The Transit Technique and Gravitational Microlensing

The transit technique was used to observe a small and repeated dimming in the light from a star named HD 209458 caused when a planet orbiting the star passed in front of, or transited, the star. This discovery was reported in November 1999 by teams of astronomers led by David Charbonneau of the Harvard-Smithsonian Center for Astrophysics and Gregory Henry of Tennessee State University in Nashville. Both groups had been looking for transits of HD 209458 because the Doppler-shift method had already shown the existence of a planet orbiting the star. The confirmation was important because it proved that the transit method could be used to discover extrasolar planets. The effective use of the transit technique was also significant because with this technique, astronomers can learn the size of the planet.

Observations of planetary transits are difficult, however, because the amount of dimming is small—about 1.5 percent of the star's normal brightness in the case of HD 209458. Also, many stars vary in brightness for other reasons. For astronomers to conclude that a star's dimming is caused by a planet, they must observe the star's light growing dimmer and brighter at regular intervals.

The magnifying-glass technique, called gravitational microlensing, enabled researchers to infer the presence of a small planet orbiting a star in the direction of the center of the Milky Way Galaxy. This finding was reported in January 1999 by astronomers David Bennett and Sun Hong Rhie at Notre Dame University in Indiana. Gravitational microlensing is based on the general theory of relativity proposed in 1916 by Einstein. According to Einstein, the gravitational field of a massive object such as a star bends the space around it, much as a bowling ball resting on a rubber mat causes a depression in the mat. Just as a marble rolling across the mat would curve toward the depression in the mat if it passed close to the bowling ball, so a beam of light is bent by the curved space near a star. If one star lies precisely in the line of sight between the Earth and a more distant star, the gravity of the nearer star can act as a celestial magnifying glass, bending and focusing the light from the more distant star. Then the distant star appears brighter to observers on Earth because more of its light reaches Earth.

Opportunities to catch two stars in precise alignment are rare. In addition, the brightening of the distant star is temporary, lasting only as long as it takes for the nearer star to cross in front of the one farther away. Bennett and Rhie surveyed millions of stars throughout the late 1990's before they found an instance of microlensing in which they noted a pattern of brightening that might indicate the presence of a planet. The astronomers used a ground-based telescope to make repeated images of the same patch of sky. Then they used a computer to compare images, looking for any stars that appeared brighter than normal for a period of time. A star that had planets would display a large peak in brightness as well as smaller peaks caused by the additional gravitational lensing effects of the planets.