Our discovery of the interesting disk around Beta Pictoris dates back all the way to 1984, marking the first time a star was known to host a circumstellar ring of dust and debris. But it’s interesting how far back thinking on such disks extends. Immanuel Kant’s Universal Natural History and Theory of the Heavens (1755) proposed a model of rotating gas clouds that condensed and flattened because of gravity, one that would explain how planets form around stars. Pierre-Simon Laplace developed a similar model independently, proposing it in 1796, after which the idea of gaseous clouds in the plane of the disk continued to be debated as alternative theories on planet formation emerged.
Today we can view debris disks directly and learn from their interactions. Out of the Beta Pictoris discovery have grown numerous observations including the new visible-light Hubble images shown below. The beauty of this disk is that we see it edge-on and, because of the large amount of light-scattering dust here, we see it very well indeed. Beta Pictoris, a young star about twenty million years old, is a relatively close 63 light years from Earth. It also offers the only directly imaged debris disk that is known to have a giant planet, imaged in infrared wavelengths by the European Southern Observatory’s Very Large Telescope in 2009.
Image: The photo at the bottom is the most detailed picture to date of a large, edge-on, gas-and-dust disk encircling the 20-million-year-old star Beta Pictoris. The new visible-light Hubble image traces the disk in closer to the star to within about 1 billion kilometers of the star (which is inside the radius of Saturn’s orbit about the Sun). When comparing the latest images to Hubble images taken in 1997 (top), 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. The Hubble Space Telescope photo has been artificially colored to bring out detail in the disk’s structure. Credit: NASA, ESA, and D. Apai and G. Schneider (University of Arizona).
As the paper on this work notes, the new images give us the most detailed view of the disk at optical wavelengths that we’ve ever had, with the opportunity as in the image above to study its characteristics over a fifteen-year period. This is helpful because the estimated orbital period of the planet here is between 18 and 22 years, giving astronomers the ability to study a large degree of planetary and disk motion in a relatively small timeframe. The comparison shows that the dust distribution has changed little over the past fifteen years despite the disk’s rotation. A key issue is how the disk is distorted by the presence of the massive planet embedded within it.
The new high-contrast images provide an inner working angle that is smaller than earlier images by a factor of 2, allowing astronomers to image the disk at the location where the gas giant planet was located in 2012. Changes in brightness within the disk indicate an asymmetry that may be the mark of an inner inclined disk projecting into the outer disk material. The image below combines data from Hubble and the ALMA array, highlighting the dust and gas asymmetry.
Image: This is a color composite image of the disk encircling Beta Pictoris. The image shows a curious asymmetry in the dust and gas distribution. This may be due to a planetary collision within the disk, which may have pulverized the bodies. 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). Credit for Hubble Data: NASA, ESA, D. Apai and G. Schneider (University of Arizona). Credit for ALMA Data: NRAO and W.R.F. Dent (ALMA, Santiago, Chile).
The disk asymmetry issue will be explored by future Hubble work as well as by observations from the James Webb Space Telescope, which should give us a better indication of planet/disk interactions in the system. There is much to learn here in relation to disk warping and the origins of the planet’s orbital inclination, a tilt from the main disk that previous work had estimated at about 1 degree. The paper refines this estimate:
The warped disk – as seen in projection – subtends an angle larger than the best-fit orbital inclination of β Pic b, suggesting that planetesimals may be perturbed to higher inclinations than that of the perturbing giant planet. This finding is consistent with the predictions of dynamical simulations of a planetesimal system influenced by secular perturbations of a planet on inclined orbits… The fact that the warp is seen at angles 4◦, would taken on face value, then suggests that β Pic b’s inclination is – within uncertainties – underestimated by current measurements and it may be close to ∼ 2◦.
Image: Key structures in the β Pic system, as derived from multi-wavelength imaging. Credit: Daniel Apai et al. (Figure 15 from the paper).
We need to learn how the massive gas giant in the Beta Pictoris system ended up with an inclined orbit, and what has caused the asymmetry in the disk itself. Learning how the process works around this nearby star will help us detect the evidence of exoplanets in other circumstellar disks. While we continue to use the dusty Beta Pictoris system as the model for debris disks around young stars, we’re learning that the structure of disks may be intimately related to the planets moving within them. Thus each circumstellar disk will likely have its own signature. “The Beta Pictoris disk is the prototype for circumstellar debris systems, but it may not be a good archetype,” says co-author Glenn Schneider (University of Arizona).
The paper is Apai et al., “The Inner Disk Structure, Disk-Planet Interactions, and Temporal Evolution in the β Pictoris System: A Two-Epoch HST/STIS Coronagraphic Study,” in press at the Astrophysical Journal (preprint).