NASA Scientist Finds World With Triple Sunsets

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CALIFORNIA INSTITUTE OF TECHNOLOGY
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Whitney Clavin (818) 354-4673
Jet Propulsion Laboratory, Pasadena, Calif.

News Release: 2005-115 July 13, 2005

NASA Scientist Finds World With Triple Sunsets

A NASA-funded astronomer has discovered a world where
the sun sets over the horizon, followed by a second sun
and then a third. The new planet, called HD 188753 Ab,
is the first known to reside in a classic triple-star
system.

"The sky view from this planet would be spectacular,
with an occasional triple sunset," said Dr. Maciej
Konacki (MATCH-ee Konn-ATZ-kee) of the California
Institute of Technology, Pasadena, Calif., who found
the planet using the Keck I telescope atop Mauna Kea
mountain in Hawaii. "Before now, we had no clues about
whether planets could form in such gravitationally
complex systems."

The finding, reported in this week's issue of Nature,
suggests that planets are more robust than previously
believed.

"This is good news for planets," said Dr. Shri Kulkarni,
who oversees Konacki's research at Caltech. "Planets may
live in all sorts of interesting neighborhoods that,
until now, have gone largely unexplored." Kulkarni is the
interdisciplinary scientist for NASA's planned SIM
PlanetQuest mission, which will search for signs of
Earth-like worlds.

Systems with multiple stars are widespread throughout
the universe, accounting for more than half of all stars.
Our Sun's closest star, Alpha Centauri, is a member of a
trio.

"Multiple-star systems have not been popular planet-hunting
grounds," said Konacki. "They are difficult to observe and
were believed to be inhospitable to planets."

The new planet belongs to a common class of extrasolar
planets called "hot Jupiters," which are gas giants that zip
closely around their parent stars. In this case, the planet
whips every 3.3 days around a star that is circled every 25.7
years by a pirouetting pair of stars locked in a 156-day orbit.

The circus-like trio of stars is a cramped bunch, fitting
into the same amount of space as the distance between Saturn
and our Sun. Such tight living quarters throw theories of hot
Jupiter formation into question. Astronomers had thought that
hot Jupiters formed far away from their parent stars, before
migrating inward.

"In this close-knit system, there would be no room at the
outskirts of the parent star system for a planet to grow," said
Konacki.

Previously, astronomers had identified planets around about
20 binary stars and one set of triple stars. But the stars in
those systems had a lot of space between them. Most
multiple-star arrangements are crowded together and difficult to
study.

Konacki overcame this challenge using a modified version of the
radial velocity, or "wobble," planet-hunting technique. In the
traditional wobble method, a planet's presence is inferred by
the gravitational tug, or wobble, it induces in its parent star.
The strategy works well for single stars or far-apart binary
and triple stars, but could not be applied to close-star
systems because the stars' light blends together.

By developing detailed models of close-star systems, Konacki
was able to tease apart the tangled starlight. This allowed
him to pinpoint, for the first time, the tug of a planet on a
star snuggled next to other stars. Of 20 systems examined so
far, HD 188753, located 149 light-years away, was the only
one found to harbor a planet.

Hot Jupiters are believed to form out of thick disks, or
"doughnuts," of material that swirl around the outer fringes
of young stars. The disk material clumps together to form a
solid core, then pulls gas onto it. Eventually, the gas giant
drifts inward. The discovery of a world under three suns
contradicts this scenario. HD 188753 would have sported a
truncated disk in its youth, due to the disruptive presence of
its stellar companions. That leaves no room for HD 188753's
planet to form, and raises a host of new questions.

The masses of the three stars in HD 188753 system range from
two-thirds to about the same mass as our Sun. The planet is
slightly more massive than Jupiter.

For artist's concepts and other graphics, visit

http://planetquest.jpl.nasa.gov/ .

For information about NASA and agency programs on the Web,
visit

http://www.nasa.gov/home/index.html .

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