http://discovermagazine.com/2008/nov/10-how-long-until-we-find-a-second-earth

 How Long Until We Find a Second Earth?

Researchers are racing to find the first planet that might support life as we know it.
by Robert Kunzig

published online October 10, 2008


The 55-inch mirror on the Kepler space telescope will focus starlight on its
detectors. Scheduled for launch next April 10, Kepler will seek slight
variations in a star's brightness, a signal that a planet is crossing in front
of it.

Courtesy of Ball Aerospace

Gliese 876 is a modest star, just one-third the mass of our sun and only 15
light-years away, but it has a history-making planetary system all its own. In
1998 a team led by Geoff Marcy of the University of California at Berkeley
detected the first sign of something interesting there: a giant planet, twice
the mass of Jupiter, circling Gliese 876 once every two months, its gravity
yanking the star back and forth at the speed of a jet plane. Three years later
the same group found a second planet, half the mass of Jupiter and closer in,
pulling the star around at the speed of a race car. Although the planets are too
faint to be seen directly, their motions cause the star\u2019s spectrum to
wobble back and forth across the digital detector of an astronomical telescope.

In the past decade, announcements of Jupiter-size planets have become
commonplace; about 300 of them have been found so far. In 2005, however, with
the help of improved detection software, Marcy\u2019s team turned up something
else orbiting Gliese 876\u2014something truly new. This invisible object added
one more regular component to the star\u2019s motion, like the third note, faint
and high, of a piano chord. It was another planet, orbiting in just two days and
pulling on the star much more gently, not at jet plane or race car speeds but at
a speed a man could run. This planet, dubbed Gliese 876 d, is clearly no
Jupiter, Marcy realized. It is no more than seven or eight times as massive as
our own: a \u201csuper-Earth.\u201d Until then, all the known exoplanets
(planets circling other stars) were big and gaseous, but this one is probably
made of rocky materials\u2014the first world like ours found in an alien solar
system.

Gliese\u2019s super-Earth lies so close to its star that it has just about no
chance of being inhabited. If it has an atmosphere at all, it probably consists
of dense steam, says Greg Laughlin of the University of California at Santa
Cruz, a member of the discovery team. But if we can find one rocky, Earth-like
planet right in our galactic backyard, surely there must be many more. Already,
the Swiss astronomers who in 1995 discovered the first Jupiter-like
exoplanet\u2014and who are the great rivals of the California group in the
exoplanet hunt\u2014said in June that they had identified not one but three
super-Earths orbiting a single star 40 light-years away. The smallest is just
four times as massive as Earth. \u201cWe\u2019ll find an Earth-mass planet by
2010,\u201d Laughlin predicts, \u201cand an Earth-mass planet that\u2019s
potentially habitable by 2012.\u201d

And yet we still won\u2019t have found a true second Earth. The hallmark of
Earth, after all, is not its mass, nor its rockiness, nor the fact that it is
potentially habitable. The hallmark is that it is actually inhabited. These
days, nobody doubts that there are other reasonably cool, rocky planets out
there among the 100 billion stars in the Milky Way. Everything astronomers have
learned about how stars and planets form says there must be. But is there life
on any of those rocks, and if so, can we detect it? \u201cThat\u2019s not going
to happen from Earth,\u201d Laughlin says. \u201cIt has to happen from
space.\u201d

    Finding life on other planets cannot happen on Earth. It has to happen from
space.

In space, above our atmosphere, stars do not twinkle; in space a telescope is
also beyond day and night and can thus stare at the same star for weeks on end,
gradually teasing from its light the barely perceptible but regular flickers
caused by a small orbiting planet. A French satellite called Corot, the first
space telescope devoted primarily to looking for rocky planets, is in orbit now.
An even more capable American mission, Kepler, will be launched in April. It is
expected to find hundreds of Earths, including the first ones orbiting stars
like the sun at distances like that of our own Earth. Then, in 2013, NASA will
launch a giant infrared telescope called the James Webb Space Telescope. An
all-purpose observatory, the Webb was not designed to follow up on the
discoveries of Corot and Kepler. But if pushed to the limit, it just might be
able to provide the first indication of life\u2014a telltale molecule, such as
oxygen, in the planet\u2019s atmosphere\u2014on a super-Earth circling another
star. By 2014 headlines could be announcing the first tentative evidence of life
beyond our solar system.

