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Planetary News: Extrasolar Planets (2006)Scientists Discover a Very Familiar-Looking Planetary SystemBy Amir Alexander30 May 2006 Scientists searching for planets beyond the solar system have not yet found another "Earth" among the roughly 170 planets discovered so far. However, according to an article published in the May 18 issue of Nature magazine, they have found a planetary system that in many ways closely resembles our own solar system. Two years of close observations of the star HD 69830 have revealed that it is orbited by no less than three low-mass planets, with a minimum mass only 10-18 times that of Earth. Furthermore, scientists strongly suspect that HD 69830 is also host to an asteroid belt. The system around HD 69830, the authors explain, is unlike any of the other 17 planetary systems discovered so far. All other known systems are dominated by at least one giant Jupiter-sized planet, with a mass hundreds of times that of the Earth. In contrast, the newly discovered system is composed of planets of roughly equal size - all with masses similar to that of our neighbor Neptune. The innermost of the three planets orbits its star in just under 9 days, whereas the two outer ones complete each revolution in 32 days and 197 days respectively. Intriguingly, this places the outermost planet just inside the star's "habitable zone" – the region in space in which liquid water is stable.
HD 69830 is an old main sequence star 41 light years away, with a mass estimated at 86% that of the Sun. Recent observations by the Spitzer Space Telescope revealed an excess radiation from the star at the infrared part of the spectrum. The surplus heat radiation indicates the presence of tiny grains of crystalline silicates around the star, heated to around 400 degrees Kelvin (260 degrees Fahrenheit). These, in all likelihood, are the telltale signs of a dense and massive asteroid belt around the star, where repeated collisions among asteroids continuously replenish the supply of dust grains. The relatively high temperature of the particles further indicates that they are no further than 1 Astronomical Unit (AU) from the star, with each AU representing the average distance of the Earth from the Sun. An alternative explanation for the presence of the crystalline grains, the authors note, is the presence of a massive supercomet recently captured by the star into close orbit, and shedding particles at a high rate. But according to the authors, such a scenario is unlikely. Led by Christophe Lovis of the Geneva Observatory, the authors made their discovery with the ultra sensitive HARPS spectrometer, installed on the 3.6 meter (11.8 foot) telescope at the European Southern Observatory at La Silla in Chile. Using the radial velocity method, which is responsible for the discovery of almost all extrasolar planets found so far, the authors looked for the slight shifts in the spectrum of a star as it moves back and forth to the tug of an orbiting planet. According to the authors, HARPS is capable of detecting the movement of a star at speeds as low as 1 meter (3.3 feet) per second, which is about the speed of a leisurely stroll. This, they say, makes HARPS the most sensitive spectroscope of this type anywhere. HD69830 is one of a batch of stars the team has been tracking closely with HARPS for the past two years. The accumulation of data over such an extended period allowed the scientists to observe some long-term patterns which would not have been noticeable in shorter observation runs. Very quickly it became clear that the star is rocking back and forward every 9 days or so, indicating the presence of short-orbit planetary companion. The scientists then noted that the specific maxima and minima of the star's speed when rocking back and forth varied over time: sometimes the star would be moving faster towards the Earth, but slower when moving away, and sometimes the opposite was the case. Since these variations were also periodic, Lovis and his colleagues began to suspect that they indicated the presence of another planet, which tugged on the star as it orbited every 30 days or so.
When the authors then calculated the likely orbits of the two planets, they got a good fit with the data, but not quite good enough. There seemed to be another periodically recurring signal, which could not be explained by the presence of only two planets. But once a third planet with a period of around 200 days was added to the mix, things seemed to fall into place. Further calculations gave the exact orbits of the planets at 8.7, 31.6, and 197 days respectively. To determine whether such a configuration of Neptune-mass planets was likely, the authors decided to test whether the system was dynamically stable, i.e., that the planets would continue in their orbits for very long periods of time, without interfering with each other. By contrast, in an unstable system a planet's gravity would soon interfere with its neighbors' orbits, and could send them careening towards the star or out to interstellar space. To determine this, Lovis and his colleagues ran a series of computer simulations and concluded that the system was stable: the planets would remain where they were, unperturbed, for at least a billion years. But where, in this configuration, would an asteroid belt fit in? For even if the planets would not much disturb each other, any one of them was nevertheless likely to clear out the rocks and dust in its own vicinity. Such a process is, in fact, a stage in the development of a planetary system, when the relatively massive planets clear out the ancient protoplanetary disk of gas and that originally gave them birth. They could easily do the same to any remaining asteroid belt. The Spitzer observations had already determined that the asteroid belt likely resides within 1 AU of its star. Within this range, the authors found that there were two distinct regions in which an asteroid belt would survive in the long-term: one was close in, between 0.3 and 0.5 AU of the star; the other was out beyond the orbit of the outermost known planet, at 0.8 AU from the star or more. Based on the relatively high temperature of the crystalline particles detected by Spitzer, Lovis and his colleagues concluded that a close-in asteroid belt was more likely. The emerging picture is of a complex planetary system, with three low-mass planets orbiting within 1 AU of a Sun-like star, and an asteroid belt probably positioned between the second and third planets. When you add to this the fact that the third planet from HD69380 is in the habitable zone, one cannot escape a nagging feeling of deja-vu: unavoidably, the system around HD69830 reminds us of our own familiar solar system All this gives scientists much to chew on. "The planetary system around HD 69830 clearly represents a Rosetta stone in our understanding of how planets form" said Michel Mayor of the Geneva Observatory, one of the article's authors and the man who was also responsible for the discovery of the very first extrasolar planet 11 years ago. "No doubt it will help us better understand the huge diversity we have observed since the first extrasolar planet was found ." |
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