New Mini-Planet Found; "Shot in the Arm" for ET Search

Anne Minard
for National Geographic News

June 2, 2008

A newfound extrasolar planet is the smallest yet discovered orbiting a star smaller than our sun, astronomers announced today.

The find may increase the chances of finding life-supporting "exoplanets," they added.

The planet, dubbed MOA-2007-BLG-192Lb, is just three times more massive than Earth and orbits what is most likely a brown dwarf—a "failed" star that is so small its core may not be massive enough to maintain nuclear reactions for very long.

The planet is 3,000 light-years from Earth and has a close-in orbit similar to Venus's. But because the newfound body's parent is so much cooler than our sun, MOA-2007-BLG-192Lb is most likely to be even colder than Pluto.

Even so—and despite almost no solar heating—there's a slim chance that the planet could maintain a habitable temperature if the atmosphere is as thick with molecular hydrogen as researchers think it could be, according to study leader David Bennett.

The record-breaking planet—the only smaller one has been found orbiting a pulsar, a rapidly spinning neutron star—was announced Monday at an American Astronomical Society meeting in St. Louis, Missouri.

(Related: "Mysterious 'Super Earth' Is Smallest Known Exoplanet?" [April 11, 2008].)

Bennett said the discovery is a shot in the arm for the search for habitable planets outside our solar system.

"The fact that we're finding outer planets around low-mass objects is an indication that planets are forming in these low-mass systems," the University of Notre Dame physicist said.

Many such systems are relatively nearby and would be easy places to take the next step—the search for life.

Brown or Red?

MOA-2007-BLG-192 is the seventh exoplanet to be discovered using a technique called gravitational microlensing. But it's the first for which researchers didn't have to rely on follow-up observations to confirm the planet's presence.

Microlensing takes advantage of an effect predicted by Albert Einstein in the early 1900s.

When a star passes precisely in front of another star, the gravity of the star in front bends and amplifies the light from the background star. Any planets circling the star in front create detectable anomalies.

As for the speed and decisiveness of the discovery, the researchers credit the wide field of view of the MOA-II telescope in New Zealand, which allows the Milky Way's entire galactic bulge to be imaged every hour.

But the process leaves some details unanswered, such as the exact mass of the host star.

The star could be 6 percent the mass of our sun, and thus a brown dwarf—or it could be 9 percent, which would make it a red dwarf. Red dwarfs are about a hundred times brighter than brown dwarfs but a thousand times fainter than the sun.

Bennett said future observations will be aimed at teasing out more about the host, including whether it has additional planets in its habitable zone.

Illuminating Brown Dwarfs

Also at the conference, Michael Liu, an astronomer at the University of Hawaii, said that the ground-based W.M. Keck Observatory and the Hubble Space Telescope are allowing new views of brown dwarfs as small as 3 percent the mass of the sun.

"They're easy to study in great detail, because they're floating close to Earth," Liu said.

Brown dwarfs hold insights into the nature of gas giants like Jupiter and Saturn, because they are the missing links between the lowest-mass stars and gas-giant planets, he said.

Liu said the latest telescope optics are allowing razor-sharp views of these prohibitively faint objects.

"If your eyes were as sharp as the Keck telescope," he said during the briefing, "you would be able to read a magazine pasted on the St. Louis arch, about a mile [1.6 kilometers] away" from the conference site.

That clarity is shedding light not just on brown dwarfs, but on the flaws in the science surrounding them, he said.

In short, the current models are off, and the telescopes are sending theorists back to the drawing board.

The current models, for example, are either "not predicting the right temperature and/or the right amount of energy," Liu said.