One rainy Tuesday afternoon last November, Annie Baglin, the chief scientist of
Corot, sat at the window of a café near the Place Denfert-Rochereau in Paris,
drinking tea. It had not been an easy day. French railway workers, striking over
their retirement benefits, had shut down the commuter trains, preventing Baglin
from reaching her office on the suburban campus of the Paris Observatory. The
railway workers and other civil servants were marching down the Boulevard
Montparnasse, near Baglin\u2019s home, brandishing bright red flares that filled
the air with chlorine-scented smoke; off the boulevard, platoons of
shield-toting, armor-wearing riot police stood nervously at the ready. Arriving
late for her rendezvous at the café, wearing the dark pink coat she had said
would make her recognizable, Baglin explained that her car had been
towed\u2014apparently the traffic wardens were not on strike. From the café
Baglin would be proceeding to her dentist to have a tooth extracted. On the plus
side, her spacecraft was performing beautifully.

As Baglin launched? into the story of the little spacecraft that could, in
principle, find many rocky planets, her high, thin voice sometimes disappeared
into the noise of the sirens outside. She is a shortish woman of 70, with
close-cropped gray hair and a warm, no-nonsense demeanor\u2014her parents were
both schoolteachers. A brief profile of her on the Paris Observatory Web site is
titled \u201cAnnie Baglin\u2014Never Say Die.\u201d Getting Corot to the
launchpad, she explained, was a long, hard slog, marked by bureaucratic
near-death experiences.

She never intended to be a planet hunter. In the mid-1980s she and her
colleagues proposed a space telescope to do stellar seismology\u2014to study the
inner workings of stars from vibrations on their surface, much as seismologists
study Earth\u2019s interior by analyzing earthquakes. The French and European
space agencies were noncommittal about the idea. Then came 1995 and the
announcement of the discovery of the first exoplanet, by Michel Mayor and his
colleagues at the Geneva Observatory. Baglin and everyone else immediately
realized that a spacecraft designed to detect the light fluctuations caused by
starquakes might also be able to detect a planet. Suddenly, Baglin says,
\u201cwe were very much sought after. In hindsight, one can say that if it
hadn\u2019t been for the discovery of exoplanets, we never would have been
approved to do Corot. That\u2019s what sold it.\u201d

Launched in December 2006, Corot is thus a 1,300-pound spacecraft that does two
very different things. No telescope yet exists that can take a picture of even a
giant exoplanet; astronomers compare the task to taking a picture of a firefly
next to a searchlight thousands of miles away. Mayor and his colleagues showed
instead that it was possible, through a technique called astrometry, to detect
the slight wobble in a star\u2019s light caused by the gravitational pull of an
orbiting planet. Most of the 300-some exoplanets discovered since have been
found that way. But Corot relies on a different technique that has lately come
to the fore in ground-based searches as well. Called photometry, it detects the
slight but regular dimming in a star\u2019s light when a planet transits in
front of it.

What the search for planetary transits has in common with the observation of
starquakes is the need to stare at the same stars for a long time\u2014long
enough to detect very slow vibrations or to detect at least three transits of a
planet. Otherwise, you can\u2019t be sure it was really a starquake or a planet
you saw, and not random fluctuations in the starlight. Corot stares at the same
spot in the sky for 150 days before switching to another. \u201cCorot is
Zen,\u201d Baglin says. \u201cOnce we\u2019re set up, we don\u2019t move. We
don\u2019t even breathe.\u201d

The spacecraft\u2019s 27-centimeter (10.6-inch) telescope monitors up to 12,000
sunlike stars at once. Getting a big sample is crucial because only one in a
hundred of those stars that do have planets will be oriented so that the passage
of the planet in front of the star is visible from Earth. The precision of the
telescope\u2019s measurements has exceeded its makers\u2019 hopes. \u201cIf
Corot were to observe the million lightbulbs that shine along the Champs-Elysées
at Christmas,\u201d said a press release from the Paris Observatory a few days
before Christmas in 2007, \u201cit would be able to detect whether a single bulb
was flashing.\u201d That parts-per-million sensitivity should allow Corot to
detect the dips in a star\u2019s light caused by a transiting planet with a
radius just twice that of Earth\u2014and perhaps an even smaller one, provided
its orbit is tighter than Mercury\u2019s, so that the planet completes three
transits during the 150-day viewing period.

Not long after the launch, the Corot science team, including Baglin, published a
description of the mission. It concluded with this prediction: \u201cThe first
confirmed terrestrial planets are expected in the spring of 2008.\u201d By last
spring, however, the Corot team had announced only two new \u201chot
Jupiters\u201d and one unconfirmed super-Earth, with 40 more candidates in the
pipeline. Seeing transits is not enough; periodic dips in a star\u2019s light
could be caused by a small companion star too dim to detect directly. To confirm
a planet, Corot\u2019s candidates have to be observed from the ground using the
wobble technique, which determines the mass of the transiting object; a planet
will be much lighter than a companion star. But competition for telescope time
on the ground is fierce, especially with so many planet hunters around.
\u201cIt\u2019s a real bottleneck,\u201d Laughlin says.

Baglin has little patience for impatience or for the pressure on her to announce
discoveries quickly. \u201cFinding these things isn\u2019t like finding the nose
on your face!\u201d she exclaimed, shortly before leaving the café last November
to head for the dentist. The Swiss astronomer Mayor gathered data for 20 years,
she pointed out, before announcing his first exoplanet. \u201cSo when people
tell me, \u2018You haven\u2019t got any results,\u2019 when we\u2019ve only been
in orbit for a year, I say, \u2018Stop! Have mercy!\u2019 In three years
we\u2019ll have results indicating how common small planets are. Big planets we
know we\u2019re going to find. We\u2019re looking for the little ones. Are they
there, or aren\u2019t they?\u201d

What about figuring out if one is inhabited? \u201cTo me, that\u2019s not the
big question,\u201d Baglin said. \u201cUnderstanding the universe in its
totality interests me more than looking for life on a planet like Earth around a
star like the sun, which is the declared goal of our competitors. That\u2019s
not uninteresting, but it\u2019s not what excites me. I find that very
anthropomorphic.\u201d

Corot\u2019s competition is Kepler, and Baglin\u2019s is an astrophysicist named
William Borucki at NASA\u2019s Ames Research Center in Mountain View,
California. In 1984 Borucki published his first description of the transit
technique they would both end up using. At that time Baglin was having her first
glimmerings of what would become Corot but had no notion of looking for planets.
Nonetheless, Bag­lin, the planet hunter in spite of herself, beat Borucki into
space with it. Now he is nipping at her heels. Kepler\u2019s bureaucratic
history was even more tortured than Corot\u2019s, but the spacecraft is headed
for launch on April 10, 2009, and astronomers are counting on it to settle the
question of just how common Earths are\u2014a result that will guide the whole
future search for life in the universe.

Borucki well remembers the effect he had with that 1984 paper, published in the
journal Icarus. \u201cThere was no effect,\u201d he says. \u201cIt was pretty
much ignored.\u201d At that time most researchers thought the way to look for
other planets was through astrometry. Borucki was convinced that looking for
planetary transits through photometry would be simpler and cheaper. Measuring
the brightness of a star over time, he reasoned, would require a much smaller
space telescope than trying to take a picture sharp enough to resolve a planet
or a tiny loop in the star\u2019s trajectory.

Borucki\u2019s peers were skeptical, though\u2014first, that a transit of an
Earth would even be distinguishable against the background noise of the
star\u2019s fluctuating light, and second, that it was possible to monitor 5,000
sunlike stars at once, as he proposed to do. Research on the sun during the
1980s laid the first objection to rest; starlight turned out to be a lot less
noisy than astronomers had thought. Over periods of days, which is how long it
would take an Earth to pass in front of it, the sun\u2019s output varies by only
around 10 parts per million, whereas the passage of an Earth would dim it by 84
parts per million.

So such a transit was in principle detectable, but Borucki\u2019s initial idea
for a detector still raised eyebrows. He wanted to drill 5,000 holes, one for
each star, into a metal template and put it near the focal plane of the
telescope, with an individual photodiode and integrated circuit behind each
hole. \u201cPeople in the industry refused to even talk to us about that
design,\u201d recalls David Koch, the deputy principal investigator, whom
Borucki roped in to his quest in 1992. Borucki\u2019s basic concept was rescued
by the emergence of the charge-coupled device, or CCD\u2014the light-sensing
chip that was then new and is now in hundreds of millions of digital cameras. A
CCD can record the brightness of many stars at once, thus eliminating the need
for thousands of photodiodes.

Borucki and Koch first proposed their mission to NASA in 1994; they called it
FRESIP, for \u201cFrequency of Earth-Size Inner Planets.\u201d They proposed it
again in 1996, 1998, and 2000. \u201cEach time they came back with a list of
reasons why we weren\u2019t selected,\u201d Koch says. \u201cIt won\u2019t work
because of this, it won\u2019t work because of that, they said. And we went back
and worked on it until we eliminated every reason they couldn\u2019t select
us.\u201d The researchers made sure, for instance, that the minute vibrations of
the spinning gyroscopes that kept the telescope pointed at its target would not
drown out the signal from a planet. The name of the spacecraft was an easier
sell. After the first proposal, Koch suggested naming it Kepler, after the
discoverer of the laws of planetary motion. In 2001 NASA finally approved the
mission. Over in Europe Corot was getting the go-ahead at around the same time,
albeit at a much lower budget.

Though the French won the race into orbit, Kepler will have a telescope
measuring 95 centimeters (37.4 inches), 3.5 times the diameter of Corot\u2019s,
with a field of view more than 10 times as large. Above all, it will be in orbit
around the sun, trailing behind Earth, whereas Corot is in low Earth orbit over
the poles. To avoid looking too close either to the sun or to Earth, Corot has
to turn 180 degrees twice a year, which is why it can\u2019t look longer than
five months at one set of stars. As it orbits the sun, Kepler will stare at the
same spot for its entire mission\u2014which will allow it to detect true Earth
analogues, those circling stars like the sun in orbits lasting about a year.
\u201cThe French will find the first terrestrial-size planet,\u201d Borucki
predicts. \u201cBut those planets won\u2019t be in the habitable zone.
We\u2019ll find the first one in the habitable zone.\u201d

Actually, he expects to find several hundred. Kepler\u2019s field of view covers
100 square degrees, around 0.25 percent of the sky, or as Koch puts it,
\u201cabout two scoops of the Big Dipper.\u201d Koch chose the most star-rich
patch he could find in the U.S. Naval Observatory star catalog. It lies just out
of the plane of the Milky Way, between the bright stars Vega and Deneb. Of the
13 million stars cataloged in that field of view, Kepler will monitor a subset
that are most like the sun in size, mass, and age\u2014stars that are quiet and
sedate like our own. Its 42 CCD detectors lie just outside the telescope\u2019s
focus. \u201cYou don\u2019t have to have a sharp image,\u201d Borucki explains.
\u201cIn fact, a fuzzy image is better\u201d\u2014the starlight is spread over
more pixels, which keeps them from saturating as quickly. Because it has CCDs,
Kepler will be able to monitor 100,000 stars simultaneously. It will spot
variations as small as 10 parts per million in their light output.

When Kepler lifts off from Cape Canaveral on April 10, Borucki will have spent
17 years on his idea. \u201cYou don\u2019t spend this long and then quit 90
percent of the way through,\u201d he says. \u201cI want to see the answers. I
want to hold the data in my hand. I want to write the paper that says,
\u2018This is how many Earths there are.\u2019\u201d He may find none\u2014which
would be the most surprising result of all. Neither Borucki nor anyone in the
planet-hunting business expects or wants that. It would mean Earths are
extremely rare at best. It would mean we really might be alone in the galaxy, if
not the universe.

By the time Kepler reports its first results, the exoplanet landscape will no
doubt have changed yet again. Astronomers all over the world are hunting a
second Earth. In May Drake Deming of NASA was collecting data he hoped might
reveal a super-Earth in the habitable zone of a red dwarf (a small and
relatively cool star) called Gliese 436; NASA had allowed him to use a
spacecraft called Epoxi, which is on its way to a rendezvous with a comet, to
observe several stars that are already known to have planets. Also, last May
Debra Fischer of San Francisco State University began training a small telescope
in Chile on Alpha Centauri, a pair of sunlike stars that are the closest ones to
Earth. Following up on an idea of Greg Laughlin\u2019s, she believes that by
observing it steadily for several years, she might detect not only an Earth but
a Mercury and a Venus as well. \u201cThat\u2019s what we\u2019re going
for\u2014not just the first Earth but the first terrestrial planetary
system,\u201d she says. \u201cIt\u2019s a pretty bold and crazy plan.\u201d

Although Kepler and Corot are focusing on sunlike stars that could support true
analogues of Earth, much of the action at ground-based telescopes is
concentrating on red dwarf stars, for the simple reason that planets are easier
to find there. An Earth-like planet would cause a bigger wobble and a darker
transit in a red dwarf than in a sun, and the effect would be even more
pronounced if the planet were in the habitable zone\u2014because the habitable
zone, where liquid water can exist, lies closer to a cool red dwarf. In the fall
of 2007 David Charbonneau of Harvard began deploying a network of small
telescopes in Arizona that will be focused on detecting transiting super-Earths
in the habitable zones of red dwarf stars. The Swiss and California teams think
they can do the same with the wobble technique. Of the super-Earths they\u2019ve
discovered so far, some\u2014including the one around Gliese 876\u2014orbit red
dwarfs, though none lie in the habitable zone.

    Is anything living out there? A hint of an answer could come in the next few
years.

No one knows for sure whether a rocky planet in a red dwarf\u2019s habitable
zone would truly be habitable. Until recently, in fact, astronomers assumed it
wouldn\u2019t be. A planet so close to a star might have been blasted and
sterilized, early in its existence, by flares from the star. It might be
gravitationally locked, with one face always pointing toward the star, the way
the moon always points the same face toward Earth. In that case the whole
atmosphere might have frozen and snowed out on the dark side, leaving the planet
airless and barren. But after having some of their preconceptions shattered by
the discovery of Jupiter-size planets orbiting their stars in less than two
days, planet hunters are no longer so confident of the others. Life might emerge
on a red dwarf planet, some now think, after the star has aged and its flares
have settled down; winds on the planet might transport heat from one hemisphere
to the other, keeping the atmosphere from freezing. After a workshop on red
dwarfs in 2005, Jill Tarter of the SETI Institute\u2014a leading thinker on
alien life\u2014and her colleagues published an analysis that convinced many
researchers that red dwarfs are worthy targets for Earth hunters. That\u2019s a
happy conclusion, given that red dwarfs are the most common stars in the galaxy
and also the easiest targets for ground-based telescopes.

But even if a habitable Earth-like world is found first from the ground, it will
most likely take a space observatory to search for the chemical signals that
tell us what we really want to know: Is anything living out there? If the planet
is one that can be observed transiting, it just might be possible to provide a
hint of an answer in the next few years. As a transiting planet passes in front
of its star, some starlight passes through the planet\u2019s atmosphere and
continues on toward Earth\u2014minus certain spectral frequencies that have been
absorbed by molecules in the atmosphere. In 2001, using Hubble, Charbonneau and
his colleagues detected the first exoplanetary atmosphere that way; it belonged
to a hot Jupiter called HD 209458 b, and it contained sodium, they said. Three
years later Charbonneau found himself locked in a race with Deming to be the
first to detect the flip side of a planetary transit\u2014the moment, called
secondary eclipse, when a planet passes behind its star. This time it was Deming
who was observing HD 209458 b, with the Spitzer Space Telescope, an orbiting
infrared observatory. Charbonneau, he knew, had collected data on a different
hot Jupiter a month earlier. \u201cWe didn\u2019t want to be second,\u201d
Deming recalls. \u201cI was analyzing data while I was eating Christmas dinner.
I had to catch Dave.\u201d In the end they published papers simultaneously and
held a joint press conference. advertisement | article continues below

