Using a relatively new planet-hunting technique that can spot worlds one-tenth the mass of our own, researchers have discovered a potentially rocky, icy body that may be the smallest planet yet found orbiting a star outside our solar system.
The discovery suggests the technique, gravitational microlensing, may be an exceptional technology for finding distant planets with traits that could support life.
Image at right: Artist's concept of the newfound rocky planet circling a distant star.
"This important research, partly funded by NASA, is providing us the opportunity to search for planets in habitable environments," said Zlatan Tsvetanov, Terrestrial Planet Finder program scientist at NASA Headquarters, Washington. "The results successfully demonstrate the power of gravitational microlensing, currently the only ground-based technique with the sensitivity to detect extrasolar Earth-size planets on Earth-like orbits, and provide an important clue of the ubiquity of small planets."
Located more than 20,000 light years away in the constellation Sagittarius, close to the center of our Milky Way galaxy, planet OGLE-2005-BLG-390Lb is approximately five-and-a-half times the mass of Earth.
Orbiting a star one-fifth the mass of the sun at a distance almost three times that of Earth's orbit, the newly discovered planet is frigid -- the estimated surface temperature is -364 degrees Fahrenheit.
Although astronomers doubt this cold body could sustain organisms, researchers believe gravitational microlensing will bring opportunities for observing other rocky planets in the "habitable zones" of stars where temperatures are perfect for maintaining liquid water and spawning life.
The discovery, authored by 73 collaborators from 32 institutions, appears in the January 26 issue of the journal Nature.
The Optical Gravitational Lensing Experiment (OGLE) project telescopes first observed the lensing event on July 11, 2005. In an attempt to catch microlensing events as they occur, OGLE scans most of the central Milky Way each night, discovering more than 500 microlensing events per year. But to detect the signature of low-mass planets, astronomers must observe these events much more frequently than OGLE’s one survey per night.
So, when OGLE detected the July 11 lensing, its early warning system alerted fellow astronomers across the globe to microlensing event OGLE-2005-BLG-390 (for the 390th Galactic bulge OGLE discovered in 2005). At that point, though, no one knew a planet would emerge.
"The only way to realize the full scientific benefit of our observations is to share the data with our competition," said co-author Bohdan Paczynski of Princeton University, Princeton, New Jersey, and one of the OGLE co-founders.
The telescopes of Probing Lensing Anomalies NETwork (PLANET) and RoboNet tracked the July 11 episode to completion, providing the data that confirmed the presence of a previously unknown planet. The telescopes collect observations more frequently in an attempt to detect the microlensing signature of planets.
"This discovery was possible because the sun never rises on the PLANET collaboration," said lead author and PLANET researcher Jean-Philippe Beaulieu of the Institut d’Astrophysique de Paris, France. "The global nature of the PLANET collaboration was crucial for obtaining data throughout the 24-hour planetary signal," he said.
Ironically, when preparing the final report, the researchers discovered that during its test runs, the new Microlensing Observations in Astrophysics (MOA) telescope, MOA-2, had taken additional measurements of the lensing event. The 6-foot aperture telescope has a wider field-of-view than the OGLE telescope, enabling it to observe 100 million stars many times per night. MOA-2 is one of several recent and future advancements that gravitational microlensing proponents hope will greatly increase the number of Earth-like planet discoveries.
"The new discovery provides a strong hint that low-mass planets may be much more common than Jupiters," said co-author and PLANET researcher David Bennett of the University of Notre Dame, South Bend, Indiana. Until recently, most extrasolar planets researchers have found have been Jupiter-like gas giants. "Microlensing should have discovered dozens of Jupiters by now if they were as common as these five-Earth-mass planets. This illustrates the primary strength of the gravitational microlensing method: its ability to find planets of low-mass."
Low-mass planets can yield signals that are too weak to detect with other methods. With microlensing, the signals of low-mass planets are rare but not weak. Thus, scientists say, the rate of low-mass planet discoveries should increase dramatically if more microlensing events can be searched for planetary signals.
What is microlensing?
The gravitational microlensing technique is based on a concept first discussed by Albert Einstein in the early 20th century. When astronomers observe a star, the light waves generally come straight from the star to the telescope. However, if another star passes directly in between, even if great distances separate the two, the gravity of the nearer object acts like a lens and magnifies the incoming light.
Telescopes cannot resolve the details of the magnified image, but they do notice a peak in light intensity, and when a planet is present around the closer star, the planet's gravity adds an extra little peak or trough of its own.
Astronomers can use those features to determine how large the planet is and how far away it is from its star. This method is 50 times more likely to detect planets of approximately twice the mass of Jupiter than it is to detect planets closer to the mass of Earth. Even relatively tiny, low-mass objects can give a strong peak signal if alignment is perfect.
Of the more than 150 planets discovered to date, almost all were discovered using a different technique, the radial velocity method. Stars with planets can wobble from the effect of their planets’ gravity, and astronomers can use the wobble to determine the size and orbit of the objects. While incredibly effective, existing technology utilizing this method can find only large, Jupiter-like planets or smaller planets that orbit too close to their host stars to harbor life.
Because this is only the third planet astronomers have detected using gravitational microlensing and already they have found a small, rocky body, the researchers believe there is a strong likelihood that rocky planets may be even more common than their gas-giant brethren. This prediction would agree with one of the models for solar system formation, core accretion, which suggests that small, rocky, "failed Jupiters" should be far more common than the massive gaseous planets.
