Apr. 15, 1999: Astronomers from four research institutions have discov
ered
strong evidence for a trio of Jupiter-sized extrasolar planets that or
bit
the star Upsilon Andromedae. In a paper submitted to The Astrophysical
Journal, scientists announce the first multiple planet system ever fou
nd
around a normal star, other than the nine planets in our Solar System.
The
closest planet in the Upsilon Andromedae system was detected in 1996 b
y
astronomers Geoffrey Marcy and R. Paul Butler from San Francisco State
University. Now, after 11 years of telescope observations at Lick
Observatory near San Jose, CA, the signals of two additional planets h
ave
emerged from the data.
Astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA)
in
Cambridge, MA, and the High Altitude Observatory (HAO) in Boulder, CO
have
independently found the two outer planets around Upsilon Andromedae. T
his
team has been studying the star for more than four years at the
Smithsonian's Whipple Observatory near Tucson, AZ.
This first planetary system, found from a survey of 107 stars, offers
the
first suggestion that planetary systems like our own are abundant in o
ur
Milky Way Galaxy, which contains approximately 200 billion stars. SFSU
researcher Debra Fischer said, "It implies that planets can form more
easily than we ever imagined, and that our Milky Way is teeming with
planetary systems."
Upsilon Andromedae is a bright star that is visible to the naked eye f
rom
the Northern Hemisphere, starting roughly in June. It is located about
44
light-years from Earth, and is roughly 3 billion years old, about
two-thirds the age of the Sun.
The innermost (and previously known) of the three planets contains at
least three-quarters of the mass of Jupiter and orbits only 0.06 AU (8
.9
million km) from the star. (One AU is the distance between the Earth a
nd
the Sun, approximately 93,000,000 miles or 149,000,000 km.) It travers
es a
circular orbit every 4.6 days. The middle planet contains at least twi
ce
the mass of Jupiter and takes 242 days to orbit the star once. It resi
des
approximately 0.83 AU from the star, similar to the orbital distance o
f
Venus. The outermost planet has a mass of at least four Jupiters, and
completes one orbit every 3.5 to 4 years, placing it 2.5 AU from the s
tar.
The two outer planets are both new discoveries and have elliptical orb
its,
a characteristic of the nine other extrasolar planets in distant orbit
s
around their stars.
No current theory predicted that so many giant worlds would form aroun
d a
star. "I am mystified at how such a system of Jupiter-like planets mig
ht
have been created," said Marcy.
"This will shake up the theory of planet formation," Robert Noyes, a
professor of astronomy at Harvard-Smithsonian CfA and a member of the
CfA-HAO team said. "A nagging question was whether the massive bodies
orbiting in apparent isolation around stars really are planets, but no
w
that we see three around the same star, it is hard to imagine anything
else."
Scientists had suspected that there was something strange about Upsilo
n
Andromedae. The velocity variations that revealed the closest planet t
o
the star in 1996 had an unusual amount of scatter. Not until early thi
s
year had enough observations been made of the star to confirm the pres
ence
of an additional planet, which explained some of the confusing pattern
in
the data. But another object still seemed to be tugging on the star. B
oth
teams of astronomers considered astrophysical effects that could mimic
the
velocity signature from these planets, but no such effects are visible
. A
computer simulation by Greg Laughlin of U.C. Berkeley suggest that the
se
three giant planets could co-exist in stable orbits.
One big question left to answer is how such a solar system arose. "The
usual picture is that gas giant planets can only form at least four AU
away from a star, where temperatures are low enough for ice to condens
e
and begin the process of planet formation," said Timothy Brown of the
HAO
team. "But all three giant planets around Upsilon Andromedae now resid
e
inside this theoretical ice boundary." The planets may have formed clo
se
to the host star, or, like balls on a billiard table, the planets may
have
scattered off each other, migrating into their current orbits from a m
ore
distant place of origin.
The discovery of this multiple planet system suggests a new paradigm f
or
planet formation where many small planets known as 'planetesimals' mig
ht
develop in the disk of matter surrounding a star. Those planets that g
row
fastest would engage in a gravitational "tug of war" that weeds out so
me
of the smaller worlds and determines which planets ultimately remain i
n
orbit. "The Upsilon Andromedae system suggests that gravitational
interactions between Jupiter-mass planets can play a powerful role in
sculpting solar systems," said Butler.
If these Jupiter-mass planets are like our own Jupiter, they would not
be
expected to have solid Earth-like surfaces. However these observations
cannot rule-out Earth-sized planets, as their signature would be too w
eak
to detect with current instrumentation.
--
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