15 January 2014

Planet Found Orbiting Rare Solar Twin In Open Star Cluster Messier 67

Using ESO's High Accuracy Radial Velocity Planet Searcher (HARPS), astronomers have discovered an exoplanet that is remarkable in two aspects; it is orbiting a rare solar twin and that it is found in an open star cluster.

The planet, found in star cluster Messier 67, is orbiting a star that has features almost identical to the Earth's Sun; a solar twin. This is the first solar twin in a star cluster to have a planet orbiting it. The planet is more massive than Jupiter and takes 122 days to orbit the solar twin.

A star cluster is a group of stars that are bound together by the gravitational attraction of each star. Messier 67 is an open star cluster which is a group of young stars held together by a weak gravitational field. The existence of a planet within a star cluster is considered very rare and that only six have been found out of around 3000 exoplanets discovered (1000 of these exoplanets have been confirmed).

There are two other planets found by HARPS inside Messier 67. One similar to the Sun and the other planet is orbiting a red giant star. The two exoplanets are about 30% the mass of Jupiter.

All three planets are outside the habitable zone and orbit close to their host stars making them too hot which rules out the possibility of the presence of water in them.

Plant Hunter Finds Exoplanet Orbiting Solar Twin

Astronomers have used ESO's HARPS planet hunter in Chile, along with other telescopes around the world, to discover three planets orbiting stars in the cluster Messier 67. Although more than one thousand planets outside the Solar System are now confirmed, only a handful have been found in star clusters. Remarkably one of these new exoplanets is orbiting a star that is a rare solar twin — a star that is almost identical to the Sun in all respects.

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. But, up to now, very few planets have been found inside star clusters. This is particularly odd as it is known that 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.

Anna Brucalassi (Max Planck Institute for Extraterrestrial Physics, Garching, Germany), lead author of the new study, and her team wanted to find out more. “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.”

The team used the HARPS planet-finding instrument on ESO's 3.6-metre 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 Messier 67 over a period of six years to look for the tiny telltale motions of the stars towards and away from Earth that reveal the presence of orbiting planets.

Video: HARPS Find Planet Orbiting Solar Twin in Star Cluster Messier 67

This cluster lies about 2500 light-years away in the constellation of 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 hence 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,” adds Luca Pasquini (ESO, Garching, Germany), 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.”


European Southern Observatory
Max Planck Institut for Extraterrestrial Physics
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