Press Release

Benchmark Survey Shows that Giant Outer Extrasolar Planets are Rare

By SpaceRef Editor
July 11, 2007
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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 at Heidelberg,
Italy’s Arcetri Observatory, the W.M. Keck Observatory and the
Harvard-Smithsonian Center for Astrophysics just concluded a benchmark
3-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 5 astronomical units (AU), or the distance between
Jupiter and the sun. (One AU is the distance between Earth and the sun.)

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 in 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.

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 5 AU so
they can determine what the “average” planetary system looks like — and
whether ours is a typical solar system.

The 3-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, Heidelberg, 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 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 UA’s Steward Observatory agreed. “We achieved contrasts high
enough to find these super Jupiters, but didn’t.” 20 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 6.5-meter, UA/Smithsonian
MMT Observatory on Mount Hopkins, Ariz.

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. (An arc second is the
diameter of a dime seen from two miles away.) A related previous news
release about SDI is online at http://uanews.org/spots/sci-12395.html

The survey was funded by NASA and the National Science Foundation. It is
being published in the Astrophysical Journal. Preprints are online at
http://arxiv.org/abs/0705.0066 and http://arxiv.org/abs/0706.4331

SpaceRef staff editor.