Astronomers have found that there are far more so-called hot-Jupiter planets – gas giants that orbit very close to their parent stars – than expected in the dense open star cluster Messier 67.

Messier 67, also known as NGC 2682 and M67, is located in the constellation of Cancer, approximately 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 billion years.

An international team of astronomers, led by Dr. Roberto Saglia from the Max-Planck-Institut für extraterrestrische Physik in Germany and Dr. Luca Pasquini from ESO, has spent more than seven years collecting high-precision measurements of stars in Messier 67.

The scientists used the HARPS spectrograph at ESO’s La Silla Observatory in Chile, along with other instruments, to look for the signatures of giant planets on short-period orbits, hoping to see the tell-tale ‘wobble’ of a star caused by the presence of a hot-Jupiter exoplanet.

This hot-Jupiter signature has now been found for the main-sequence star YBP401 and two other stars in the cluster alongside earlier evidence for several other planets.

“We want to use an open star cluster as laboratory to explore the properties of exoplanets and theories of planet formation,” Dr. Saglia said.

“Here we have not only many stars possibly hosting planets, but also a dense environment, in which they must have formed.”

The team found that hot-Jupiters are more common around stars in Messier 67 than is the case for stars outside of clusters.

“Our sample of 66 main-sequence and turnoff stars includes 3 hot-Jupiters, which indicates a high rate of hot-Jupiters in this cluster (5.6% for single stars and 4.5% for the full sample) – far more than in comparable studies of stars not in clusters, where the rate is more like 1%,” the astronomers said.

They think it highly unlikely that these exotic giants actually formed where we now find them, as conditions so close to the host star would not initially have been suitable for the formation of Jupiter-like planets.

Rather, it is thought that they formed further out, as Jupiter probably did, and then moved closer to the star.

“No hot Jupiters at all had been detected in open clusters until a few years ago,” Dr. Pasquini said.

“In three years the paradigm has shifted from a total absence of such planets – to an excess.”

This research was reported in an advance online article in the journal Astronomy & Astrophysics (arXiv.org preprint).

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A. Brucalassi et al. Search for giant planets in M67. III. Excess of hot Jupiters in dense open clusters. A&A, published online June 17, 2016; doi: 10.1051/0004-6361/201527561