Has the universe always existed? How did it become a place that could harbor life? What was the birth of our planet like? Are we alone, or are there alien worlds waiting to be discovered?
Dynamic astrophysicist Neil deGrasse Tyson, Director of the Hayden Planetarium at the American Museum of Natural History. Tyson leads viewers on a cosmic journey to the beginning of time and into the distant reaches of the universe, searching for life's first stirrings and its traces on other worlds.
Origins: Where Are the Aliens? (Part 1)
"Origins: Where are the Aliens?," Tyson explores such provocative questions as: would "ETs" resemble us or the creatures of science fiction? Are there "aliens" already amongst us on Planet Earth's brainy creatures whose intelligence is very different from our own? And are planets on which life can flourish rare or common in our universe?
Extraterrestrial life, also called alien life (or, if it is a sentient or relatively complex individual, an "extraterrestrial" or "alien"), is life that does not originate from Earth. These hypothetical life forms may range from simple single-celled organisms to beings with civilizations far more advanced than humanity. Although many scientists expect extraterrestrial life to exist in some form, there is no evidence for its existence to date. The Drake equation speculates about the existence of intelligent life elsewhere in the universe. The science of extraterrestrial life in all its forms is known as exobiology.
The science of astrobiology considers life on Earth as well, and in the broader astronomical context. In 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia, when the young Earth was about 400 million years old. According to one of the researchers, "If life arose relatively quickly on Earth, then it could be common in the universe."
Since the mid-20th century, there has been an ongoing search for signs of extraterrestrial intelligence, from radios used to detect possible extraterrestrial signals, to telescopes used to search for potentially habitable extrasolar planets. It has also played a major role in works of science fiction. Over the years, science fiction works have increased the public's interest in the possibility of extraterrestrial life. Some encourage aggressive methods to try for contact with intelligent extraterrestrial life, whereas others argue that it might be dangerous to actively call attention to Earth.
Origins: Where Are the Aliens? (Part 2)
Alien life, such as microorganisms, has been hypothesized to exist in the Solar System and throughout the universe. This hypothesis relies on the vast size and consistent physical laws of the observable universe. According to this argument, made by scientists, such as Carl Sagan and Stephen Hawking, as well as well-regarded thinkers, such as Winston Churchill, it would be improbable for life not to exist somewhere other than Earth. This argument is embodied in the Copernican principle, which states that Earth does not occupy a unique position in the Universe, and the mediocrity principle, which states that there is nothing special about life on Earth. The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the universe was only 10–17 million years old. Life may have emerged independently at many places throughout the universe. Alternatively, life may have formed less frequently, then spread—by meteoroids, for example—between habitable planets in a process called panspermia. In any case, complex organic molecules may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of Earth. According to these studies, this process may occur outside Earth on several planets and moons of the Solar System and on planets of other stars.
Since the 1950s, scientists have argued the idea that "habitable zones" around stars are the most likely places to find life. Numerous discoveries in these zones since 2007 have generated estimations of frequencies of Earth-like planets —in terms of composition— numbering in the many billions though as of 2013, only a small number of planets have been discovered in these zones. Nonetheless, on 4 November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way, 11 billion of which may be orbiting Sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists. Astrobiologists have also considered a "follow the energy" view of potential habitats.
Life on Earth requires water as its solvent in which biochemical reactions take place. Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on terrestrial planets with a chemical make-up and temperature range similar to that of Earth. More generally, life based on ammonia (rather than water) has been suggested, though this solvent appears less suitable than water. It is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon, such as methane, ethane or propane.
About 29 chemical elements play an active positive role in living organisms on Earth. About 95% of this living matter is built upon only six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. These six elements form the basic building blocks of virtually all life on Earth, whereas most of the remaining elements are found only in trace amounts. The unique characteristics of carbon make it unlikely that it could be replaced, even on another planet, to generate the biochemistry necessary for life. The carbon atom has the unique ability to make four strong chemical bonds with other atoms, including other carbon atoms. These covalent bonds have a direction in space, so that carbon atoms can form the skeletons of complex 3-dimensional structures with definite architectures such as nucleic acids and proteins. Carbon forms more compounds than all other elements combined. The great versatility of the carbon atom makes it the element most likely to provide the bases—even exotic ones—for the chemical composition of life on other planets.
Some major international efforts to search for extraterrestrial life. Robotic exploration of the Solar System (image: Curiosity rover on Mars)
Planetary habitability in the Solar System
Some bodies in the Solar System have the potential for an environment in which extraterrestrial life can live, particularly those with possible subsurface oceans. Should life be discovered elsewhere in the Solar System, astrobiologists suggest that it will more likely be in the form of extremophile microorganisms.
