OGLE-TR-56

The newfound planet is shrouded in clouds of iron atoms. Astronomers have found the most distant planet orbiting another star. It is also the fastest yet found - its year lasts a mere 29 hours. The planet is 5,000 light years away, towards the centre of our Milky Way, and 20 times further than the previous record holder.

The planet, called OGLE-TR-56b, is fourteen times closer to its star than Mercury is to the Sun, its discoverers told this week's American Astronomical Society meeting in Seattle. "It's a very strange orbit," says Dimitar Sasselov of the Harvard-Smithsonian Center for Astrophysics, who led the team. The discovery might open the way for finding planets similar to Earth.

OGLE-TR-56b is the first planet astrophysicists have discovered by watching it pass in front of the star that it orbits, a phenomenon called transiting. Based on the 1% dimming in the star's brightness, the team believe the new planet is about the size of Jupiter. Only planets in solar systems that are side-on to us will have orbits that obscure our view of their stars.

One other planet has been shown to do this, but it was first spotted using the traditional method of measuring its gravitational pull on its parent star. Transiting should be much more sensitive than the gravitational technique, but several years of searching had yielded nothing.

29 hours is a year on OGLE-TR-56b Most distant planet orbiting another star spotted in passing.

Location Constellation: Sagittarius Right Ascension: 17 56 35.51 Declination: -29 32 21.2 Distance from Sol: 4892.45 Light Years Basic Data Spectral Type: G Apparent Magnitude: 16.6 Mass: 1.04 Solar Masses Habitability Inner Edge of Habitability Zone: Unknown Outer Edge of Habitability Zone: Unknown

OGLE-TR-56-ORBIT B

OGLE-TR-56-ORBIT B Statistics

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 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.

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.

Planetary Orbit Of OGLE-TR-56-b