Why Does This Star Cluster Have So Many Hot Jupiters?

Artist's impression of a hot Jupiter circling a Sun-like star in Merssier 67. ESO/L. Calçada

A study of the open cluster Messier 67 has found an excess of the planets known as “hot Jupiters”, gas giants that orbit very close to their parent stars. The discovery suggests this category of planets can be produced by interactions between nearby stars, possibly resolving a major debate in planetary studies.

Back when the only system of planets we knew was our own, it all seemed so simple. Rocky planets like Earth and Venus were close to the Sun, farther out lay the gas giants like Jupiter and Saturn. Big planets were dependent on the accumulation of ice grains beyond the “frost line”.

However, once our telescopes became sensitive enough to detect the influence of planets around other stars, we got a surprise. Explorations repeatedly turned up giant planets, too massive to be solid, orbiting close in. Although hot Jupiters are easier to detect than smaller or more distant worlds, their abundance remains a puzzle.

In Astronomy and Astrophysics (preprint on ArXiv) a team led by Dr Anna Brucalassi of the Max Planck Institute for Extraterrestrial Physics has revealed that hot Jupiters are even more common in the star cluster known as Messier 67.


M67 contains more than 500 stars. A study of 66 of these found three hot Jupiters. Two Micron All Sky Survey

Containing more than 500 stars and located between 2,500 and 3,000 light-years away in the constellation of Cancer, M67 is one of the oldest known clusters within the Milky Way, although still far younger than the globular clusters that orbit the galaxy. Being the closest example of an old cluster, it has been intensively studied for hints of stellar evolution. The discovery of planets within it, or indeed other open clusters, is far more recent.

"We want to use an open star cluster as a laboratory to explore the properties of exoplanets and theories of planet formation," said co-author Dr Roberto Saglia in a statement. The team examined 66 stars within the cluster, finding three hot Jupiters, all orbiting stars with masses similar to the Sun.

“There are hot Jupiters around some 5 percent of the Messier 67 stars studied – far more than in comparable studies of stars not in clusters, where the rate is more like 1 percent," said Brucalassi.

Astronomical consensus remains that hot Jupiters form beyond the frost line, but something causes them to migrate inwards, probably causing chaos among planets that started off on their inside track.

Plenty of theories have been thrown around as to what might trigger such an odyssey. Considering M67's distinctive feature is how closely the stars within it are packed, close encounters between different stars are likely to disrupt the orbit of outer planets, sending them spiraling towards their star. Alternatively, when stars get too close their planets may affect each other.

The paper notes that alternative theories on hot Jupiter formation, such as high metal content in the star-forming disk, are not applicable in this case.


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