Ken Tapping, April 22, 2009

For at least a century astronomers have believed there are planets orbiting other stars. After all, our Sun is a typical yellow dwarf star, and there are countless similar stars out there. In addition, our ideas on how stars form more or less lead to the presence of planets. However, stars are very bright, and planets are only visible because they reflect a fraction of the light hitting them. Finding them would be like spotting a moth flying around a distant searchlight. Directly seeing worlds orbiting other stars seemed to be a non-starter, and astronomers started experimenting with more indirect methods.

Even though stars are a lot more massive than planets, stars with planets tend to wobble slightly, like someone dancing with an invisible partner; sometimes the visible dancer is approaching you and at other times receding, so you can deduce the presence of an invisible partner. Searching for planets in this way is difficult. Firstly the effect is tiny and secondly we are trying to make measurements from the surface of a spinning ball that is moving around the Sun, which in turn is orbiting the centre of our galaxy. However, it does work. Luckily though, some ingenious people found another, easier method. We could detect planets by the minute decrease in light level that occurs when a planet moves in front of the star. The sensitivity of modern instruments has helped this method work very well. In fact we can observe a patch of stars in the sky all at the same time, waiting for one of them to dim very slightly as a planet moves in front. Advanced amateur astronomers have got in on the act, and are successfully discovering planets too. Now we know of a few hundred planets orbiting other stars. However, to this point all the detection had been indirect; we had never actually seen any non-solar planets. That is, until now.

By the time the light from a distant star has arrived at our instrument, it has suffered en route. For most of its journey through empty space, nothing much happened to it, but the last bit of its journey takes it through our turbulent atmosphere, which makes the image blur, dance and shimmer. Then no matter how good our telescope is, it adds further distortions. Any star other than the Sun should appear in our telescopes as a point of light. However, thanks to the telescope and atmosphere, it appears as a shimmering circular blob surrounded by rings that would hide any faint objects lying close by. Fortunately, if we can measure the atmospheric and telescope distortions, we can now to a surprising extent remove them.

Last year Christian Marois, an astrophysicist at the NRC Herzberg Institute for Astrophysics in Victoria, did just that. He was looking at a star called HR 8799, an unremarkable star lying 128 light years away, in the constellation of Pegasus. Because it is never possible to completely remove effects from data unless one has absolutely all the information, the glare from the star was reduced to a patch of coloured shreds. However, clearly visible in the image were three planets. After decades of struggling with indirect evidence, this was the first time anyone had actually seen planets orbiting another star. This observation breaks new ground in astronomy and adds a new dimension to the study of planetary systems beyond our own.

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Ken Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca