In the basement of the astronomy department at the University of California-Berkeley, a half-dozen researchers sit behind computers three or four nights each month and look for planets beyond our solar system. Using an internet interface and a video link, they remotely control one of the largest telescopes in the world-a 270-ton instrument at the W.M. Keck Observatory in Hawaii, where two round telescopes sit perched on the summit of Mauna Kea like eight-story snowballs.
As the astronomers analyze the starlight data collected by Keck, they look for repetitious shifts in the wavelengths of individual stars. Those shifts could mean a star is being orbited by its own planet.
"There's no sort of magic time-you know, a eureka moment-when suddenly, 'Ah-hah, it's there!'" says Andrew Howard, a member of the Berkeley team, explaining how a star suspected of hosting a planet must be measured over and over. "It's pretty exciting when you discover that there's a very good candidate: 'Wow! If this thing pans out it'll be the second smallest planet ever discovered!'"
The search for extrasolar planets-or exoplanets-is now one of the hottest pursuits in astronomy. Since the existence of exoplanets was confirmed in the 1990s, astronomers have tallied over 400 of them. It's a number that's growing quickly as planet-hunting technology expands and astronomers endeavor to find something out there comparable to Earth.
Howard's candidate did pan out: After magnifying a star inconveniently named HD156668 about 90 separate times over five years, the Berkeley team concluded it was being orbited by a planet about four times as heavy as the Earth-the second smallest exoplanet known to orbit a star similar to our sun. Howard's research team announced the discovery at the annual meeting of the American Astronomical Society (AAS) in January.
It was a significant discovery because of the technical difficulty of finding smaller exoplanets (it was "a nice sort of trophy" for the team, said Howard), and also because most exoplanets found so far have been comparable to the size of Jupiter, at 300 times the mass of the Earth, or even larger.
But finding an Earth-size planet is the gem many exoplanet hunters are looking for, and it's the chief goal of NASA's Kepler mission, which announced its own discovery of five new exoplanets at January's AAS meeting. Launched from Florida's Cape Canaveral in March of last year, the cylinder-shaped Kepler spacecraft orbits the sun rather than the Earth, and stares persistently into a single swath of space about the size of your hand at arm's length. It monitors the glow of 156,000 stars and beams information home to four clusters of computers, which take note of any star that suddenly dims by a small amount-perhaps 1 percent-then brightens again several hours later.
That light pattern may be the signature of an exoplanet dozens or hundreds of light-years away. It works like this: If the plane of an exoplanet's orbit lies edge-on toward us, the planet will pass in front of its star, blocking a portion of the light, like a moth near a lamp. The dimming will occur at regular intervals, each time the planet orbits the star, perhaps every few days-or every few years.
Technically, Kepler's method for detecting exoplanets is different from the Berkeley team's method, which measures fluctuations in wavelength rather than brightness. But astronomers combine the methods to confirm their data and get an accurate calculation of an exoplanet's diameter and mass, which reveals its density. Kepler's principal science investigator, William Borucki, said knowing the density helps theoreticians understand the structures of exoplanets: "High density? Gonna be something like the Earth. Low density? We're not quite sure what it means, but it's probably Jupiter-like. Something intermediate? Probably like Neptune."
The five exoplanets Kepler discovered last year are "hot Jupiters," so called because of their large size and intense temperatures (2,200°F to 2,900°F). The largest one of them turned out to be the least dense: Its average density was one-fifth that of water, or like "a piece of Styrofoam."
All five of Kepler's exoplanets are so close to their host stars that they complete their orbits every three to five days. Planets with longer orbital periods will take more of Kepler's time: Finding an exoplanet with an orbit similar to Earth's, for instance, requires three years of observation, since the planet must pass before its star three times before its discovery can be confirmed. Borucki said Kepler has identified hundreds of potential planets, but many will turn out to be false alarms, caused by other phenomena such as eclipsing binary stars.
The total number of exoplanets in space is a risky guess, but if each star in the Milky Way were orbited by just one planet, the galaxy would contain over 100 billion.
Popular interest in the question of life beyond our solar system is helping fuel the hunt for smaller and smaller exoplanets. Astronomers are looking for Earth-size planets that may lie within the "habitable zone" of their stars-an orbital distance where water can physically exist in its liquid form.
Jason Lisle, an astrophysicist who works as a research scientist for Answers in Genesis, a creationist organization, sounded upbeat about the hunt for exoplanets and said he expected improving technology to discover smaller planets. But, "I would not expect there to be another planet out there that's like Earth in the sense of having all the right ingredients for life, and having life on its surface. And so far, observations have borne out that prediction."
Lisle noted that some planetary systems-such as those with Jupiter-size planets orbiting close to their stars-or one discovered last year with a planet orbiting in the opposite direction of its star's rotation-throw a wrench in current theories about the formation of stars and planets.
Researchers have yet to find an exoplanet that can properly be called "Earth-like." Or hospitable. The smallest exoplanet discovered so far, CoRoT-7b, is 70 percent larger than Earth and has a density similar to Earth or Mercury, most likely making it a "rocky" planet. But with surface temperatures reaching perhaps 3,600°F, "lava" planet is a more realistic description.
Kepler has found even warmer and stranger worlds: "We've got a companion that is planet-sized, but is much hotter than the star it orbits," said Borucki. The star is about 16,500°F and the companion is 21,000°F, more than double the temperature of our sun. Borucki speculates the companion could be something like a white dwarf star, but he says, "We're a little bit puzzled about some of these things too."
But the Kepler mission itself is proof that perseverance precedes discovery. Borucki pioneered the idea of measuring star brightness to detect exoplanets in the '80s, and throughout the '90s tried to convince NASA to put a special telescope in space for that purpose. NASA rejected the idea as technologically impractical for several years. "And we were laughed at for a very long time," says Borucki. But by 2000 his team had overcome the technical challenges and sold the space agency on Kepler.
"What we're talking about is a major effort by humankind. You do it one step at a time," said Borucki of the extrasolar planet search. "The technology simply didn't exist 10, 15 years ago to do these things."
If a planet passes directly between a star and an observer's line of sight, it blocks out a tiny portion of the star's light, thus reducing its apparent brightness. Sensitive instruments can detect this periodic dip in brightness. From the period and depth of the transits, the orbit and size of the planetary companions can be calculated. Smaller planets will produce a smaller effect, and vice versa. A terrestrial planet in an Earth-like orbit, for example, would produce a very small dip in stellar brightness that would last just a few hours. This is one of several ways in which astronomers can find exoplanets.