No doubt humans have struggled with the question
of whether we are alone in the universe since
the beginning of consciousness. Today, armed
with evidence that planets do indeed orbit other
stars, astronomers wonder more specifically: What are those
planets like? Of the 100 billion stars in our Milky Way galaxy,
how many harbor planets? Among those planets, how
many constitute arid deserts or frigid hydrogen balls? Do
some contain lush forests or oceans fertile with life?
For the first time in history, astronomers can now address
these questions concretely. During the past two and a half
years, researchers have detected eight planets orbiting sunlike
stars. In October 1995 Michel Mayor and Didier Queloz
of Geneva Observatory in Switzerland reported finding the
first planet. Observing the star 51 Pegasi in the constellation
Pegasus, they noticed a telltale wobble, a cyclical shifting of
its light toward the blue and red ends of the spectrum. The
timing of this Doppler shift suggests that the star wobbles
because of a closely orbiting planet, which revolves around
the star fully every 4.2 days—at a whopping speed of
482,000 kilometers (299,000 miles) an hour, more than
four times faster than Earth orbits the sun.
Another survey of 107 sunlike stars, performed by our
team at San Francisco State University and the University of
California at Berkeley, has turned up six more planets. Of
those, one planet circling the star 16 Cygni B was independently
discovered by astronomers William D. Cochran and
Artie P. Hatzes of the University of Texas McDonald Observatory
on Mount Locke in western Texas.
Detection of an eighth planet was reported in April 1997,
when a nine-member team led by Robert W. Noyes of
Harvard University detected a planet orbiting the star Rho
Coronae Borealis. A ninth large object, which orbits the star
known by its catalogue number HD114762, has also been
observed—an object first detected in 1989 by astronomer
David W. Latham of the Harvard-Smithsonian Center for
Astrophysics and his collaborators. But this bulky companion
has a mass more than 10 times that of Jupiter—large,
though not unlike another large object discovered around
the star 70 Virginis, a similar object with a mass 6.8 times
that of Jupiter. The objects orbiting both HD114762 and 70
Virginis are so large that most astronomers are not sure
whether to consider them big planets or small brown dwarfs,
entities whose masses lie between those of a planet and a star.
Detecting Extrasolar Planets
Finding extrasolar planets has taken a long time because
detecting them from Earth, even using current technology,
is extremely difficult. Unlike stars, which are fueled
by nuclear reactions, planets faintly reflect light and emit
thermal infrared radiation. In our solar system, for example,
the sun outshines its planets about one billion times in visible
light and one million times in the infrared. Because of the distant
planets’ faintness, astronomers have had to devise special
methods to locate them. The current leading approach is the
Doppler planet-detection technique, which involves analyzing
wobbles in a star’s motion.
Here’s how it works. An orbiting planet exerts a gravitational
force on its host star, a force that yanks the star around
in a circular or oval path—which mirrors in miniature the
planet’s orbit. Like two twirling dancers tugging each other
in circles, the star’s wobble reveals the presence of orbiting
planets, even though we cannot see them directly.
The trouble is that this stellar motion appears very small
from a great distance. Someone gazing at our sun from 30
light-years away would see it wobbling in a circle whose
radius measures only one seventh of one millionth of one degree.
In other words, the sun’s tiny, circular wobble appears
only as big as a quarter viewed from 10,000 kilometers away.
Yet the wobble of the star is also revealed by the Dopplereffect of the starlight. As a star sways to and fro relative to
Earth, its light waves become cyclically stretched, then compressed—
shifting alternately toward the red and blue ends of
the spectrum. From that cyclical Doppler shifting, astronomers
can retrace the path of the star’s wobble and, from
Newton’s law of motion, compute their masses, orbits and
distances from their host stars. The cyclical Doppler shift
itself remains extremely tiny: stellar light waves shrink and
expand by only about one part in 10 million because of the
pull of a large, Jupiter-like planet. The sun, for example,
wobbles with a speed of only about 12.5 meters per second,
pivoting around a point just outside its surface. To detect
planets around other stars, measurements must be highly
accurate, with errors in stellar velocities below 10 meters
per second.
Using the Doppler technique, our group can now measure
stellar motions with an accuracy of plus or minus three
meters per second—a leisurely bicycling speed. To do this,
we use an iodine absorption cell—a bottle of iodine vapor—
placed near a telescope’s focus. Starlight passing through the
iodine is stripped of specific wavelengths, revealing tiny shifts
in its remaining wavelengths. So sensitive is this technique
that we can measure wavelength changes as small as one part
in 100 million.
