51 Pegasi Star in the constellation Pegasus, 50.9 light-years from the Earth. It is orbited by the planet 51 Pegasi B, and was the first exoplanet to be identified.
astrobiology The development and testing of hypotheses concerning extraterrestrial life. Astrobiologists also research into the early development of life on Earth. Exobiology refers to the narrower study of the effects of extraterrestrial environments on hypothetical beings and the implications for extraterrestrial life.
astrophysics Study of the physics of the Universe, incorporating investigation of celestial objects’ physical properties and interactions.
carbon Element found in all known life forms. In the Universe, it is the fourth most abundant element after hydrogen, helium and oxygen. It is the product of the burning of helium in nuclear fusion reactions within stars.
Curiosity NASA vehicle that landed on Mars on August 6 2012, part of the NASA Mars Science Laboratory launched on November 26 2011.
exoplanets Planets outside the solar system.
extraterrestrial life Any form of life that does not have its origin on Earth. The term can refer to any hypothetical life-form, from an organism consisting of a few cells to a more conventionally imagined ‘alien’.
gaseous envelope Cloud of gases, held together by gravity. An envelope of gases forms a nebula. The Earth’s atmosphere is also sometimes called a gaseous envelope.
geophysics Study of the physics of the Earth and its atmosphere.
GJ 1214b An exoplanet orbiting the star GJ 1214, around 40 light-years from the Sun in the constellation Ophiuchus. GJ 1214b, discovered in 2009, is an example of a super-Earth, with a radius and mass larger than Earth but smaller than the gas giants of our solar system. Planet GJ 1214b is considered a likely candidate to be an ocean planet (one whose surface is entirely covered by water). Observations of GJ 1214b with the Hubble Space Telescope appear to indicate that water accounts for a major part of its mass.
HD 209458b A exoplanet orbiting the star HD 209458, around 150 light-years from the Sun in the constellation Pegasus. It was the first exoplanet to be observed in transit to its star, and the first exoplanet to have its atmosphere studied. Its orbit is so close to its star that it has a year lasting only 3.5 Earth days and a surface temperature estimated at 1,000°C (1,800°F).
Kepler 10b An exoplanet in orbit around the star Kepler 10, some 564 light-years away from the Sun in the constellation Draco. The NASA Kepler mission targeted Kepler 10 and discovered a planetary system containing at least two planets, named Kepler 10b and Kepler 10c.
Kepler mission NASA discovery mission number 10, a space satellite launched to detect super-Earth exoplanets in the habitable zone of their stars (area in which planets might possibly have sufficient atmospheric pressure to have liquid water on their surface, and so might harbour life).
macroscopic As opposed to microscopic. A macroscopic object can be seen with the naked eye, while a microscopic object can be seen only using a microscope.
ocean planets Hypothetical planets whose surface is entirely covered by ocean. Astronomers think it likely that some planets migrate inward towards their star during the process of formation, and that as a result an icy planet might become an ocean planet when the ice melted to water.
perturbation Movement of a planet or large body due to forces that do not originate from the gravitational attraction of a single other source. Perturbation can be the effect of planets or natural satellites, or of an atmosphere – any source other than the one that controls the body’s normal motion. Astronomers can identify exoplanets, for example, by detecting gravitational perturbations in the orbit of a star. The normal orbit around the centre of the galaxy is being disturbed by the gravitational effect of the planet; although the exoplanet is too far away to see, the astronomers can deduce its existence from the perturbation of the star.
protoplanetary disc Spinning disc of gas surrounding a newly formed star.
super-Earths Exoplanets with mass between those of the Earth (mass 5.9722 × 1024 kg) and Neptune (102.4 × 1024 kg). Neptune’s mass is 17.5 times that of Earth.
transit Passing of a planet in front of its star. During a transit astronomers using the Hubble Space Telescope can take readings indicative of the planet’s atmosphere.
Viking NASA discovery mission to Mars. Two spacecraft, Viking 1 and Viking 2, launched in August and September 1975 and landed on Mars in July and September 1976.
The more humanity has learned about space in the many centuries since ancient astronomers placed the Earth at the centre of the Universe, the more we have realized that there is nothing unusual about the solar system. The most recent milestone in this process of discovery is the identification of exoplanets, and hints that Earthlike planets are extremely common. These new insights could indicate that life as we know it, based on common atoms and molecules, may be an ordinary thing in the Universe. However, humanity has still not recorded any indication that life exists elsewhere, either within or outside our solar system, nor found any sign that our Earth has been visited by another life form. This simple fact indicates that highly advanced extraterrestrial civilizations with the desire to communicate are rare, which may appear as a paradox considering the probably extremely large number of ‘other Earths’ and the fact that technological evolution seems exponential. A possible explanation to this paradox is that the lifespan of advanced civilizations is short, a hypothesis that has a strong echo at a time where the Earth’s human population is realizing the alarming rate at which it is changing its own environment.
