By inserting genes that turn back a cell's developmental clock, researchers are gaining insights into disease and the biology of how a cell decides its fate （跟贴有详细介绍）
2 Seeing Exoplanets
Seeing might be believing, but for scientists belief rarely depends on seeing. The right squiggles coming out of an instrument are usually enough to confirm that they have caught their quarry, however infinitesimal, insubstantial, or bizarre. Astronomers searching for planets circling other stars, however, may have been getting just a tad impatient with their progress toward their ultimate goal: recognizing a habitable, even an inhabited, planet beyond our own solar system. For that, they'll need to see their target. But all exoplanet detections had been of the squiggly variety.
Now, astronomers have seen exoplanets for the first time--a half-dozen candidates have been announced in the past few months. To some, the new observations may simply have replaced squiggles with dots. But the faint pinpricks of light from far-off worlds have captured the public's imagination and will give astronomers new clues to what those distant planets are made of and how they were formed. Key to these direct detections have been big telescopes and the latest technology to pick out a vanishingly faint planet from its host star's overwhelming glare.
Previous, indirect detections of more than 300 exoplanets had provided breakthroughs of their own. For 13 years, astronomers have been finding exoplanets using ground-based telescopes to monitor the subtle wobble a planet gravitationally induces in its star. This workhorse radial-velocity technique is especially useful for finding massive "hot Jupiters" searingly close to their star. No light is seen from the planet, however. Another method, called microlensing--in which a planet's gravity momentarily brightens a background star by bending its passing light--is particularly good for detecting planets more distant from their stars and in principle could spot lightweights with masses down to that of Earth. But microlensing is a one-off event; once the fleeting alignment with the star is over, no sign of the planet will ever be seen again.
If a planet happens to orbit across the face of its star as viewed from Earth, however, the repeated tiny dimming of the total light of the star plus the planet can reveal the presence of the planet. At the same time, starlight passing through the outer planetary atmosphere can reveal clues about composition. Already, water, methane, and--just last month--carbon dioxide have been detected in transiting exoplanets. Those compounds, plus molecular oxygen, are the key markers of an inhabited planet. But only hot Jupiters--unlikely abodes of life--are liable to transit their stars and be detected using current technology.
That leaves direct detection. The chore is simple enough: Separate the light from a planet from the light of its nearby star. The hitch is that the star is millions of times brighter than any planet, and Earth's turbulent atmosphere churns the light of star and planet together. To solve the latter problem, astronomers can move their telescopes above the atmosphere to Earth orbit. Or they can correct the incoming telescopic image using so-called adaptive optics, in which precisely controlled warping of a mirror many times a second straightens out distorted light. Coping with the vast difference in brightness between planet and star requires a coronagraph in the telescope to physically block out the star or "virtual coronagraph" software to remove starlight from the image. It also helps to search for very young and therefore still hot planets at infrared wavelengths, in which case the star-planet contrast will be much smaller.
With more than 5 years of observations using the latest technology, astronomers are suddenly busting down the doors to announce candidates for directly detected exoplanets. Published last month, the most secure--and surely the most stunning--are three objects orbiting a star called HR 8799, 128 light-years from Earth. Judged to have five to 10 times the mass of Jupiter, they orbit at least 24 to 68 times farther from their star than Earth orbits from the sun. That makes them among the most massive exoplanets discovered and by far the most distant from their star. New detection techniques typically start by finding such oddballs. These are giving theorists fits; they don't see how planets could have formed that far out.
Other direct detections came one per star. Last month, another group also reported detecting a planet of roughly three Jupiter masses orbiting the star Fomalhaut, one of the brightest stars in the sky. A third group announced a single candidate exoplanet last September but must await confirmation that it is orbiting the star rather than just passing through. And a fourth group announced late last month what would be--at eight times the sun-Earth distance from its star--the imaged planet closest to its star.
作者:admin@医学,生命科学 2011-05-29 05:11