Astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, MA, 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."
|The farthest know planet ever discovered is a strange world indeed. Whizzing around its star every 29 hours, it is shrouded in clouds made not of water droplets but of iron atoms. This is a world of iron rain.
Credit: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics
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 orbiting them.
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 object.
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 center.
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.
|Artist's conception of a gas giant planet orbiting a nearby star.
Credit: NASA and G. Bacon (STScI)
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.
|Our Milky Way galaxy is packed with 400 billion stars and perhaps even more planets.
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 life-supporting orbits.
"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.
In December 2001, NASA selected the Kepler Mission, a project based at NASA Ames, as one of the next NASA Discovery missions. The Kepler Mission, scheduled for launch in 2006, will use a spaceborne telescope to search for Earth-like planets around stars beyond our solar system. A key criterion for such suitable planets would be whether they reside in habitable zones, or regions sometimes protected by gas giants but with temperate climates and liquid water.
Future missions, such as ESA's Herschel mission will search for many more and take detailed pictures of stars that might harbor dust rings. As these images become available, astronomers will be able to predict the sizes and orbits of giant planets within the alien solar system.
This research will be reported in the January 23, 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 Letters. 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.