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An unexpected excess of hot Jupiters within a star cluster

An international team of scientists, which includes a CNRS scientist at the Paris Observatory, has uncovered a surprising fact: the stellar cluster Messier 67, has apparently an unexpectedly high number of “hot Jupiter” type planets. This excess of “hot Jupiters” could be a consequence of the high stellar density of the cluster. This result was published in the June 17th 2016 issue of the journal Astronomy and Astrophysics.

The open cluster Messier 67 is at a distance of roughly 2 500 light years from the Earth, in the Crab constellation and contains on the order of 500 stars. These stars are particularly intersting, since they are as old as, and have the same chemical composition as the Sun.

Furthermore, Messier 67 is a dense environment within which have evolved stars and their planetary systems.

Using various instruments, and notably the HARPS spectrograph at ESO’s La Silla observatory in Chile as well as the SOPHIE instrument attached to the 1,93 m telescope at the Haute-Provence observatory, and international team of astronomers has been accumulating for many years very accurate data on a sample of 88 stars in Messier 67. The presence of a neighboring massive object can be inferred from an analysis of the radial velocities of the stars.

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This artist’s impression shows a hot Jupiter planet orbiting close to one of the stars in the rich old star cluster Messier 67, in the constellation of Cancer (The Crab)

Thanks to this method, two exoplanets were discovered in 2016 in the open cluster Messier 67, adding to the three others known since 2014. These Jupiter class bodies, with masses between 0,4 and twice that of our giant planet.

In their work, the scientists imagine that many other planets orbit the stars in the Messier 67 cluster, which they consider moreover as a laboratory for the analysis of the properties of exoplanets, and to test planetary formation theories.

 A surprising statistic

In the paper published in June 17th 2016 in the journal Astronomy and Astrophysics, the scientists searched particularly for the signaturres of giant planets with short orbital periods, bodies usually referred to as « hot Jupiters », since they orbit close to their host stars.

In three of the stars in the cluster, they managed to detect the signal of a hot Jupiter, with an orbital period of less than seven days. In this sense, these bodies a significantly different from “our” Jupiter, whose orbital period around the Sun is close to 12 terrestrial years and whose surface temperature is less than that of the Earth.

This work shows that stars harboring hot Jupiters are less frequent within the cluster Messier 67 than outside. This is quite surprising: 5% of the stars in Messier 67 are accompanied by hot Jupiters, and only 1% in the case of stars beyond the cluster.

 The hypothesis of a migration

According to astronomers, it is extremely unlikely that these exotic giant stars were formed in the exact location where they are observed today, since the conditions which are in the close vicinity of the host star are not favorable for the formation of Jupiter type planets. It is more likely that they were formed much farther out, as was probably the case for Jupiter, and subsequently migrated towards their host star.

Originally cold and far out, these giant planets are now much warmer. Why they migrated towards the center of their systems is a fundamental question.

There are many scenarios. Nevertheless, the authors are in favor of a gravitational interaction with neighboring stars, or with the planets of neighboring solar systems.

In effect, the close environment can have a significant impact on the evolution of a planetary system. In a cluster such as Messier 67, more compact and whose stars are closer to each other than in the case of stars which are not in a cluster, such encounters are likely to be more frequent, which could explain the particularly high number of hot Jupiters. Source : Paris Observatory