Many science fiction stories and movies involve planets orbiting other stars. We have always believed such things exist, but it was not clear we would ever find out for sure. Seeing a planet orbiting another star would be like detecting a small moth flying around a searchlight from a distance of hundreds of kilometres. However, thanks to instruments such as the Kepler spacecraft, we now know of at least 442 stars other than the Sun that have planets. In some cases we have detected more than one, bringing to total number of extra-solar planets so far to 528. Moreover, by the time you read this, that number is likely to have grown some more. The Kepler spacecraft can monitor a field of many stars, watching for one of them to dim very slightly as one of its planets moves in front of it. In addition, from the duration and depth of the dimming, and the time interval between two successive dimmings, we can estimate the size of the planet, its mass and how far it lies from its parent star. The Holy Grail planets we seek are ones with masses similar to the mass of the Earth, lying at a distance from their star which would give them surface temperatures right for the existence of liquid water. That would make them suitable for the appearance of some form of life as we know it.
Actually, we have managed to get even further. If a planet with an atmosphere passes in front of its parent star, on its way to us, some of the starlight passes through the atmosphere of the planet, and in doing so acquires the spectral signatures of the gases making up that planet’s atmosphere. By comparing the spectrum of the starlight when the planet is in front of the star with the spectrum when the planet is not, we can identify the signatures of those atmospheric gases. What we would like to find are signatures of gases like oxygen or other reactive gases that only exist in a planet’s atmosphere when they are being continually replaced by living things. With the rate at which new observational data are being accumulated, we can be moderately optimistic that if such things are out there, we stand a good chance of finding them.
Amazingly, we have managed to find out all these things without ever seeing an actual image of an extra-solar planet. However, even that is becoming possible. For example, there is a new instrument under construction that is planned to start making observations in 2012 or so. It is called the Gemini Planetary Imager (GPI), and is intended for use on the Gemini South Telescope, located on a high plateau in Chile. The Gemini North Telescope is on Mauna Kea, in Hawaii, so that we have both the Northern and Southern Hemisphere covered. Canada is a participant in these facilities. Using advanced adaptive optics techniques to correct for atmospheric turbulence and other problems, in combination with advanced image processing techniques, the GPI will make it possible to actually see planets orbiting nearby stars, despite the glare. Tools like this would definitely have been in the realm of science fiction not that long ago.
Jupiter is now low in the southwestern sky after sunset, setting about 7 p.m. It is still very conspicuous. Saturn rises around 10 p.m., and Venus around 5am.The Moon will be full on the 18th.
Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.
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