An international team of astronomers has discovered three new extrasolar planets in an open star cluster known as Messier 67: two planets circling dwarf stars YBP1194 and YBP1514, and one orbiting the K3 star S364. One of these exoplanets, designated YBP1194b, is the first planet found around a Sun-like star that belongs to a stellar cluster.
This is an artist’s impression of the newly discovered exoplanet YBP1194b. Image credit: ESO / L. Calçada.
Exoplanets orbiting stars outside our Solar System are now known to be very common. These extrasolar planets 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.
Discovered in 1780, the star cluster Messier 67 (M67), also known as NGC 2682, is located in the constellation of Cancer, around 2,500 light-years from Earth.
It contains about 500 stars and is among the oldest open clusters. Estimations of its age vary between 3.2 and 5 billion years with recent valuations indicating it to be nearer to 4.0 billion years.
“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,” said Dr Anna Brucalassi of the Max Planck Institute for Extraterrestrial Physics in Germany, who is the lead author of the paper published in the journal Astronomy & Astrophysics (full paper in .pdf).
By using the HARPS instrument on ESO’s 3.6-m telescope at the La Silla Observatory, Chile, along with other telescopes, Dr Brucalassi with colleagues monitored 88 stars in Messier 67 over a period of 6 years to look for the tiny telltale motions of the stars towards and away from Earth that reveal the presence of orbiting planets.
Two of the newly discovered exoplanets, YBP1194b and YBP1514b, are orbiting Sun-like stars. The third planet, S364b, is circling a more massive red giant star, S364.
YBP1194b has a period of 6.9 days and a minimum mass of 0.34 Jupiter masses. Its parent star, YBP1194, is one of the most similar solar twins identified so far and is almost identical to the Sun.
YBP1514b orbits its star in 5.1 days and is 0.40 times the mass of Jupiter.
S364b takes 121.7 days to orbit its host and has a minimum mass of 1.54 Jupiter masses.
All three exoplanets are closer to their stars than the habitable zone where liquid water could exist.
Study co-author Dr Luca Pasquini of ESO, Germany, said: “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.”
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Brucalassi A et al. 2014. Three planetary companions around M 67 stars. A&A 561, L9; doi: 10.1051/0004-6361/201322584
