November 16, 1999
Shadow Confirms Existence of Extrasolar Planets
By JOHN NOBLE WILFORD
dark shadow like an ink spot drifting across the face of a
star 153 light-years away has given astronomers the first direct
observations that they said confirmed the existence of planets
around stars outside Earth's solar system.
Five years of indirect observations had yielded the detection of
more than 25 extrasolar planets orbiting relatively nearby stars.
Their presence was inferred from ever-so-slight wobbles of the
stars, presumably caused by the gravitational pull of large objects
in their orbit.
Last week, telescopes recorded for the first time the distinct
dimming of light as a planet crossed in front of a sunlike star, in
this case HD 209458 in the constellation Pegasus.
The direct observation was made by Dr. Greg Henry, an astronomer
at Tennessee State University in Nashville, operating remotely
automated telescopes in the mountains of southern Arizona.
"This is a spectacular discovery," said Dr. Geoffrey Marcy of
the University of California at Berkeley, the leading seeker of
planets around other stars. "There's just no question that the
planets we have been detecting from indirect evidence really are
Dr. Paul Butler of the Carnegie Institution in Washington, who
collaborates with Marcy, said that not only did the new findings
confirm the presence of extrasolar planets by independent means but
the observed shadow "provides us with wonderful new information"
about the physical properties of this particular planet. Indirect
observations of a star's wobbles had yielded only gross estimates
of the planets' mass and size.
Henry said the "amount of dimming of the star's light during
the transit gives us the first-ever measure of the size and density
of an extrasolar planet."
From early calculations, the astronomers reported that the
planet was traveling a circular orbit around the star once every
3.523 days. The planet's mass is only 63 percent of that of
Jupiter, the solar system's giant. But because the planet is so
close to its star, it is extremely hot (perhaps 3,000 degrees
Fahrenheit) and the sphere of gases is thus expanded to a bloated
shape. Its radius is 60 percent greater than that of Jupiter.
What the astronomers found fit neatly with recent theories,
developed by Dr. Adam Burrows of the University of Arizona in
Tucson, that predicted the puffed-out shapes of gaseous planets so
close to their stars. If Jupiter happened to be as close as that to
the Sun, it would be much more bloated. The theorists had foreseen
that such a close-orbiting planet would be at least 50 percent
bigger than Jupiter.
Knowing the planet's size and mass, astronomers calculated its
density to be about 0.2 grams per cubic centimeter. This tells them
that it, like Jupiter, is a huge gas giant, probably composed
mostly of the lightest elements, hydrogen and helium. In fact, the
extrasolar planet's density appeared to be a little less than that
of Saturn, the solar system's least dense planet.
For perspective, the diameter of Jupiter at its equator is 11
times that of Earth and its mass is 318 times Earth's.
The latest discovery about extrasolar planets stemmed from a
frustrating night of bad weather at the Keck Observatory on Mauna
Kea in Hawaii. When clouds rolled in on the night of Nov. 5, Marcy
and Butler spent some time going over previous observational data
and noticed the telltale signs of a star's wobble, something they
had overlooked before. It was their 19th extrasolar planet.
"It popped right out of the analysis," Marcy said Monday. "We
immediately e-mailed Henry, as we always do, exactly when and where
to look for the planet crossing in front of the star."
Henry took up the search on the night of Nov. 7. He noted at one
point a 1.7 percent dip in the star's brightness, a phenomenon that
lasted about two hours.
"This planetary transit occurred at exactly the time predicted
from Marcy's observations, confirming absolutely the presence of a
companion," Henry said. "We've essentially seen the shadow of the
planet and used it to measure the planet's size."
An effort to repeat the observations on Sunday night failed
because of rain at a telescope in northern California and rare
clouds at the Arizona telescopes.
Because the star is dropping from view at these observatories,
astronomers hope to continue research at Hawaii and other sites
around the world. One objective is to study light leaking through
the fringes of the planet's shadow, which could reveal the
composition of the planet's atmosphere and other physical
"This is what we've been waiting for," Marcy said, reacting to
the discovery and prospects for further research.
The first indirect evidence for a planet around another ordinary
star was found in 1995 by Dr. Michel Mayor of the Geneva
Observatory in Switzerland.
Until earlier this year, only single Jupiter-size planets could
be detected around any of the stars. Then Marcy and Butler detected
the gravitational effects of two large planets orbiting Upsilon
Andromedae, 44 light-years away. It is only a matter of time, most
astronomers think, before they will be able to compare and contrast
the solar system with many other planetary systems.