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, and proof of other solar systems (stars with more than one planet orbiting them) in 1999.
Even then, we didn’t see any solar systems that looked much like ours. The giant planets (many of them larger than Jupiter) we discovered orbited so closely around their stars that their systems simply couldn’t hold little rocky planets like Earth–the kind of planets most likely to harbor life.
But last week astronomers announced the discovery of the first planet found outside our solar system with an orbit similar to Jupiter’s, meaning there could be smaller planets inside its orbit. They also announced the discovery, in another star system, of the least massive planet so far discovered, just 40 times the mass of Earth (which may sound like a lot, but that’s only 15 percent the mass of Jupiter) again, a good sign that solar systems come in many configurations.
The planet with a Jupiter-like orbit is between 3.5 and five times as massive as Jupiter. It orbits a star called 55 Cancri every 13 years (Jupiter’s orbit takes 11.86 years). A second planet, slightly less massive than Jupiter, has also been found orbiting 55 Cancri, but its orbit only takes 14.6 days. Computer models of planetary formation developed by Gregory Laughlin of the University of California, Santa Cruz, confirm that an Earth-sized planet could exist in an Earth-sized orbit within such a system, although there’s currently no way to tell if such a planet is really there.
The planets were discovered by a team led by Geoffrey Marcy of the University of California, Berkeley, and Paul Butler of the Carnegie Institution of Washington. In all, they announced the discovery of 11 new worlds at a press conference at NASA headquarters, which brings the total number of known extrasolar planets to 98.
Marcy and Butler used the “wobble method,” which looks for the tiny but detectable wobble in a star caused by the gravitational pull of the planets orbiting it. It’s great for finding very massive planets, but it’s useless for finding planets the size of Earth, which don’t exert enough gravity.
There’s another method for finding planets called the transit method, based on detecting the dimming of a star’s light as a planet passes between us and it. Laurance Doyle, a researcher at the SETI (Search for Extraterrestrial Intelligence) Institute, has yet to discover a planet using this method, but has used it to detect the atmosphere of a previously discovered planet. He says it should be able to discover a planet only twice as big as Earth (roughly eight times as massive), which might be able to harbor life.
We have to use indirect methods of detecting planets around other stars because stars are so far away–even the closest is so distant light from it takes four years to reach us at 300,000 kilometres per second–that not even the Hubble Space Telescope can directly photograph planets around them.
Detecting Earth-sized planets around other stars will still have to wait a few years for the launch of next-generation space observatories. NASA is planning one called the Space Interferometry Mission, which will be able to determine the positions and distances of stars several hundred times more accurately than we can now, and another called the Visible Light Coronagraph, a large optical telescope (the mirror would be three to four times larger and at least 10 times more precise than the one on the Hubble Space Telescope) that blocks out starlight but lets light from around the star come in..
There are no firm dates for either mission, but both would follow the Kepler mission, currently set for 2007. This space telescope will use the transit method to detect and count Earth-sized planets around other stars (if they exist), but it won’t be able to photograph them or analyze their atmospheres.
Learning about other solar systems will tell us more about our own, but what will be most exciting will be the confirmation, if it comes, that the universe is packed with solar systems much like ours, making it unlikely that our little planet is the only one to harbor life–maybe even intelligent life.
Science fiction readers and writers like me have never believed we’re alone in the universe. It’s nice that science is finally catching up with us.
