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Finding planets is a task that is far harder than it would appear; despite the size of our solar system's largest planets, even mighty Jupiter is a runt-sized upstart compared to our Sun, at one tenth the diameter and about a thousandth its volume. It's a small enough speck in the sky at a few astronomical units' distance, so consider how hard it would be to find at, say 20 light years - it can be done if you have really good eyes and extremely good measurements of the light of the star (the transit method); or can detect the doppler shift in the star's light as a result of the planet's gravitational pull; or can look for gravitational microslensing. (It helps to have all the tools that you can muster when hunting for planets!) Some scientists hope to get direct images of stars via a “coronagraph” that blocks out the star's light in order to keep it from obscuring a view of any planets in its system or other direct imaging techniques, but that has yet to happen.
Astrometry is an additional planet finding technique that astronomers had worked out years ago but had not actually used successfully until now. In this technique, astronomers observe the star and look for the change in motion associated with a tiny companion. Extreme patience is required, as the recent discovery of a planet around the star VB 10, twenty light-years away in the constellation Aquila (Eagle, in English) demonstrates.
Two researchers at the Palomar Observatory, after monitoring the motion of 30 stars for over a decade, finally succeeded in detecting the motion indicative of a planet. Calculations indicate that this planet is around six times the mass of Jupiter and in a similar position, though its star is much cooler than our own (as spectral classes go, it's an M-class, or dwarf, star). This is one reason that the scientists' discovery is so exciting; while there are other star finding techniques, the vast majority of the planets found to date are “hot Jupiters” located around stars significantly more massive than our Sun. The ability to detect planets around stars smaller than our own sun makes the eventual find of a Earth-class planet that much more likely.
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Image courtesy of NASA