Did Our Solar System Originate in a Distant Star Cluster?

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By Yidir K. Published on September 30, 2014 15:42
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Is it possible that our original home exists as a great collection of stars, a star cluster known as Messier 67 (shown above), a gathering of suns and stellar remnants some 2,700 light-years distant that contains more than a hundred stars that bear a striking resemblance to the Sun. Astronomers have searched for star clusters in our galaxy whose members come close to matching the Sun’s elemental composition and age. This past January, astronomers using ESO's HARPS planet hunter in Chile, along with other telescopes around the world, discovered three planets orbiting stars in the cluster Messier 67 shown above. 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.
Star clusters come in two main types. Open clusters are groups of stars that have formed together from a single cloud of gas and dust in the recent past. They are mostly found in the spiral arms of a galaxy like the Milky Way. On the other hand globular clusters are much bigger spherical collections of much older stars that orbit around the centre of a galaxy. Despite careful searches, no planets have been found in a globular cluster and less than six in open clusters. Exoplanets have also been found in the past two years in the clusters NGC 6811 and Messier 44, and even more recently one has also been detected in the bright and nearby Hyades cluster.

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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. 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.

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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. Solar twins, solar analogues and solar-type stars are categories of stars according to their similarity to the Sun. Solar twins are the most similar to the Sun, as they have very similar masses, temperatures, and chemical abundances. Solar twins are very rare, but the other classes, where the similarity is less precise, are much more common.

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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."

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To get back to our opening question of which stars could have shared our original region of the galaxy is limited by our precision in measuring such enormous distances and the specific motions of stars, as well as by the sheer number of objects to sift through. Recent computer simulations of the motions of stars in Messier 67 have examined the projected path that our solar system would have had to take if it were ejected from the star cluster and it appears that it would require a very rare alignment of no less than two or three massive stars in Messier 67 to provide the gravitational ejector seat to place our Solar System to its present location. Additionally, in the process of this ejection, gravitational tides would likely have ripped our nascent planetary system apart.

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However, Caleb Scharf, Caleb points out in his new book The Copernicus Complex, "this conclusion itself rests on assumptions about the configuration of the Milky Way’s great spiral arms of stellar objects. If these change more than we thought over billions of years, it’s possible that Messier 67 could have let the Sun go in a less dramatic, and more plausible, fashion. So the jury is still out on where our solar system originated, but the radioisotope clues and the events unfolding in other nebulae leave us in little doubt that, one way or the other, we have been orphaned."

The Daily Galaxy via ESO

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