For four years, the historic planet hunting
mission, Kepler, starred at a group of 150,000 stars located in a region extending three
thousand light years away from earth. The data collected by this spacecraft has
brought a turning point in the long search for other planets like earth. Is ours one of countless life-bearing worlds
strewn about the galaxy; or is it a rare garden of eden in a barren universe? What are we learning about our place in the
cosmos, from the search for earthlike planets? Tens of thousands of years ago, humans began
to fan out across the planet, following unknown pathways, crossing unmeasured distances. We traced coastlines, and sailed uncertain
seas. We crossed ocean straits drained by an ice
age. Into every corner of Earth we ventured, looking
for places to put down our roots, to raise our families, or just to see what was there. Today, it’s the final frontier that fires
our imaginations. With so many stars in our galaxy, we make
a simple extrapolation, that the cosmos must be filled with worlds like ours, with life,
even intelligent life. This so-called “many worlds” view goes
back to ancient times, to China, India, Greece and Egypt. The Qur’an, the Talmud, and many Hindu texts
all imagined a universe full of living beings. It wasn’t until the 16th century that the
idea became grounded in concrete notions of the physical universe. Astronomer and mathematician Nikolas Copernicus
declared that Earth revolves around the Sun. That opened the way for the Italian friar,
Giordano Bruno, a natural philosopher who believed that the universe is eternal and
without end. He held that there is a multitude of worlds
with diverse life forms, including intelligent beings. Bruno’s outspoken challenges to church doctrine
got him executed in the year 1600. His ideas gained support when Galileo Galilei
used his telescope to show that our Sun is just one among countless other stars. By the modern era, the “many worlds” view
held sway in scientific circles. New telescope technologies gave us a view
of vast star populations within our galaxy. As the astronomer and author Carl Sagan noted,
given the sheer number and diversity of stars in our galaxy, it’s “far more likely that
the universe is brimming over with life.” In 1961, the astronomer Frank Drake sought
to lay out the odds of finding advanced alien civilizations. The Drake equation took into account, The rate of star formation in our galaxy. The fraction of stars with planets. Of planets that might support life. That might develop intelligent life. or radio communications, which we could perhaps
detect. Even as astronomers began to scour the heavens
for alien signals, another view of the galaxy gained momentum. It started with the Greek philosophers Aristotle
and Ptolemy. They believed that humanity and Earth are
unique. With the spread of Christianity, this Ptolemaic
system became embodied in the “rare Earth” hypothesis. In religious doctrine, it was taken to mean
that mankind and earth were specially created by God, in his image. In science, it implied that the circumstances
that allowed life to unfold on Earth are so particular and fortuitious, that the odds
are slim we’ll find another place like it. Where does the debate stand today, with information
about Earth and the cosmos pouring in from ever more advanced technologies? On the “many worlds” side, modern theories
hold that planet formation is a common byproduct of star formation. As we’ve seen on this planet, life is persistent,
adaptable, relentless. Millions of species grace the landscapes and
oceans of our planet, from simple one-celled plants to complex mega fauna. In terms of mass and sheer numbers, none of
it holds a candle to a simple, hardy form. Bacteria have been documented in fossils dating
back some three and a half billion years. Bacteria are found in habitats ranging from
hot springs and volcanoes,. the digestive systems of animals, the soil, or the sulfurous
environments of deep sea hydrothermal vents. They are part of a much larger global ecosystem
that suffuses the Earth’s crust, down where heat and chemicals from Earth’s interior
fuel their growth. Not even the worst climate catastrophes of
the past could dislodge this biological storehouse, from widespread volcanic eruptions to episodes
of global glaciation. It’s easy to imagine life gaining a foothold
on a wide variety of worlds. Comets and asteroids, for example, have been
found to deliver a steady rain of interplanetary dust to Earth, including organic compounds
and water, that could have supplied the building blocks of life. Analyzing a type of carbon-rich meteorite,
