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Tucson, Arizona – July 11, 2007:
Astronomers who used powerful telescopes in Arizona and Chile
in a survey for planets around nearby stars have discovered
that extrasolar planets more massive than Jupiter are extremely rare in other outer solar systems.
University of Arizona
astronomers and their collaborators from the
European Southern Observatory,
Max Planck Institute for Astronomy
in Heidelberg, Italy's
W.M. Keck Observatory
Harvard-Smithsonian Center for Astrophysics
just concluded a benchmark three-year survey using
direct detection techniques sensitive to planets farther from their stars.
The survey looked at 54 young, nearby stars that were
among the best candidates for having detectable giant Jupiter-like planets
at distances beyond five astronomical units (AU),
or the distance between Jupiter and the Sun.
One AU is the distance between Earth and the Sun.
Since 1995, astronomers have found more than 230 "super Jupiters" orbiting very close to their parent stars using the radial velocity method. This indirect planet-detecting technique measures the slight back-and-forth motion of the star as it is tugged by an
unseen planet's gravity.
Scientists have written more
than 2,000 scholarly papers about these giant Jupiter-like planets
within a few Earth-to-sun distances of their stars.
However, the radial velocity method presently used is most sensitive to planets close to their stars. The technique reveals little about extrasolar planets farther out in nearby solar systems.
Astronomers need other techniques to map extrasolar planets beyond five AU
so they can determine what the "average" planetary system looks like –
and whether ours is a typical solar system.
The three-year survey didn't turn up even one giant extrasolar planet in the outer part of any nearby solar system.
"We certainly had the ability to detect outer super Jupiter planets at 10 AU, and farther out, around young sun-like stars," said UA astronomy Professor Laird Close. Close, along with Rainer Lenzen of the Max Planck Institute for Astronomy and Don McCarthy of The University of Arizona, designed the unique, methane-planet sensitive imagers used on two powerful telescopes for the survey.
"The odds are extremely slight that planets larger than four to five Jupiter masses exist at distances
Comparison of images taken with SDI on and off.
A number of fake planets (at separations of 0.55", 0.85", and 1.15" from the star)
were added to this data, which was then analyzed first using the SDI method and,
second, using standard adaptive optics techniques.
The simulated planets, each seen as a pair
of black-and-white dots 33 degrees apart in the SDI image,
are easily detected yet are 10,000 times fainter
than the central star in the standard adaptive optics analysis.
Image: University of Arizona
greater than 20 AU from these stars,"
concluded graduating doctoral student Beth Biller of the
UA Steward Observatory.
Biller is lead author on the first scholarly paper reporting direct-imaging results
for farther-out massive Jupiters from this survey, the most sensitive to date.
"There is no 'planet oasis' between 20 and 100 AU," doctoral student Eric Nielsen of Steward Observatory agreed. "We achieved contrasts high enough to find these super Jupiters, but didn't." Twenty AU is the orbital distance of the planet Uranus in our own solar system.
Astronomers were surprised in the early days of planet finding to discover a population of planets more massive than Jupiter, within the orbit of Mercury, taking only a few days to orbit their host star, Biller said. "Now that we know there aren't large numbers of giant planets lurking at large distances from their stars, astronomers have a more complete picture, and can better constrain how planets are formed," she said.
The team used Close's novel Simultaneous Differential Imager (SDI) for observations made with the European Southern Observatory's (ESO) 8.2-meter Very Large Telescope's (VLT) in Chile and with the
One SDI instrument was used with Lenzen's CONICA adaptive optics camera on the VLT, and another SDI instrument was used with McCarthy's ARIES adaptive optics camera on the MMT. The SDI devices made the highest contrast astronomical images ever made from ground or space of methane-rich companions within an arc second of their stars.
The survey was funded by NASA and the National Science Foundation.
B.A. Biller, L.M. Close, E. Masciadri, E. Nielsen, R. Lenzen, W. Brandner, D. McCarthy, M. Hartung, S. Kellner,
E. Mamajek, T. Henning, D. Miller, M. Kenworthy and C. Kulesa,
"An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with the Simultaneous Differential Imager at the Very Large Telescope and MMT," Ap. J. Supp. 173, 143–165 (2007) (ArXiv e-print)
E.L. Nielsen, L.M. Close, B.A. Biller, E. Masciadri and R. Lenzen,
"Constraints on Extrasolar Planet Populations from VLT NACO/SDI and MMT SDI and Direct Adaptive Optics Imaging Surveys: Giant Planets are Rare at Large Separations," Ap. J. 674, 466–481 (2008) (ArXiv e-print)
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