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51 Pegasi was the first star
similar to our sun found
to have a planet orbiting
around it.
14 imagine
ince the ancient past, humanity has looked up at the
night sky and wondered if those faint stars are in fact
bright suns shining on distant worlds. From the ancient
Greeks to Enlightenment philosophers to Hollywood
producers, it seems like everyone has had their own
speculations about what other planets may be like. But only
in the last few years has it become possible to move beyond
speculation to true knowledge. It doesn’t take the starship
Enterprise to seek out new worlds: Armed with powerful
telescopes and the latest digital detectors, astronomers
are now capable not only of detecting the presence of
planets orbiting other stars, but of measuring their physical
properties and even in some cases chemical makeups, all
from right here on Earth.
In fact, since the first distant planets were discovered
in the mid-1990s, the study of such “exoplanets” has
blossomed into one of the hottest and most active areas
of modern astronomy. The total number of planets
known around other stars is now about 300—and that
number increases by one or two new worlds every week.
It’s a very exciting time to be an astronomer!
Wobble Wobble, Little Star
The challenge of detecting distant planets is that they are
tiny and dim, almost impossibly faint compared to their
parent stars. Recall that we see our own solar system’s
planets in reflected sunlight. If we try to do the same
for distant worlds, we find that the starlight they reflect
is literally millions or even billions of times fainter than
the direct glare of their parent stars. To see such faint
pinpricks of light right next to a blazing star is a daunting
challenge indeed.
Thus the first, and still far most successful, method
of detecting exoplanets relies on a clever indirect route.
As a planet orbits a star, its gravitational pull tugs on the
star and causes it to “wobble” back and forth slightly.
That slight wobble causes an almost imperceptible
Doppler shift in the star’s light, a tiny effect but one that
can be measured using careful techniques and special
instruments on some of the world’s largest telescopes.
These measurements can reveal the masses, distances
from the star, and orbital periods of planets that remain
hidden from direct sight.
November/December 2008
Image: ©Lynette Cook
by Marshall Perrin, Ph.D.
This “radial velocity” method was pioneered in the
1990s by two teams of astronomers, one led by Michel
Mayor and Didier Queloz in Switzerland, and one led
by Geoff Marcy and Paul Butler in California. Mayor
and Queloz stunned the scientific world with their first
discovery in October 1995, of a planet now called 51
Pegasi b, but Marcy and Butler’s team quickly followed
with discoveries of their own. In the years since then, the
mostly good-natured competition among these teams and
others has led to science at its finest. Each success spurs
another team to improve techniques still further, allowing
ever smaller or more distant worlds to be found.
Strange Neighbors
All these discoveries have allowed us to piece together
an increasingly detailed tourist’s guide to the solar
neighborhood. Perhaps the biggest surprise of the
exoplanet revolution has been the tremendous diversity
of our neighboring systems. With only our own solar
system as a model, scientists once assumed that most
solar systems would consist of small rocky planets near
the star and massive gas giants at greater distances, which
took decades to round the star in their ponderous orbits.
Yet the very first discovery was of a Jupiter-mass planet
in a four-day orbit, whipping around at a rocket’s pace
just barely above the stellar surface, ten times closer than
Mercury is to our own sun. To the naked eye, it would
glow like an ember, shrouded in clouds of molten metal
and rock vapor. The surprising 51 Peg b proved to be just
the first of many such “hot Jupiters,” and today dozens of
such worlds are known.
Detecting worlds through the radial velocity
technique required watching at least one full orbit, so
naturally these fast-orbiting worlds were the quickest
to find. It took patient years of searching to find more
distant planets on their slow Jupiter-like orbits, but
eventually such worlds started to be found. Today they
dominate the catalogs, and hot Jupiters seem to be the
exception, not the rule. The majority of giant planets
are located far from their parent stars, just as in our
own solar system. Yet there were other surprises in store:
Unlike the stately circular orbits of our own planets,
the vast majority of exoplanets have highly elliptical
orbits. Some worlds swing from beyond the Earth’s
orbital distance all the way in to hot Jupiter territory,
and back out again, in a never-ending bake/freeze cycle.
Others come in pairs locked in the gravitational dance
called orbital resonance. Only a few percent of systems
contain massive planets in circular orbits relatively
November/December 2008
Marshall Perrin in front of the
Gemini North observatory on
Mauna Kea in Hawaii.
far from their stars, like our own solar system. Recent
surveys have also taught us a lot about where planets
aren’t, indicating that no more than one in five stars has
planets as massive as those in our solar system.
Theoretical models are still struggling to explain all
this diversity. It seems likely that the formation of solar
systems is a complicated and chaotic process, and one
that depends very sensitively on the initial conditions
around young stars. Like a galactic Goldilocks tale, stars
that begin their lives too heavily surrounded by dust
and gas probably give birth to hot Jupiters or planets
forced into crazy orbits, while those with too little dust
are barren and planetless. Most likely, only a very narrow
range of conditions is just right for producing solar
systems like our own—but this question remains far
from settled.
