www.astrosociety.org/uitc
No.
19 - Winter 1991-92
©
1992, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco,
CA 94112
The
Search for Planets Around Other Stars
by Andrew Fraknoi,
Astronomical Society of the Pacific
The question of
whether there are planets outside our Solar System has intrigued scientists,
science fiction writers and poets for years. But how can we know if any really
exist? We devote this issue of The Universe in the Classroom to
the search for planets around other stars.
Stars are huge luminous
balls of gas powered by nuclear reactions at their centers. The enormously high
temperatures and pressures in the core of a star force atoms of hydrogen to fuse
together and become helium atoms, releasing tremendous amounts of energy in the
process. Planets are much smaller with core temperatures and pressures too low
for nuclear fusion to occur. Thus they emit no light of their own. When you see
Venus or Jupiter in the night sky, you're really seeing sunlight reflected by
those planets back to you.
Some planets,
like Earth and Mars are solid rocky bodies, but others, like Jupiter and Saturn
are mostly gas and liquid. Jupiter, the largest planet in our solar system,
is roughly 300 times more massive than the Earth, but only one-thousandth the
mass of the Sun. However, had Jupiter been 75 times more massive, it would just
have been large enough for the pressures and temperatures at its core to ignite
nuclear fusion, and the Earth would have had two Suns in our skies.
We think our own Solar
System formed as a natural by-product of the formation of the Sun. About five
billion years ago, a huge, amorphous cloud of gas and dust, thousands of times
larger than the present Solar System, began to contract. The exact reason why
the contraction began is not clear; one idea is that a nearby exploding star gave
it a push. But once started, the cloud collapsed under its own gravity, with most
of the gas and dust falling to the center to form the Sun. The remaining material
fell into a broad, flattened disk. Throughout the disk, dust grains orbiting the
proto-Sun collided with one another, occasionally sticking together. Small clumps
joined together to make ever larger ones, eventually forming the planets. This
process of accumulating material is called accretion. According to
this scenario, planets are a natural by-product of the formation of the Sun. Thus
astronomers think many stars like the Sun should have planets.
Because planets are
small, appear to lie close to their parent star, and shine only by reflected starlight,
the faint glimmer of a planet is lost in the brilliant glare from its parent star.
Imagine suspending a grain of rice an inch or two from a lighted 100 Watt light
bulb. Someone standing at the end of a long dark hall would see only the light
bulb, not the grain of rice. It's the same with planets and stars. Consider the
case of Jupiter and the Sun. Jupiter is only a tenth the size and has one hundredth
the surface area of the Sun. As seen from the nearest star, Alpha Centauri, mighty
Jupiter would appear extremely faint, a billionth as bright as the Sun.
Jupiter would also appear extremely close to the Sun, a mere four arc seconds
away (an arc second is a unit of angular measurement equal to the apparent size
of a U.S. quarter from a distance of five kilometers or three miles). An Alpha
Centauran, with equipment similar to our best instruments on our largest telescopes,
would simply not be able to see Jupiter in the glare of the Sun. Since most stars
are much farther away than Alpha Centauri, there is little chance of seeing or
photographing individual planets around other stars.
 |
| The star moving in
a straight line has no planets; the one which "wobbles" around
a straight path is being influenced by an unseen planet's orbital motion.
(Diagram courtesy David C. Black, NASA Ames Research Center) |
Although we cannot
see the planet itself, we can see the effect of the gravitational tug the planet
exerts on its parent star. As the planet revolves around the star, it pulls the
star first one way, then the other. The more massive the planet, the more noticeable
its effect on the star will be. As the star moves through space, the planet's
tugs show up as tiny deviations from a straight-line path. That's because the
star and the planet actually move around the center of mass of the star-planet
system, the point where one would balance a seesaw holding the star on one end
and the planet on the other. For example, the Sun is a thousand times more massive
than Jupiter, so the center of mass of the Sun-Jupiter system lies very close
to the Sun. Nevertheless, an extraterrestrial observer measuring the Sun's motion
through space would detect a slight wobble in the Sun's path, a wobble with a
period of twelve years, the same time it takes Jupiter to orbit once around the
center of mass. Smaller planets like the Earth also cause perturbations on the
Sun's orbit, but they are so tiny they couldn't be detected across interstellar
distances. Analysis of the wobbles can give information about the planet's mass,
orbit, period and distance from the star.