Mars may have niche subsurface environments where microbial life might exist. A subsurface marine environment on Jupiter's moon Europa might be the most likely habitat in the Solar System, outside Earth, for extremophile microorganisms.
The panspermia hypothesis proposes that life elsewhere in the Solar System may have a common origin. If extraterrestrial life was found on another body in the Solar System, it could have originated from Earth just as life on Earth could have been seeded from elsewhere (exogenesis). The first known mention of the term 'panspermia' was in the writings of the 5th century BC Greek philosopher Anaxagoras. In the 19th century it was again revived in modern form by several scientists, including Jöns Jacob Berzelius (1834), Kelvin (1871), Hermann von Helmholtz (1879) and, somewhat later, by Svante Arrhenius (1903). Sir Fred Hoyle (1915–2001) and Chandra Wickramasinghe (born 1939) are important proponents of the hypothesis who further contended that life forms continue to enter Earth's atmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution.
Directed panspermia concerns the deliberate transport of microorganisms in space, sent to Earth to start life here, or sent from Earth to seed new stellar systems with life. The Nobel prize winner Francis Crick, along with Leslie Orgel proposed that seeds of life may have been purposely spread by an advanced extraterrestrial civilization, but considering an early "RNA world" Crick noted later that life may have originated on Earth.
In the early 20th century, Venus was often thought to be similar to Earth in terms of habitability, but observations since the beginning of the Space Age have revealed that Venus's surface is inhospitable to Earth-like life. However, between an altitude of 50 and 65 kilometers, the pressure and temperature are Earth-like, and it has been hypothesised that aerial microbial life could exist. Furthermore, Venus likely had liquid water on its surface for at least a few million years after its formation.
Life on Mars has been long speculated. Liquid water is widely thought to have existed on Mars in the past, and now can occasionally be found as low-volume liquid brines in shallow Martian soil. The origin of the potential biosignature of methane observed in Mars' atmosphere is unexplained, although hypotheses not involving life have also been proposed.
There is evidence that Mars had a warmer and wetter past: dried-up river beds, polar ice caps, volcanos, and minerals that form in the presence of water have all been found. Nevertheless, present conditions on Mars' subsurface may support life. Evidence obtained by the Curiosity rover studying Aeolis Palus, Gale Crater in 2013 strongly suggest an ancient freshwater lake that could have been a hospitable environment for microbial life.
Current studies on Mars by the Curiosity and Opportunity rovers are now searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic and/or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on Mars is now a primary NASA objective.
Ceres, the only dwarf planet in the asteroid belt, has a thin water-vapor atmosphere. Frost on the surface may also have been detected in the form of bright spots. The presence of water on Ceres has led to speculation that life may be possible there.
Carl Sagan and others in the 1960s and 1970s computed conditions for hypothetical microorganisms living in the atmosphere of Jupiter. The intense radiation and other conditions, however, do not appear to permit encapsulation and molecular biochemistry, so life there is thought unlikely. In contrast, some of Jupiter's moons may have habitats capable of sustaining life. Scientists have indications that heated subsurface oceans of liquid water may exist deep under the crusts of the three outer Galilean moons—Europa, Ganymede, and Callisto. The EJSM/Laplace mission is planned to determine the habitability of these environments.
Jupiter's moon Europa has been subject to speculation about the existence of life due to the strong possibility of a liquid water ocean beneath its ice surface. Hydrothermal vents on the bottom of the ocean, if they exist, may warm the ice and could be capable of supporting multicellular microorganisms. It is also possible that Europa could support aerobic macrofauna using oxygen created by cosmic rays impacting its surface ice.
The case for life on Europa was greatly enhanced in 2011 when it was discovered that vast lakes exist within Europa's thick, icy shell. Scientists found that ice shelves surrounding the lakes appear to be collapsing into them, thereby providing a mechanism through which life-forming chemicals created in sunlit areas on Europa's surface could be transferred to its interior.
On 11 December 2013, NASA reported the detection of "clay-like minerals" (specifically, phyllosilicates), often associated with organic materials, on the icy crust of Europa. The presence of the minerals may have been the result of a collision with an asteroid or comet according to the scientists. The Europa Multiple-Flyby Mission, which would assess the habitability of Europa, is planned for launch in 2025. Europa's subsurface ocean is considered the best target for the discovery of life.
Internal structure of Europa. The blue is a subsurface ocean. Such subsurface oceans could possibly harbor life.