The idea that planets orbit most of the stars in the universe has such a firm hold on our imagination, thanks to Star Trek and Star Wars, that most people are surprised to hear we only found the first planet outside our solar system in 1995. Only this past week have we confirmed the existence of another solar system: that is, another star with more than one planet orbiting it.
Other stars are so far away–even the closest is so distant light from it takes almost four years to reach us at 300,000 kilometres per second–that not even the Hubble Space Telescope can directly photograph planets in orbit around them.
Nevertheless, we’ve always assumed they’re there, simply because the sun is just an ordinary star among 200 billion in this galaxy–and the Milky Way is just an ordinary galaxy among 80 billion others. It was hard to imagine only our star would have planets.
Confirmation, however, had to wait until October of 1995, when Michel Mayor and Dider Queloz of Geneva Observatory in Switzerland announced discovery of the first planet outside of our solar system. Essentially, they noticed that the light of a star called 51 Pegasi changed color slightly, in a regular pattern. This “Doppler shift” indicated the star was wobbling, and the best explanation for that wobbling was the gravitational tug of a large planet hurtling around the 51 Pegasi in a very close orbit, making a complete circle every 4.2 days.
“Doppler shifting” sounds exotic, but we experience it every day. When a car passes you, the sound it makes deepens. That’s because while it’s approaching, the soundwaves are slightly compressed; as it moves away from you, they get longer. The same thing happens with light: as a star wobbles toward us, its color changes slightly toward blue; when it wobbles away, its color changes slightly toward red.
Since 1995, scientists have turned up around 20 extrasolar planets, but in every case, they were singular–which caused some to wonder if they were planets at all, or some kind of small, dark star. Then, last week, two teams of astronomers, one from San Francisco State University and the other from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and the National Center for Atmospheric Research in Boulder, Colorado, announced they had independently determined that the wobbling of Upsilon Andromedae, a star similar to the sun 44 light years away, could best be explained by the existence of three large planets in orbit around it.
Two of the planets are several times more massive than Jupiter, which itself is 318 times more massive than Earth. One is about as far away from the star as the asteroid belt is from our sun, while the other orbits at about the same distance as Venus. The third planet, at least three-quarters as massive as Jupiter, orbits so close it whips around it in 4.6 Earth days.
One such object might be a dark star, but three, astronomers say, have to be planets–which makes it likely the other objects discovered are also planets. It also makes it likely the universe is full of solar systems. After all, we know of two, now, very close together in astronomical terms.
The Upsilon Andromedae system is unlikely to contain Earth-like planets, but there’s no way to be sure, because Earth-sized planets are too small to cause much of a wobble in their stars. However, detecting Earth-sized planets may become possible in a very few years, thanks to an upcoming NASA project.
Set for launch in 2005, the Space Interferometry Mission will be able to determine the positions and distances of stars several hundred times more accurately than we can now, and will also pioneer a new technique to block out the light of bright stars to allow us take images of nearby objects. This may let us find Earth-sized planets. Eventually, we may even be able to analyze the atmospheres of such planets, which could tell us if they can support life.
Learning about other solar systems will tell us more about our own, but what’s most exciting about this discovery is that it points to a universe packed with such systems. That makes it almost inconceivable that only our little planet has life, and highly unlikely that only our planet has intelligent life.
Science fiction fans like me have never believed we’re alone in the universe. It’s nice to know that science is finally catching up with us.