The disk is fainter than the star because, at the visible wavelengths measured, its dust only reflects light. To see the faint disk, astronomers used Hubble's Advanced Camera for Surveys' coronagraph, which blocked the light from Beta Pictoris. The images clearly show a distinct secondary disk that is tilted by about four degrees from the main disk. The secondary disk is visible out to roughly 24 billion miles from the star, and probably extends even farther. The finding appears in the June 2006 issue of the Astronomical Journal.
The best explanation for the observations is that a suspected unseen planet, up to 20 times the mass of Jupiter and in an orbit within the secondary disk, is using gravity to sweep up material from the
primary disk.
"The Hubble observation shows that it is not simply a warp in the dust disk but two concentrations of dust in two separate disks," said lead astronomer David Golimowski of Johns Hopkins University in Baltimore. "The finding suggests that planets could be forming in two different planes. We know this can happen because the planets in our solar system are typically inclined to Earth's orbit by several degrees.
Perhaps stars forming more than one dust disk may be the norm in the formative years of a star system."
Computer models by David Mouillet and Jean-Charles Augereau of Grenoble Observatory in France suggest how a secondary dust disk can form. A massive planet in an inclined orbit gravitationally attracts
small bodies of rock and/or ice, called planetesimals, from the main disk, and moves them into an orbit aligned with that of the planet.
These perturbed planetesimals then collide with each other, producing the tilted dust disk seen in the new Hubble images. "The actual lifetime of a dust grain is relatively short, maybe a few hundred thousand years," Golimowski said. "So the fact that we can still see these disks around a 10- to 20-million-year-old star means that the dust is being replenished by collisions between planetesimals." |
