General Meeting Report, June 2006
Syllabus Item: Extrasolar Planets, the story so far. Speaker, Dr Rosemary Mardling, Senior Lecturer, Centre for Stellar and Planetary Astrophysics, Monash University.
I well remember the excitement when we first heard about extra solar planets. ‘We are no longer alone in the Universe!’ the headlines shouted. Little green men and UFOs had a new lease of life. After an initial scratchy start 17 years ago (the first proposal happened in 1989), the list of verified extra solar planets is now growing rapidly. At the last count it was 194 planets in 166 separate planetary systems. The definition of “Planet” in this list is any companion to a star with less than 13 Jupiter masses. Beyond that they are considered stars. The 1989 candidate was confirmed earlier this year with a mass of 11 Jupiters and a rotational period of 84 days at a distance from Earth of 28pc.
When Dr Mardling last spoke at an ASV Meeting in 2002, the list had already grown to 80 planets. But data on them was still sketchy enough for Andrew Prentice to dismiss the science as “wishful thinking” (I recall he actually used stronger words to that effect), “the mathematics are all against it”. His sceptism then was based on the anomalous orbit to mass relationship of those proposed extrasolar planets. Since then, with measurement accuracy increasing all the time, the data can no longer be ignored and, no doubt, Dr Prentice will come up with a theory that will explain this anomaly.
You have to remember here the incredible precision required for a reliable detection of these objects. To detect the wobble of the sun, for example, caused by the gravitational pull of Jupiter in our solar system, means you would have to measure a spectroscopic Doppler shift on a linear movement of 3 metres a second, which is at the limit of today’s technology. For the Earth this would be only 3cm/sec. Such “reflex velocity” detection is limited by an inherent “stellar jitter”, typically 3m/s, due to surface instability motion of the star. The existence of the 1998 candidate above was confirmed in 2005 by measuring a sideways displacement of the respective star of 0.010714 arcseconds. Put into perspective, this figure is the equivalent of measuring the width of a 10cent piece at a distance of 200km, or the width of a human hair at 25m, with an error margin of less than 0.1%.
Spectroscopy is of course only one of several possible tools to detect celestial movement. Of the 194 extrasolar planets known so far, their respective and means of discovery and confirmation were:
! 181 via the Doppler wobble of a star (radial velocity measurements)
! 4 via microlensing surveys
! 6 by direct imaging
! 5 as transits during microlensing surveys
! 4 by Pulsar radio pulse “time of arrival” method
! 9 by Photometric transit measurements
These are predominantly short orbit period planets (the shortest only 1.2 days). 76 are below the orbit time for Mercury, 88days, and only the microlensing objects exceeds Jupiter’s period of 12 Earth-years (one of them registered at 100years +).
Copernicus, Kepler and Newton gave our Solar System a scientific basis, and for 300 years it was assumed to be unique in the Universe. We have now detected other planetary systems, but to confirm the existence of another, similar star-system will require the detection of a Jupiter-Mass Planet, with a 12 year orbit. While there is no way known yet to detect life elsewhere in the Cosmos, hints of gaseous atmospheres have shown up on photometric transit measurements, giving hope to spectroscopic analysis of the gaseous envelopes. Gravitational lensing (The Einstein Rings) allows us to study the distribution of unseen matter in an intervening object or detect light from more distant object that would otherwise be unseen. Precision Micro-lensing uses the same principle, where a star with a planet passing between a more distant star and us, causes a gradual rise plus a momentary blip in the brightness of the star, caused by its planet. This is contrary to the standard transit measurement on nearby objects, which are designed to detect a drop in the brightness of the star. The event time depends mainly on how far the lensing object is away from us. Microlensing has the advantage that the objects can be arbitrarily far away. The four objects listed above are all more that 8,000ly away.
Two planetary candidates have been detection in star forming clusters by direct infrared measurements. One even seems to have a circumstellar disc from which moon may form.
And what is our vision the future? Dedicated space telescopes to search for Earth-mass transit planets; radial velocity method search for planets with infra-red spectroscopy around low mass stars; infrared searches for planets forming in dusty discs; spectroscopic analysis of atmospheres of Earth-like transiting planets, THE SIGNATURES OF LIFE.
Dr Rosemary Mardling is an astrophysicist in the School of Mathematical Sciences at Monash University. She uses mathematics to figure out all kinds of cool stuff about how stars and planets and galaxies move around the Universe. She has a wide field of activity to apply her talent:
Astronomy; tides in stars and planets; tidal capture; the dynamical evolution of stellar and planetary systems; dynamics of stars in galactic nuclei, in clusters and extrasolar planetary systems; the stability of three-body systems and chaos theory; formation of low-mass X-ray binaries and Pulsar binaries.
Oh, she also teaches students mathematics and astrophysics. Alfred Klink