When composer Gustav Holst penned "The Planets" early this century, he only had eight to deal with - all in our own solar system.
Tackling a similar project today, he might throw up his hands in despair; astronomers looking beyond the sun's cluster of companions are finding alien worlds orbiting distant stars with striking regularity.
This week, three teams of astronomers at an international meeting in Manchester, England, added nine more candidates. This brings to 50 the number of known planets beyond our solar system since the first unambiguous detection in 1995.
"We're now at a stage where we are finding planets faster than we can investigate them," says Geoffrey Marcy, an astronomer with the University of California at Berkeley and leader of one of the teams reporting the new finds. "It's wonderful. Planet hunting has morphed from the marvelous to the mundane."
Mundane to the hunters, perhaps, but not to astronomers looking for patterns in planetary systems that would yield clues about the likelihood that our galaxy holds other Earthlike planets hospitable to organic life.
The new planets are comparable to our solar system's outer planets - those beyond Mars - and range from about 1/10th Jupiter's mass to about five times its mass.
The distribution and gravity of our outer planets played what Southwest Research Institute planetary scientist Harold Levison calls "a critical role" in determining the size, number, and suitability for life of the first four rocks from the sun. Earth, in particular, benefited from the asteroids and comets the outer planets slung its way. These are thought to have provided much of Earth's water, organic molecules, and gases, Dr. Levison notes. He and colleagues from the University of Colorado and Canada's Queen's University in Kingston, Ontario, are using planet discoveries and computer simulations of planet formation to uncover the range of outer-planet dynamics that would foster the formation of terrestrial planets capable of supporting life.
To find planets, astronomers rely on starlight. By using powerful telescopes, researchers measure a star's "wobble," traced by a shift in its spectrum from blue (moving toward Earth) to red (moving away) and back. The shift comes as an orbiting planet's gravity tugs on the star. The red shift and blue shift give scientists the object's orbital period and the change in velocity it imparts to the star. These serve as a basis to calculate the object's mass, distance from the star, and other characteristics.
This approach also may underestimate the number of stars hosting planets, since it works only on systems seen more or less edge-on. This bias has prompted scientists to explore different techniques.
A team at NASA's Goddard Space Flight Institute in Greenbelt, Md., for example, is looking for the signature the planets' gravity would trace in dust disks surrounding young stars.
Looking ahead, astronomers plan to combine images from three large telescopes in two hemispheres to find ever smaller "wobble" velocities, which imply smaller planets or larger ones at greater distances from the host star.
(c) Copyright 2000. The Christian Science Publishing Society