47 Tucanae

Copyright : NASA/HST

In 2000, the Hubble Space Telescope (HST) made the first systematic attempt at the detection of exoplanets in the globular cluster 47 Tucanae. This cluster, at a distance of 12 000 light-years (4 kiloparsecs), had an advantage : gathering a large number of stars in the telescope field.

The aim of this program was to detect transits of massive exoplanets orbiting close to their host stars. To achieve this project, the HST observed 35 000 stars over the course of 8 days.

With a number of planets similar to that of the solar neighbourhood, about 20 exoplanets should have been detected. But none had been observed.

There are several explanations for this : the Tucanae cluster was formed 12 billion years ago. It does not contain gas anymore, and stars do not form anymore. The stars of this cluster are all evolved giants, which is the final stage in the life of these stars. On the other hand, the solar neighbourhood is made of young stars and gas enriched with "metals" (the atoms that make up solid matter), owing to the succession of several star generations.

• The stars of the Tucanae cluster are metal-deficient. The lack of solid matter prevented the formation of planets.
• The stars of the cluster are very close together : the circumstellar disks were maybe nonexistent or not massive enough to form planets.
• The massive planets close to their host stars, observed in the proximity of the Sun, could possibly be a transitory phenomenon : maybe some planets were present in the Tucanae cluster, but disappeared, swallowed by the star or burned by the UV emission of nearby stars.
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