February General Meeting Report
Guest Speaker, Dr Sarah Maddison, Centre for Astrophysics and Supercomputing, Swinburne University, with: “ Understanding Planetary Formation”.
It wasn’t so much “Planetary Formation” we had on our mind going to the February Meeting as was our planet Earth’s current calamitous behaviour. The Sun hung like a red ball in the sky in the haze from the tragic firestorm of Victoria’s worst-ever natural disaster. Living not far from the Bunyip State Park East of Melbourne where the air had an acrid smell and was filled with flying black ash, we kept our fingers crossed all the while attending Sarah Maddison’s talk,
Dr Sarah Maddison has been studying Planetary Formation and the evolution of Solar Systems for many years. A quick Google on her activities shows that she has written (or been associated in the publication of) some 35 Papers on this or related subjects since 1996. And she is no stranger to the members of the ASV. Her last appearance (albeit by proxy in the form of college Dr James Murray) was in April 2003 with “the Search for Extra Solar Planets” (Crux Vol 21 No 3). The first extra solar planet, 51 Peg b, was found by a Swiss team way back in 1995. While the number of confirmed discoveries has since risen to 340, their formation within protoplanetary disks around new stars (proplyds) is still a hotly debated subject. We know proplyds are prolific; the Hubble Space Telescope has already imaged 100s of these in the Orion Nebula. Particle size is the stumbling block. The initial reservoir of solid material for planet formation is made up of micron-sized particles of rocky or icy dust, which makes up about 1% of the mass of a typical proplyd. Some dust grains travel with the gas when a portion of a molecular cloud collapses on the way to form a star, while more dust condenses from the gas phase within the disk. The dynamics of dust within a disk is dominated by gravity from the forming star and aerodynamic forces from the gas. In contrast, gravitational interactions between small bodies are very weak (the escape velocity from a 1 metre diameter rock is less than 0.1 cm/s). Aerodynamic forces remain dominant until bodies grow to over one km in size. Such bodies, referred to as planetesimals, are massive enough that their gravitational interactions are significant, while their small surface area to volume ratio means they are only weakly affected by aerodynamic forces. Now, how do you get from micron size dust particles to boulder sized rocks in the relatively short time before radiation pressure and solar wind from the newly formed star blows the dust away. It is estimated that the time for this cannot exceed 106 years. “What about Dr Prentice’s theory of Supersonic Turbulence?” a voice booms from the audience. “Ahem, yes, hum, ho, you know...” Sarah launches into an apodictic, bona fide imitation of the normally inimitable mannerism of Andrew Prentice; but then responds more soberly that Andrew has had some spectacular success in predicting the composition of planets and their moons, but his theory of their formation via supersonic turbulence seems to work only for him. No one else has been able to confirm the process. Dust grains grow by colliding with one another and sticking together by electrostatic forces. Small particles also physically embed themselves in larger aggregates during high-speed collisions. Particles larger than 1mm develop significant velocities relative to the gas because gas orbits the star somewhat more slowly than a solid body due to an outward pressure gradient in the disk. This velocity differential causes particles to migrate radially towards the star and to settle vertically towards the mid-plane of the disk. The inward migration time scale is particularly rapid for cm to metre sized bodies - at 1 AU from the star it can be as short as 102 years, which means that growth through this size range must be rapid, or else much of the solid material in the disk would evaporate when it enters the hot regions close to the star. Alternatively, planetesimals may form via the gravitational collapse of regions containing dense concentrations of solid particles, but this requires a substantially larger than normal dust fraction, and / or larger disk mass, in order that collapse is not disrupted by turbulence in the particle-laden flow. Computer modelling plays a critical part in this research. At Swinburne’s Centre for Astrophysics and Supercomputing Sarah Maddison and her colleges can have their theories tested, conditions simulated and programs run to look for realistic and sensible answers to these cosmic quandaries.
To date most Extra Solar Planets found are very large (Jupiter size) and have small orbits, because current detection by the radial and proper motion method is naturally biassed that way. 51 Peg b for instance has a period of just 4.2 days and yet is 0.46 the size of Jupiter. The direct detection method is extremely difficult because of the glare of the parent star. The first such detection was a 5 Jupiter mass object separated by 0.8 arc-seconds from dwarf star 2M1207 in April 2004. Photometric detection techniques can be used to detect a planet transit (passing in front of) its star causing a minute dip in its brightness . The event “light-curve” then tells us about the physical and orbital properties of the unseen planet. The first exo-planetary transit observed was HD209458 in September 1999, since confirmed by Doppler Shift. NASA's new Kepler spacecraft is about to begin an unprecedented journey of discovery measuring photometric transits. It is the first mission with the ability to find planets like Earth in the “comfort” zone where liquid water could be maintained on the surface. Specifically designed to detect transiting planets the mission is to spend three and a half years surveying more than 100,000 sun-like stars in the Cygnus-Lyra region of our Milky Way. Kepler’s cameras can register changes in brightness down to 20 parts per million. This is equivalent to watching a street lamp 1km away as you move your head back by 1cm. The statistics so compiled will help understand what type of planets are formed and perhaps chart a course toward one day imaging a pale blue dot like our planet, orbiting another star in our galaxy.
The vote of appreciation was given by the President, Perry Vlahos, with the traditional Reds in a planetary shopping bag. Alfred Klink