Researchers believe hot Jupiters form farther away and later migrate closer to their host stars. Photo by ESO/L. Calçada

GARCHING, Germany, June 17 (UPI) -- A team of astronomers have discovered a surprisingly large concentration of hot Jupiters inside the star cluster Messier 67. Hot Jupiters are exoplanets roughly the size and mass of Jupiter but which orbit their host stars at closer distances.

Messier 67 hosts 88 stars and is roughly the same age as our sun. Scientists believe the solar system was born of conditions similar to those found in Messier 67, which is why astronomers have been studying the cluster for several years.


"We want to use an open star cluster as laboratory to explore the properties of exoplanets and theories of planet formation," lead researcher Roberto Saglia, an astronomer at the Max Planck Institute for Extraterrestrial Physics, explained in a news release. "Here we have not only many stars possibly hosting planets, but also a dense environment, in which they must have formed."

Researchers have so far found evidence of hot Jupiters around three of the 88 stars in Messier 67 -- a rate of 5 percent. The rate of hot Jupiters among stars outside of clusters is less than 1 percent.

"This is really a striking result," said Anna Brucalassi, who led the analysis of Messier 67 data collected by HARPS, a planet-searching instrument installed on a telescope at the La Silla Observatory in Chile.

The findings of Brucalassi and her colleagues are detailed in a new paper, published this week in the journal Astronomy & Astrophysics.

Astronomers believe hot Jupiters likely formed elsewhere and migrated closer to their host stars at some point during their evolution. But researchers aren't sure what causes their migration.

Some astronomers hypothesize stellar flybys likely throw hot Jupiters off orbit and push them closer to their host stars. Messier 67 features a high density of stars, which means stars interact with each other rather frequently.

Researchers say further exploration of star clusters offers astronomers the best chance for explaining the evolution of hot Jupiters.

"No hot Jupiters at all had been detected in open clusters until a few years ago," said study co-author Luca Pasquini, an astronomer at the European Southern Observatory. "In three years the paradigm has shifted from a total absence of such planets -- to an excess!"