Beta Pictoris Disk Hides Giant Elliptical Ring System

Date: January 15, 2000

The planetary dust disk around the star Beta Pictoris is dynamically "ringing like a bell," according to astronomers investigating NASA Hubble Space Telescope images. The "clapper" is the gravitational wallop of a star that passed near Beta Pictoris some 100,000 years ago.

The surprising findings, presented at the 195th Meeting of the American Astronomical Society, show that a close encounter with a neighboring star can severely disrupt the evolution and appearance of thin disks, which are the nurseries of planetary systems. Similar fly-bys of our solar system long ago may have reshuffled the comets that now populate our Oort cloud and Kuiper belt.

Discovered in 1983, the dust disk around the nearby star Beta Pictoris — long suspected to harbor a planetary system — has puzzled astronomers because it contains more dust grains than any other comparable system. Also, the dust spreads over a huge 65-billion-mile-diameter area. Yet, one side of the disk is 20 percent longer and thinner than the other side.

In these latest findings, Hubble astronomers carefully studied the appearance of the disk using 10 years of archival data from the Hubble Space Telescope and from ground-based telescopes in Hawaii and Chile. Hidden within the densest part of the disk are clumps of dust that are present only on the long, thin side of the disk. Because the disk is tilted edge-on to our line-of-sight, the astronomers inferred that the clumps might represent rings if the disk was viewed face-on. They hypothesized that these rings must be highly elliptical if they appear only on one side of the disk, and this could arise if another massive object, like a passing star, recently disturbed the entire system.

To test their ideas the researchers asked theorist John Larwood of Queen Mary and Westfield College (London, United Kingdom) to create a computer simulation of a quiescent disk made of one million test particles orbiting a virtual star. The simulation explored what would happen if another star zipped by it in a near-collision trajectory. In the simulation, the gravity of the passing star rearranged the orbit of each particle, setting up an elliptical ring system 100,000 years after the almost catastrophic event. The model also reproduced the 20 percent asymmetry in the disk, which has mystified astronomers since the Beta Pictoris disk was first seen 16 years ago.

The astronomers are continuing their detective work, searching for the intruder star among 186 suspects near Beta Pictoris. Their simulations predict it might be only a fraction of the mass of our Sun (a class called an M-dwarf star). The present results will be published in a future issue of Astrophysical Journal Letters.

How do the images show that Beta Pictoris was blindsided by a passing star?

Hubble astronomers carefully studied the appearance of the disk using 10 years of archival data from the Hubble telescope and from ground-based telescopes in Hawaii and Chile. Hidden within the densest part of the disk are clumps of dust that are present only on the long, thin side of the disk. (One side of the disk is 20 percent longer and thinner than the other side.) Because the disk is tilted edge-on to our line-of-sight, the astronomers inferred that the clumps might represent rings if the disk were viewed face-on. They hypothesized that these rings must be highly elliptical if they appear only on one side of the disk, and this could arise if another massive object, like a passing star, recently disturbed the entire system.