Rare Exoplanet Found in Cluster, Orbits Sun's 'Twin'

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Three new exoplanets have been discovered inside a star cluster, which is a rare find as only a handful of such exoplanets are known to exist. However, one of the three new finds is even more remarkable — it orbits a star that appears to be “an almost perfect solar twin.”

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The discovery was made by astronomers using the European Southern Observatory’s HARPS exoplanet-hunting instrument attached to the 3.6-meter telescope at the La Silla Observatory in Chile and was confirmed by other collaborating observatories. The astronomers’ attention was focused on the Messier 67 open star cluster, which is located approximately 2,600 light-years away in the constellation Cancer.

It is believed that all stars originated from within some kind of stellar cluster, including our sun. Clusters of stars are a consequence of a brood of stars emerging from a stellar nursery and, throughout their stellar evolution, remain gravitationally bound.

However, there is a mysterious lack of exoplanetary discoveries inside star clusters, leading astrophysicists to hypothesize that perhaps the planet-forming rules inside clusters are somehow different from stars that have gone on to disassociate themselves from their clusters. This is what inspired the focus on this particular star cluster.

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“In the Messier 67 star cluster the stars are all about the same age and composition as the sun,” said Anna Brucalassi of the Max Planck Institute for Extraterrestrial Physics, Garching, Germany, in an ESO press release. “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 cluster is composed of around 500 stars, of which HARPS monitored 88 for slight “wobbles” over six years. These wobbles betray the gravitational presence of orbiting exoplanets — as the alien worlds swing around their host stars, they are massive enough to exert a gravitational tug, shifting the star slightly off-center, allowing HARPS to detect a slight Doppler shifting of starlight received from that star. This exoplanet-hunting technique is known as the radial velocity method.

But at distance of over 2,500 light-years, the challenge to detect the slight wobble in the faint starlight was formidable.

Two of the exoplanets are approximately one-third the mass of Jupiter and orbit their sun-like stars in five and seven days. These compact orbits ensure that the worlds aren’t remotely “Earth-like”; they are “hot-Jupiters”, hellish worlds that are baked by their host stars. The third world to be discovered is more massive than Jupiter and orbits a red giant star, taking 122 days to complete one orbit.

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It is one of the two hot-Jupiters that orbits a star that appears to be the same size, age and composition as our sun.

“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 of the ESO, Garching, Germany and co-author of the research. “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.”

This new discovery will add to our understanding about how and where planets form and clusters like Messier 67 could be the planetary “Petri dish” to provide us with some answers as to how planetary systems like our solar system form.