researchers have found amino acids, molecules used to make proteins. They also found components used to make DNA,
along with sugar-related organic compounds that are a basic part of living cells. But primitive life forms don’t necessarily
evolve to more advanced forms. The rare earth view points to a complex, and
fortuitous, chain of circumstances on this planet. It began when simple bacteria gave rise to
one-celled organisms called eukaryotes. They evolved specialized internal organs to
regulate processes such as photosynthesis. These organisms began to regulate surface
temperatures by taking in carbon dioxide, a greenhouse gas, while releasing oxygen. Volcanism and other geological processes released
more CO2 in the air. Ocean and land plants, along with chemical
reactions in rocks called weathering, pulled CO2 back out of the air. A global carbon cycle developed that kept
surface temperatures within a relatively narrow range. A stable climate allowed the planet to retain
its stores of water. Water, in turn, has helped drive the movement
of earth’s crustal plates, a process that releases and buries CO2. There were other factors as well, a nearly
circular orbit has helped keep seasonal extremes in check. The moon stabilized the day-night cycle. Earth is also at a distance from the sun that
allows surface temperatures to hover between the freezing and boiling points of water,
the so-called “Habitable Zone.” Some scientists also believe we live in a
“Galactic Habitable Zone.” We’re close enough to the galactic center
to be infused with heavy elements generated by countless stellar explosions over the eons, But far enough away from deadly gamma radiation
that can roar out of the center. At the same time, Earth has been able to survive
a range of other natural hazards. Some researchers, for example, have linked
mass extinctions in the past to the Sun’s passage through one of the spiral arms, where
gamma radiation sources lie in wait. So too, we’ve made it through asteroid impacts,
climate changes, and solar eruptions. Now we wonder, are there kindred spirits,
somewhere out there, to share our survival stories with? Or is Earth alone amid the wastelands of a
barren galaxy? This image shows, in stark relief, the biggest
obstacle faced by planet hunters. We’re looking at Earth, as photographed
by the Voyager spacecraft, from a distance of 3.7 billion miles. Our mighty world occupies only about one tenth
of one pixel. Try seeing something this small at hundreds
of thousands of times that distance. And try seeing it through the bright glare
of a star. Still, astronomers have made extraordinary
progress. In 1995, Swiss astronomers announced the discovery
of a planet orbiting the star, 51 Pegasi. They found it by carefully charting the star’s
wobble, caused by the gravitational tug of an orbiting planet. What they found is no Earth. It has about half the mass of Jupiter, but
orbits at a distance closer than our own Mercury is to the Sun. Most of the planets discovered with this method
are gas giants, so called hot jupiters that swing in close to their host star. 51 Peg is a G-type dwarf star, like our sun. It is brighter and more massive than 85% all
other stars in the galaxy. But there are only about 500 others like it
within a hundred light years of Earth. Scientists have turned their wobble method
on a more plentiful breed, called M Stars. One of them, 20 light years from Earth, is
too dim to see with your naked eye. Gliese 581, in the southern constellation
Libra, is a red dwarf with 31% of the Sun’s mass, but only 1.3% of its luminosity. Using the wobble method, the Swiss team detected
an entire solar system, with up to six rocky planets, ranging from 2 to 18 times the mass
of Earth. The most enticing is Planet G, whose presence
is still unconfirmed. It’s within the life zone. But a star like this gives off so little energy
that a planet would have to orbit close just to get enough heat to power its climate. That subjects it to solar flares, common in
small stars. So too, a close orbit increases the tendency
of the star’s gravity to halt any spinning motion. That makes it more difficult for heat and
moisture to circulate, and a habitable climate to form. If planet hunters operating on ground-based
observatories have told us anything, it’s that planets and solar systems are highly
diverse. The search for earth-like planets requires
a larger sample. Enter the Kepler space telescope, launched
in the year 2009. For nearly four years, astronomers aimed its
precision instruments at a tiny patch of sky, with 150,000 stars at a distance of up to
