Technique used could lead to finding planets that potentially harbor life

Astronomers from Princeton and other institutions have discovered the smallest planet found outside of our solar system using a technique that researchers believe will uncover others that potentially harbor life.

The rocky, icy planet is about five-and-a-half times the mass of Earth and is located more than 20,000 light years away in the constellation Sagittarius, close to the center of our Milky Way galaxy. The discovery, detailed in the Jan. 26 issue of Nature, was made by a collaboration of astronomers worldwide, including the Optical Gravitational Lensing Experiment (OGLE) group co-founded by Princeton’s Bohdan Paczynski.

Researchers do not believe that life could be sustained on the frigid new planet, which has an estimated surface temperature of minus 364 degrees Fahrenheit — similar to Pluto’s. The planet orbits its star at a distance of more than three times that of the Earth orbiting the sun; its star is about one-fifth the mass of our sun.

But astronomers are optimistic that the technique used in the finding — known as gravitational microlensing — will lead to the discovery of planets that are closer in size to Earth, or even much smaller, and that may exist in “habitable zones” around stars where temperatures could potentially sustain liquid water on their surfaces.

“We may predict with reasonable probability that gravitational microlensing will discover planets with masses like that of Earth at a similar distance from their stars and with comparable surface temperature,” Paczynski said.

Three planets have been found in the past two years using gravitational microlensing, which allows astronomers to detect changes in the brightness of a star when a massive object in space — such as a planet, another star or even a black hole — crosses in front of it. The object’s strong gravitational pull bends the light rays from the distant star and magnifies them like a lens. By analyzing the patterns of the brightening of the distant star’s light rays, researchers can identify the object passing in front of it.

Previously, more than 150 planets were discovered outside our solar system with a technique known as the radial velocity method, which observes a wobble in stars caused by the planet’s gravitational effects.

“For radial velocity changes to be measurable, the planet has to be either massive or on a close orbit, so most planets are massive, like Jupiter, or hot,” Paczynski said. “Only gravitational microlensing is sensitive to low-mass planets at large separation from their stars.”

Paczynski co-founded the OGLE project with Andrzej Udalski of Warsaw University, who designed and leads the effort to survey millions of stars. OGLE researchers first detected the microlensing event that led to the new planet discovery in July with its telescope at the Las Campanas Observatory in Chile. The OGLE team collaborated with several other international research groups to confirm the presence of the new planet. In total, the effort included 73 researchers from 32 institutions.

Two planets previously discovered using gravitational microlensing were much larger than the most recent finding. Both were roughly three to five times the size of Jupiter, the largest planet in our solar system with a mass 318 times that of Earth’s. Because gravitational microlensing enables researchers to monitor the light signal of stars with great precision, they may eventually be able to find planets similar to or even much smaller than Earth, perhaps one-tenth of its size.

Paczynski, who is Princeton’s Lyman Spitzer Professor of Theoretical Astrophysics, first proposed using gravitational microlensing to search for planets in 1991 along with his graduate student Shude Mao, now a professor at Manchester University in England. Paczynski predicts that as OGLE and other research teams employ the technique to find smaller planets very far from Earth, their work will aid NASA’s efforts to make similar discoveries closer to our solar system.

“Unfortunately, gravitational microlensing detects stars at very large distances — many thousands of light years — so it is not possible to image those planets,” he said. “That hopefully will be done with the future Terrestrial Planet Finder planned by NASA, which is a multibillion-dollar undertaking. It is sensible to find out if there are planets with the Earth’s mass in orbits similar to ours — this search can be done with gravitational microlensing, spending far smaller funds. Once we know such planets are common, it will make sense for NASA to look for them.”

More information about the new planet discovery can be found on the National Science Foundation’s Web site.