Technique finds planets with lower masses, larger orbits
Posted: October 23, 2002

A new extrasolar planet has been discovered using a new technique that will allow astronomers to detect planets no other current method can. Planets around other stars have been previously detected only by the effect they have on their parent star, limiting the observations to large, Jupiter-like planets and those in very tight orbits. The new method uses the patterns created in the dust surrounding a star to discern the presence of a planet that could be as small as Earth or in an orbit so wide that it would take hundreds of years to observe its effect on its star.

The research by Alice Quillen, assistant professor of physics and astronomy at the University of Rochester, and undergraduate student Stephen Thorndike, appears in the current issue of The Astrophysical Journal Letters.

"We're very excited because this will open up the possibility of finding planets that we'd probably never detect just looking at the parent star," says Quillen. "We can confirm the presence of certain planets in five years instead of the two centuries it would otherwise take."

The new planet was discovered orbiting the star Epsilon Eridani about 10 light years from Earth. It is one of the lowest mass planets yet discovered around another star and has by far the longest, largest orbit of any yet discovered. Epsilon Eridani already has one discovered planet, the size of Jupiter (our solar system's largest planet) and orbiting around the star about every five years. By contrast, the new planet is roughly a tenth of Jupiter's mass and completes an orbit once every 280 years.

Traditional planet-detection methods cannot reveal the new planet, tentatively named "Epsilon Eridani C," because those methods watch for the effect a planet has on it's parent star, and low-mass planets or those in very large orbits do not dramatically effect their star. The method that has detected most of the 100+ extrasolar planets so far measures how much the parent star "wobbles" as the planet's gravity tugs on it throughout its orbit. A newer method watches for planets as they pass in front of a star and slightly dims its light.

Unlike current methods, Quillen's technique does not use direct light from the star, but rather light radiating from the dust surrounding it. Not all stars have large concentrations of dust, but those that do, like Epsilon Eridani, can display certain telltale patterns in their dust fields. These patterns can betray the existence of a planet.

Quillen started her research by running computer simulations of how a planet might sculpt the dust surrounding a star. Instead of using a simple, circular orbit like most planets in our own solar system follow, she decided to experiment with highly eccentric orbits-orbits where the planet sometimes swings very close to the star and then moves very far away. She found that for certain situations where the planet orbited the star three times for every two times the dust orbited, or five times for every three dust orbits, the dust would settle into definable clumps in a ring around the star. These clumps formed as the planet swung to its farthest point from the star and its gravity pulled the dust into the patterned clumps. After finding this pattern in her simulations, Quillen turned to the heavens to see if she could find a star surrounded with dust with these patterns. She found Epsilon Eridani.

"The fact that the dust around this star closely matches what we expected to see if a planet were present doesn't mean we know for sure that a planet is really there," says Quillen. "The images of Epsilon Eridani that we matched with our model are five years old. If Epsilon Eridani were re-observed then the clumps should have moved. The rate that they move will pin down the likely location of the planet."

Quillen plans to find more planets and work out new simulations to determine if patterns could emerge from other kinds of planetary orbits. She's hoping to find if a change in the light emitted from the dust fields could help signal the presence of a planet, as well as what other kinds of patterns might form from the dust, such as rings or swaths of orbiting dust-free zones. She's also planning to learn where the disk of dust comes from, if it comes from frequently colliding planetesimals as she expects. If she pins down how the dust forms, she may be able to estimate the number of planetesimals needed to create the dust.

The research was funded in part by the National Science Foundation through its Research Experience for Undergraduates (REU) program. The program supports highly qualified students who undertake research at the University for 10 weeks each summer.

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