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Planetary News: Extrasolar Planets (2006)Discovery of Small Distant Planet Suggests Many "Earths" Not Far BehindBy Amir Alexander26 January, 2006 The latest addition to the strange menagerie of planets outside our solar system resides in the Milky Way's galactic bulge, 20,000 light years away. It orbits a red dwarf about one fifth the mass of our Sun, and its surface temperature hovers around 50 kelvins (–220 degrees Celsius or –370 degrees Fahrenheit). Why is it then that scientists consider its discovery to be a major step towards the detection of a second Earth? One reason is the new planet's mass. At around five times the mass of Earth, the new planet, designated OGLE–2005-BLG-390Lb, is the lowest-mass planet ever detected outside the solar system. And when one considers that the vast majority of the approximately 170 extrasolar planets detected so far have been Jupiter-like gas giants, dozens or hundreds of times the mass of Earth, the discovery of a planet of only five Earth masses is indeed good news. But there is more: astronomers believe that the detection of OGLE–2005-BLG-390Lb, announced in the January 26 issue of the journal Nature, is a strong indication that terrestrial-type planets are plentiful, probably much more so than gas giants. And if this is so, then it is quite possible that among these numerous rocky planets there are some that resemble the Earth.
Bending StarlightThe source of this optimism, based as it is on a single discovery, has its roots in the unique method used to detect the planet -- microlensing. The vast majority of the extrasolar planets discovered so far have been detected by the spectroscopic method -- the slight changes in the light spectrum of a star as it wobbles to the tug of an orbiting planet. Spectroscopy, also known as the "radial velocity" method, is most sensitive to massive planets orbiting close to their stars. Because of its limited sensitivity, and the fact that it depends on a planet completing several orbits, spectroscopy cannot detect small rocky planets orbiting far from their star. This explains why most of the planets found so far are "hot Jupiters." Even if Earth-like planets are, in fact, abundant in our galaxy, spectroscopy would never find them. Microlensing operates by a completely different principle, based on Einstein's General Theory of Relativity. According to Einstein, when the light emanating from a star passes very close to another star on its way to an observer on Earth, the gravity of the intermediary star will slightly bend the light rays from the source star, causing the two stars to appear farther apart than they normally would. This effect was used by Sir Arthur Eddington in 1919 to provide the first empirical evidence for General Relativity. Now, if the source star is positioned not just close to the intermediary star when seen from Earth, but precisely behind it, this effect is doubled. Light rays from the source star pass on both sides of the intermediary, or "lensing" star. Since both light streams are bent by the lensing star's gravity, the source star should appear doubled from Earth -- once on each side of the lensing star. In reality, even the most powerful Earth-bound telescope cannot resolve the separate images of the source star and the lensing star between them, seeing instead a single giant disk of light, known as the "Einstein disk," where a star had previously been. The resulting effect is a sudden dramatic increase in the brightness of the lensing star, by as much as 1,000 times. This typically lasts for a few weeks or months before the source star moves out of alignment with the lensing star and the brightness subsides. While this is the normal pattern of a microlensing event, things are substantially different when the lensing star has a smaller companion. If a planet is positioned close enough to the lensing star so that it crosses one of the two light streams emanating from the source star, the planet's own gravity bends the light stream and temporarily produces a third image of the source star. When measured from Earth, this effect appears as a temporary spike of brightness, lasting several hours to several days, superimposed upon the regular pattern of the microlensing event. For planet hunters, such spikes are the telltale signs of the presence of a planet.
The microlensing event that led to the discovery of the new planet was first observed by the Poland-based international group OGLE, the Optical Gravitational Lensing Experiment, led by Andrzej Udalski of Warsaw University. OGLE makes use of the 1.3 meter "Warsaw" telescope at Las Campanas, Chile to search for microlensing events, which are rare and unpredictable. Every night the telescope is pointed toward the same dense field of 100 million stars in the vicinity of the galactic bulge, while the telescope's complex CCD cameras note any change in brightness of any point in the star-field. Every year OGLE detects around 500 microlensing events, and at first, the event observed on July 11, 2005 seemed no different than others. A star was passing in front of another as seen from Earth, lensing its light. Whenever OGLE detects a microlensing event, it contacts a network of telescopes that specialize in searching for signs of the presence of a planet. The networks, known as PLANET (Probing Lensing Anomalies) and Robonet, include 1 and 2 meter telescopes in La Silla (Chile), Hobart (Tasmania, Australia), Perth (Australia), Boyden (South Africa), Sutherland (Australia), La Palma (Spain), and Haleakala (Hawaii). The networks tracked the event carefully for several weeks, as the star's brightness peaked on July 31 and then began to wane. Then, on August 10, PLANET member Pascal Fouque, observing at the Danish 1.54 meter European Southern Observatory telescope at La Silla, noticed a slight disturbance in the smooth light graph. Over the next 12 hours, telescopes at Perth, Australia and Las Campanas, Chile confirmed Fouque's observation: the lensing star had a planet.
Microlensing Earths?OGLE –2005-BLG-390Lb is the third planet discovered by microlensing. The previous two are gas giants similar in type to the ones detected by spectroscopy, only much farther away (the planets detected through spectroscopy are all within 100 light years of Earth). But OGLE –2005-BLG-390Lb is different: the direct evidence of its microlensing light-curve, combined with statistical calculation based on the type of stars that are likely to be found in that region of the galaxy, tell astronomers a great deal about this distant world. At five Earth masses it is smaller than any previously discovered extrasolar planet and is almost certainly composed of rock and ices. It takes about 10 years to complete each orbit around its star at an average distance of 2.6. astromonical units (AU), each AU being the average distance of Earth to the Sun. Because of this distance, and because its star is only 20-25% the mass of the Sun and much cooler, OGLE –2005-BLG-390Lb is extremely cold -- only 50 kelvins above absolute zero. This makes it not only the lowest mass planet ever detected outside the solar system but also the coldest. But perhaps the most significant aspect of planet OGLE –2005-BLG-390Lb is that it was discovered at all. According to the paper published in Nature, terrestrial-sized planets are very difficult to detect through microlensing. Since the microlensing signal of a planet is proportionate to its mass, the paper's authors estimate that it is 50 times more likely to detect a gas giant than a terrestrial planet. The fact that it was found and is one of only 3 planets ever detected through microlensing might therefore be significant: it seems to suggest that terrestrial-type planets are far more common than their massive gas-giant cousins.
"The finding means that Earth-mass planets are not that uncommon," said Kailash Sahu of the Space Telescope Science Institute who co-authored the Nature article. "If we found one, there must be more." Added Jean-Philippe Beaulieu of the Institute d'Astrophysique de Paris, the article's lead author: "the discovery of a five Earth-mass planet -- though much harder to detect than more massive ones -- is a strong hint that these low mass objects are very common." OGLE –2005-BLG-390Lb will never be seen again. Once the rare microlensing event was over, that cold lonely world receded back into the dense star field of the galactic bulge from which it had briefly emerged. But OGLE –2005-BLG-390Lb will not be forgotten. Those few hours on August 10, 2005, when it was observed from our planet, have left a deep mark on Earthlings' understanding of their place in the universe. For if scientists' estimates prove true, OGLE –2005-BLG-390Lb is but the harbinger of many more terrestrial and Earth-like planets waiting to be discovered. |
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