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Whether extraterrestrial life exists is a defining question for humanity, which remains unanswered despite recent indications that life might not be uncommon in the Universe.
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It is difficult to predict what alien life could look like. Life is usually defined as a system that is self-organizing, reproducing, responsive to its environment and evolving in successive generations. The easiest way to accomplish this seems to be based on the chemistry of the carbon atom in liquid water, but this idea is, of course, inspired by life as we know it.
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ENRICO FERMI
1901–54
Italian-born American physicist
FRANK DRAKE
1930–
American astronomer and astrophysicist
JILL TARTER
1944–
American astronomer and astrobiologist
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François Fressin
Life on Earth is based on the DNA molecule. Would extraterrestrial life also develop from macromolecule replication?
Astronomer, astrophysicist, cosmologist and prolific writer, Carl Sagan was an unashamed popularizer of what he loved best, and he enjoyed a high public profile (to the annoyance and even chagrin of some of his peers). Captivated by stars at the age of five, according to his own memoirs, he combined a sense of awe at the cosmos with an unswerving commitment to the rigours of the scientific method: keep your mind open, but constantly test what comes into it. He devised the ‘Baloney Detection Kit’, a set of mental tools to help anyone with a lively mind debunk junk science and disrobe charlatans, and urged its use at all times.
Sagan actively embraced the popular media, producing an internationally successful TV series in the 1980s, explaining the cosmos as we then knew it, and wrote or worked on more than 20 books aimed at the nonscientific reader. At the same time, he maintained a successful ‘hard-science’ career – at Cornell University, New York, in the main. It was perhaps lucky for him that his career took off at about the same time as the NASA space programme; he was an adviser throughout, from the 1950s (when he was still a PhD student) onwards, briefing the Apollo astronauts and designing experiments to be carried out by robotic spacecraft. In his astronomer’s chair, he theorized (and was proved largely correct) about the surface temperatures of Jupiter and Venus, seasonal changes on Mars, and the likelihood of water on Titan (a moon of Saturn) and Europa (a moon of Jupiter). He was also an early whistle-blower on the dangers of climate change, and (during the Cold War) on the catastrophic possibility of a nuclear winter, if the war ever turned hot.
Sagan is probably best known for his trailblazing work on exobiology (the study of nonterrestrial biological conditions) and the search for extraterrestrial life. He encouraged the use of radio telescopes to detect signs of life, and devised the plaques sent out with the Pioneer and Voyager space probes designed to be decoded by intelligent life forms. True to his precepts, he applied stringent tests to all UFO sightings or unverifiable stories of alien abduction and concluded towards the end of his life that it was very unlikely that the Earth had ever been visited by extraterrestrial intelligence: but that did not mean to say it was not out there, somewhere.
Born in Brooklyn, New York
1954
Graduated from the University of Chicago as Bachelor of Arts
1955
Bachelor of Science in Physics at the University of Chicago (and Masters in 1956)
1960
PhD in Astronomy and Astrophysics at the University of Chicago
1960–62
Miller Fellow at the University of California
1962
NASA space probe Mariner 2 supports his hypothesis that the surface of Venus is very dry and hot
1962–68
Worked at the Smithsonian Astrophysical Observatory in Cambridge, Mass, and lectured at Harvard
1971
Worked at Cornell University, Ithaca, New York
1972
Became full professor at Cornell, Director of Planetary Studies, and (until 1981) Associate Director of the Center for Radio Physics and Space Research
1972
NASA space probe Pioneer 10 launched, carrying Sagan-designed communication plaque
1977
Awarded NASA’s Distinguished Public Service medal
1978
Won Pulitzer Prize for nonfiction with The Dragons of Eden: Speculations on the Evolution of Human Intelligence (1977)
1979
Wrote Broca’s Brain: Reflections on the Romance of Science
1980
Co-wrote and narrated award-winning TV series Cosmos: A Personal Voyage
1982
Organized petition advocating the establishment of the SETI Institute (Search for Extraterrestrial Intelligence) in the journal Science
1984
SETI Institute set up; became a member of the Board of Trustees
1985
Wrote Contact, later made into a movie (1997)
1990
Awarded Oersted medal
1994
Awarded the National Academy of Sciences’s Public Welfare Medal
1995
