Most stars are essentially enormous nuclear fusion generators that emit energy in the electromagnetic spectrum, lighting up the night sky. The planets of our solar system reflect the light emitted by our star, the sun or Sol, enabling us to see them through telescopes even on the Earth’s surface. But that reflected light is minuscule in comparison to that generated by Sol or any other star. We need another method to determine the presence of planets orbiting distant stars. Such planets were initially referred to as extrasolar planets but that has generally been abbreviated to exoplanets nowadays.
The wobble method detects very large planets orbiting closely around their star. These are generally referred to as “hot Jupiters” because they are of comparative size to Jupiter, the largest planet in our solar system, but much closer to the star they circle and therefore hotter. We don’t actually see the planet itself, we infer its presence by its gravitational effect on its star. That is where the “wobble” comes in. While the star exerts far more gravitational force on the planet, causing it to orbit the star, the planet still has a gravitational effect on the star as well. This causes the star to wobble slightly around its center of mass, something that stars without a planetary system or some other type of orbiting body will not do.
Binary star systems will display wobble as well, whether paired with another normal star, a neutron star or a stellar mass black hole. But these can be determined to be what they are, so when wobble is displayed where these alternatives have been discounted, to the best of our current astronomical knowledge, we presume the star in question has a hot Jupiter planet. As we advance in our astrological abilities we can hope to be able to detect cold Jupiter planetary bodies, those further out from their stars, and even smaller planets, eventually even those Earth sized.
Currently in 2009 however, we can only detect Earth sized planets that orbit pulsars. In fact, the first extrasolar planets discovered in 1991 orbited a pulsar, it was not until 1995 that the discovery of an exoplanet orbiting an “ordinary” star was announced. The realization that planets orbiting a pulsar had been detected astounded the astronomical community, and for good reason. Pulsars result from stars that have gone supernova, violently exploding beyond our ability to conceive. It was presumed all planets would be incinerated, totally destroyed by such an event, and yet large and even Earth sized planets have been detected orbiting pulsars as close as Mercury orbits our sun. How this can be is open to conjecture and debate and some hypotheses, but we don’t really know.
As far as Sol type stars go, we can only detect large, close in planets at this time. It does tell us that other stars have planets but it doesn’t necessarily tell us which might have inhabitable planets. It might be reasonable to think that a Jupiter sized planet close to the star would actually eliminate the possibility of an Earth sized planet orbiting at a suitable distance for our colonization. If we actually launched a slowboat colony ship towards another star in the next couple of hundred years it might be better to aim for one where we haven’t detected a “hot Jupiter” rather than one we have. Hopefully we will have improved our detection capabilities to the extent we can detect and analyze an Earth-sized planet in an appropriate orbit before we have reached the level of technological ability and the political will to launch such an adventure!
