Best View Yet of a Young Planet's Dusty Home
New images show a complex structure in the inner disk, previously unexplored in visible light, and even hint at the possible aftermath of the pulverization of a Mars-size body in a giant collision.
Using NASA's Hubble Space Telescope, a team led by UA astronomers Daniel Apai and Glenn Schneider has obtained the most detailed picture to date of a large, edge-on, gas-and-dust disk encircling the 20-million-years-old star Beta Pictoris. This gigantic disk extends more than 20 times the diameter of the planetary orbits in our solar system and has been known for its complex structure, possibly shaped by one or more massive planets.
The new images show a complex disk structure in the inner disk, previously unexplored in visible light. The images allow astronomers to study the structure of the dust disk down to the close-in orbit of the planet, which is important to test whether the planet has shaped the disk. The UA astronomers find that the inner disk structure is similar to the predictions of numerical models, in which a single planet deforms the disk.
Beta Pictoris remains the only directly imaged debris disk that has a giant planet, discovered in 2009. Because the planet's orbital period is comparatively short — estimated to be between 18 and 22 years — astronomers can see large motion in just a few years. This allows scientists to study how the Beta Pictoris disk is distorted by the presence of a massive planet embedded within the disk.
The new visible-light Hubble image traces the disk closer to the star to within about 650 million miles of the star (which is inside the radius of Saturn's orbit about the sun).
"Some computer simulations predicted a complicated structure for the inner disk due to the gravitational pull by the short-period giant planet," said Apai, an assistant professor with joint appointments in the UA Department of Astronomy/Steward Observatory and UA's Lunar and Planetary Laboratory. "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."
The gas-giant planet in the Beta Pictoris system was directly imaged in infrared light by the European 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 orbiting the star like a carousel. This means the disk's structure is smoothly continuous in the direction of its rotation on the timescale, roughly, of the accompanying planet's orbital period.
In 1984, Beta Pictoris was the very first star discovered to host a bright disk of light-scattering circumstellar dust and debris. 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 because of a very large amount of starlight-scattering dust. What's more, at 63 light-years, Beta Pictoris is closer to Earth than most of the other known disk systems.
Although nearly all of the approximately two dozen known light-scattering circumstellar disks have been viewed by Hubble to date, Beta Pictoris is the first and best example of what a young planetary system looks like, according to the researchers.
One thing astronomers recently have learned about circumstellar debris 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 systems.
"The Beta Pictoris disk is the prototype for circumstellar debris systems, but it may not be a good archetype," said co-author Schneider.
For one thing, the Beta Pictoris disk is exceptionally dusty. This may be due to recent major collisions among unseen planetary-size and asteroid-size 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 pulverization of a Mars-size body in a giant collision.
Both the 1997 and 2012 images were taken in visible light with Hubble's Space Telescope Imaging Spectrograph in its coronagraphic imaging mode. A coronagraph blocks out the glare of the central star so that the disk can be seen.
Follow Daniel Apai on "Distant Earths," his blog about exoplanets, astrobiology and astrophysics.
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University of Arizona in the News