Professor discovers smallest planet

Late last July, a University astrophysicist helped discover the smallest planet ever detected outside the solar system — without even seeing it.

Using a planet-finding technique called "gravitational microlensing", a process theorized by Professor Bohdan Paczynski, astronomers were able to detect a planet with a mass about 5.5 times the mass of Earth.

"The phenomenon is produced by the near-perfect alignment of a planet and a star. The gravity of the planet causes a small perturbation in the star's brightness," said Paczynski, co-founder of OGLE (Optical Gravitational Lensing Experiment), one of the groups responsible for the discovery.

This "wink" in a star's brightness allows the researchers to calculate the mass of the planet, and the duration of the perturbation event relates to the distance between the celestial bodies, Paczynski said.

Because the planet — not so cleverly named OGLE-2005-BLG-390Lb — orbits a star about one-fifth the mass of the Sun at about three times the distance from Earth to the Sun. Researchers estimate that its temperature is near that of Pluto, about negative 220 degrees Celsius.

"NASA is planning to spend billions of dollars on a project to find small planets around nearby stars," Scott Tremaine GS '75, chair of the astrophysical sciences department, said. "The best way for this to work is to know what you can expect to find, which is what the [planetary] data from the OGLE survey should help with."

OGLE is based out of the Las Campanas Observatory in Chile, but works in collaboration with other groups taking data in Australia, South America, and New Zealand.

"This is an excellent example of international collaboration," Paczynski said, "one of the few cases where collaboration is not only helpful but necessary. The night never ends in this collaboration."

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Only about one out of a million stars is microlensed in the best areas of the sky, and "planetary microlensing is much more rare" said Andrzej Udalski, a professor at Warsaw University in Poland and the other founder of OGLE.

"We monitor about 150 million stars each night, looking for brightening resembling the microlensing," Udalski said. "When we find such [brightening] we alert other groups like PLANET so they could also monitor the discovered events."

Udalski said that collaboration is especially important in this field because of the short duration of the microlensing events, which may last from a few hours to several days and only occur once.

Paczynski proposed gravitational microlensing in 1991 and began observational surveys with OGLE the next year, he said.

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"My contributions were originally theoretical," Paczynski said. "My [current] contribution is to supervise many collaborating groups."

Data for these experiments are recorded with cameras from telescope images, a process which provides information with limited accuracy, Tremaine said.

"The number of pixels [in the image] is increasing all the time," Tremaine said. "It is easier to build a better camera than a telescope."

"The future is extremely promising," he added.