The Tiniest Eclipses
While the tried-and-true radial velocity technique has
revealed hundreds of planets, there is a limit to what
those measurements can tell us. For one thing, we still
can’t detect planets as small as the Earth; the wobbles
are just too small. For another, radial velocities tell us
only planets’ masses and orbits, but nothing about their
sizes or compositions. For those, we must turn instead to
another method called planetary transits.
A small fraction of exoplanets have orbital planes that
line up precisely with the Earth so that every so often
the planets pass in front of their parent stars, causing
tiny partial eclipses. Because planets are so much smaller
➜ 42 Hunters
imagine 15
©Lynette Cook
➜ 15 Hunters
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than stars, typically only one
optics coronagraph is under
ten-thousandth of a star’s light is
construction right now by a team
blocked. Measuring the amount
of astronomers, of which I am a
of this slight dimming lets us
part, from California, New York,
directly see a planet’s size. Similar
and Canada. Our instrument, the
measurements in the infrared
Gemini Planet Imager, should
can reveal a planet’s temperature.
be operational by early 2011.
Better yet, measuring transits
Its improved sensitivity will let
in multiple wavelengths of light
us observe Jupiter-mass planets
lets us detect gases in planets’
much closer to their parent stars;
atmospheres based on how
we will be able to see planets with
different gases absorb different
masses and orbits comparable to
wavelengths. When NASA’s
Gliese 581 c, shown from the perspective of a possible moon, is thought to be a rocky world
Jupiter and Saturn in our own
Hubble and Spitzer Space
about 1.5 times the radius of Earth. Its temperature is uncertain, yet it resides near the star’s
solar system. Just as with radial
Telescopes were designed, such
habitable zone, making the existence of oceans and lakes a possibility.
velocities, there is a scientific race
measurements were beyond
spurring us onwards, with a team
astronomers’ wildest dreams—yet those two telescopes have proven
of our European colleagues and friends building a similar instrument
champions at the extraordinarily precise measurements needed to turn
of their own. If all goes well, in a few years these instruments will be
distant eclipses into newfound knowledge. snapping photos and measuring the atmospheres of planets by the
This coming spring, those two spacecraft will be joined by a new
hundred.
space telescope called Kepler, a special-purpose craft optimized
Beyond that, the future gets hazier. NASA is today planning a series
precisely for measuring planetary transits for many more stars far more of Exoplanet Probe missions over the next few years, building toward
carefully than ever before. If all goes according to plan, Kepler will
an ambitious Terrestrial Planet Finder program around perhaps 2020
be able to detect the one-part-in-a-million microeclipses caused by
or beyond. That hypothetical craft would use techniques similar to
an Earth-like planet passing in front of a star. By surveying hundreds
coronagraphs to directly image Earth-like planets in search of alien
of thousands of stars over the next few years, Kepler boldly aims
life signs, but the precise details of how to actually build such a thing
to produce a galactic census of Earth-mass planets orbiting in the
remain far from clear. We seek no less than to answer at long last those
“habitable zone” where temperatures are right for life like ours.
age old questions: How common are worlds like our own? Do any
distant suns shine down on worlds teeming with alien life, perhaps
Toward Pale Blue Dots
even intelligent life? The chances are greater than ever that these
These and other indirect methods have unmasked for us the secrets
questions will be answered within our lifetimes. Doing so will take the
of several hundred worlds, but astronomers have never given up the
combined efforts of many hard-working scientists and brilliant young
dream of seeing such worlds directly. Such observations would let
planet hunters. Maybe they’ll even be answered by some of you! i
us find new planets with a single snapshot, instead of the long years
of waiting required for radial velocity or transit work, and would
Marshall Perrin is an astronomer working at UCLA. Originally from
open the way to much more detailed studies of atmospheres and
Annapolis, MD, he attended CTY Lancaster from 1993–1995. After gradcompositions. Over the past decade, we have nurtured adaptive optics
uating from Harvard, he headed west to California and earned a Ph.D.
technology, which can correct for the blurring and twinkling caused
in astronomy from UC Berkeley in 2006. His other interests include sciby the Earth’s turbulent atmosphere, and developed instruments called ence education, biking and backpacking, environmental conservation,
coronagraphs that can block out almost 100 percent of a star’s light
and science fiction. For more of his writing, check out his bimonthly
while still letting an orbiting planet be seen. For several years, such
column in the online magazine Strange Horizons (http://strange
instruments have surveyed nearby stars to no avail—until now.
horizons.com ).
On the very day that I am writing this, a team of Canadian
astronomers using the Gemini Telescope’s
adaptive optics system has just presented their
very first direct image of a probable planet
Word Wise Solution
orbiting a young sunlike star. It’s a weird object,
“Seen for the first time through
eight times more massive than Jupiter but
a backyard telescope, ringed
orbiting shockingly far from its parent star, ten
Saturn, icon of the otherworldly,
times Neptune’s distance from our sun, where
is the vision most likely to turn
the greater separation from the star made it
an unsuspecting viewer into an
easier to detect. astronomer forever.”
This pale dot is but the tip of the
—Dava Sobel, The Planets
iceberg. A much more advanced adaptive
42 imagine
November/December 2008