If we make extremely
detailed measurements of a star's position, accurate to one-thousandth of an
arc second, we might be able to see wobbles in its motion due to a large unseen
companion. Current techniques in astrometry, the branch of astronomy
that deals with measuring positions of stars, are becoming capable of detecting
Jupiter-sized planets around nearby stars.
Several stars do indeed
seem to follow slightly wavy paths through space. Some astronomers have reported
that a few stars have companions with masses similar to that of Jupiter (perhaps
the most famous is Barnard's Star), but other astronomers have been unable
to confirm these claims. Astrometric observations are extremely difficult to make
since the sought-after wiggles are very small, about one-thousandth the size of
a star's image on an astronomical photograph. One problem is that the errors inherent
in making the observations and measurements are about the same size as the planetary-induced
wiggles astronomers are seeking, making it very difficult to be sure if a measured
wobble is real. So far there have been no uncontested detections of planets from
stellar wobbles.
Barnard's Star is
a faint, red star about two-tenths as massive as the Sun. It is six light years
away (fourth closest star to the Sun), and has the largest proper motion (angular
motion across the sky) of any known star. In 1963, Peter van de Kamp, then director
of Swarthmore College's Sproul Observatory, announced that, based on an analysis
of its motion, Barnard's Star had an unseen companion. Van de Kamp estimated that
the companion was 50 percent heavier than Jupiter, much too small to be a star.
Six years later, van de Kamp revised his analysis and declared that Barnard's
Star actually had two planetary companions, one 0.7 times the mass of Jupiter,
the other half Jupiter's mass. It seemed that the first real planetary system
had been found around another star.
But other astronomers,
using different telescopes, didn't see any evidence of van de Kamp's perturbations
when they studied the motion of Barnard's Star. Critics questioned his procedures
and charged that he had not properly corrected for small changes in his telescope
over time, especially when its lenses were cleaned and reinstalled. So far,
no one has been able to duplicate his results. Van de Kamp still believes in
his perturbations and his two-planet interpretation. But most astronomers today
doubt they are real.
A planet's gravitational
tug on a star can also be seen in measurements of the star's radial velocity,
its motion toward or away from us along the line of sight from Earth to the star.
As the star orbits the system's center of mass, it alternately moves toward, then
away from us. Features in the star's rainbow-like spectrum are Doppler shifted
slightly toward the blue end of the spectrum when the star is approaching and
toward the red end when it is receding. It's the same principle that causes the
sound waves in a police car's siren to change pitch as it approaches you, and
then recedes from you. Because a star is much more massive than a planet, the
size of the Doppler shift is extremely small, requiring very sophisticated instruments
to measure it.
Bruce Campbell
of the Dominion Astrophysical Observatory in Victoria, British Columbia has
studied a number of nearby stars, looking for subtle shifts in radial velocity.
About half show velocity variations indicative of possible planet-sized companions
from one to ten times Jupiter's mass. But variations in the star itself, pulsations,
for example, could also cause small radial velocity changes like those observed.
If these pulsations are periodic, they could easily be mistaken for planetary
companions. There are ways to tell planets from pulsations, but they require
years of painstaking observations and analysis, which have not yet been completed.
Nevertheless, these observations remain some of the more promising candidates
for extrasolar planetary systems.
In 1983, the Infra-Red
Astronomical Satellite (IRAS) surveyed the sky, measuring
the heat given off by astronomical objects. Among its many discoveries was that
several nearby stars, including the bright stars Vega and Fomalhaut,
are surrounded by shells or disks of orbiting solid particles. Most of the disks
stretch several hundred AU from their parent stars (one Astronomical Unit, or
AU, is the distance from the Sun to the Earth, about 150 million kilometers or
93 million miles). In the case of Vega (the brightest star in the constellation
Lyra) the disk extends out 7.4 billion miles from the star, or about twice the
distance from the Sun to Pluto, our farthest planet. Astronomers think that the
disks are remnants of the formation of the star, and possibly an early stage in
the formation of a planetary system.
Astronomers have
also found disks of material around a class of very young stars called T
Tauri stars (named after the prototype star in the constellation of Taurus).
