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Hubble Telescope Sees Planet Orbiting Distant Star

By | November 17, 2008

      The Hubble Space Telescope for the first time snapped a visible-light picture of a planet circling another star, NASA announced.

      The planet is called Formalhut b.

      While the Hubble and other observers have detected exoplanets circling other stars previously, they often have done so through indirection, such as noting a slight wobble in a star that can be explained by the gravitational pull of a planet circling the star.

      This latest find, actually seeing visible light reflected from another planet outside the solar system, is just the sort of contribution Hubble can make that prompted Congress to fund an extra space shuttle mission to rescue and reburbish the orbiting observatory, so it will last into the next decade, when the James Webb Space Telescope will be launched.

      A major legislative force that won the Hubble rescue mission by Space Shuttle Atlantis next year is Sen. Barbara A. Mikulski (D-Md.), who chairs the Senate Appropriations Committee commerce, justice, science and related agencies subcommittee that provides funds for NASA. She has been a tireless fighter for NASA, and in recent years has sought an extra $1 billion for the space agency to compensate it for costs of recovering from the Space Shuttle Columbia mission.

      "Hubble continues to amaze us," Mikulski said. "This discovery is the first time we’ve ever gotten a picture of a planet orbiting a distant star."

      If this is what Hubble can do just after recovering from a systems glitch that for weeks sidelined the orbiting telescope, then think what an improved Hubble can do, she added.

      "I share the worldwide excitement about the upcoming servicing mission to extend Hubble’s life and give it an even more powerful view into the universe," she said. "Hubble is the telescope that could, and its best years are still ahead."

      NASA described the location of this newly observed planet and its sun.

      That sun, Formalhaut, is bright in the southern sky of Earth, and is relatively close star, just 25 light-years away in the constellation Piscis Australis, or the "Southern Fish."

      Fomalhaut has been a candidate for planet hunting ever since an excess of dust was discovered around the star in the early 1980s by an American space asset, the Infrared Astronomy Satellite, IRAS.

      In 2004, the coronagraph in the High Resolution Camera on the Hubble Advanced Camera for Surveys produced the first-ever resolved visible-light image of the region around Fomalhaut. It clearly showed a ring of protoplanetary debris approximately 21.5 billion miles across and having a sharp inner edge.

      This large debris disk is similar to the Kuiper Belt, which encircles the solar system and contains a range of icy bodies from dust grains to objects the size of dwarf planets, such as Pluto.

      Hubble astronomer Paul Kalas, of the University of California at Berkeley, and team members proposed in 2005 that the ring was being gravitationally modified by a planet lying between the star and the ring’s inner edge.

      Circumstantial evidence came from Hubble’s confirmation that the ring is offset from the center of the star. The sharp inner edge of the ring is also consistent with the presence of a planet that gravitationally shepherds ring particles. Independent researchers have subsequently reached similar conclusions.

      Finally, Hubble actually photographed a point source of light lying 1.8 billion miles inside the ring’s inner edge. These results are being reported in the Nov. 14 issue of Science magazine.

      "Our Hubble observations were incredibly demanding. Fomalhaut b is 1 billion times fainter than the star. We began this program in 2001, and our persistence finally paid off," Kalas says.

      "Fomalhaut is the gift that keeps on giving. Following the unexpected discovery of its dust ring, we have now found an exoplanet at a location suggested by analysis of the dust ring’s shape. The lesson for exoplanet hunters is ‘follow the dust,’" said team member Mark Clampin of Goddard Space Flight Center in Greenbelt, Md.

      Observations taken 21 months apart by the ACS coronagraph show that the object is moving along a path around the star, and is therefore gravitationally bound to it. The planet is 10.7 billion miles from the star, or about 10 times the distance of the planet Saturn from our sun.

      Formalhut b is brighter than expected for an object of three Jupiter masses. Jupiter is the largest planet in the solar system, where Earth is a moderate-sized planet. One possibility is that it has a Saturn-like ring of ice and dust reflecting starlight. The ring might eventually coalesce to form moons. The ring’s estimated size is comparable to the region around Jupiter and its four largest orbiting satellites.

      Kalas and his team first used Hubble to photograph Fomalhaut in 2004, and made the unexpected discovery of its debris disk, which scatters Fomalhaut’s starlight. At the time they noted a few bright sources in the image as planet candidates. A follow-up image in 2006 showed that one of the objects is moving through space with Fomalhaut but changed position relative to the ring since the 2004 exposure. The amount of displacement between the two exposures corresponds to an 872-year-long orbit as calculated from Kepler’s laws of planetary motion.

      Future observations will attempt to see the planet in infrared light and will look for evidence of water vapor clouds in the atmosphere.

      This would yield clues to the evolution of a comparatively newborn 100-million-year-old planet. Astrometric measurements of the planet’s orbit will provide enough precision to yield an accurate mass.

      The James Webb Space Telescope, scheduled to launch in 2013, will be able to make coronagraphic observations of Fomalhaut in the near- and mid-infrared. Webb will be able to hunt for other planets in the system and probe the region interior to the dust ring for structures such as an inner asteroid belt

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