HST ACS/HRC wide view of Fomalhaut system; click here for more images.

Two teams of astronomers have succeeded in finding an astronomical holy grail of sorts, taking the first images of planets orbiting other stars. Using different imaging techniques, they have found four such planets circling a pair of stars.

Although more than 300 extrasolar planets have been found since the mid-1990s, astronomers had not been able to image them until now. Previous discoveries had all been by indirect methods such as measuring the spectra of stars in search of minute changes in their velocity to indicate that an unseen body was pulling at them, or periodic decreases in a star's brightness caused by a planet crossing the face of its disk.

Direct detection has been hampered by the immense distances even to the nearest stars as well as the fact that a planet's relatively feeble light is overwhelmed by the glare of its parent star. These obstacles have at last been overcome, thanks to the resourceful use of imaging techniques by the astronomers involved.

Fomalhaut, a relatively nearby star at 25 light years distance, is the southernmost first-magnitude star (star of the highest tier of brightness) visible from the latitude of New York; it twinkles low in the south on autumn evenings. In 1983, the Infrared Astronomy Satellite (IRAS) discovered a dust ring encircling the star.

By comparing images of Fomalhaut's dust ring taken with the Hubble Space Telescope in 2004 and 2006, astronomers led by Paul Kalas at the University of California, Berkeley detected a small, relatively bright spot moving counterclockwise within the dust ring--a planet less than 3 times the mass of Jupiter.

The images were taken using a coronograph--a device that masks a star, in effect creating an artificial eclipse to allow, fainter, normally invisible objects in its vicinity to be detected. A suspiciously sharp inner edge to Fomalhaut's dust ring had led Kalas to suspect that a planet might be shaping the ring.

Another team, headed by Christian Marois of the Herzberg Institute of Astrophysics in British Columbia, has found three planets in orbit around HR 8799, a star about 130 light years away within the Great Square of Pegasus. The team has been using the Gemini and Keck telescopes on Mauna Kea in its search for exoplanets, aided by these telescopes' use of adaptive optics.

Adaptive optics is a method of correcting for atmospheric instability that can smear or blur the image of a star or planet. It combines a hardware (the rapid flexing or jiggling of the telescope's mirror) and software solution to compensate for unsteady air. After 8 years of imaging sunlike stars with no success, they turned their attention to younger, hot stars like HR 8799. The three worlds they found orbiting that star are very large, between 6 and 10 times the mass of Jupiter.

Post by Tony Hoffman