One of the astronomical activities that excites me most is the search for other planets outside our solar system. I’ve mentioned the Kepler probe before as one example – and, so far, a very successful one – of this planet-hunting.
But how do we find these planets? Although we have some pretty powerful telescopes planets are, relative to their distance in other star systems, very small. Experiments like Kepler find planets by making use of the fact that these planets – like the planets in our solar system – orbit around their stars. Sometimes those orbits line up so that when we’re looking at that system’s star the planet moves through our field of vision, across the front of our view of the star. If it does we can’t see the planet itself (it’s still too small) but we can see a tiny drop in the brightness of the star. But measuring how much the star’s brightness diminishes we can get an idea of how big it is. By measuring how frequently it transits in front of its star we can get an idea of how long its orbit takes. They can still only detect fairly large planets this way, though.
Now scientists in Germany, Bulgaria and Poland have developed another method to find even smaller planets. If there are other, smaller planets in a system where they’ve already detected a large planet using the transit method, then the smaller planets will make their presence felt – via gravity – on the large planet. The tugs of gravity will affect the orbit of the large planet and we’ll observe variations in the timing of the large planet’s orbit. Using computer models we can then infer the presence of the small planets, despite being far too small to see directly.