CfA Press Release
Release No.: 03-01 |
For release: EMBARGOED until 12:30 p.m. PST, Monday, January 6, 2003
Farthest Known Planet Opens the Door For
Finding New Earths
Seattle, WA-Astronomers from the Harvard-Smithsonian Center for
Astrophysics (CfA) in Cambridge, MA, announced today they have
detected the most distant extrasolar planet (OGLE-TR-56b) ever found
in the constellation Sagittarius using a new method that could lead
to the discovery of Earth-like worlds around nearby stars. Their
achievement establishes the transit technique as the most accurate
tool available to astronomers today with the potential for finding
Earth-like planets in the future.
"We stand on the threshold of a new era of exploration and discovery.
It only happens once in the history of an intelligent species and we
are closing in on it," said Harvard astronomer and CfA team leader
Dimitar Sasselov. "As in the Golden Age of Exploration in the 16th
century, we have found a better way to detect new worlds in our own
Milky Way galaxy that paves the way for future planetary discoveries."
Extrasolar planets are hard to detect because of their great
distances and because they do not produce any light of their own. The
feeble sunlight they do reflect back into space is lost in the glare
of their sun.
In the past, astronomers have used radial velocity Doppler
measurements of nearby 'wobbling' stars to deduce the existence of
giant planets. They have also used astrometric measurements to detect
the slight "to and fro" motion of stars caused by giant planets
In transit searches, astronomers look for systems where, from our
point of view, a planet passes directly in front of the parent star
it is orbiting. The planet blocks a tiny fraction of the star's
light, causing the star to periodically dim. The effect is small,
like a mosquito flying in front of a searchlight two hundred miles
away, but still detectable. These measurements yield more accurate
information regarding the size of the planet and its orbital
characteristics than is possible using any other current method. It
also extends the stellar search field from 40 thousand current
stellar candidates to 100 million or more. While one other extrasolar
planet (HD 209458b) is known to transit its parent star, that planet
was first discovered using the radial velocity technique, which
detected the slight gravitational tug the planet exerted on its star.
First Success for a Transit Search
This discovery marks the first success of a search program looking
for transiting planets. Astronomers have conducted such searches for
a number of years. While surveys that monitor the brightnesses of
thousands of stars have turned up dozens of candidate systems, this
is the first such system proven to harbor a planet-sized companion.
"Finding planetary candidates through photometric monitoring of stars
is a relatively easy and straightforward task which does not require
large telescopes. However, for the first time around, confirming that
we had indeed found a new planet was a much more challenging task,"
said Caltech astronomer Maciej Konacki, lead author on the paper
announcing the discovery.
The researchers' success came from studying 59 candidates identified
by the Optical Gravitational Lensing Experiment (OGLE) survey. The
OGLE project searches for "dark matter" objects by monitoring
thousands of stars for a brightness change caused by an object
passing between the star and the Earth.
Sasselov's team succeeded in discovering the transiting planet by
systematically eliminating imposters. They first examined the 59 OGLE
candidates spectroscopically using the 1.5-meter telescope at Fred L.
Whipple Observatory, Arizona, and the 6.5-meter Magellan telescope at
Las Campanas Observatory, Chile. Most of the systems were found to be
binary star systems where the companion was a faint, stellar-mass
Five candidates remained as potential planetary systems because they
showed small or undetectable radial velocity variations. Konacki and
the CfA team then examined those candidates more closely using the
HIRES instrument (High Resolution Echelle Spectrometer) at the Keck
Observatory on Mauna Kea, Hawaii. The HIRES observations confirmed
that the star designated OGLE-TR-56 was a single star orbited by a
Jupiter-sized planet and a strange one indeed.
"Our success depended on efficiently eliminating binary stars using
smaller telescopes," said Konacki. "The remaining planetary
candidates were then confirmed using the largest optical telescope in
the world, the 10-meter Keck I telescope in Hawaii. Our time on Keck
was critical to achieving this discovery."
A Distant Planet "On The Edge"
The planet OGLE-TR-56b found by Sasselov's team is quite unique among
the approximately 100 known extrasolar planets. Firstly, it is more
than 20 times farther away than any currently known planet orbiting a
normal star. In fact, it is the first planetary system found outside
our local neighborhood - the Orion spiral arm that contains the Sun.
