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Location: UFOUpDatesList.Com > 1999 > Apr > Apr 15

Multiple Planets Found Around A Sun-Like Star

From: Stig Agermose <stig.agermose@get2net.dk>
Date: Fri, 16 Apr 1999 00:01:01
Fwd Date: Thu, 15 Apr 1999 20:38:25 -0400
Subject: Multiple Planets Found Around A Sun-Like Star

Source: Astronomy Now's webpage on breaking news:




First system of multiple planets found around a sun-like star


April 15, 1999


SAN FRANCISCO, April 15, 1999 Astronomers from four research
institutions have discovered strong evidence for a trio of
extrasolar planets that orbit the star Upsilon Andromedae. This
is the first multiple planet system ever found 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 by San Francisco State University (SFSU) astronomers
Geoffrey Marcy and R. Paul Butler. Now, after 11 years of
telescope observations at Lick Observatory near San Jose, CA,
the signals of two additional planets have emerged from the
data. Therefore, Upsilon Andromedae harbors the first planetary
system that is reminiscent of our own Solar System.

In parallel, 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. This tea'as 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 our Milky Way Galaxy, which contains 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."

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 from the star. (One "AU" equals the distance
from the Earth to the Sun). It traverses a circular orbit every
4.6 days. The middle planet contains at least twice the mass of
Jupiter and takes 242 days to orbit the star once. It resides
approximately 0.83 AU from the star, similar to the orbital
distance of 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 star. The two outer planets are both
new discoveries and have elliptical (oval) orbits, a
characteristic of the nine other extrasolar planets in distant
orbits around their stars.

No current theory predicted that so many giant worlds would form
around a star. "I am mystified at how such a system of
Jupiter-like planets might have been created," said Marcy,
SFSU's Distinguished Professor of Science. "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 now that we see three around the same star, it is hard to
imagine anything else."

Currently a staff astronomer at the Anglo-Australian
Observatory, Butler, an American, is the lead author of the
paper, submitted to the Astrophysical Journal, announcing the
triple planet system. Along with Marcy, Fischer, and Noyes, the
authors include Sylvain Korzennik, Peter Nisenson, and Adam
Contos of the Harvard-Smithsonian CfA, and Timothy Brown of the
HAO. "Both of our groups found essentially the same size and
shape for the orbits of the companions," said Korzennik. The
chances of this happening by accident are infinitesimal." Added
Fischer, "This is an extraordinary finding and it demands
extraordinary evidence. Having two completely independent sets
of observations gives us confidence in this detection."

Marcy and Butler had suspected that there was something strange
about Upsilon Andromedae. The velocity variations that revealed
the closest planet to the star in 1996 had an unusual amount of
scatter. Not until early this year had enough observations been
made of the star to confirm the presence 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.
"We looked at the two planet solution that we had been expecting
and there was still too much extra noise," said Fischer. "We
arrived at the conclusion that the extra observed wobble could
only be explained by the presence of a third planet." Both teams
of astronomers considered astrophysical effects that could mimic
the velocity signature from these planets, but no such effects
are viable. A computer simulation by Greg Laughlin of U.C.
Berkeley suggests that these 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 condense and begin the process of planet
formation," said Brown. "But all three giant planets around
Upsilon Andromedae now reside inside this theoretical ice
boundary." The planets may have formed close to the host star,
or, like balls on a billiard table, the planets may have
scattered off of each other, migrating into their current orbits
from a more distant place of origin.

The discovery of this multiple planet system suggests a new
paradigm for planet formation where many small seed planets
known as planetesimals might develop in the disk of matter
surrounding a star. Those planets that grow fastest would engage
in a gravitational tug of war that weeds out some of the smaller
worlds and determines which planets ultimately remain in 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. But,
Nisenson noted, "Our observations can't rule out Earth-sized
planets as well in this planetary system, because their gravity
would be too weak for them to be detectable with present

A bright star visible to the naked eye starting this June,
Upsilon Andromedae is 44 light-years away from Earth, and it is
roughly 3 billion years old, two-thirds the age of the Sun. This
star should make an ideal target for NASA's upcoming Space
Interferometry Mission (SIM). Expected to launch in 2005, SIM
will spend five years probing nearby stars for Earth-sized
planets and will test technology slated for future
planet-searching telescopes. The ongoing ground-based planet
search will enable SIM to home in on those stars most likely to
harbor small planets.

San Francisco State University is a highly diverse community of
27,000 students and 3,500 faculty and staff. It is one of the
largest campuses in the nationally recognized 23-campus
California State University system. Founded in 1899, the
University is celebrating its 100th year of service to San
Francisco, the Bay Area and beyond. The Harvard-Smithsonian
Center for Astrophysics, located in Cambridge, MA, is a joint
collaboration between the Smithsonian Astrophysical Observatory
and the Harvard College Observatory.


*A schematic diagram of the orbits for the 3 planets detected
around Ups Andromedae. The red dots mark the orbits of
components b,c and d. The dashed circles show the orbits of
Mercury, Venus, Earth and Mars (inside to outside, respectively)
overplotted to give a sense of the scale of the extrasolar
planetary orbits. Jupiter, at 5 AU would be outside the
boundaries of this plot.

zoom (link)

*An image of the star Upsilon Andromedae. The newly discovered
planets are not able to be seen, because of the overpowering
brightness of the star. Image: Digital Sky Survey

Copyright 1999 Pole Star Publications Ltd

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