Keith Tritton – “Extrasolar Planets”

Wednesday 13 January 2010

The New Year programme got off to a flying start with a fascinating talk by Keith Tritton on extra solar planets. Keith of course is a founder member of the Club which began its first meetings in 1994. He introduced his talk by noting that at that time there was no evidence of planets orbiting other stars, but since then there have been over 400 discoveries reported, a number that is increasing almost daily.

It is important to know how our own solar system was formed in order to understand how planets may form around other stars. Keith showed diagrams of the large clouds of gas circling the early sun in spirals that slowly condensed into the rocky Earth-like planets and the gas giants such as Jupiter. The division between these two types of planet is known as the Frost Line. In our solar system, the giant planets are kept at a healthy distance from the sun. However, as was pointed out later, there are many extra solar planets of Jupiter mass that are significantly closer to their parent star.

How were these planets discovered? Keith creatively used a number of “props” to show us how these objects have been detected by inference rather than direct observation which is very challenging. Any object that orbits a star will induce a slight wobble in the star (illustrated with a small rubber ball and football respectively). The star’s wobble will cause a shift in its spectrum due to the Doppler effect – modern spectroscopy is highly sensitive and can detect this slight movement without too much difficulty. So the first extra solar planet (orbiting 51 Pegasi) was detected in 1995. Many more have been discovered using this technique since then. Another approach is to measure the slight dimming of the star’s light when a planet transits. This requires extremely accurate observations to detect a tiny drop in starlight, but it works and has even been reproduced by amateurs. It would of course be desirable to image the planets directly and there have been some reports of just that, with convincing movement of tiny dots of light around artificially obscured starlight detected by imaging over several months. This is more readily achieved using infrared light where the balance between the brightness of the planet and star is more favourable than in visible light.

As mentioned before, the number of discoveries is increasing almost daily and can be checked on the Extrasolar Planets Encyclopaedia website, which is a rich source of information on extra solar planets.

The launch of NASA’s Kepler space telescope last year has already yielded a new crop of extra solar planets with many more to come. Most importantly it will be possible to detect planets with a similar mass to our earth. Currently most of the planets have been Jupiter sized or greater and as mentioned earlier, are sometimes found very close to their parent star. This is in stark contrast to our own solar system, prompting some discussion about how typical we are in this respect. It also raises the question of whether our model of solar system formation is actually correct.

This talk certainly gave us food for thought. Since this is such a hot area of astronomy, we fully expect there to be many more fascinating discoveries by this time next year.

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