Greenbelt - August 10, 2000 - Some mystics claim to divine the future in the swirl of tea leaves at the bottom of a cup, but astronomers may be able to perform an equally impressive feat -- read the patterns imprinted in dust disks around nearby stars to find hidden planets.
According to a team of NASA and university researchers, the gravitational influence of newborn planets weaves patterns in the dust disk from which they were formed, and the type of pattern depends on the planet's mass and orbital characteristics. If confirmed, the new disk analysis method promises to allow the discovery of extrasolar planets undetectable with current methods.
"Dust blocks our view and makes it hard to definitively detect planets," said lead author Dr. Nick Gorkavyi, a National Academy of Sciences National Research Council Senior Research Associate at NASA's Goddard Space Flight Center, Greenbelt, Md.
"We turned this problem into an advantage -- by reading the patterns imprinted on the dust disk, we can determine whether a planet is hiding there. The planet is still hidden, but it writes its signature in the dust."
The scientists applied their method to observations of dust disks around three nearby stars: Beta Pictoris, Epsilon Eridani, and Vega. The researchers estimate Beta Pictoris has a planet 10 times the mass of Earth orbiting about 6.5 billion miles from the star, while Epsilon Eridani has a 0.2 Jupiter-mass planet about 5.5 billion miles away, and Vega has a planet twice the mass of Jupiter in an orbit about 5 billion miles away. These distances from the parent star are larger than any of the planets in our solar system.
The research will be presented August 7 during the International Astronomical Union General Assembly meeting at the University of Manchester, Manchester, UK, and was published in the July 10 issue of the Astrophysical Journal Letters (Vega and Epsilon Eridani models only).
Astronomers believe a solar system is born when a cloud of gas and dust in interstellar space collapses. The densest region at the center of the cloud becomes a new star, while the outer regions form a surrounding disk of material, called a circumstellar disk. The disk is unstable, and portions collapse further under their own gravity, forming planets, asteroids, and comets.
According to the new method, a planet's gravitational influence redistributes dust in the disk, forming beautiful features including swirls, arcs, voids, warps, and clumps. Because the pattern depends on the planet's mass and orbital characteristics, determining the kind of pattern reveals this information about the planet.
"These patterns persist for a long time, because the dust becomes trapped in these special orbital patterns by the planet's gravity," said Dr. Gorkavyi.
"If there were no planets present, radiation and particle emissions from the star would slow the orbital velocity of the surrounding dust, which would spiral into the star relatively quickly, causing the disk to vanish. So, we believe these special patterns are the signature of a planet."
If confirmed by further observations, the new method could be applied to analyze circumstellar dust disks and identify planets where it is difficult or impossible using other methods.
For example, a common planet detection method is to use the wobble produced in a star's motion by the gravitational pull of unseen massive planets orbiting it.
The wobble causes light emitted by the star to change color very slightly. (The change is too small to be noticed by the human eye.) By analyzing this change with a special instrument called a spectrograph, astronomers can deduce the unseen planet's mass and orbit. This method has a few significant limitations, however.
First, the star's wobble must have some component that is directed towards the Earth, or the color change will not be seen. Thus, alien worlds whose orbital plane happens to be tilted perpendicularly to Earth's orbit will remain unseen, because that kind of orbit does not pull the star toward or away from the Earth.
Also, planets whose orbits are remote from the star will not be seen because their pull is too slight to produce a noticeable color change in the star's light.
Even if a remote planet is sufficiently massive to exert a detectable pull, it will be many years before they are identified -- the color change will occur very slowly because it takes centuries for them to complete their huge orbits.
The new method overcomes these limitations; however, the star must be close enough for the pattern in its disk to be identified. With current telescopes, the scientists estimate their method is good for stars within approximately one hundred light years from Earth.
Although the patterns are long-lived, the dust still disperses over time, so the method works best for relatively young stars, which have more circumstellar dust.
The researchers include Leonid M. Ozernoy of George Mason University, Fairfax, Va., Nick N. Gorkavyi, John C. Mather, and Sara R. Heap of NASA's Goddard Space Flight Center, and Tanya A. Taidakova of Crimean Astrophysical Observatory, Ukraine.
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