Wrote Pale Blue Dot: A Vision of the Human Future in Space
1996
Wrote The Demon-Haunted World: Science as a Candle in the Dark
20 December 1996
Died in Seattle
1997
Billions and Billions: Thoughts on Life and Death at the Brink of the Millennium published posthumously
The existence of worlds orbiting stars other than the Sun has been imagined for centuries, but no such worlds were known to humanity until the end of the twentieth century. In 1995, Swiss astrophysicist Michel Mayor and his student Didier Queloz discovered the gravitational perturbation of an object as massive as Jupiter orbiting the star named 51 Pegasi. Since that date, astronomers have identified thousands of exoplanets. The diversity of planets they have found seems to be limited only by the technological possibilities of the telescopes used to find them. Some planets have been discovered orbiting binary stars; others are not bound to any star and are free-floating in space. There are scorched worlds with molten surfaces, giant planets with huge rocky cores several dozen times the mass of the Earth, planets darker than black paint, and water worlds that are probably covered completely with an ocean. On the path towards the discovery of a true analogue of the Earth a field of research has emerged. Combining astrophysics, geophysics and biology, the study of the habitability of exoplanets evaluates their suitability for hosting life.
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Exoplanets, or extrasolar planets, are planets outside the solar system; their recent discovery intensified the search for other life forms in the Universe.
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Detecting ‘other Earths’ is extremely challenging. Our Earth is already practically invisible from our own probes in the outskirts of the solar system. However, astronomers can find the signature of an exoplanet by studying its gravitational influence on the motion of the star it orbits, or by detecting a temporary drop of a small amount of light due to a planet eclipsing a small part of a star.
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GIORDANO BRUNO
1548–1600
Italian astronomer burned at the stake by the Roman Inquisition for claiming the Universe may contain many other inhabited worlds
MICHEL MAYOR & DIDIER QUELOZ
1942– & 1966–
Swiss astrophysicists and pioneers of the search for exoplanets
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François Fressin
51 Pegasi was the first star known to host an exoplanet, a planet as large as Jupiter but orbiting it in just four days.
Imagine taking planet Jupiter and dragging it inward from the outer regions of the solar system, all the way past Mercury, until it hovered a few solar radii above the surface of the Sun. At such a small distance from the Sun, this giant would zoom around its tiny orbit once every few days. Strong gravitational forces would squeeze and tug on this planet until it rotated synchronously with its orbit, keeping one side always in light and the other always in darkness. The incredible flux of energy from the blazing star would heat its atmosphere to 1,000°C (1,800°F – as hot as fire) and launch powerful winds to distribute this energy to the planet’s permanent night side. No such planets exist in our solar system, and for years astronomers thought they couldn’t exist at all, until they discovered them around other stars. The mellifluously named exoplanet HD 209458b, a hot Jupiter, was the first exoplanet ever found to ‘transit’, or pass in front of its host star. Transits provided not only confirmation that exoplanets were actually real, but also a way to determine an exoplanet’s size, by measuring how much light it blocks during transit.
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As big as the Sun’s biggest planet and hotter than its hottest planet, these hellish worlds were among the first extrasolar planets ever discovered.
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If an exoplanet transits its host star, astronomers can do more than just measure its size; they can see its atmosphere, too. A tiny fraction of starlight will filter through the planet’s atmosphere before reaching Earth. The atmosphere’s molecular composition and structure will leave its spectroscopic fingerprint on this starlight. By observing this fingerprint through big telescopes, astronomers can learn about the atmospheres of planets too far away for us to visit in person.
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TIMOTHY BROWN
1950–
American astronomer, who, with David Charbonneau, first detected the transit and measured the atmosphere of a hot Jupiter, HD 209458b
DAVID CHARBONNEAU
1974–
Canadian-American astronomer
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Zachory K. Berta
This example of a hot Jupiter is depicted to scale (both in terms of size and distance) with its parent star.
