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Small planets discovered at a faster pace

Anybody out there? Astronomers announce recent discoveries of two Earth-like planets.

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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.

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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.

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