Innovative Technique Detects Planets With Lower Masses and Larger Orbits Than Any Current Method
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.
