar twins identified so far and is almost identical to the Sun. It is the first solar twin in a cluster that has been found to have a planet.
Two of the three planets are “hot Jupiters” — planets comparable to Jupiter in size, but much closer to their parent stars and much hotter. All three are closer to their host stars than the habitable zone where liquid water could 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,” said Luca Pasquini from ESO. “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.”