April General Meeting Report
The Search for Extra-Solar Planets
A talk prepared by Dr Sarah Maddison and presented by her college Dr James Murray
Centre for Astrophysics & Supercomputing Swinburne University
Life is a paradox. When you want to study human consciousness you don’t test humans, you experiment with baboons, with rats and with fruit-flies. When you want a better understanding of the greenhouse effect and the resultant threat to our weather here on Earth, you go to Mars, to Venus or even Titan to search for answers. To learn how our Galaxy was formed you look for clues in quasars and black holes billions of light years away. And what you do when you want to learn all about our Sun’s planetary system? You guessed it, you look for extra-solar planets. It appears our home-grown theories of how the solar system formed, and our knowledge how the local planets came to be where they are, no longer measure up to the advances in science. As James Murray pointed out, we do not have a definitive model of star or planet formation to fit all the given parameters any more, it needs a bigger picture to put things in perspective and we must look to extra solar planets to fill the gaps.
This new-found uncertainty, combined with the natural curiosity we humans are renowned for, probably is what drives the current resurgent interest in planetary evolution. After millennia of musing and half a century of false claims, it’s almost embarrassing that we still don’t know. This, together with the apparent ubiquity of planetary systems out there, pushes the need to explain their origin to the forefront. There are now over 50 ongoing programs worldwide dedicated to the research into extra-solar planets and planet formation, and another 20 in the planning stage. Dr Sarah Maddison (who is also the Swinburne Astronomy Online Coordinator) has in the last few years published several theses on the subject (some of them together with Dr James Murray) ranging from “Building Planets with Dusty Gas”; “Galactic Cannibalism”; “Are High-Velocity Clouds the Building Blocks” and “SPH Simulations of Accretion Disks”. By programming the supercomputer at Swinburne’s Centre of Astrophysics they can readily simulate models of dust clouds condensing into small particles, but on an astronomical scale the time involved for the coalition of these particles into large rocks and planets may exceed the time available for them to do so. Unless the dust can form into metre sized boulders before the central cloud-mass becomes a star, chances are the solar wind of the newly born star will blow it all away. The people at Swinburne University Centre for Astrophysics make use of information from telescope facilities around the globe. Even the Old Melbourne Telescope at Mt Stromlo Observatory (before the bushfires) was put to use in the search for gravitational Microlensing
After an initial scratchy start 10 years ago, the list of verified extra solar planets is now growing rapidly. At the last count it was 107 planets in 93 separate planetary systems. The definition of “Planet” in this list is any companion to a star, with less than 13 Jupiter masses. There is of course a natural tendency towards the detection of large, rapid orbit objects; they are easier and quicker to find. So the list has a predominance of large Jupiter-sized objects with orbit times from as short as a few days to a maximum of 5 years. As the time frame for this research becomes longer so will no doubt do the confirmed orbit times of extra-solar planetary objects. To separate out multiple planet systems (12 candidates are already on the list) requires at least one complete revolution of the longest period orbit. Our Jupiter orbit is about twelve Earth years.
The methods of detecting extra-solar planets are varied, and, as the objects themselves are too faint to be seen directly with even the most powerful telescopes, always depend on secondary effects. Of these the main one’s are: a gravitational wobble of the parent star resulting in position- or Doppler shifts, eclipses or transits, and microlensing. But the list is growing (See attached chart). Dr Murray in his impressive PowerPoint Presentation showed some simulated sequences of such detections and their details: Doppler shift measurement on light received from objects moving towards or away from us (orbits seen edge-on) is so sensitive that such motions of the parent star in response to the planet’s orbit can be detected at speeds as low as 1metre per second. For face-on orbits Interferometry can these days resolve displacements down to micro arc-seconds. As a comparison astrometric displacement of the Sun due to Jupiter’s mass as seen from 30 light-years away would amount to about 0.001 arc seconds, well within the limits of modern equipment. Eclipses or transit detection also require a sensitivity to changes in light below one part per million. To again put this in perspective, look at a street lamp 1 km away and then move your eye 0.5 mm closer to it. The street lamp is now about 1 ppm brighter to your eye.
How will all this new awareness help us to better understand our own solar system? Does it explain for instance the Bode-Titius planetary distance law? Or perhaps the relationship between the five regular solids that fit concentric into a sphere, and the six known planets at Kepler’s time? Is there any significance in the fact that the solar planet that failed to form in the asteroid belt happens to be at the border between the solid and the gaseous planets? What about Dr Andrew Prentice’s planetary theory of Supersonic Turbulence, did that not help to predict the four additional moons of Neptune? James Murray smiled and, carefully worded, explained that Prentice had been one of his lecturers at Monash University, and that the enthusiasm that Andrew brought into astrophysics motivated many of his fellow students to consider the subject seriously, as a profession.
While we may not yet be able to make sense of the vast amount of extra-solar planetary data collected so far, there is little doubt in the astronomical community that this will drastically improve within the next decade, and with it our appreciation of Kepler’s music of the spheres:
“The heavenly motions are nothing but a continuous song for many voices, perceived not by the ear but by the intellect, a figured music which sets landmarks in the immeasurable flow of time.”