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Dienstag, 15. Dezember 2009 um 00:00 Uhr |
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A super-Earth is defined as a planet between one and ten times the mass of the Earth (i.e. still less massive than Uranus and Neptune) - and three new such cases have been announced in December in rapid succession. For two of them, orbiting the stars 61 Vir and HD 1461, only lower mass limits of 5.5 and 7.5 Earth masses can be given as their orbital inclination is not known; these planets were detected through tedious radial velocity measurements.
They are remarkable for orbiting to stars that are very much like our Sun: This may indicate that low-mass planets may be common around nearby stars. And one researcher involved now boldly predicts that "the discovery of potentially habitable nearby worlds may be just a few years away." 61 Vir is one of the best twins of the Sun in terms of age, mass and other properties and has actually at least three planets: They range in mass from roughly five to 25 times the mass of Earth and orbit the star in four, 38 and 124 days.
The planet of GJ1214 has the advantage that we know its mass - 6.5 Earths - because it transists the disk of his star which is a red dwarf (spectral class M). This is actually how it was discovered: by an array of amateur-style telescopes monitoring the brightness of 2000 such stars. GJ1214b orbits its star once every 38 hours at a distance of only 2 million km. Astronomers estimate the planet's temperature to be about 200°C: Although warm as an oven, it is still cooler than any other known transiting planet because it orbits a very dim star. Since GJ1214b crosses in front of its star, its radius could be measured as 2.7 times that of Earth: This makes GJ1214b one of the two smallest transiting worlds astronomers have discovered (the other being famous CoRoT-7-b). The resulting density suggests that GJ1214b is composed of about three-fourths water and other ices, and one-fourth rock. There are also tantalizing hints that the planet has a gaseous atmosphere.
You can find Daniel Fischers Cosmic Mirror here. |