As recorded by spectrometers and analyzed by computers,
a star’s light reveals the telltale wobble produced by its orbiting
companions. For example, Jupiter, the largest planet in
our solar system, is one thousandth the mass of the sun.
Therefore, every 11.8 years (the span of Jupiter’s orbital
period) the sun oscillates in a circle that is one thousandth
the size of Jupiter’s orbit. The other eight planets also cause
the sun to wobble, albeit by smaller amounts. Take Earth,
having a mass 1/318 that of Jupiter and an orbit five times
closer: it causes the sun to move a mere nine centimeters
a second.
Yet some uncertainty about each extrasolar planet’s
mass remains. Orbital planes that astronomers view
edge-on will give the true mass of the planet. But
tilted orbital planes reduce the Doppler shift because of
a smaller to-and-fro motion, as witnessed from Earth.
This effect can make the mass appear smaller than it
is. Without knowing a planet’s orbital inclination,
astronomers can compute only the least possible mass
for the planet; the actual mass could be larger.
Thus, using the Doppler technique to analyze light
from about 300 stars similar to the sun—all within
50 light-years of Earth—astronomers have turned up
eight planets similar in size and mass to Jupiter and
Saturn. Specifically, their masses range from about a
half to seven times that of Jupiter, their orbital periods
span 3.3 days to three years, and their distances from
their host stars extend from less than one twentieth
of Earth’s distance to the sun to more than twice that
distance [see illustration on opposite page].
To our surprise, the eight newly found planets
exhibit two unexpected characteristics. First, unlike
planets in our solar system, which display circular
orbits, two of the new planets move in eccentric, oval
orbits around their hosts. Second, five of the new
planets orbit very near their stars—closer, in fact, than
Mercury orbits the sun. Exactly why these huge
planets orbit so closely—some skim just over their
star’s blazing coronal gases—remains unclear. These
findings are mysterious, given that the radius of Jupiter’s
orbit is five times larger than that of Earth. These
observations, in turn, provoke questions about our own
solar system’s origin, prompting some astronomers to
revise the standard explanation of planet formation.
Reconsidering How Planets Form
What we have learned about the nine planets in our
own solar system has constituted the basis for the
conventional theory of planet formation. The theory
holds that planets form from a flat, spinning disk of gas and
dust that bulges out of a star’s equatorial plane, much as
pizza dough flattens when it is tossed and spun. This model
shows the disk’s material orbiting circularly in the same
direction and plane as our nine planets do today. Based on
this theory, planets cannot form too close to the star, because
there is too little disk material, which is also too hot to coalesce.
Nor do planets clump extremely far from the star, because
the material is too cold and sparse.
Considering what we now know, such expectations about
planets in the rest of the universe seem narrow-minded.
The planet orbiting the star 47 Ursae Majoris in the Big
Dipper constellation stands as the only one resembling what
we expected, with a minimum bulk of 2.4 Jupiter-masses
and a circular orbit with a radius of 2.1 astronomical units
(AU)—1 AU representing the 150-million-kilometer distance
from Earth to the sun. Only a bit more massive than Jupiter,
this planet orbits in a circle farther from its star than Mars
does from the sun. If placed in our solar system, this new
planet might appear as Jupiter’s big brother.
But the remaining planetary companions around other
stars baffle us. The two planets with oval orbits have eccentricities of 0.68 and 0.40. (An eccentricity of zero is a perfect
circle, whereas an eccentricity of 1.0 is a long, slender oval.)
In contrast, in our solar system the greatest eccentricities
appear in the orbits of Mercury and Pluto, both about 0.2;
all other planets show nearly circular orbits (eccentricities
less than 0.1).
These eccentric orbits have prodded astronomers to scratch
their heads and revise their theories. Within two months of
the first planet sighting, theorists hatched new ideas and adjusted
the standard planet formation theory.
For instance, astronomers Pawel Artymowicz of the University
of Stockholm and Patrick M. Cassen of the National
Aeronautics and Space Administration Ames Research
Center recalculated the gravitational forces at work when
planets emerge from disks of gas and dust seen swirling
around young, sunlike stars. Their calculations show that
gravitational forces exerted by protoplanets—planets in the
process of forming—on the gaseous, dusty disks create alternating
spiral “density waves.” Resembling the “arms” of
spiral galaxies, these waves exert forces back on the forming
planets, driving them from circular motion. Over millions of
years, planets can easily wander from circular orbits into eccentric,
oval ones.
A second theory also accounts for large orbital eccentricities.