Of the planets that orbit our Sun, none have sizes between those of Earth and the ice-giant Neptune. Yet, because astronomers can determine the sizes of planets orbiting other stars by measuring how much starlight they block, we know that our galaxy seems to be teeming with planets in this size range. Although astronomers call these planets ‘super-Earths’ because of their size, many might be nothing like Earth. One such exoplanet is Kepler 10b, which is so much denser than Earth that it must be made entirely out of molten rock and iron. In contrast, the exoplanet GJ 1214b is far less dense than Earth and could be composed largely of steamy water vapour mixed with other gases – a water-world or ocean planet. Kepler 10b and GJ 1214b are both close to their host stars and very hot, but super-Earth planets in cooler orbits could have barren rock crusts, or puffy hydrogen atmospheres, or shifting continents like Earth, or maybe even worldwide oceans hundreds of miles deep. As astronomers study the masses, sizes and atmospheres of more super-Earths, we will learn more about the processes governing the formation and evolution of these exoplanets.
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Exoplanets slightly bigger than the Earth could have a variety of compositions; astronomers are eager to unravel the mysteries of these new worlds of possibility.
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If a super-Earth grew a little too big in its youth, it would start a process called runaway accretion, quickly sweeping up huge swaths of gas from the protoplanetary disc and becoming a gas giant. It would be difficult for life to form on a planet with such a deep gaseous envelope. How big a planet can be and still be hospitable to biological life is an active area of research in exoplanetary science.
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SARA SEAGER
1971–
Canadian-American astrophysicist and a leading researcher into super-Earth planets
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Zachory K. Berta
All we know about many recently discovered super-Earths is their size – bigger than our Earth but far smaller than planets such as Neptune or Jupiter.
Recent technological advances have made it possible for astronomers to identify the first Earth-sized planets orbiting other stars, but we do not yet know how common these planets are, nor what fraction of them could sustain life. There are two techniques used to look for these faint and extremely distant objects – which are no more than tiny rocks orbiting fireballs one million times more massive than they are, but are seen together as a speck of light in telescopic images. The dynamical technique consists of identifying the motion of the planet around the star by looking for the reflex motion of the star pulled by the planet or the dimming of light when the planet eclipses a small part of the star during its orbit. The direct-imaging technique requires blocking the light coming from the star to see its surroundings, which otherwise would be invisible in the star’s glare. Once telescopes have identified and gathered enough light, they can study the planet’s atmospheric features and investigate how similar they are to those of the Earth. As the twenty-first century proceeds, we envisage imaging these planets and mapping these other worlds – and looking for seasonal changes and direct signs of life.
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Astronomers now have the technology to find Earth-sized planets around other stars, with the goal of finding out how similar to the Earth they are.
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The Alpha Centauri system is the closest to the Sun, and by 2020 we could have proof that there is another Earth in that system. The next step could be to send a probe to take high-resolution pictures. Such a challenging project would probably involve several international generations of scientists by the time the probe reached Alpha Centauri. Running the project and sharing its results would be a unifying experience.
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BERNARD LYOT
1897–1952
French astronomer
GEOFFREY MARCY
1954–
American astronomer, pioneer in the discovery of exoplanets
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François Fressin
Alpha Centauri A, the constellation Centaurus’ brightest star, is the same type as our Sun, fuelling speculation that it might contain planets that harbour life.
The search for life on another planet can be considered to have started in the late nineteenth century, when observers thought they could see straight ‘canals’ on Mars that later proved to be optical illusions due to the instruments they were using. In the 1970s, the Viking spacecraft carried biological experiments to the surface of Mars but could not identify any conclusive sign of life. Despite many claims of unidentified flying objects (UFOs) visiting the Earth since, the only clear picture of a ‘flying saucer’ we have is one of the shell carrying the Curiosity rover that NASA sent to Mars in 2011–12, with the aim of studying the planet’s habitability and whether it could have ever supported life. Astronomers are also investigating whether life could exist on other objects in the solar system, such as on Jupiter’s moon Europa. However, our best chance of discovering another macroscopic life form is the investigation of ‘other Earths’ orbiting distant stars. By probing the atmospheric composition of such planets, astronomers search for molecules more likely to have been produced by living organisms (such as the oxygen and methane on Earth) than ordinary chemistry.
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Astrobiology investigates whether life exists beyond Earth, and in what likely conditions, as well as how humans can detect it if it does.
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The search for extraterrestrial intelligence aims at directly finding an advanced extraterrestrial civilization, or letting it find us. The most important effort involves the use of large radio telescopes to identify the fingerprint of another civilization’s telecommunications, but such a signal has not yet been identified. Another question is how we could leave detectable fingerprints and what ‘language’ would be intelligible to an alien civilization.
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CARL SAGAN
1934–96
American astronomer, astrophysicist and writer
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François Fressin
Jupiter’s satellite Europa shows cracks in its icy surface, indicative of a possible subsurface ocean where life could have developed.
