Using an armada of telescopes, an international team of astronomers has found the smallest planet ever detected around a normal star outside our solar system.

The extrasolar planet is five times as massive as Earth and orbits a red dwarf, a relatively cool star, every 10 years. The distance between the planet, designated OGLE-2005-BLG-390Lb, and its host is about three times greater than that between the Earth and the Sun. The planet’s large orbit and its dim parent star make its likely surface temperature a frigid minus 364 degrees Fahrenheit (minus 220 degrees Celsius). This temperature is similar to that of Pluto, but the newly found planet is about one-tenth closer to its star than Pluto is to the Sun.

Its detection, however, opens a new window in the search for Earth-like worlds.

“This finding means that Earth-mass planets are not that uncommon,” said Kailash Sahu of the Space Telescope Science Institute in Baltimore, Md., and a founding member of the Probing Lensing Anomalies Network team (PLANET) that helped detect the new planet. “If we found one, there must be more.”

The result will be reported tomorrow in a Letter to the journal Nature.

The finding also supports theories for how our solar system formed. “The favored theory proposes that planets were created from material accreting around a star,” explained Bohdan Paczynski of Princeton University and a member of the Optical Gravitational Lensing Experiment (OGLE), a group that also helped to discover the planet. Paczynski and Shude Mao proposed the idea of using gravitational microlensing to discover planets in 1991. “Around red dwarfs, the theory predicts Earth- and Neptune-sized planets to be more common than Jupiter-sized planets. The planets would be located between 0.1 and 10 times the Earth-Sun distance from their stars.”

Astronomers discovered the planet indirectly with a technique called gravitational microlensing. The technique takes advantage of the random motions of stars, which are generally too small to be noticed. If one star, however, passes precisely in front of another star, the gravity of the foreground (“lens”) star bends the light from the background (“source”) star. The foreground star, therefore, acts like a giant lens, amplifying the light from the background star, a phenomenon called gravitational microlensing. A planetary companion around the foreground star can produce additional brightening of the background star. This additional brightening can thus reveal the planet, which is otherwise too faint to be seen by telescopes.

The higher the mass of the “lensing” star, the longer is the duration of the microlensing event. So, while a microlensing event due to a star lasts many days, the extra brightening due to a planet lasts a few hours to a couple of days. In the case of the newly found planet, the extra brightening lasted only about 12 hours.

Using the microlensing technique, astronomers determined the planet’s mass. This method, however, does not reveal any clues about an object’s composition. Astronomers think the planet is composed of ice and rock. Its estimated mass suggests that it is a giant version of terrestrial planets like Earth and Mars. The planet orbits the most common star in our Milky Way Galaxy, a red dwarf five times less massive than the Sun. The pair is located about 20,000 light-years away in the constellation Sagittarius, not far from the central bulge of our galaxy.

None of the roughly 160 planets found outside our solar system has been imaged directly. The planets are too dim and too close to their stars to be seen. Astronomers have discovered most of them by detecting the gravitational tugs the unseen planets exert on their parent stars. This popular technique, however, favors finding large planets orbiting very close to their hosts.

“Microlensing is a promising technique to find Earth-mass planets because other current planet-hunting techniques are not sensitive to discovering low-mass planets like Earth,” Sahu explained.

Because microlensing events are unpredictable and rare, astronomers improve their chances of observing one by looking at many stars at once. To catch a microlensing event, monitoring teams such as OGLE watch 100 million stars every night in the crowded bulge of our galaxy.

In the past decade, the OGLE team has found more than 1,000 microlensing events. OGLE’s monitoring team, however, may not notice the short-duration brightening caused by a planet.

PLANET has a dedicated army of 1-meter class telescopes, including the European Space Observatory’s Danish 1.54-meter telescope at La Silla in Chile, to specifically look for planetary signatures through continuous monitoring of ongoing microlensing events. The group has monitored about 200 microlensing events during its 10-year campaign. So far, three planets have been discovered with gravitational microlensing, including OGLE-2005-BLG-390Lb. The newly found planet is the smallest of the three and marks the first planet found by the PLANET team. To increase its chances of finding planets, the PLANET team joined forces in 2005 with RoboNet, a network of 2-meter robotic telescopes operated by the United Kingdom.

PLANET, RoboNet, OGLE, and other monitoring teams discovered the planet in 2005. The microlensing event was first spotted on June 11, 2005, by the OGLE search team. The OGLE-2005-BLG-390 event triggered the PLANET telescopes to start collecting data. On Aug. 10, round-the-clock monitoring by the PLANET team revealed an additional brightening that led to the discovery of the planet. An OGLE telescope from the same night also detected the planetary signature. The Microlensing Observations in Astrophysics (MOA) team later identified the background source star on its images and also confirmed the planetary signature.

Release Date: 1:00PM (EST) January 25, 2006
Release Number: STScI-2006-06


Donna Weaver
Space Telescope Science Institute, Baltimore, Md.
(Phone: 410/338-4493; E-mail:

Kailash Sahu
Space Telescope Science Institute, Baltimore, Md.
(Phone: 410/338-4930; E-mail:

Josh Chamot
National Science Foundation, Arlington, Va,
(Phone: 703/292-7730; E-mail: