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Geoff Marcy remembers the hair standing up on the back of his neck.
Paul Butler remembers being dead tired. The two men had just made
history: the first confirmation of a planet orbiting another star.
The groundbreaking discovery had been announced less than a week
earlier by the European team of Michel Mayor and Didier Queloz. But the
news was met with some initial skepticism in the astronomical community.
By a stroke of good luck, Marcy and Butler happened to have previously
scheduled observation time on a 120-inch telescope at the Lick
Observatory, atop California's Mount Hamilton.
The scientists, who would become two of the world's most famous
planet hunters, remember driving down the mountainside together in
October 1995. They'd spent four straight nights making their
observations. And while further processing would be needed to make the
scientific case, their data seemed clear and unmistakable -- and almost
impossible. A huge planet, at least half the size of Jupiter, was not
only orbiting its host star more tightly than Mercury hugs the sun. It
was racing around that star, making a complete orbit in just four days.
The planet, called 51 Pegasi b, would open a new era in humanity's
exploration of our galactic neighborhood. It would be the first in a
series of "hot Jupiters" -- giant planets in fast, tight orbits --
discovered in rapid succession. The rush of new worlds would propel
Marcy, Butler and their research team into the media spotlight, and
forever change our view of the cosmos.
'A spine-tingling experience'
But for the moment, on that solemn drive down the mountainside, Marcy
and Butler were alone with their world-altering news. "We knew we were
the only people on the planet to be sure that 51 Peg, the planet, really
did exist," Marcy said recently. "It was exhilarating. We were
absolutely thrilled to know an historic moment in science history was
happening before our eyes. It was a truly spine-tingling experience."
Still, the astronomical pioneers had a few struggles ahead to gain
the acceptance of the scientific community. The hunt for extrasolar
planets -- exoplanets, for short -- had a poor track record, with
decades' worth of false detections. Among them was the thrilling
discovery of a planet orbiting Barnard's star in the 1960s; it turned
out to be an unnoticed shift of a telescope lens. Once the shift was
accounted for, the "planet" disappeared.
The early '90s had seen the actual detection of "pulsar planets," but
these seemed too strange to count, orbiting a rapidly spinning,
radiation-spewing stellar remnant called a pulsar. Most scientists would
reserve the "first" designation for a planet orbiting a normal star.
"The whole field had a snake-oil sort of feel to it," Butler said in a
recent interview. "For the previous fifty years or so, there were many
announcements, all proved to be wrong. If we went to a meeting and said
we were looking for extrasolar planets, we might as well have said we
were looking for little green men."
Even Marcy greeted the announcement of 51 Peg, made at a scientific
conference in Florence, Italy, by Mayor and Queloz, with a bit of a yawn
-- at first.
"This claim on October 6, 1995, of the first planet ever discovered
was sort of business as usual," he said. "Here's another false claim.
This one is more obviously a false claim. The orbital period is claimed
to be 4.3 days. Nobody in their right mind thought planets could orbit
so close to a star."
But the four nights of observations at the Lick Observatory --
perfectly coinciding with 51 Peg's four-day orbit -- changed all that.
Both the Mayor and Marcy teams had been trying to develop a
planet-hunting technique based on wobbling stars. The wobbles, known as
the star's "radial velocity," were induced by the gravitational tugs of
orbiting planets. The starlight wavelength was compressed, then
stretched, as the star moved toward and away from astronomers'
Now, Mayor and Queloz had proven that the technique worked. And a few
days later, Marcy and Butler validated both the method used by Mayor's
team and their own very similar detection method.
But Marcy and his team realized something more. The only thing that
had kept them from beating Mayor's group to that first detection was a
perfectly reasonable assumption: that big planets moved in stately
orbits, like the 12 years it took Jupiter to take one lap around the
Either they would have to watch stars for a very long time, or they
would have to refine their wobble detector until it could pick up the
very tiny shifts in a star's position caused by small planets in
tighter, faster orbits.
They were working on just this type of refinement when Mayor
announced his discovery. More importantly, they had been recording
observations with their wobble-detecting device, known as a
spectrograph. Sure enough, when they took another look, big,
star-hugging planets began popping out of their data.
