These are heady times for planet hunters. During the past decade, astronomers have uncovered at least 125 confirmed planets in 98 solar systems in our corner of the Milky Way. But generally these planets have been huge - up to 17 times more massive than Jupiter. Earth-like planets - the "holy grail" of planet hunting - are too small to detect.
Tuesday, however, two teams of astronomers in the US reported breaking the size barrier with the discovery of two of the smallest planets yet, orbiting two nearby stars. The announcement follows a similar report last week from European astronomers of another small planet orbiting a third star. Where past discoveries have detected planets the size of Jupiter and larger, these new finds are closer in scale to Neptune and Uranus.
In addition, a fourth team announced last week that they had found a Jupiter-class planet using a new network of telescopes.
The discovery of the three smaller planets in particular - combined with a new, exquisitely sensitive camera bolted to the back end of one of the world's most powerful telescopes at Keck Observatory in Hawaii - appears to boost the prospects for discovering Earth-size planets within the next few years.
Such finds not only would be groundbreaking in their own right, astronomers say. They also would provide a set of early targets for a suite of space-based planet-hunting telescopes, the first of which is scheduled for launch in 2007. And the new finds are presenting a fresh set of challenges to theories about how solar systems form.
The pace of discovery for smaller planets "is picking up," says Johns Hopkins University astronomer Zlatan Tsvetanov, who is the program scientist for NASA's Terrestrial Planet Finder project. "It's really an exciting time."
Underpinning these efforts is a question that has captivated humanity since it first understood the true nature of the night sky: Are we alone in the galaxy?
"It's easy to get carried away and ask questions you have no way of answering," says Timothy Brown, a researcher at the National Center for Atmospheric Research in Boulder, Colo., and co-leader of one of the four planet-hunting teams. But, he adds, humans do have the capability to begin grappling with the question of how common Earth-like planets may be. "The only way to answer that is to go out and look," he says.
At a press briefing at NASA headquarters Tuesday, two teams described their new planets.
An international group led by Barbara McArthur, an astronomer at the University of Texas, discovered a planet only 14 times more massive than Earth orbiting a sun-like star nearly 44 light-years away in the constellation Cancer. (Jupiter, by contrast, is roughly 318 times more massive than Earth.) The new planet orbits so close to its parent star that its "year" - a complete orbit around its star - lasts 2.8 Earth days. The discovery brings to four the number of planets orbiting the star, known in some circles as "rho Cancri A."
A second group, led by Paul Butler of the Carnegie Institution of Washington, discovered a planet around a dwarf star with the prosaic name GJ 436, which lies 33.4 light-years from Earth. This planet is at least 21 times more massive than Earth, and enjoys a year that lasts only 2.6 Earth days.
Last week, a team from Europe announced the detection of yet another planet 14 times as massive as Earth. This planet orbits a sun-like star, 50 light-years from Earth, in the southern constellation Altar, once every 9.5 days.
Planets this small are difficult to detect. All three groups used an indirect technique that measures subtle changes in a star's spectrum as a planet tugs on its host as it orbits. The star's change in velocity as it "wobbles" toward and then away from an earthbound observer is tiny - roughly equivalent to the speed of a sprinting cheetah.
Dr. McArthur's group was able to get the most precise mass measurement for its planet because the team also used data from the Hubble Space Telescope to measure rho Cancri A's "wobble" against more distant background stars. These measurements gave them additional information.
Dr. Butler's group, in contrast, found their planet orbiting a type of star known as an M dwarf. These are stars are small - GJ 436 has only about 40 percent of the sun's mass - and very faint. But M dwarfs also are thought to dominate the galaxy's star population. The team has been monitoring 150 nearby M dwarfs. Three years ago they discovered one that hosted planets. Until Tuesday's announcement, it was the only one.
"This led to a suspicion that these low-mass stars might not harbor planets - at least not the large planets" the team had been detecting elsewhere, Dr. Marcy says.
Now it appears that M dwarfs might be very good hosts for small planets. And an M dwarf's "habitable zone" - where the amount of stellar light and heat hitting a planet is just right to allow liquid water to accumulate - would fall much closer than it would at a star like our sun. The new planet orbits too close to GJ 436 to fall into its habitable zone, Marcy says.
But calculations suggest the planet always shows the same face to its "sun." Thus, he adds, it's likely that the planet has a narrow temperate "twilight" zone separating its day side from its night side where water could accumulate.
Yet trying to determine the rough composition of these planets - rocky, gaseous, or icy - is tricky; it relies on a growing but still shaky understanding of how solar systems form, researchers say.
One technique that helps resolve the issue is to look for planets that eclipse their host star. This gives researchers a good estimate of the planet's radius. Combined with data from radial velocity measurements, which yield a mass estimate, astronomers can estimate the object's density - and roughly its composition.
Last week, Dr. Brown's team announced the discovery of a Jupiter-class planet orbiting a star 500 light-years away using this "transit" method. It's the first detection for a network of three telescopes located in the Canary Islands, Arizona, and California. Follow-up observations should allow the team to glean information about the planet's atmosphere.
If nothing else, researchers say, these discoveries illustrate the power of the combined techniques - astrometry, radial velocity, and transit studies - to unlock the secrets of distant planets.
"We're on the way to finding our first extra-solar Earth, and that's an exciting highway to be on," McArthur says.