Mon, 24 Aug 2009
Planet of the Week: Planet X ?
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Up on arxiv.org today is a paper, "Title: The perihelion precession of Saturn, planet X/Nemesis and MOND", by Lorenzo Iorio.
The perihelion is the point of the planets orbit where it is closest to the Sun. Over the millennia, this point moves, or precesses, under the gravitational pull of other objects. This is a tiny effect, spotted because we have accurate measurements for Saturn thanks to measurements with the Cassini probe. (We have no such measurements further out, for Neptune and Uranus; but we have a more famous case: the Pioneer Anomaly, unexplained changes in the path of the Pioneer probes). Iorio comes up with measurements for a potential planet X to explain this; an Earth-sized planet around 80-150 AU out, up to a Jupiter-sized object at 1000 AU. (1000 times the Earth-Sun distance).I've written before about this: such a planet is remotely feasible to detect by ground-based systems (or perhaps Herschel). It would certainly help explain the architecture of the outer solar system. Perhaps we should start seriously looking for such an object ?
The Brown Dwarf possibility is also interesting; Centauri Dreams has an article on the Wide-field Infrared Survey Explorer (WISE) mission, due for November launch, thats expected to be capable of finding many cold brown dwarfs nearby.
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Thu, 20 Aug 2009
In this weeks Astronomy & Astrophysics was a paper by M. Reidemeister et al., A possible architecture of the planetary system HR 8799.
This revisits HR 8799, one of the few planetary systems seen by direct imaging, actually observing the planets.
These are believed to be three Jupiter-sized planets (of about 5, 7 and 7 times the mass of Jupiter: the exact numbers depend on the precise viewing angle from Earth; Reidmeister think this is nearly pole-on). They were discovered last year by Marois et al..Summarizing all available data, they decide that HR 8799 is less that 50 Million years old, practically still forming. Their proposed layout of the system has at least three dust rings, still coalescing and cooling. By comparison, our Solar system at that stage had formed the main planets but the four outer giants were still moving into position, working their way through the remains of a dusty, gassy disk : another 500 million years or so before Jupiter and Saturn settled into their current orbits, and bombarding the inner system as they did so, leading to the cratering we see on the moon and resurfacing Mars, Earth and Venus.
What makes the HR 8799 system so interesting is that its a good test for theories of planetary or stellar formation. Its unclear whether it formed as a 'planetary system', with the planets formed in the debris disk of the star, or as a "multiple star" system; for the latter. Modelling the formation of HR 8799 will be a good test of Alan Boss's 'Gravitational Instability' theory, vs the 'core accretion' model which has been gaining ground in recent years.
In the meantime for non-theorists, its pole-on orientation gives us a good view of planets in formation. Roll-on the clearer images of the James Webb Space Telescope.