General Meeting Report
13/2/2002
EXTRA SOLAR PLANETS
by Dr. Rosemary Mardling, Monash University
Centre for Stellar and Planetary Astrophysics

If, as they tell us, the Universe is 99 percent empty, how come it is so often impossible to find a parking spot at the National Herbarium on General Meeting Night? Is that what astronomers mean when they refer to mysterious dark matter, which could be the key to explain the missing mass problem? While this simile may not revolutionise cosmologist=s thinking, planets, extra solar planets in particular and dark companions of other solar systems, certainly are still an unknown factor in the make-up of galactic systems in our Universe.
Dr Rosemary Mardling specialises in this field and, looking at her recent activities, may very well have gravitated towards this speciality along similar lines of thought. Papers on "The Structure and Dynamical Evolution of Globular Clusters" and AChaotic Tides in Binary Stars and the Three-Body Problem" inevitably led to AA Parametric Study of the Stability of Extrasolar Planets@.
The study of extra-solar planets is a very new science. New in the sense that the technology used to detect their presence was just not available, even 5 years ago. It takes very sophisticated (read that as expensive) equipment to capture and measure the tiny variations in brightness or spectrum of the distant stars caused by the presence of a planet. This information is hidden in the radiation/light we receive from such a star, detected these days mainly with charge-coupled devices (CCD), analysed by astrometry (wobble), photometry (photon count) and spectroscopy (analysis of the spectrum).
The technical requirements for detecting even a Jupiter-sized planet light-years away are mind-boggling. If you have seen pictures of the transit of Venus or Mercury across the disc of the sun, just a black dot against the face of the sun (no bigger than a fly-spec), you can visualise the problem of measuring the resultant dip in the Sun=s overall brightness, or detecting a wobble in the Sun=s gravitational response to the orbiting planet. Such a wobble can be detected by measuring the red-shift (or blue-shift) of known absorption lines in the spectrum as the source of light moves away or towards us respectively. Dr. Mardling showed graphs of the figures we are talking about here. For a Jupiter-sized planet the wobble would amount to a linear displacement of the star of no more than 3 metres a second, for the Earth only 3cm/sec. The first item is now technically just feasible, the second is definitely not.
So all of the 80 positive findings so far are of Jupiter size (or bigger) planets/companions to stars within a 100 light-years from us. The chart shows the respective mass distribution. A curious anomaly of these bodies seems to be their short orbital periods, some as short as 3.5 days. While one logical explanation for this prevalence might be that shorter periods are more easily detected than orbital periods of years, nevertheless this fact has raised some heated controversy with suggestions that thermal instability in stars may also be the cause of such fluctuations.