April 16, 1999
At Long Last, Another Sun With a Family of Planets
In New Discoveries, a Planetary Mystery
By JOHN NOBLE WILFORD
he solar system is not alone as an array of planets orbiting in
the gravitational embrace of a shining star.
Laying to rest any lingering notion of the Sun's family being a
singular phenomenon in the universe, astronomers announced on
Thursday the detection of three large planets around Upsilon
Andromedae, a solar-type star 44 light-years away.
Solitary planets had been observed around several other stars in
the last four years, but this is the first clear evidence showing
another star accompanied by multiple planets in a stable system
bearing some resemblance to the Sun's.
|An image of Upsilon Andromedae, a solar-type star with three large planets orbiting it. |
Two of the planets are several times more massive than Jupiter,
the solar system's giant, which is 318 times heftier than Earth.
The third planet, with at least three-quarters the Jovian mass, is
so close to the star that it completes a full orbit -- its year --
every 4.6 Earth days. Astronomers said that they would not be
surprised if they eventually find other, more distant objects
around the star.
Other astronomers greeted the discovery with unbridled
enthusiasm. They called it a major milestone in planetary science.
Here, finally, was what they had eagerly been seeking: another
planetary system to compare with their own. They expected further
study of the Upsilon Andromedae system to challenge some theories
of planet formation and evolution, and probably hatch new ones.
Of even greater philosophical as well as scientific importance,
the discovery encouraged astronomers in their growing belief that
the universe abounds in stars with planetary systems. This, in
turn, increased the probability that some of them include habitable
worlds, scientists said, though no such claim is being made for the
newly discovered system.
"The single planets we found around other stars was a glorious
discovery, but the architecture of other planetary systems had been
missing," Dr. Geoffrey Marcy, a leader of the discovery team, said
in an interview. "Here for the first time, we can see a kinship
between these planets and our own solar system."
Dr. Alan P. Boss, a theorist of planetary systems at the
Carnegie Institution of Washington, who must come to grips with the
implications of the findings, said simply, "This is exciting
The discovery was made independently by two teams, one from San
Francisco State University and the other from the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.,
and the National Center for Atmospheric Research in Boulder, Colo.
They joined in announcing the results at a news conference in San
Astronomers have detected three large planets orbiting the star Upsilon Andromedae, marking the first time that a multi-planet solar system has been observed around a normal star, other than the Sun. As seen in the diagram below, the innermost of these giants orbits much closer to its star than Mercury does to the Sun, completing an orbit every 4.6 Earth days.
The New York Times
|Source: San Francisco State University|
A full report, which has already been reviewed by more than a
dozen independent astronomers, has been submitted for publication
in The Astrophysical Journal.
"Having two completely independent sets of observations gives
us confidence in this detection," said Dr. Debra Fischer, of the
San Francisco team. And Marcy, the team leader and most prolific
discoverer of extra-solar planets, said, "I would bet my house on
Although Upsilon Andromedae is a nearby bright star visible to
the unaided eye, the three planets cannot be seen even with the
most powerful telescopes. Astronomers infer their existence, orbits
and minimum masses from years of careful study of their
gravitational effects, characterized as reflex motions, on the host
star. In their orbital courses, the planets tug first one way and
then the other on the star, causing ever-so-slight changes in the
This observational technique has been responsible for the
detection of 18 Jupiter-class extra-solar planets since 1995, when
Swiss astronomers found the first planet around another normal
star, 51 Pegasi.
Dr. Robert Noyes, a Harvard-Smithsonian astronomer, said the new
observations should dispel any doubts that these objects are true
"A nagging question was whether the massive bodies orbiting in
apparent isolation around stars really are planets," Noyes said in
a statement. "But now that we see three around the same star, it
is hard to imagine anything else."
