Earthlike Planet Spied in Distant Solar System

John Roach
for National Geographic News
January 26, 2006
Yesterday astronomers announced the discovery of the most Earthlike planet ever detected outside of our solar system.

The discovery raises the prospect that the Milky Way galaxy (see photo) is full of planets that could harbor life, the scientists say.

Five times as massive as Earth, the newfound planet orbits its parent star once every ten years. The planet is as about three times as far from its star as Earth is from the sun.

The newfound planet's host star is a red dwarf with about one-fifth the mass of our sun. Red dwarfs have cooler surface temperatures and are smaller, dimmer, and less massive than the sun. They're also the most common stars in our galaxy.

The planet, named OGLE-2005-BLG-390Lb, is located more than 20,000 light-years away in the constellation Sagittarius, near the center of the Milky Way.

The discovery team says the mass of the planet, its distance from its star, and the brightness of its star suggest that the planet has a rocky surface buried beneath frozen oceans. The distant world also likely has a thin atmosphere.

Scientists believe the surface of OGLE-2005-BLG-390Lb is a frigid –364°F (–220°C). That's about as cold as the surface of Pluto.

"The only substances able to form a planet of that size, at that temperature, are rock and ice," Andrew Williams, an astronomer at the Perth Observatory in Australia, said in an email interview.

"And during formation, the rock, being heavier, would have sunk to the middle. Pluto and many of Saturn's and Jupiter's moons are similar," he added.

Williams is one of 73 co-authors who describe the discovery in the January 26 issue of the science journal Nature.

New Technique

The planet is likely too cold to harbor life, but the technique researchers used to find it may one day detect a habitable Earthlike planet, according to the astronomers.

To date, more than 150 planets have been found in orbit around distant stars using a technique that detects the gravitational tug the unseen planets exert on their parent stars.

The method favors detection of large planets that orbit close to their hosts.

But the newfound planet is the third to be revealed using a different technique known as gravitational microlensing, which is based on an effect predicted by Albert Einstein in 1915.

It works like this: When a star passes precisely in front of another star, the gravity of the star in front bends the light from the background star.

The bending effect acts like a lens that amplifies the light of the background star, making it brighter.

The extra radiance can last from a few days to a year. If the forward star happens to have a companion planet, it will produce a defect in the brightening that usually lasts from hours to days.

Scientists analyze this defect to calculate the mass and orbit of the companion planet.

To catch and characterize planets with this technique requires continuous round-the-clock monitoring of ongoing microlensing events. Such an effort demands cooperation of astronomers around the world.

The event that uncovered OGLE-2005-BLG-390Lb was first detected on July 11 by the Optical Gravitational Lensing Experiment (OGLE) search team, led by Andrzej Udalski of Warsaw University in Poland.

The team monitors the brightness of millions of Milky Way stars with a telescope in Chile. When the event was detected, the OGLE search team alerted astronomers around the world.

In turn, the newfound planet was first detected by astronomers on the Probing Lensing Anomalies Network (PLANET) team, led by Jean-Philippe Beaulieu of the Institut d'Astrophysique de Paris, on August 10. Other groups later verified the find.

Model Fit

To date, gravitational microlensing has yielded three planets in orbit around distant stars. The first two planets have masses a few times greater than Jupiter's.

"This [new] planet is actually the first and only planet that has been discovered so far that is in agreement with the theories for how our solar system formed," team member Uffe Gråe Jøgensen of the Niels Bohr Institute in Copenhagen, Denmark, said in a media statement.

According to those theories, the bits of dust swirling around a newborn star glom together and form rocks, asteroids, and ultimately small planets. A few planets go on to pull in gas, becoming giant planets.

Around a red dwarf, the type of star OGLE-2005-BLG-390Lb orbits, the model suggests there should be lots of small planets and few large ones.

Williams, the Perth Observatory astronomer, says the discovery of a planet using the microlensing technique confirms the belief that Earthlike planets may be common in the universe.

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