OGLE-2005-BLG-390Lb is a 'super-Earth' extrasolar planet orbiting the star OGLE-2005-BLG-390L, which is situated 21,500 ± 3,300 light years away from Earth, near the center of the Milky Way galaxy. On January 25 2006, Probing Lensing Anomalies NETwork/Robotic Telescope Network (PLANET/Robonet), Optical Gravitational Lensing Experiment (OGLE), and Microlensing Observations in Astrophysics (MOA) made a joint announcement of the discovery. The planet does not appear to meet conditions presumed necessary to support life.
OGLE-2005-BLG-390Lb orbits around its star at an average distance of 2.0 to 4.1 AU, or an orbit that would fall between the orbits of Mars and Jupiter in our own solar system. The variation in distance is the range of error in measurement and calculation; it does not represent its orbital eccentricity, as the planet's orbital elements are not known. Until this discovery, no small exoplanet had been found farther than 0.15 AUs from a main sequence star. The planet takes approximately 10 Earth years to orbit its star, OGLE-2005-BLG-390L.
OGLE-2005-BLG-390Lb's sun (located in the constellation Scorpius, RA 17:54:19.2, Dec −30°22′38″, J2000, 6.6 ± 1.0 kpc distance) is thought to likely be a cool red dwarf (95% probability), or a white dwarf (4% probability), with a very slight chance that it is a neutron star or black hole (<1% probability). No matter which of these OGLE-2005-BLG-390L is, the radiant energy output would be significantly less than that of the Sun.
The planet is estimated to be about five times Earth's mass (5.5 ME). Some astronomers have speculated that it may have a rocky core like Earth, with a thin atmosphere. Its distance from the star, and the star's relatively low temperature, means that the planet's likely surface temperature is around 50 K (−220 °C, −364 °F). If it is a rocky world, this temperature would make it likely that the surface would be made of frozen volatiles, substances which would be liquids or gases on Earth: water, ammonia, methane and nitrogen would all be frozen solid. If it is not a rocky planet, it would more closely resemble an icy gas planet like Uranus, although much smaller.
The planet is not so much notable for its size, or possible composition — although these are unusual — but for the fact that such a relatively small exoplanet was detected at such a relatively large distance from its star. Prior to this, "small" exoplanets, such as Gliese 876 d which has a "year" of less than 2 Earth-days, were detected very close to their stars. OGLE-2005-BLG-390Lb shows a combination of size and orbit which would not make it out of place in Earth's own solar system.
"The team has discovered the most Earthlike planet yet," said Michael Turner, assistant director for the mathematical and physical sciences directorate at the National Science Foundation, which supported the work. At 5.5 Earth masses, the planet is less massive than the previous candidate for lowest-mass extrasolar planet around a main sequence star, the 7.5 Earth mass Gliese 876 d. Earth-sized or smaller planets have been detected, but as of January 2006, only around millisecond pulsars like PSR B1257+12.
OGLE-2005-BLG-390Lb's signature was first detected on August 10 2005 by observations at the Danish 1.54-m telescope at ESO La Silla Observatory in Chile. The telescope was part of a network of telescopes used by the PLANET/RoboNet gravitational microlensing campaign. Much of the follow-up observational data was gathered by 0.6-m telescope at the Perth Observatory in Western Australia.
Gravitational lensing occurs when light from a distant star is bent and magnified by the gravitational field of a foreground star. A gravitational microlensing event occurs when a planet accompanying this foreground star can cause an additional small increase in the intensity of magnified light as it passes between the background star and the observer as well.
The PLANET/RoboNet campaign regularly investigates promising microlensing event alerts that are issued by the Polish OGLE or the Japanese-New Zealand MOA survey. The observation of just such an event led to the discovery of OGLE-2005-BLG-390Lb. OGLE detected the microlensing effect produced by the star OGLE-2005-BLG-390L, and it was the PLANET team's follow up observations and analysis which uncovered evidence of the planet itself.
The PLANET team conducted close observation of the OGLE-2005-BLG-390 microlensing event over a period of about two weeks. During this series of observations, a 15% "spike" in intensity occurred, lasting approximately 12 hours long. From the intensity of the increase, and its length, the PLANET astronomers were able to derive the planet's mass, and its approximate displacement from the star.