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 M_E). 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 only 3 Earth-days, were detected very close to their stars. Unlike previously discovered "small" exoplanets, 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. Prior to the discovery, the smallest extrasolar planet found around a main sequence star — Gliese 876 d — was about 7.5 Earth masses. Earth-sized or smaller planets have been detected, but as of January 2006, only around millisecond pulsars like PSR B1257+12.

Discovery

OGLE-2005-BLG-390Lb's signature was first detected on 10 August 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 passed on to it 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-390L 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.

The paper submitted to Nature bears the names of all members of PLANET, RoboNet, OGLE, and MOA.

References