3,000 light years away from Earth. Kepler used what’s called the transit method,
to record subtle dips in the star’s light caused by a planet passing in front of it. By analyzing the light as it dipped, scientists
are able to estimate the planet’s mass, radius, and the distance from its parent star. Combining Kepler and ground-based observations,
there are now 3,841 planetary candidates. 1075 have been confirmed by further telescope
or computer analysis. The vast majority of candidates orbit in the
hot zone of their parent star, and most have a mass equivalent to Neptune or Jupiter. There is a smaller cadre of planets out in
the warm or habitable zone. These too are mostly large gas planets. About a dozen, though, have masses that are
smaller than Neptune but larger than Earth, At the higher end of this range, they are
known as mini-Neptunes or gas dwarfs. At the lower end, are rocky worlds called
Super Earths. How Earth-like are they? Most super earths are thought to have dense,
inhospitable atmospheres. That’s because their intense gravity is
able to hold onto stores of gas drawn to it in the early days of solar system formation. If a slightly smaller planet can avoid this
fate, it may succumb to another. Consider the star Kepler 62, a relatively
sun-like star with 69% the mass of our sun. Two of its five known planets are most likely
solid like Earth, but with large amounts of surface water. 62F, on the outer rim of the habitable zone,
could well be frozen over. 62E, farther in, is likely inundated. The thinking is that the density and internal
heat of a planet this size prevents surface water from migrating down into the mantle. With land areas kept to a minimum, a super
earth would not develop the carbon cycle necessary for regulating a climate. It may be too early to write these worlds
off. A recent study showed that the weight of their
oceans may be enough to push large amounts of water down into the mantle, where it could
help fuel volcanism and plate tectonics. These geological processes would create land
masses, allowing a carbon cycle and a climate to take hold. So far, we have not seen what we’re looking
for, Earth 2.0. A malfunctioning gyroscope on the Kepler satellite
ended its observations after four productive years in space. The data it captured remains a mother lode
that scientists continue to mine. Based on a statistical analysis of all the
Kepler observations, a new study says, one in five sun-like stars do have planets about
the size of Earth, with surface temperatures conducive to life. Given that about 20 percent of stars in the
galaxy are sun-like, that would amount to several tens of billions of potentially habitable,
Earth-size planets. And there may well be a whole other population
of habitable worlds we haven’t considered. 620 light years away, in the constellation
of Cygnus the swan, is a star that’s slightly cooler and smaller than our sun, Kepler 22 has a planet squarely in the habitable
zone. But at around 6 times the mass of Earth, it’s
probably enshrouded by a dense atmosphere. Its large enough that it could have pulled
another planet into orbit, one that’s large enough to maintain an atmosphere and liquid
water on its surface. If any of the confirmed exoplanets do have
a moon, it would leave a subtle but distinctive imprint on the star’s light. Kepler came on line in a new age of massive
data gathering and supercomputer analysis. That will define future planet-finding missions
as well. The Transiting Exoplanet Survey Satellite,
TESS, for example, is scheduled for launch in 2017. It will focus on about half a million bright
sunlike G and K stars, looking for telltale dips in their light as orbiting planets pass. The James Webb Space Telescope, in active
development since 1996, is currently slated for launch in 2018. Astronomers hope to use it to peer into planetary
atmospheres to detect carbon dioxide, oxygen, methane, and other indicators of climates
or even life. Optimism that we’ll find other worlds like
ours is driving these increasingly sophisticated efforts. It’s tempered by what we’ve learned of
our own world and solar system, that there are so many factors that can derail a planet’s
evolution. In the future, if our itch to explore becomes
unbearable, or if somehow things don’t work out on this planet, There is bound to be some available galactic
real estate out there. We’ll have to develop advanced transport
to get there, twenty, a hundred, a thousand light years away. We may find that we’re not exactly adapted
to its gravity, its mix of land and water,. its atmosphere. We may not prefer its climate. The question is, will it be worth the trip? 7