Suppose, for instance, that Saturn had grown much
larger than it actually is. Conceivably, all four giant planets
in our solar system—Jupiter, Saturn, Uranus and Neptune—
could have swelled into bigger balls if our original protoplanetary
disk had contained more mass or had existed
longer. In this case, the solar system would contain four
superplanets, exerting gravitational forces on one another,
perturbing one another’s orbits and causing them to intersect.
Eventually, some of the superplanets might be gravitationally
thrust inward,others outward, an unlucky few even ejected from the planetary system.Like balls ricocheting
on a billiards table,the scattered giant planets might adopt extremelyeccentric orbits, as we
now observe for three of the new planets. Interestingly,this billiards model for eccentric planets shows that we should beable to detect the massive planets causing eccentric orbits—planets perhaps
orbiting farther out than the planets we have detected thus far. A variation on this theme suggests that a companion
star, rather than other
planets, might gravitationally
scatter planet
orbits.
The most bizarre of the
new planets are the four
so-called 51 Peg planets,
which show orbital periods
shorter than 15 days. The four members of this class are
51 Peg itself, Tau Bootis, 55 Cancri and Upsilon Andromedae,
which have orbital periods of just 4.2, 3.3, 14.7 and 4.6 days,
respectively.
These orbits are all small, with radii less than one tenth the
distance between Earth and the sun—indeed, less than one
third of Mercury’s distance from the sun. Yet these planets
are as big as, or bigger than, the largest planet in our solar
system. They range in mass from 0.44 of Jupiter’s mass for
51 Peg to 3.64 of Jupiter’s mass for Tau Bootis. Their
Doppler shifts suggest that these planets orbit in circles.
Mysterious 51 Pegasi–Type Planets
The 51 Peg planets defy conventional planet formation
theory, which predicts that giant planets such as Jupiter,
Saturn, Uranus or Neptune would form in the cooler
outskirts of a protoplanetary disk, at least five times the
distance from Earth to the sun.
To account for these planetary oddities, a revised planet
formation theory is making the rounds in theorists’ circles.
Astronomers Douglas N. C. Lin and Peter Bodenheimer, both
of the University of California at Santa Cruz, and Derek C.
Richardson of the University of Washington extend the
standard model by arguing that a young protoplanet precipitating
out of a massive protoplanetary disk will carve a
groove in the disk, separating it into inner and outer sections.
According to their theory, the inner disk dissipates energy
because of dynamical friction, causing the disk material and
the protoplanet to spiral inward and eventually plunge into
the host star.
A planet’s salvation stems from the young star’s rapid
rotation, spinning every five to 10 days. Approaching its star,a planet would cause tides on the star to rise, just as the
moon raises tides on Earth. With the young star rotating
faster than the protoplanet orbiting the star, the star would
tend to sprout a bulge whose gravity would tug the planet
forward. This effect would tend to whip the protoplanet into
a larger orbit, halting its deathly inward spiral.
In this model, the protoplanet hangs poised in a stable
orbit, delicately balanced between the disk’s drag and the
rotating star’s forward tug. Even before the discovery of the
51 Peg planets, Lin predicted that Jupiter should have spiraled
into the sun during its formation. If this were so, then
why did Jupiter survive? Perhaps our solar system contained
previous “Jupiters” that did indeed spiral into the sun, leaving
our Jupiter as the sole survivor.
Why, we wonder, does no large 51 Peg–like planet orbit
close to our sun? Perhaps Jupiter formed near the end of our
protoplanetary disk’s lifetime. Or the protoplanetary disk
may have lacked enough gas and dust to exert sufficient
tidal drag. Perhaps protoplanetary disks come in a wide
range of masses, from a few Jupiter-masses to hundreds of
Jupiter-masses. In that case, the diversity of new planets
may correspond to different disk masses or disk lifetimes,
perhaps even to different environments, including the presence
or absence of nearby radiation-emitting stars.
On the other hand, astronomer David F. Gray of the University
of Western Ontario in Canada has challenged the existence
of the 51 Peg planets altogether. Gray argues that the alleged
planet-bearing stars are themselves oscillating—almost like
wobbling water balloons. In his view, the cyclical Doppler
shifts in these stars stem from inherent stellar wobbles, not
planets tugging at stars.
Armed with new data, astronomers now largely dismiss the
existence of the oscillations. The strongest argument against
the oscillations stems from the single period and frequency
seen in the Doppler variations from the star. Most oscillating
systems, such as tuning forks, display a set of harmonics, or
several different oscillations occurring at different frequencies,
rather than just one frequency. But the 51 Peg stars show
only one period each, quite unlike harmonic oscillations.