Dr. Douglas Lin, a theorist at the University of California at
Santa Cruz who has sought to explain how such huge planets could
exist so close to their stars, much closer than Jupiter is to the
Sun, said the new detections should enable scientists to evaluate
their various theories. They are struggling to understand if
systems with several super-Jupiter planets, traveling eccentric
orbits close to their stars, are more typical than the solar
system, with its gaseous giant planets all traveling circular
orbits at great distance from the Sun.
"This is a very, very important discovery," Lin said in an
interview. "It tells us that planetary systems are quite
ubiquitous, and some of them are quite stable. It also tells us
that the existence of habitable planets is highly probable."
Although Earth-size planets could exist in the Upsilon
Andromedae system, astronomers said, they would be undetectable
with current search methods. In any event, they would be unlikely
to exist in what astronomers think of as the habitable zone of a
planetary system, close enough to be warmed by the star, like
Earth, but not so close as to be baked like Mercury and Venus.
Also, the gravitational forces of Jupiter class objects in that
vicinity would more than likely have cleared it any small planets.
The discovery teams calculated that the middle planet in the
Upsilon Andromedae system is in an orbit corresponding to the
distance of Venus from the Sun. It is at least twice the mass of
Jupiter, making the complete circuit every 242 days. The outermost
known planet, at least four times more massive than Jupiter, orbits
the star about once every four earth years at a distance comparable
to the region between Mars and Jupiter.
Astronomers suspect that these giant planets are, like Jupiter
and Saturn, huge spheres of gas without a solid surface. But, also
like Jupiter and Saturn, they could have many large moons.
Possibly, Lin speculated, on one of these moons there could be
liquid water, and atmosphere and other conditions conducive to
"Who knows," Lin remarked, "on one of those satellites, I
would probably have a beach-front property."
The planet closest to Upsilon Andromedae had already been
discovered by Marcy and Dr. R. Paul Butler in 1996. At the time,
they detected additional motions of the star suggesting other
companions, but only with repeated observations and careful
analysis could they be sure.
Astronomers were less surprised by the discovery than relieved.
For several months, they had generally assumed the existence of
extra-solar planetary systems. After all, the Sun is a common type
of star, one of 200 billion in the Milky Way alone, and beyond lie
more than 80 billion other galaxies. It hardly seemed likely that
the Sun's planets were unique.
The latest discovery is expected to tax the ingenuity of
theorists. It had been thought that such giant gas bodies could
only form at great distances from a star, out where temperatures
are low enough for ice to condense and begin the process of
planetary formation as gaseous spheres. Finding the single
Jupiter-class object near stars had forced scientists to invent
migration theories, explaining how the planets might have formed at
greater distances and then worked their way in closer to the star.
"I am mystified at how such a system of Jupiter-like planets
might have been created," Marcy said of the Upsilon Andromedae
system. "This will shake up the theory of planetary formation."
Boss of the Carnegie Institution said the discovery could upset
conventional ideas explaining giant Jupiter-class planets. "Maybe
nature has many ways of making giant planets," he said.
The next milestone, astronomers said, would be finding evidence
of another system with a Jupiter-class planet out at a distance
from its star corresponding to Jupiter's from the Sun.
"Until we do," Marcy said, "there will always be the question
of whether the solar system is a cosmic freak."
Other members of the Harvard-Smithsonian team were Dr. Sylvain
Korzennik, Dr. Peter Nisenson and Adam Contos, a graduate student.
Dr. Timothy Brown of the National Center for Atmospheric Research
worked with them. Their observations were made at the Whipple
Observatory near Tucson, Ariz. The San Francisco team used the Lick
Observatory near San Jose, Calif. Butler, formerly at San Francisco
State and now at the Anglo-Australian Observatory near Sydney, is
the lead author of the journal report on the detections.
It may be a decade or more before spacecraft are in place to
look for Earth-size planets of other stars.
After years of searching and speculation, both fanciful and
educated, the discovery of multiple objects orbiting Upsilon
Andromedae marked the beginning of the science of comparative