The new planet orbits a star located in the Sagittarius arm, which is
a spiral arm of stars adjacent to ours and closer to the Galaxy
The newfound planet is also unique because it orbits closer to its
star than any other known planet, only four stellar radii away, or 50
times closer than the Earth is to the Sun. This Jupiter-sized world
whips around its star every 29 hours (as compared to the 88-day orbit
of Mercury and the 365-day orbit of Earth) and is baked to a
temperature of 3,100 degrees Fahrenheit (2,000 Kelvin).
A handful of "hot Jupiters" have been found, the closest taking only
3 days to revolve around its parent star. However, finding a still
closer-in planet was a surprise. Theorists have explained the
existence of "hot Jupiters" by hypothesizing that the planet forms
farther out in the disk of primordial material surrounding a newborn
star. The gas giant then migrates inward, pulled by disk matter
closer to the star and pushed by disk matter farther out. Any planet
that moved too far inward was expected to be pushed completely into
the star, where it would be swallowed up and destroyed.
Sasselov explains the existence of this newfound world by invoking
mass transfer. When the planetary system was forming about 4 billion
years ago, the planet migrated inward so close to the star that some
of the planet's atmosphere was pulled off into the star. After losing
about half of its original mass, the planet spiraled back outward to
its current, stable location. This "dance" between the planet and its
star lasted for about a million years. By the time the planet reached
its current orbit, the protoplanetary disk from which it formed had
dissipated, so there was nothing left to push the lucky survivor in
to its final destruction.
By measuring the system's velocity wobble, the astronomers derived a
mass for the planet of 0.9 Jupiter masses. The magnitude of dimming
during transits showed that the planet's size (diameter) is about 1.3
times that of Jupiter, showing that the planet is a gas giant,
similar in density to Saturn.
Intriguingly, the temperature of OGLE-TR-56b's upper atmosphere is
theoretically just right to form clouds, not of water vapor, but of
iron atoms. Earlier this year, astronomers reported evidence for iron
rain on brown dwarfs. However, such storms only occur over a short
portion of a brown dwarf's lifetime, while the newly discovered 4
billion year-old OGLE-TR-56b should still be experiencing this exotic
weather, thanks to strong heating from the nearby star.
The Most Promising Way to Find New Earths
Seeking planets by looking for transits offers several advantages
over radial velocity and astrometric studies. Transit searches offer
greater efficiency, enabling astronomers to examine many more stars
in a shorter period of time. It also opens the door for studying
hundreds of thousands of new very distant stars like OGLE-TR-56
located 5,000 light-years away. Transit searches also can detect
smaller planets and help measure their sizes and densities.
Radial velocity searches, on the other hand, are approaching the
limit of current technology. These searches are limited to nearby,
bright stars within a hundred or so light-years by the need to
collect large amounts of light. Researchers cannot study farther,
fainter stars until larger telescopes are built. Nor can they detect
planets much smaller than Neptune because the velocity shifts due to
the planet are masked by noise in the velocity shifts from the star
itself. These techniques will not find smaller Earth-like planets in
"Yes, we are excited," says Sasselov. "We are at the leading edge of
extrasolar planet research and we are getting closer and closer to
finding new habitable worlds like our own. Here at the CfA we are
currently conducting three more transit searches that use
complementary strategies for locating new planets. Undoubtedly, more
discoveries will come in the near future."
In the next ten years, ground-based transit searches will be
complemented by space-based searches. For example, NASA's planned
Kepler mission will monitor thousands of stars over a four-year
period, searching for transiting planets. Kepler will be sensitive
enough to detect Earth-sized worlds, if any exist, around several
hundred nearby stars. These studies will then lead to the ambitious
Terrestrial Planet Finder mission, which will examine extrasolar
planets for signs of life. CfA astronomers, like Lewis and Clark, are
contributing to the Kepler mission to be launched in 2006 by scouting
out new candidates for future exploration and making initial
observations of them. CfA researchers also are developing new
instrument technologies that may be used on NASA's Terrestrial Planet
Finder Mission to be launched between 2012-15.
This research will be reported in the January 30, 2003 issue of the
scientific journal Nature. In addition to Sasselov and Konacki,
participating researchers were Guillermo Torres of CfA and Saurabh
Jha of UC Berkeley. A paper on the formation and nature of
OGLE-TR-56b will appear separately in The Astrophysical Journal
Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian
Center for Astrophysics (CfA) is a joint collaboration between the
Smithsonian Astrophysical Observatory and the Harvard College
Observatory. CfA scientists organized into seven research divisions
study the origin, evolution, and ultimate fate of the universe.
An image to accompany this release can be found at http://www.cfa.harvard.edu/news/archive/pr0301_image.html.
For more information and list of extra-solar planetary experts, contact:
David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016