OGLE-2005-BLG-390Lb
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OGLE-2005-BLG-390L b.
OGLE-2005-BLG-390L b (OGLE-05-390L b) is an extrasolar planet orbiting the star OGLE-2005-BLG-390L, which is situated 20,000 light years [1] away from Earth in the constellation Sagittarius, near the centre of the Milky Way galaxy. At the time of its discovery, it was the exoplanet most similar to Earth. The discovery of the planet by the Polish Optical Gravitational Lensing Experiment was announced on January 25 2006.
Physical characteristics
| Orbital characteristics | |
|---|---|
| 2.6? | |
| Eccentricity | ? |
| 3800? | |
| Inclination | ? |
| Physical characteristics | |
| Mass | 5.5 ME, 0.018 |
Mean density | ? |
GLE-05-390L b orbits around a cool red dwarf star at a distance of 2.5 astronomical units, or about 380 million kilometers, slightly less than half the distance between Jupiter and the Sun. Until this discovery, no small exoplanet had been found farther than 0.15 AU from its parent star. The planet takes approximately ten years to orbit its star.
The planet is estimated to be about five times Earth's mass (with an uncertainty factor of two). 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 53 Kelvins (-220 degrees Celsius, -364 degrees Fahrenheit). 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.
"The team has discovered the most Earthlike planet yet," [2] 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 normal star — Gliese 876 d — was about 7.5 Earth masses. Earth-sized planets have been detected, but only around dying neutron stars.
Discovery
At the time of its discovery, OGLE-2005-BLG-390L b was the most Earth-like exoplanet to be discovered. The planet was found using a network of telescopes scattered across the globe, including the Danish 1.54-meter telescope at ESO La Silla in Chile. The technique involved the phenomenon of "gravitational microlensing", in which light from a distant star is bent and magnified by the gravitational field of a foreground star. The presence of a planet around the foreground star causes light from the distant star to become momentarily brighter.
Astronomers found most of the more than 150 planets discovered so far using the Doppler technique, which detects wobbles in a star's motion caused by the gravitational pull of a planet. This method has uncovered dozens of huge worlds (with masses comparable to Jupiter), but it cannot spot small planets that are far from their stars. Microlensing, on the other hand, can detect small planets. "Microlensing should have discovered dozens of Jupiters by now if they were as common as these five-Earth-mass planets," said study co-author David Bennett.
This suggests that most of our galaxy's planets are small and rocky, a prediction which agrees with the standard model for solar system formation, known as the "core accretion" model. In this model, dust around newborn stars forms clumps that stick together and eventually become asteroids, comets and planet precursors. In this scheme, relatively few planets successfully become gas giants, and they are outnumbered by small, rocky worlds.
Unfortunately, the microlensing technique has its own shortcomings. Since star alignments are unique events, a microlensing experiment can never be repeated. Todd Henry, an astronomer at Georgia State University who was not involved in the study, said the discovery was an "intriguing result from this particular technique, but unfortunately you can't follow it up."
Many astronomers view the lack of repeatability as an acceptable trade-off, however, because thousands of star systems can be screened in a relatively short period of time compared with other techniques. "You can't learn a whole lot about the details of individual systems... but it's a wonderful alternative for learning about what the mass distribution of extrasolar planets might be and the frequency at which they occur," said David Latham, an astronomer at the Harvard-Smithsonian Center for Astrophysics who was also not part of the study.
