NASA's Hubble Space Telescope has provided strong evidence for the existence of a roughly Jupiter-sized planet orbiting the star Beta Pictoris.
Detailed Hubble images of the inner region of the 200-billion mile diameter dust disk encircling the star reveal an unexpected warp. Researchers say the warp can be best explained as caused by the gravitational pull of an unseen planet.
The suspected planet would dwell within a five-billion mile wide clear zone in the center of the disk. This zone has long been suspected of harboring planets that swept it clear of debris, but the Hubble discovery provides more definitive evidence that a planet is there. (Alternative theories suggest the clear zone is empty because it is too warm for ice particles to exist.)
"We were surprised to find that the innermost region of the disk is orbiting in a different plane from the rest of the disk," says Chris Burrows (Space Telescope Science Institute, Baltimore, Maryland, and the European Space Agency) who is presenting his results at the meeting of the American Astronomical Society in San Antonio, Texas. As he analyzed Hubble images, taken in January 1995 with the Wide Field Planetary Camera 2, Burrows discovered an unusual bulge in the nearly edge-on disk, which was mirrored on the other side of the star. "Such a warp cannot last for very long," says Burrows. "This means that something is still twisting the disk and keeping out of a basic flat shape."
"The presence of the warp is strong though indirect evidence for the existence of planets in this system. If Beta Pictoris had a solar system like ours, it would produce a warp like the one we see." Burrows concludes, "The Beta Pictoris system seems to contain at least one planet not too dissimilar from Jupiter in size and orbit. Rocky planets like Earth might circle Beta Pictoris as well. However, there is no evidence for these yet. Any planet will be at least a billion- times fainter than the star, and presently impossible to view directly, even with Hubble."
An alterative explanation of the warp is that the disk could have been perturbed by a passing star However this is very unlikely because only the inner region of the disk is affected. Burrows estimates that there is a one in 400,000 chance for Beta Pictoris to have such a close encounter with another star. "Though Beta Pictoris is probably at least 100 million years old, other explanations for the warp do not allow it to last for very long."
The size of the warp allows Burrows to roughly measure the mass of the orbiting body. "It must lie well within the warp, probably within the clear zone that exists around Beta Pictoris." On the other hand, he points out, it cannot be too close to the star because its gravitational pull would cause the star to "jiggle," and such radial velocity variations have never been seen in Beta Pictoris.
Burrows estimates the planet is from one-twentieth to twenty times the mass of Jupiter. The planet must lie within the range of distances typical of planetary distances within our solar system – from about Earth's distance from the Sun to about Pluto's distance from the Sun (Pluto is roughly 30 times father from the Sun than Earth.)
If the suspected planet were as far from Beta Pictoris as Jupiter is from our Sun, it also would have about the same mass as Jupiter. The planet's orbit must be inclined by about three degrees to the plane of the Beta Pictoris disk, and this is typical of the inclinations of the orbits of the planets in our solar system.
The star is located 50 light-years away in the southern constellation Pictor (Painter's Easel). Though its precise age is not known, Beta Pictoris is generally considered a mature, main sequence star, slightly hotter than our Sun.
Detections of substellar objects orbiting nearby stars have recently been reported for two other normal (i.e., main sequence) stars – Gliese 229 and 51 Pegasus. However, Beta Pictoris is the only candidate that looks like it might possess a planetary system similar to our own.
Beta Pictoris also is the only known star with a circumstellar disk of gas and dust that can be optically imaged. Despite the presence of dust around approximately one-third of the brightest nearby stars – as deduced from NASA's Infrared Astronomy Satellite (IRAS) data – ground-based telescope imaging has not detected other disks.
Several Hubble programs are currently in progress to search for these disks. The NICMOS (Near Infrared Camera and Multi-Object Spectrometer), to be installed on Hubble during the February 1997 servicing mission, will provide a near-infrared capability needed for this type of search.
SCIENCE BACKGROUND: THE DYNAMICS OF THE BETA PICTORIS DISK Why Hasn't The Warp Been Detected Previously?
Though the Beta Pictoris Disk has been studied intensively for more than a decade the inner region of the disk is very hard to see with ground-based telescopes because of the glare from the central star. Also, the visible disk is faint because it consists of microscopic grains of ice and dust that shine only by reflecting light from the star. Hubble Space Telescope concentrates the star's light and produces an image that is ten times sharper than can be obtained from the ground under good conditions.
How Is The Lifetime of The Warp Determined?
Because the microscopic dust particles in the disk collide, in about a million years they either fall into the star or get broken up and blown out of the system by radiation pressure. Any warp in the visible disk will straighten out in even less time because of the same viscous processes. This means some continuous source of both particles and the warp must be in operation. The particles are probably the result of collisions within a belt of unseen larger comet-like objects which are tens of kilometers in diameter (like objects in the Kuiper belt around our own Solar System). In the absence of a planetary perturbation, gravity would straighten out any warp in this "Kupier belt" region within less than ten million years.
How Exactly Does a Planet Warp the Disk?
In 100 million years, a Jupiter-sized planet in a Jupiter-sized orbit would produce and maintain the warp Hubble sees. The Hubble results predict the planet's orbital plane is inclined by about three degrees to the outer disk. The comet-like bodies within the warped area precess, or wobble, around the planet's orbital plane and this leads to the inner disk being fattened and aligned with the planet's orbit. Material outside that radius has not time for its orbit to precess significantly, so appears in its original plane.
Why Is There a Central Clear Area in the Beta Pictoris Disk?
The central clear area, approximately the diameter of our Solar System, has long been suspected of harboring one or more planets which coalesced out of the disk. After planets form, they are expected to rapidly clear the visible disk in their vicinity. However, an alternative explanation was that the clear zone is the result of ices melting (sublimation).
What Was Known Previously about the Beta Pictoris Disk?
Discovered in 1983, the Beta Pictoris disk has long ben been considered a relic of planet formation. In 1775 philosopher Immanuel Kant proposed the nebular hypothesis of planet formation to explain the fact that the orbits of the planets almost lie in the same plane. He considered these coplanar orbits a "skeleton" of a primordial disk where the planets grew from smaller particles that stuck together to "snowball" into larger bodies – a process called agglomeration. (Hubble observations of the Orion star forming region find these disks are common in early stages of star formation.)
The disk about beta Pictoris was deduced from infrared observations obtained with NASA's Infrared Astronomical Satellite (IRAS). The discovery image was obtained by Brad Smith (University of Arizona) and Richard Terrile (JPL) in 1984 using a Charge Coupled Device (CCD) electronic camera with a coronagraph to block out the light from Beta Pictoris to reveal the faint disk. Such ground-based telescopic images of Beta Pictoris have revealed a nearly edge-on disk extending at least 100 billion miles from the star (1,000 times the distance between the Earth and Sun).
