These bursts and their less violent cousins, ordinary supernovae, spread heavy elements throughout their host galaxies, including carbon, oxygen, silicon, magnesium, and iron—the stuff of new planets. In the Milky Way, it's clear that past generations of dying stars seeded our galactic neighborhood with these raw materials. Astronomers detected the first planets circling other stars like our sun in 1995. Now there are more than 300, and that number is soaring. Most of the alien worlds spotted by telescopes so far are bloated balls of gas like Jupiter. But as time goes on, we're finding smaller ones. The smallest to date, dubbed super Earths, are about five times the mass of our planet. They probably have solid surfaces, and some may have the right temperatures for water to flow. Many extrasolar planets come in family sets; the star 55 Cancri, in the constellation Cancer, hosts at least five sibling planets.
If this sounds comforting and familiar, it should. Planetary scientists think it's only a matter of time before we find a solar system resembling our own. The Hubble Space Telescope has spied disks of gas and dust around many young stars. The images suggest that planets are assembling within the disks, clearing out gaps as their gravity gathers more material. The Spitzer Space Telescope, which uses infrared light to look deeply into clouds of dust where stars form, also sees evidence of planetary embryos nearly everywhere it looks. Star birth and planetary formation apparently go hand in hand.
We don't yet know whether planets like ours are common denizens of the galaxy. We can't actually see planets around other stars, at least not the kinds of planets we're familiar with. In the glare of a star's light, they're far too faint. Rather, scientists infer the presence of each planet, either by the slight back and forth gravitational tug it exerts on its star as it orbits, or by a tiny eclipse in the star's light each time the planet crosses in front of it. Because an Earth-size world is so small, these signals are tough to discern. NASA will launch a mission in 2009 called Kepler to monitor nearly 200,000 stars for the shadows of planets the size of Earth.
Let's say we find dozens or hundreds of Earths, or even thousands, as some astronomers predict. What then? We surely won't visit any of them in the foreseeable future. We'll listen for signs of intelligent life with our radio telescopes, but it's far more likely that we'll first find signs of slime. Bacteria, algae, and other microorganisms alter the atmosphere of a planet by using carbon dioxide and producing unstable gases like oxygen and methane. These gases leave distinct imprints in the heat radiated by the planet—imprints that a special orbiting observatory could detect. NASA had long planned to build such a mission, called Terrestrial Planet Finder. But it's complex and so costly that NASA now has committed only to developing the technology in modest steps.