What each had done for the first time was detect an exoplanet\u2019s photons. No
telescope yet can spatially distinguish an exoplanet from its star; the distance
between them is too small and the brightness contrast too large. A Jupiter adds
about a billionth to the visible light of a sunlike star, and about a
ten-thousandth to the star\u2019s infrared glow (planets give off more heat than
they do reflected starlight). By observing the combined infrared radiation of
star and planet with Spitzer and then subtracting the radiation recorded from
the star alone when it hid the planet, Deming and Charbonneau had detected the
heat of the planet itself. From that they could calculate its temperature;
Charbonneau\u2019s team has since been able to create a crude weather map of
their exoplanet, HD 189733 b, which showed that fierce winds must be spreading
heat around its surface. Others using the secondary eclipse technique have
detected evidence for water vapor and methane in the atmosphere of HD 189733 b.

These findings are trials for the far tougher task of picking apart the light of
an Earth-like planet, much smaller and farther from its star (and thus far
dimmer) than the hot Jupiters studied to date. Spitzer wasn\u2019t designed to
measure the spectrum of hot Jupiters, but it did. And the James Webb Space
Telescope, which is slated to replace Spitzer in 2013, has not been designed to
detect the spectrum of Earth-like exoplanets\u2014but with its 6.5-meter
(21.3-foot) mirror, nearly eight times the diameter of Spitzer\u2019s, Deming
and Charbonneau think it might. Other astronomers are more cautious. \u201cI
certainly wouldn\u2019t claim JWST is going to prove habitability, because
it\u2019s not,\u201d says Mark Clampin, project scientist for the observatory.

Sara Seager of MIT, who collaborates with Deming, is trying to figure out which
spectral signatures in a planet\u2019s atmosphere would provide the best
evidence for signs of life. Water vapor is indicative of liquid surface water,
which is necessary but not sufficient for life as we know it. Oxygen, which
would quickly react out of Earth\u2019s atmosphere if it weren\u2019t
continually produced by plants, is closer to a smoking gun, especially if it
were seen together with methane. Then there is what Seager has dubbed
\u201cvegetation\u2019s red edge\u201d: At wavelengths of 700 to 750 nanometers,
at the red end of the visible range, the reflectance of leafy green plants
sharply increases, to four or five times what it is even at green wavelengths.

Whether the Webb can detect such a signature is not yet known, and even if it
can, the data probably won\u2019t be definitive. Deming and Charbonneau\u2019s
secondary eclipse technique, ingenious as it is, lacks the power to distinguish
between life and something else. Making that distinction will require a new kind
of space telescope. That\u2019s where Corot and especially Kepler come in. They
won\u2019t provide targets for that future space telescope, unfortunately. To
monitor many stars and maximize its chances of finding Earths, Kepler is forced
to monitor distant ones; any Earths it finds will most likely be about 300
light-years away, too far for any currently imaginable space telescopes to take
a spectrum from. What Kepler will do is tell astronomers\u2014and NASA and
ESA\u2014what sort of space telescope it will take. If nearly every sunlike star
has an Earth, we might find life around a relatively nearby star, with a
relatively small telescope. If Earths are rare, the next telescope will have to
be big.

Eventually such a telescope will get built\u2014and if the pace of discovery
remains as rapid as it is now, that day will come sooner rather than later.
Finding convincing evidence for extraterrestrial life may take decades, but that
is not a long time given the stakes. \u201cThroughout recorded history
we\u2019ve had this question: Are we alone?\u201d Tarter says. \u201cFor
millennia, all we could do was ask the philosophers. Suddenly we have a way of
looking for an answer that is not based on a belief system. I live in the first
generation of humans that is able to do that. I think that\u2019s extremely
exciting.\u201d

Other astronomers too feel acutely the historic nature of their quest. One of
the last things to be mounted on Kepler this fall, before it makes the journey
to Canaveral for the launch, will be a metal plaque engraved with the names of
all 2,000 scientists, engineers, and managers who contributed to its mission,
which will run through 2012. Kepler will follow a 53-week orbit around the sun,
meaning that it will steadily drift farther behind Earth. \u201cIt loses a week
a year,\u201d Borucki says. \u201cSo 53 years after launch, it will come back to
Earth. At that point, I expect, people will go up and pick up the spacecraft and
put it in the Smithsonian. I know that sounds far-fetched. But I really think it
will happen.\u201d