Moreover, ordinary physical models predict that the
strongest wobbles would occur at higher frequencies than
those of the observed oscillations of these stars. In addition,
the 51 Peg stars show no variations in brightness, suggesting
that their sizes and shapes are not changing.
Planetary Comparisons
Although we are tempted to compare the eight new
planets with our own nine, the comparison is, unfortunately,
quite challenging. No one can draw firm
conclusions from only eight new planets. So far our ability
to spot other types of planets remains limited. At present,
our instruments cannot even detect Earth-size companions.
Although the extrasolar planets found to date have orbital
periods no longer than three years, this finding does not
necessarily represent planetary systems in general. Rather it
arises from the fact that astronomers have searched for
other planets with better techniques for only about a decade. With more time and improved Doppler precision,
more planets with longer orbital periods may be found.
Curiously, finding these new planets proves that our own
history could easily have played out quite differently. Suppose
that gravitational scattering of planets occurs commonly in
planetary systems. We see in our own solar system evidence
that during its first billion years, planetesimals—fragmentary
bodies of rock and ice—hurtled through space. Our cratered
moon and Uranus’s highly tilted axis—nearly perpendicular
to the axes of all its neighbors—show that collisions were
common, some involving planet-size objects. The neatly
carved orbits of our now stable solar system emerged from
the collision-happy orbits of its youth.
We should consider ourselves lucky that Jupiter ended up
in a nearly circular orbit. If it had careened into an oval orbit,
Jupiter might have scattered Earth, thwacking it out of the
solar system. Without stable orbits for Earth and Jupiter, life
might never have emerged.
The Future of Planet Hunting
In July 1996 we began a second Doppler survey of 400
stars, using the 10-meter Keck telescope at Mauna Kea
Observatory in Hawaii. Mayor and Queloz of Geneva
Observatory recently tripled the size of their Northern Hemisphere
Doppler survey to about 400 stars, and soon they will
begin a Southern Hemisphere survey of 500 more stars.
Within the next year, Doppler surveys of several hundred
additional stars will begin at the nine-meter Hobby-Eberly
Telescope located at McDonald Observatory.
By the year 2000 two Keck telescopes on Mauna Kea and
a binocular telescope at the University of Arizona will become
optical interferometers, precise enough to image extrasolar
planets. NASA plans to launch at least three spaceborne
telescopes to detect planets in infrared light.
One proposed NASA space-based interferometer,
a second-generation telescope known
as the Terrestrial Planet Finder, should obtain
pictures of candidate habitable planets orbiting
distant stars. Arguably the greatest telescope
ever conceived, Planet Finder could spot other
Earths, starting in about 2010. Using a
spectrometer, it could analyze light from faroff
planets to determine the chemical makeup
of their atmospheres—data to determine if
biological activity is proceeding. This monumental,
spaceborne telescope would span a
football field and sport four huge mirrors.
Drawing from the data on planets found so
far, we believe other planets orbit similar stars,
many the size of Jupiter, some the size of Earth.
It may be that as many as 10 percent of all
stars in our galaxy host planetary companions.
Based on this estimate, 10 billion planets
would exist in our Milky Way galaxy alone.
Seeking the ideal Earth-like planet on which
life could flourish, astronomers will search for
planets that are neither too cold nor too hot,
temperate enough to sustain liquid water to
serve as the mixer and solvent for organic
chemistry and biochemistry. Planets with the
perfect blend of molecular constituents
orbiting at just the right distance from the sun
enjoy what astronomers call a “Goldilocks” orbit.
Seeing such a planet would spawn an endless stream of
questions: Does its atmosphere contain oxygen, nitrogen,
and carbon dioxide, like Earth’s, or sulfuric acid and CO2,
the deadly combination on Venus? Is there a protective
ozone layer, or is the surface scorched by harmful ultraviolet
rays? Even if a planet has oceans, does the water have a pH
neutral enough to permit cells to grow?
There may even exist some other biology that thrives on
sulfuric acid—even starves without it. Indeed, if primitive life
does arise on another Earth, does it always evolve toward
intelligence, or is our human technology some fluke of
Darwinian luck? Are we humans a rare quirk of nature,
destined to appear on Earth-like planets only once in a
universe that otherwise teems with primitive life?
Amazing as it seems, answers to some of these questions
may arise during our lifetimes, using tools such as telescopes
already in existence or on the drawing board. We can only
barely imagine what the next generation will see in our
reconnaissance of the galactic neighborhood. Human
destiny lies in exploring the galaxy and finding our roots,
biologically and chemically, out among the stars.
0 comments:
Post a Comment