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.
For previous Monthly News items, click here.
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