The shadow of a planet passing before a distant star has for the first time provided direct and visible evidence to human eyes that other worlds exist outside our solar system.
The stunning confirmation came last week from astronomer Greg Henry of Tennessee State University, who detected a subtle dimming in a sun-like star 153 light-years away. The dimming occurred when a massive Jupiter-like planet passed between the distant star and Earth.
Unlike past indirect evidence of extrasolar planets, this discovery gives scientists the precise mass and radius of the body, as well as the ability to study its chemical composition. At a more basic level, the find is a leap toward better understanding how planetary systems evolve and, by extension, where to find worlds that can sustain life.
"This is a moment that I hope everyone can enjoy, because our Earth and its little planetary system has been placed, for the first time, in direct comparison with the rest of the universe," says astronomer Geoff Marcy of the University of California at Berkeley, who collaborated with Dr. Henry.
The debate over the existence of other worlds has been a contentious affair. In the late 16th century, Italian philosopher Giordano Bruno penned, "Innumerable suns exist; innumerable earths revolve around these suns in a manner similar to the way the seven planets revolve around our sun. Living beings inhabit these worlds." He was imprisoned and burned at the stake for his astronomical heresy.
In modern times, the famous astronomer Carl Sagan suggested there were billions of planets within the Milky Way Galaxy alone. For this, he was viciously ridiculed in the astronomical community.
But five years ago, Swiss astronomers Michel Mayor and Didier Queloz appeared to vindicate Bruno and Sagan when they found the first evidence of an extrasolar planet. Dr. Mayor and Dr. Queloz used a complex detection method called radial velocity, measuring slight wobbles in a star's rotation supposedly caused by the gravitational tug of a large orbiting body. Using radial velocity detection, scientists have discovered at least 19 alleged planets.
Without direct evidence, however, some critics claimed that the supposed extrasolar planets were actually brown dwarfs, stars so cool and dark that they masquerade as planets. Others said the wobbles were magnetic pulsations that blurred and bent radial-velocity measurements.
Furthermore, according tothese measurements, the planets were about as big as Jupiter, but they orbited much closer to their stars than Jupiter orbits the sun.
"Before people began to find these short-period planets, we assumed what we were looking for was analogous to the solar system,... small rocky planets in short orbits and gas giants in distant orbits," says Henry. "The discovery of these hot Jupiters has really upset the ideas of how solar systems form."
Planet-finders countered these objections in scientific journals, but they needed hard evidence. That evidence came from Henry, who uses a sensitive digital camera to measure fluctuations in the light of nearby stars. He began collaborating with Dr. Marcy and his partner R. Paul Butler, who have found more planetary candidates than any other team of astronomers.
From their radial-velocity measurements, Marcy and Dr. Butler could predict when a planet should transit directly between Earth and the star. After several attempts, the collaboration paid off Nov. 7, when Henry, responding to a tip from Marcy and Butler, observed a slight darkening in a star very similar to the sun.
"It's more like seeing a shadow rather than the actual object but in some sense it is a direct detection," says Henry. "It's neat to have been the first person to see that."
The new find is already providing valuable insights. Scientists are now virtually certain that the planet, which has a much larger radius than Jupiter but a smaller mass, is composed mostly of liquid metallic hydrogen and orbits its host star every 3.5 days. The combination of large radius and small mass confirms that hot Jupiters are not rocky orbs but rather are gas giants that swell considerably when they absorb intense stellar radiation.
On a broader scale, the find appears to validate new theories of planetary evolution, in which large Jovian bodies migrate toward their host star.
"If these Jovian planets formed at a large distance from the star and migrated inward, they may have taken a lot of the terrestrial planets with them," says Adam Burrows, a theoretical astrophysicist at the University of Arizona in Tuscon. "That may indicate what types of systems don't have the proper environment for life as we know it to arise."
Soon, scientists hope to use sensitive telescopes to capture evidence of the planet's spectroscopic footprint and thereby divine its precise chemical composition. A similar study of Earth-like planets might not be far off.
Innovative ways to link large telescopes could put these bodies within reach in five years. And a speculative NASA project that would put a detection system similar to Henry's in orbit would greatly speed up the planet search.
(c) Copyright 1999. The Christian Science Publishing Society