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RSAA News of the Month: January 2005

Where have all the planets gone?
RSAA student shows that Extrasolar Planets are much rarer in Globular Clusters

 

A large-scale search for giant planets in the globular cluster 47 Tucanae has recently been completed at Mount Stromlo, and has shown that they are much less frequent in such regions of the Universe. ANU PhD student David Weldrake observed 22,000 sun-like stars in detail inside the giant stellar cluster, searching for a tell-tale brightness signature of an orbiting planet.

At least seven planets were expected but none were found. This is the main result of David’s PhD thesis, due for submission on the 21st January. It places a strong constraint on the giant planet frequency in the outer parts of 47 Tucanae.

The project involved using the ANU 40-inch telescope fitted with the Wide Field Imager at Siding Spring Observatory in July and August 2002. By measuring how the brightness of the 22,000 stars changed over a month, David hoped to see a periodic two hour dimming of some of the stars as any orbiting planets periodically pass in front.

This ‘transit’ method has been used to find giant planets in the Sun’s neighbourhood and is the subject of many ongoing searches for giant planets orbiting close to their parent stars. This method is most sensitive to the so-called ‘Hot Jupiters’ with orbital periods of only a few days, clearly unlike anything in our Solar System.

Left: The globular cluster 47 Tucanae, as seen with the 40-inch telescope at Siding Spring Observatory.
By repeatedly measuring the brightness of 22,000 sun-like stars in this cluster, the project hoped to detect seven planets.
Picture by D.Weldrake

Right: How the brightness of a star changes as a planet passes in front.
It produces a temporary drop in brightness which lasts for a couple of hours and can be readily seen with small telescopes.
Charbonneau et al 2000

Theoretical work predicts that planetary systems are effectively destroyed in the cores of the dense globular star clusters. However, in the uncrowded outer fringes of the cluster, where this search was most sensitive, planets are mostly unaffected by stellar encounters, increasing the prospects of a detection.

By finding no planets, even in this region, the conclusion is that the lack of heavy elements in 47 Tuc’s ancient stars, rather than crowding, is responsible. It seems that the planets never existed in the first place.

This result is the first time that this dependence of heavy element abundance on planet formation has been seen outside the local Solar Neighbourhood. It also suggests that planet formation was much rarer in the earlier Universe, with planets being a relatively recent phenomenon.

Despite finding no planets, David did detect 100 variable stars, with 69 of them being new discoveries. Comprising many eclipsing binaries and pulsating stars, they allowed the distance of the cluster to be found as well as providing new results and objects for other researchers to study. One star seems to be caught in the act of throwing off it’s outer atmosphere, forming a so-called ‘planetary nebula’, and another displays currently unexplained behaviour.

Examples of eclipsing binary star systems discovered in the project, systems with two stars orbiting around each-other.
These plots show how the brightness varies with time for these systems, with their identification number marked and orbital period given in days.
These systems have allowed the distance to 47 Tuc to be found and present evidence that the component stars are interacting with each-other,
important for theoretical studies into how binary stars evolve with time.

David plans to continue his work on Extrasolar Planets as a postdoctoral researcher, studying how the different conditions in the birth-places of stars affects how many planets are formed there. This shall be done by looking for transiting giant planets in the massive cluster Omega Centauri, as well as in the nearby galactic disk, with results due next year.

The research was carried out entirely at Mount Stromlo, with the principal supervisors of the project being Prof Penny Sackett and Prof Ken Freeman. Dr Terry Bridges of Queen’s University in Ontario, Canada also served as project advisor. Initial results were presented at the Astronomical Society of Australia (ASA) meeting in Brisbane in July 2004. The final analysis will appear in the February 2005 edition of the Astrophysical Journal.

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