At a meeting of the American Astronomical Society in January 1996,
Marcy announced two more planet discoveries: 70 Virginis and 47 Ursae
Majoris. The first had a 116-day orbit -- far more reasonable than 51
Peg's scorching four days -- and its orbit was elliptical, making it
unlikely to be anything but a planet. The orbit of 47 Ursae Majoris was
more reasonable still: 2.5 years. Together, they provided a "bridge" to
our own solar system, Marcy said, with planets behaving themselves as
proper planets should.
The discoveries vaulted Marcy and his team into scientific celebrity
status, with appearances on nationwide nightly news shows; their new
planets even made the cover of Time magazine.
And the Marcy-Butler team was just warming up. The floodgates were
opened. They discovered at least 70 of the first 100 exoplanets that
were found in the years that followed. Their pioneering, planet-hunting
safari went on for a decade. Soon, however, the landscape would change
The gold rush of planet finding kicked into high gear with the launch
of the Kepler Space Telescope in 2009. This spacecraft nestled into an
Earth-trailing orbit, then fixed its eye on a small patch of sky -- and
kept it there for four years.
Within that patch were more than 150,000 stars, a kind of
cross-section of an arm of our own Milky Way galaxy, as if Kepler were
shining a searchlight into deep space. Kepler was looking for planetary
transits -- the infinitesimally tiny dip in starlight that occurs when a
planet crosses the face of the star it is orbiting.
The method only works for distant solar systems whose planets'
orbits, from our perspective, are seen edge-on. This way, an exoplanet
is silhouetted as it passes between Kepler and its host star, reducing
the starlight measured by Kepler.
The fifth time's the charm
Kepler was the brainchild of William Borucki of the NASA Ames
Research Center in Moffet Field, California. Borucki, who retired in
early July 2015, doggedly pressed his case for Kepler. During the '90s,
his proposed designs were rejected no less than four times. He finally
won approval from NASA in 2001.
But no one knew what Kepler might find, or even if it would find anything at all.
"We launched Kepler, to some extent, like Magellan or Columbus went
to sea, not knowing quite what we were going to encounter," said James
Fanson, deputy manager in the Instruments Division at NASA's Jet
Propulsion Laboratory in Pasadena, California. Fanson was Kepler's
project manager when the spacecraft was launched.
"We knew we were going to make history," he said. "We just didn't know what history we were going to make."
Kepler's transit watch paid off, however, identifying more than 4,600
candidate planets hundreds to thousands of light-years distant. So far,
1,028 of those have been confirmed -- some of them Earth-sized planets
that orbit within their star's so-called habitable zone, where liquid
water can exist on a planet. Scientists are still mining Kepler data,
regularly turning up new planetary candidates and confirming earlier
Kepler itself ended its initial mission in 2013, when two of four
reaction wheels used to keep the spacecraft in a stable position failed.
But the Kepler science team developed clever ways to continue squeezing
useful data out of the space telescope, relying on the subtle pressure
of sunlight to stabilize it on one axis. Kepler is now in its second
phase of life, and it's still discovering planets.
Preceding Kepler was the groundbreaking COROT satellite, a European
venture launched in 2006 that discovered numerous planets before it
ceased functioning in 2012 -- including the first rocky planet found to
orbit a sun-like star. COROT used the transit method to detect
exoplanets, and was the first space mission dedicated to that purpose.
The prolific discoveries still flowing from the Hubble Space
Telescope include not only exoplanets, but characterizations of
exoplanet atmospheres, identifying a variety of gases. And the Spitzer
Space Telescope has found water vapor in exoplanetary atmospheres as
well as weather patterns.
Both the wobble and transit methods, relied upon by the exoplanet
pioneers, are still in use today, along with several other techniques.
And 20 years after the first discovery, the exoplanet total is up to
more than 5,000 candidates, with more than 1,800 of those confirmed.
A new reality
The galaxy, it seems, is crowded with planets. Yet we are not yet able to answer the big question: Are we alone?
A new generation of telescopes in the years and decades ahead, on the
ground and in space, will continue to search for an answer. One
critical tool will be the same one pioneered by Marcy and the other
early planet hunters: spectroscopy. They used this method to dissect the
light coming from distant stars, revealing their back-and-forth,
planet-induced wobbling as the starlight was stretched and compressed;
the newest generation of instruments will do the same thing to the light
from the atmospheres of exoplanets. Splitting this planetary light into
its constituent parts, a little like the rainbow colors of sunlight
shining through a prism, should reveal which gases and chemicals are
present in those alien skies.
And one day, some of those atmospheric constituents might suggest the presence of life far beyond planet Earth.