Astronomers Have Found A New Twist In A Suspected Protoplanetary Disk

A telltale new warp uncovered in a vast, thin disk of dust encircling the star Beta Pictoris may be caused by the gravitational tug of a bypassing star or companion brown dwarf, say Hubble Space Telescope astronomers. These conclusions are based on unprecedented detail in Hubble images taken with the Wide Field Planetary Camera 2, which reveal the dim outermost reaches of the disk, which are 7 billion miles from the central star.

Though other Hubble teams have seen a warp in the inner edge of the disk and attributed it to the gravitational tug of unseen planets, the new WFPC2 images show that the warp in the outer edge of the disk is too great to be easily explained by the effects of planets, say researchers.

These results were presented at the 191st meeting of the American Astronomical Society in Washington, DC, by team leader Al Schultz of Computer Sciences Corporation (CSC) at the Space Telescope Science Institute in Baltimore, MD, and Fred Bruhweiler of The Catholic University of America (CUA) in Washington DC. (Other team members include Cherie Miskey, Brendan Smith, Jeff Silvis and Michael DiSanti of CUA, Helen Hart of CSC/STScI, Glenn Schneider of Steward Observatory, in Tucson, and Kent Reinhard of Doane College in Nebraska).

According to Bruhweiler, "The distortions we are seeing may have been caused by the passing of a nearby star within the past few 100 million years since the disk was formed. The culprit could easily be a thousand light-years away by now. We probably will never know who did it."

On the other hand, team lead Al Schultz favors the idea that the warp could be caused by a small faint brown dwarf star which may be circling Beta Pictoris at large distances. He suggests a search for faint stellar companions might find such a star.

Schultz and Bruhweiler agree their findings do not necessarily rule out the presence of one or more planets circling Beta Pictoris at closer distances.

Previous Hubble observations, as well as new images using the Space Telescope Imaging Spectrograph aboard Hubble, show warping or bending of the disk close to Beta Pictoris at distances within 750 million miles (80 astronomical units) of the star.

In 1996, Chris Burrows of the Space Telescope Science Institute originally proposed this warping could be due to a massive planet orbiting Beta Pictoris at angles out of the plane of the disk. This was proposed to explain the warp in the disk on opposite sides of the star, like a twist in an airplane propeller.

A Young Planetary System?

Located 60 light-years away in the southern constellation Pictor the star Beta Pictoris has been considered the best example of how a star would be expected to look if it possessed a planetary system still forming. "The new Hubble pictures show many new indications that this disk may be the outer reaches of a solar system around Beta Pictoris," said Bruhweiler, "It could be what our solar system looked like four billion years ago."

This huge dust disk seems to have an analogue in our own solar system and appears to be similar to the primordial disk dating from the time of the formation of the solar system. The comets of our solar system have their origins in a similar disk or cloud still surrounding the Sun.

Astronomers theorize that when the solar system formed some 4.6 billion years ago, all the gas and dust quickly settled into a flat disk. This disk is thought to have been quite large, some 200,000 times the distance between the Earth and the Sun, and extended a significant distance to the nearest star. Over time, the orbits of dust and comets in the extreme outreaches of this disk, were transformed by the faint tugs of gravity from passing nearby stars. The successive tugs of these passing stars altered the shape of the disk until the outermost comets of this disk formed an almost spherical, or possibly even football shaped, halo around the Sun (called the Oort's Cloud after the Dutch astronomer who hypothesized its existence). The inner region of the solar system's originally flat disk has been perturbed very little by the gravitational tug of nearby stars and still seems to have remained essentially flat. This inner region is called the Kuiper Belt.

"What is so surprising is that it appears that we are looking at the disk of Beta Pictoris at an angle that is almost exactly edge-on. The probability of this happening is very small," said team member Helen Hart. The images show a sharp, bright, straight ridge extending over the entire length of the disk, as well as the increased thickness or "flaring" of the disk close in toward the star.