In an important breakthrough, astronomers have found two distant planets significantly smaller than Jupiter.
Up until now, planet hunters have found about 30 alien worlds - most of them very close to their parent stars and more massive than the largest planet in our solar system.
These newly discovered planets could be an important step toward finding distant worlds more like Earth. While they are also too close to their stars to harbor any known form of life, they suggest that many stars may have smaller planets circling them.
"Now we are confident we are seeing a distinctly different population of bodies," said Geoffrey Marcy of the University of California at Berkeley.
The smaller planets were only one part of an eventful week for planet hunters. Two British astronomers also announced that they've found 13 planets that are floating through space, unattached to any star.
Only two such "wanderers" (the original meaning of the term "planet") were known before. Japanese astronomers found them in the Chamaeleon Nebula.
The British astronomers' free-floaters - each weighing in at eight to 13 times the mass of Jupiter - are all located in a star-forming part of the Orion Nebula. Along with new stars, they seem to have formed from the dust and gas of the nebula.
"The discovery of 13 more in one cluster suggests that they might be very common," notes Britain's Royal Astronomical Society.
Philip Lucas at the University of Hertfordshire and Patrick Roche at the University of Oxford found the free-floaters by detecting heat left over from their formation. That means they are very young in astronomical terms - about one million years old. They will eventually cool to Earth-like temperatures. But the society says "it is unlikely they could ever sustain life."
It's a different story for star-orbiting planets. As astronomers begin to pick up smaller planets, the hunt is on for candidates that are in a star's so-called habitable zone. That's the region that is neither too close to the star, nor too far away to be reasonably expected to sustain life.
In announcing this first discovery of smaller planets, NASA says planet hunting has "crossed an important threshold."
The find reinforces a long-held theory that planets form by a snowball effect of growth from small ones to large. That means many different kinds of worlds could exist.
This latest discovery was made by Dr. Marcy, together with Paul Butler of the Carnegie Institution in Washington and Steven Vogt of the University of California at Santa Cruz.
One planet, with at least 80 percent of the mass of Saturn, orbits a star known as HD 46375, 109 light-years from Earth in the constellation Monoceros. The other planet, with 70 percent of Saturn's mass, accompanies the star 79 Ceti, 117 light-years distant in the constellation Cetus.
What the planets tell us
Although much larger than Earth, these planets are considered relatively small, since Saturn is three times less massive than Jupiter. And while they are also too close to their parent stars - and thus too hot - for life, scientists think the planets formed farther out, where they accumulated mass from cool gases. Then they migrated inward.
Such migration of giant planets would disrupt any smaller bodies such as Earth. But, as with our solar system, there should be cases where giants and small bodies coexist. Alternatively, some giants might be in a habitable zone, where some of their moons could support life.
Finding habitable worlds requires new ways of searching. Most hunters find planets by "wobbles." Planets' gravity affect their parent stars' motions, making the stars appear to wobble. But because only large planets can significantly impact their stars' motion, this technique favors giant-planet detection.
Last December, though, NASA's Ames Research Center at Mountain View, Calif., reported the first detection of a planet by the way it dims its star's light when crossing in front of the star. William Borucki, principle investigator for the project, said that technique, used from a space observatory planned for 2004, should allow a small telescope to pick out Earth-size planets.
Also last December, Andrew Cameron at the University of St. Andrews in North Haugh, Scotland, reported direct observation of a giant planet orbiting the star Tau Botis. Other astronomers have not confirmed the observation, but this may be the first time an alien planet has been seen by the reflection of light from its star.
Indeed, detection of Earth-size planets may only be a decade or two away. NASA hopes to orbit a succession of increasingly powerful observatories, beginning in 2006, that would ultimately allow us to gather detailed information from alien worlds.
(c) Copyright 2000. The Christian Science Publishing Society