Free Astronomy Magazine May-June 2015

SPACE CHRONICLES T his is a color composite image of the large, edge-on, gas-and-dust disk encircling the 20-million-year-old star Beta Pictoris. The image shows a curious asymmetry in the dust and gas distribution. This may be due to a planetary col- lision within the disk, which may have pulverized the bodies. The new Hubble Space Telescope Imaging Spectrograph visible-light image (shown in blue) traces the disk in closer to the star to within about 650 million miles of the star (which is inside Saturn's orbit about the Sun). Radio data from the Atacama Large Millimeter/submillimeter Array (ALMA) shows the dust (1.3 millimeter is colored green) and carbon monoxide gas (colored red). [Hubble data: NASA, ESA, and D. Apai and G. Schneider (University of Arizona); ALMA Data: NRAO and W.R.F. Dent (ALMA, Santiago, Chile)] disks is that their structure, and amount of dust, is incredibly diverse and may be related to the locations and masses of planets in those sys- tems. "The Beta Pictoris disk is the prototype for circumstellar debris systems, but it may not be a good archetype," said co-author Glenn Schneider of the University of Ari- zona. For one thing the Beta Pictoris disk is exceptionally dusty. This may be due to recent major collisions among unseen planetary-sized and aster- oid-sized bodies embedded within it. In particular, a bright lobe of dust and gas on the southwestern side of the disk may be the result of the pul- verization of a Mars-sized body in a giant collision. Both the 1997 and 2012 images were taken in visible light with Hubble's Space Telescope Imaging Spectro- graph in its coronagraphic imaging mode. A coronagraph blocks out the glare of the central star so that the disk can be seen. ly, of the accompanying planet's or- bital period. In 1984 Beta Pictoris was the very first star discovered to host a bright disk of light-scattering cir- cumstellar dust and debris. Ever since then Beta Pictoris has been an object of intensive scrutiny with Hubble and with ground-based telescopes. Hubble spectroscopic observations in 1991 found evidence for extrasolar comets frequently falling into the star. The disk is easily seen because it is tilted edge-on and is especially bright due to a very large amount of starlight-scattering dust. What's more, Beta Pictoris is closer to Earth (63 light-years) than most of the other known disk systems. Though nearly all of the approxi- mately two-dozen known light-scat- tering circumstellar disks have been viewed by Hubble to date, Beta Pic- toris is the first and best example of what a young planetary system looks like, say researchers. One thing astronomers have recent- ly learned about circumstellar debris the radius of Saturn's orbit about the Sun). "Some computer simulations predic- ted a complicated structure for the inner disk due to the gravitational pull by the short-period giant planet. The new images reveal the inner disk and confirm the predicted structures. This finding validates models, which will help us to deduce the presence of other exoplanets in other disks," said Daniel Apai of the University of Arizona. The gas-giant planet in the Beta Pictoris system was directly im- aged in infrared light by the Euro- pean Southern Observatory's Very Large Telescope six years ago. When comparing the latest Hubble images to Hubble images taken in 1997, astronomers find that the disk's dust distribution has barely changed over 15 years despite the fact that the entire structure is or- biting the star like a carousel. This means the disk's structure is smoothly continuous in the direction of its rotation on the timescale, rough- n