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Physics News Update -- Number 658, October 21, 2003
[ from sci.physics ] Physics News Update -- Number 658, October 21, 2003 by Phillip F. Schewe, Ben Stein, and James Riordon Direct Imaging of Extrasolar Planets Direct imaging of extrasolar planets might be easier than astronomers thought, a new study shows. Evidence for the existence of planets around nearby stars comes mostly in the form of tiny Doppler shifts in the star's spectra as one or more orbiting planets tug on the star. In a few cases the transit of a planet across the face of a star can be detected from a minute dimming of the star's emission. These approaches are indirect. The problem of imaging extrasolar planets directly is that the planet is far outshone by the nearby star. One proposed way of getting around this glare problem is to use nulling interferometry. In ordinary interferometry the light waves from two or more telescopes are added together in such a way that the resulting observation is equivalent to one made with a single telescope with a much wider diameter than any of the component scopes. But instead of maximizing the composite signal from the distant object, it can be minimized (see past Update item). By doing this, a weaker nearby object, like a planet, might suddenly emerge from what had been irrepressible glare. In a new paper, William Danchi (Goddard Space Flight Center) and his colleagues have performed extensive studies of the interferometry nulling technique, especially the way in which increasing the precision of component detectors increases the degree to which the star's image is truly nulled, the better to see either smaller planets or planets that are closer in toward their parent star. Both the smaller and closer criteria are pertinent when searching for earth-like extrasolar planets. Danchi (301-286-4586) says that the new study shows that with the right configuration of detectors, the spatial resolution of the overall interferometer (which is related to its size) can be less than have been thought, an important consideration for what would be an orbiting space-based observatory. Danchi envisions that a first-round nulling interferometer using two half-meter-sized telescopes separated by a 12-meter boom could observe already discovered extrasolar planets (including spectroscopic studies of atmospheres). With a later, larger version of the nulling interferometer one could hope to search for earthlike planets harboring characteristic molecules such as ozone, and/or oxygen, plus carbon dioxide, water, and methane. Detecting these molecules could help determine the age of the planet and what life processes might be occurring there. (Danchi, Deming, Kuchner, and Seager, Astrophysical Journal Letters, 1 November 2003; preprint astro-ph/0309361) Evidence for an Unusually Active Sune Evidence for an unusally active sun since the 1940s comes from a new estimation of sunspots back to the ninth century. Many natural phenomena such as solar radiance and sunspots vary according to natural cycles. The variation is subject also to additional fluctuations (arising from as yet unexplained effects) which complicate any study which examines only a short time interval. The longer the baseline, the more confident one can be in drawing out historical conclusions. In the case of sunspots, the direct counting goes back to Galileo's time, around 1610. But earlier sunspot activity can be deduced from beryllium-10 traces in Greenland and Antarctic ice cores. The reasoning is as follows: more sunspots imply a more magnetically active sun which then more effectively repels the galactic cosmic rays, thus reducing their production of Be-10 atoms in the Earth's atmosphere. Be-10 atoms precipitate on Earth and can be traced in polar ice even after centuries. Using this approach, scientists at the University of Oulu in Finland (Ilya Usoskin, 358-8-553-1377) and the Max Planck Institute in Katlenburg-Lindau in Germany have reconstructed the sunspot count back to the year 850, nearly tripling the baseline for sunspot studies. They conclude that over the whole 1150 year record available, the sun has been most magnetically active (greatest number of sunspots) over the recent 60 years. (Usoskin et al., Physical Review Letters, upcoming article) Can a Single Gas Bubble Sink a Ship? Yes, according to an experimental and theoretical analysis performed by researchers at Monash University in Australia (David May and Joseph Monaghan). The ocean floor contains vast quantities of methane gas hydrates, ice-like crystals of methane surrounded by cages of water molecules. If disturbed, these methane gas hydrates can erupt from the floor and rise to the surface as gas bubbles, some of which can be very large. Copious amounts of methane hydrates exist in the North Sea, which lies in between the United Kingdom and continental Europe. At a large eruption site in the North Sea known as the Witches Hole off the coast of Aberdeen, a sonar survey recently uncovered the presence of a sunken vessel, but the cause of the wreck remains undetermined. Simple experiments have previously shown that many small bubbles rising to the surface could sink a cylinder of water (and conceivably a ship), by causing a loss of buoyancy (Denardo et al., American Journal of Physics, October 2001). But could a single large gas bubble do the trick? The Monash researchers investigated this possibility in a simple, roughly two-dimensional system. Trapping water between a pair of vertical glass plates, and launching single gas bubbles from the bottom, they used a video camera to observe a single large bubble's effect on a small piece of acrylic shaped like the hull of a boat. Along with numerical simulations of this scenario, the experiments showed that the bubble could sink the ship, if the bubble's radius was comparable to or greater than the ship's hull. Sinking would occur because a mound of water formed above the bubble as it approached the surface. As the bubble reached the surface, it would temporarily lift the ship. However, water in the mound would then flow off the sides of the bubble, forming deep troughs at either side, and the water flow would carry the boat to one of the troughs. In addition, the eventual rupture of the bubble would create high-velocity jets of fluid that moved into the troughs, creating vortices that further pulled down the boat. The researchers say that their numerical simulations could test other scenarios, including those involving multiple large bubbles, more realistic boats, and ultimately a full three-dimensional simulation. (American Journal of Physics, September 2003). Physics News Update is a digest of physics news items arising from physics meetings, physics journals, newspapers and magazines, and other news sources. Subscriptions are free as a way of broadly disseminating information about physics and physicists. Feel free to post it where others can read it; please credit the American Institute of Physics. Physics News Update appears approximately once a week. Questions? Contact the editors at . |
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