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Planets Pile Up in Star Clusters: One Orbits a ‘Solar Twin’

Image: Star and planet

This artist's impression shows one of the three newly discovered planets in the star cluster Messier 67. In this 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. Very few planets in clusters are known, and this one has the additional distinction of orbiting a solar twin — a star that is almost identical to the sun. L. Calcada / ESO

There goes another old astronomers' tale about where planets can't form.

Scientists used to have a hard time detecting alien worlds in open star clusters, leading some to wonder whether a mysterious factor was holding back planet formation in a crowd. Just in the past couple of years, readings from NASA's planet-hunting Kepler probe have turned up evidence for several cluster planets. Now a study published in the journal Astronomy & Astrophysics suggests that clusters don't suffer any handicap in the planet-making department.

"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," the European Southern Observatory's Luca Pasquini, one of the study's co-authors, said Wednesday in a news release.

The study is based on six years' worth of data from the HARPS spectrograph at ESO's La Silla facility in Chile, with an assist from other telescopes around the world. Readings were collected for 88 selected stars in the open star cluster Messier 67, which lies about 2,500 light-years away in the constellation Cancer. Astronomers watched for wobbles in the stars' back-and-forth movement, a telltale sign pointing to the gravitational influence of orbiting planets.

Three planets were detected, which is about what would be expected for run-of-the-mill stars. Two of the planets are "hot Jupiters," so close to their parent stars that they made full orbits in five to seven Earth days. The world with a seven-day year goes around a star that's so similar to our own sun that it's called a solar twin. The third giant planet has a 121-day orbit.

All three planets are too hot for liquid water to exist on the surface, which means they're outside their stars' "habitable zone." But who knows? Maybe there are as-yet-undetected planets that could harbor life as we know it. In any case, the claim that planets can't form in open star clusters now sounds as hollow as the claim that planets can't form in double-star systems. And when you consider that most stars are born in open clusters, that suggests that planets could be even more numerous in our galaxy than previously thought.

Planets found in star cluster 3:00

Here's more about the find from Pasquini, who responded to questions via email:

Q: What are the leading points of this study?

A: "Until last year, most evidence pointed to a lower rate of planet formation in clusters. Actually, this was a bit at odds with the common concept that most stars now in the field were born in clusters. This work confirms that, to the contrary, the fraction of stars hosting planets in open clusters is compatible with the field stars (a paper by Meibom et al. in 2013 found the same). Possibly, the old studies did not have good enough statistics to address this issue properly, or they underestimated the measurement errors.

"The fact that the star Y1194 is an almost perfect solar twin is really cool. Solar-type stars that host planets have been found in another cluster. The concept of solar-type stars is quite broad, however, and it covers all those stars that are cool enough to have a sub-photospheric convective zone (sorry for the technicality).

"Star Y1194 is a real solar twin, so it has all the parameters, including the chemical abundances of many species, that make it indistinguishable from the sun. Interesting enough, while our study was in press, an independent group confirmed this astonishing similarity. They did not know we just found out that this star hosts a giant exoplanet."

Q: Why has it been so difficult to detect planets in open star clusters?

A: "The main difficulty is that there are very few nearby open clusters, and none of them is old. M67 is far away, so the stars are very faint for precise radial velocity measurements. One single measurement takes a few minutes for a nearby field star, but almost one hour for the solar twin star in M67. When coupling this with the need to have a large sample, a lot of telescope time on large telescopes is required. The process also takes a long time. Stars must first be selected as belonging to the cluster, then the binaries are eliminated, then the monitoring of the remaining stars continues."

Q: What are the implications for the future of the search for exoplanets?

A: "Extrasolar planet science is a teenager, only 18 years old. I do expect that the search for planets in different environments will become more and more important, in open clusters, in star-forming regions and even in other galaxies. Open clusters have the unique advantages with respect to field stars in that we observe a 'pure' sequence of masses, since all the other parameters of the stars (such as age and metallicity) are the same.

"The second big advantage is that because the chemical composition of the stars is the same, once we have a census of stars with planets and without planets in a cluster, we can perform a direct comparison of their chemical composition and see whether the presence of the planetary system affects the stellar chemical composition. ...

"For the future: ESO is building ESPRESSO (in collaboration with a consortium led by the Geneva Observatory). ... With ESPRESSO, it will be possible to find low-mass planets around the M67 solar stars and to extend the search of giant planets to lower-mass stars. ESPRESSO will be ready in three years. But with regard to the search for extrasolar 'Earths' in habitable zones, we will be limited to the nearby field stars, at least with the radial velocity technique."