A bevy of unexpected gas giants hide in a nearby star cluster

The denser environment in a cluster will cause more frequent interactions between planets and nearby stars, which may explain the excess of hot Jupiters.
By and | Published: June 17, 2016 | Last updated on May 18, 2023
Cluster M67
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 in all respects.
ESO/L. Calçada
An international team of astronomers has found that there are far more “hot Jupiters” than expected in a cluster of stars called Messier 67 (M67). This surprising result was obtained using a number of telescopes and instruments, including the HARPS spectrograph at the European Southern Observatory’s (ESO) La Silla Observatory in Chile. The denser environment in a cluster causes more frequent interactions between planets and nearby stars, which may explain the excess of hot Jupiters.

A Chilean, Brazilian, and European team led by Roberto Saglia at the Max-Planck-Institut für Extraterrestrische Physik in Garching, Germany, and Luca Pasquini from ESO has spent several years collecting high-precision measurements of 88 stars in M67. This open star cluster is about the same age as the Sun, and it is thought that the solar system arose in a similarly dense environment.

The team used HARPS, along with other instruments, to look for the signatures of giant planets on short-period orbits, hoping to see the telltale “wobble” of a star caused by the presence of a massive object in a close orbit, a kind of planet known as a hot Jupiter. This hot Jupiter signature has now been found for a total of three stars in the cluster alongside earlier evidence for several other planets.

A hot Jupiter is a giant exoplanet orbiting close to their parent stars, with an orbital period less than 10 days in duration. That is different from our Jupiter, which has a year lasting around 12 Earth-years and is much colder than Earth.

“We want to use an open star cluster as laboratory to explore the properties of exoplanets and theories of planet formation,” said Saglia. “Here we have not only many stars possibly hosting planets, but also a dense environment in which they must have formed.”

The study found that hot Jupiters are more common around stars in M67 than is the case for stars outside of clusters. “This is really a striking result,” said Anna Brucalassi, who carried out the analysis. “The new results mean that there are hot Jupiters around some 5 percent of the M67 stars studied — far more than in comparable studies of stars not in clusters, where the rate is more like 1 percent.”

Astronomers think it highly unlikely that these exotic giants actually formed where we now find them, as conditions so close to the parent 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 parent star. What were once distant, cold, giant planets are now a good deal hotter. The question then is: What caused them to migrate inwards towards the star?

There are a number of possible answers to that question, but the authors conclude that this is most likely the result of close encounters with neighboring stars, or even with the planets in neighboring solar systems, and that the immediate environment around a solar system can have a significant impact on how it evolves.

In a cluster like M67, where stars are much closer together than the average, such encounters would be much more common, which would explain the larger numbers of hot Jupiters found there.

Pasquini from ESO looked back on the remarkable recent history of studying planets in clusters: “No hot Jupiters at all had been detected in open clusters until a few years ago. In three years, the paradigm has shifted from a total absence of such planets to an excess!”