Finding other worlds
Ken Tapping, June 20, 2012
Transits of Venus were once the means by which we could measure the Solar System. Now we have radar and spacecraft that give us more accurate numbers than transits of Venus ever could. Moreover we can use these methods at times of our choosing, rather than opportunities eight years apart and separated by more than a century. Today the transits of Venus are finding a new use in searching for worlds beyond the Solar System.
The most powerful method for finding worlds orbiting other stars is to have sensitive instruments watching stars, waiting for those tiny drops in brightness as one or more planets orbit in front of them. In short, a transit of an alien planet in front of its star. That’s where the transits of Venus come in. We can see what information we can glean from the Venus events and try to get the same information from other transits. Some things are obvious, such as the time the planet moves in front of its star and when it moves away. How long it takes to cross its star’s disc is another piece of valuable information.
Since the side of the planet facing us is dark, the depth of the drop in brightness tells us how much of its star’s disc it covers, that is, how large the disc of the planet is compared to the disc of its star. In many cases the time between successive transits will tell us how long the planet takes to orbit its star. Analyzing the light of the star by means of a spectroscope will show the tiny wobbling of the star as it and its planet circle one another. From this we can find the mass of the planet. Knowing its size we can estimate its mean density and what it’s made of, whether the planet is mostly gas or mostly rock.
There’s more. Observers of Transits of Venus noticed that on occasion a bright ring surrounds the dark silhouette of the planet. This is sunlight being refracted through Venus’s atmosphere. En route the light picks up the signatures of chemicals in the planet’s atmosphere. We know these chemicals are associated with the planet because stars are too hot for these chemicals to exist.
With a bit more difficulty we can apply this technique to other stars. We cannot actually see the transiting planet, but we can see its contributions to the starlight, and we can detect what makes up its atmosphere.
So, from observing two or more successive transits of a distant planet across the face of its star, we know its orbit, its distance from its star, its mass, what it is made of, its temperature and the chemicals in its atmosphere. We’d like to discover an alien planet about the mass of the Earth, with a temperature of about 20° C and an atmosphere containing water vapour and oxygen. The only way a planet can have oxygen in its atmosphere is if there is life something like ours present on that distant world. Is there someone out there somewhere observing the Earth transiting the face of the Sun and wondering about us?
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