Disks of material seem to be a common attribute of young stars. But they are
not planetary systems. There is no way to tell for sure if there are planets
present in addition to the disks, if the disks will some day form planets, or
if the disks are all that will ever be there. But they do indicate that solid
matter can form in a disk-like configuration very similar to the one out of
which astronomers think our Solar System condensed.
Pulsars are
compact, ultra-dense, rapidly spinning stars with strong magnetic fields, believed
to be born in the fiery debris of a supernova explosion, the enormously
powerful death throes of a giant star. As the pulsar spins on its axis several
times a second, a powerful pulse of energy sweeps by the Earth, rather like the
rotating beacon of an interstellar lighthouse. These pulses are normally very
regular, but last July, British astronomer Andrew Lyne and colleagues found that
the radio pulses from one pulsar had a puzzling variation. At first the signals
arrived a hundredth of a second earlier than average. Three months later they
were a hundredth of a second late. After another three months it was early again,
and so on. Lyne thought the radio pulse variations were Doppler shifts as the
pulsar, tugged by an unseen companion about ten times the mass of the Earth, orbited
a center of mass.
Several months
later, Alexander Wolszczan, of the Arecibo Radio Observatory in Puerto Rico,
and Dale Frail, of the National Radio Astronomy Observatory, reported semi-regular
variations in radio pulses from a different pulsar. They concluded that this
pulsar has two companions, each about 3 times the mass of the Earth. They also
reported a possible third planet about the same size as Earth.
But in January
1992, Lyne reported that his team had not properly removed the effects of the
Earth's motion around the Sun from their analysis, and, when the calculations
were redone correctly, the pulse variations disappeared. There was no planet.
The variations Wolszczan and Frail noticed are too complex to be caused by the
Earth's motion, but most astronomers are waiting for more information before
deciding if Wolszczan and Frail have indeed detected planets around a pulsar.
Summary
The search for planets
outside our Solar System is a difficult one, hampered by the extremely large distances
between stars and the inherent faintness of the objects we seek. So far, there
is no proof that any star other than the Sun has planets circling it. There are
several tantalizing hints and possible detections, but none is without controversy.
As Percival Lowell, a 19th century astronomer who saw seasonal variations in the
surface markings of Mars and convinced himself he was seeing evidence of a dying
civilization (later proven to be wrong), once said: "When dealing with the most
far-reaching scientific questions, it can be hard to separate one's science from
one's imagination." Still, in the next few years, better instruments and more
sophisticated techniques may yet tell us whether planets exist around other stars.
by Thomas Hockey,
University of Northern Iowa
Materials
- Elongated rubber
pet toy
- Two rubber or
plastic balls (of similar size but unequal mass)
- Bright sticker
(phosphorescent sticker is optional)
- Phosphorescent
ball (or a ball painted with phosphorescent paint)
Instructions
Attach the two rubber
balls to the pet toy. The "double loop" design of the toy holds the balls in place
without fasteners. When tossed, the model turns end over end; the two balls revolve
around a point on the toy, the center of mass. Ask students to estimate where
the center of mass is located. They can then determine experimentally where this
center is by placing the model on a finger - the center of mass occurs where the
model balances. Students may be surprised that this point is not situated half
way between the balls but at a point closer to the more-massive ball. Mark the
center of mass with a bright sticker. Now throw the model again. The balls will
revolve around an axis beneath the sticker. If one of the balls is exchanged with
a more or less massive one, the sticker will no longer remain "steady" in flight
but will itself revolve around the new center of mass.
To illustrate
the astrometric wobble of a star with a planetary companion, remove the two
balls from the toy. Turn off the classroom lights. Toss the phosphorescent ball
by itself and ask the students to observe the relatively simple curve of its
path (analogous to a star with no planets). Then place the phosphorescent ball
in the small loop of the toy (leaving the other loop empty, symbolizing a planet
with much less mass than its star) without the students knowing it. Toss the
model once again in darkness, and note the more complicated path of the glowing
ball. Ask the students to hypothesize why the apparent motion of the ball differed
between the two tosses. Then show them the model with the lights on, and discuss
the empty loop's role as an unseen planetary companion. Finally you can toss
the model again, this time with phosphorescent paint or a phosphorescent sticker
applied to the center of mass.
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