BASED ON A
UNIVERSITY OF CALIFORNIA BERKELEY PRESS
November 14, 1999: Last Friday, a team
led by Greg Henry (Tenn. State) and Geoff Marcy (UC Berkeley)
announced the discovery of a shadow of a planet crossing a distant
star. Little known HD 209458, a Sun-like star 150 light-years
away, had been suspected of harboring planets from a slight wobble
found in its motion. Henry et al. now find that this wobble exactly
corresponds to a planet crossing the face of the star, creating
the slight dimming effect of a partial eclipse. The astronomers
were then able to make a ground breaking estimate of the mass
and radius of the extra-solar planet, which they find to have
about two-thirds the mass of Jupiter but about 60 percent larger
Above: Artist Lynette Cook's depiction
of a planetary eclipse in the HD 209458 system. Copyright 1999,
Lynette Cook, all rights reserved. [more
information; see also NASA/Goddard's Astronomy
Picture of the Day]
"This is the first independent confirmation of a
planet discovered through changes in a star's radial velocity
and demonstrates that our indirect evidence for planets really
is due to planets," said Geoffrey Marcy, a professor of
astronomy at the University of California, Berkeley.
Marcy and his colleagues, Paul Butler of the Department of Terrestrial
Magnetism at the Carnegie Institution of Washington in Washington,
D.C., and Steve Vogt of UC Santa Cruz and Lick Observatory, first
detected a wobble in the star called HD 209458 on Nov. 5. Ascribing
the wobble to a nearby planet, they were able to estimate its
orbit and approximate mass.
As with all new planets they detect, the team immediately brought
it to the attention of collaborator Greg Henry, an astronomer
at the Tennessee State University Center of Excellence in Information
Systems in Nashville. He conducts research with several automatic
telescopes at Fairborn Observatory, a non-profit research foundation
located in the Patagonia Mountains of southern Arizona.
Henry turned one of his automatic telescopes on the star at the
time Marcy and Butler predicted the planet would cross the face
of the star if the planet's orbital plane were lucky enough to
carry it between Earth and the star. Until now, none of the 18
other extrasolar planets Marcy and Butler have discovered has
had its orbital plane oriented edge-on to Earth so that the planet
could be seen to transit the star, nor have any of the other
planets discovered by other researchers.
However, on Nov. 7, Henry observed a 1.7 percent dip in the star's
brightness. Because the planet orbits its star once every 3.523
days, he plans to repeat his observations on Sunday, Nov. 14.
"This planetary transit occurred at exactly the time predicted
from Marcy's observations, confirming absolutely the presence
of a companion," Henry said. "The amount of dimming
of the star's light during the transit also gives us the first-ever
measure of the size and density of an extrasolar planet. We've
essentially seen the shadow of the planet and used it to measure
the planet's size."
Above: Greg Henry's photometry data
showing ingress of the planet orbiting HD 209458 onto the stellar
disk. Quoting from IAU Circular #7307 (11-12-99):
Tennessee State University, G.Marcy, U.C. Berkeley, R.P.Butler,
Dept. of Terrestrial Magnetism, and S.S.Vogt, UCO Lick Observatory
report that HD 209458 (G0V) exhibits sinusoidal velocity variations
with semi-amplitude of 81 m/s, indicating presence of a companion
with Msini = 0.63 Jupiter masses and an orbital period of 3.523
Photometry reveals a transit ingress at JD 2451490.70 with depth
of 0.017 mag, consistent with the transit time predicted from
the velocities. Further measurements of transits and velocities
would be valuable. The next three predicted times of ingress
occur at UT times: 15 Nov 6:19, 18 Nov 18:53, 22 Nov 7:28, all
times uncertain by 1 hour.
If correct, the inferred mass is 0.63 Mjup and radius is 1.6
Rjup, implying a density of 0.21 g/cc."
The star HD 209458 is 47 parsecs (153 light years or 1.4 million
billion kilometers or 895,000 billion miles) away in the constellation
of Pegasus, and is about the same age, color and size as our
own Sun. It is very near the star, 51 Pegasi, around which the
first extrasolar planet was discovered in 1995.
With the orbital plane of the planet known, the astronomers for
the first time could determine precisely the mass of the planet
and, from the size of the planet measured during transit, its
Interestingly, while the planet's mass is only 63 percent of
Jupiter's mass, its radius is 60 percent bigger than that of
Jupiter. This fits with theories that predict a bloated planet
when, as here, the planet is very close to the star.
The density, about 0.2 grams per cubic centimeter, means it is
a gas giant like Jupiter. However, such gas giants could not
have formed at the distance this planet is from its star.
"This supports the theory that extrasolar planets very near
their star did not form where they are, but formed farther out
and migrated inward," Henry said.
Various groups around the world have been searching for planets
by looking for dimming of stars, or as Marcy says, "staring
at the sky and seeing if any star blinks." To date, none
of these searches has turned up a new planet.
"With this one, everything hangs together," Marcy said.
"This is what we've been waiting for."
The research described in this article was supported by the
National Aeronautics and Space Administration, the National Science
Foundation, Sun Microsystems and the Richard Lounsbery Foundation.