New Planets.

Planet hunters discover the first solar system outside our own.

Late evening last March, astronomer Geoffrey Marcy stared at his computer screen, speechless. It wasn't the latest Internet gossip that stunned him. Data from two telescopes at the University of California's Lick Observatory revealed something that would forever change scientists' view of the universe: the first solar system ever found outside our own.

Marcy's discovery squelches all speculation that ours is the only solar system in the universe.

Birth of our Solar System

Our solar system formed about 4.5 billion years ago from a nebula, a swirling cloud of gas and dust (1).The nebula's center contracted and gravity began to pull matter inward. Increasing pressure caused the center to heat up, and the sun was born (2). The nebula's outer rim continued to spin around the sun (3). Meanwhile, small whirlpools formed within the rim and attracted their own gas and dust, forming small spheres (4). The spheres' gravity pulled in more gas and dust to form the planets (5).

After all, the Milky Way, our home galaxy (large system of stars), contains at, least 200 billion stars. And at least 80 billion more galaxies stretch far beyond the Milky Way. Why wouldn't the universe teem with other planetary systems?

PLANET HUNT

Marcy and his team of planet hunters have been scanning a number of sun-like stars, looking for extrasolar (outside our solar system) planets. One star, Upsilon Andromedae, is located 44 light years (the distance light travels in one year--about 6 trillion miles or 9.5 trillion kilometers) away, and can be seen from Earth with the naked eye (see star map, p. 15). But the three planets orbiting Upsilon can't be detected with even the most powerful telescopes. So how did Marcy find them?

He relies on a method called the Doppler technique. As a planet orbits a star, its gravitational pull yanks the star around in a small oval. This "wobble," as astronomers call it, mimics the planet's own orbital path. To detect the wobble, Marcy relied on a spectrograph, an instrument that records the spectrum, colors that make up the white light that the star emits.

As the star wobbles toward Earth, the wavelengths of light it emits are shortened, or shift toward the blue end of the spectrum. (Blue has a shorter wavelength than red.) As the star moves away, its wavelengths get. longer and shift toward the red end (see diagram, p. 15). This cyclical shift in the light spectrum, known as the Doppler effect, shows the star's wobble patterns and betrays the presence of orbiting planets.

MOTHER STAR

Upsilon Andromedae's spectrograph revealed that the star was gyrating wildly in space. "I realized the star's wacky motion was simply the stun of three planetary orbits," Marcy told SW. "The star was being yanked around by the gravitational field of not one, not two, but three planets!"

The three planets are gas giants--two even larger than Jupiter, the biggest planet in our solar system. Marcy suspects the planets--like the sun, Jupiter, and most large objects in the universe---are made of hydrogen and helium gas. What puzzles astronomers is that, unlike large gas planets in our solar system that orbit far away from the sun, these giants orbit very closely to their star (see graph, p. 13).

SPACE GIANTS

How did this system of giant gas planets form so close to their star? Marcy is mystified. Current theory holds that planets materialize out of a disk of dust, gas, and ice that surrounds a star after it explodes into existence (see "Birth of our Solar System," p. 14). Until now, scientists thought Jupiter-size planets could form only at great distances from a star, where temperatures are low enough to let ice grains condense and gather together, starting the planet-forming process.

Astronomers' new theory: The planets, which are about 3 billion years old, must have taken shape farther from the star, then migrated inward after forming. Planets moving through the dust-filled disk encounter friction (resistance to an object's motion) as they collide with particles in their path. Friction might have caused the planets to slow down and spiral inward.

WHAT'S NEXT?

Scientists hope future space missions will help answer how "new" planets formed. Marcy is especially excited about NASA's Space Interferometry Mission (SIM), scheduled to launch in 2005. SIM will measure the changing position of stars as they're pulled by companion planets. Unlike the Doppler technique, which can't detect small planets--their tiny wobbles don't show up in a spectrograph--SIM should be able to find Earth-size planets. (All extrasolar planets that have been discovered so far are gas giants.)

While the latest discovery has added another piece to the grand puzzle of the universe, it raises countless questions: How common are planetary systems? Are there other planets like Earth where life exists, and which humans could colonize?

For Marcy, who has found 15 of 20 extrasolar planets discovered since 1995, the search for other earths is the ultimate mission driving his work. "I envision a foreign planet with streams and waterfalls and strange species flying above and swimming below," he says.