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  • 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-calledmany worldsview goes back to ancient times, to China, India, Greece

  • and Egypt. The Quran, 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, themany worldsview 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 therare 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 well 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 themany worldsside, modern theories hold that planet formation is a common byproduct

  • of star formation.

  • As weve 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-calledHabitable Zone.”

  • Some scientists also believe we live in a “Galactic Habitable Zone.” Were 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, weve 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. Were

  • 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