Earth: our oasis in space. Our solar system includes seven other planets and many moons and other small worlds. But only Earth has all the ingredients for life as we know it. There is no place like home—at least in our neighborhood.
Imagining Other Worlds
So far scientists have yet to find another world like Earth. But people have long imagined Earth-like planets and alien creatures living on them. They have shared their visions through books, movies, and popular culture. What would it take for people to live on another planet? How are scientists trying to find other habitable worlds?
For hundreds of years people thought life could exist nearly everywhere in our solar system. Discoveries made by spacecraft missions to the planets changed that view—but take a look at some of the imaginative ways people thought about life on other worlds in the gallery below.
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In 1835, the New York newspaper The Sun published a series of stories about astronomers discovering life on the Moon. People rushed to buy copies of the paper to read about giant winged people and other creatures. “The Great Moon Hoax” succeeded in selling a lot of newspapers. Image courtesy of the Library of Congress.
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In his 1823 book, The Christian Philosopher, Thomas Dick calculated that our solar system contained exactly 21,891,974,404,480 inhabitants. For scale, Earth has about 7+ million people—a lot less than the over 21 trillion estimated by Dick! He assumed every planet was a lot like ours. Image courtesy of Astrochemist on Wikimedia.
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The War of the Worlds (1898) by H.G. Wells was one of the first stories to depict the meeting of humans and aliens. Welles’s book helped popularize modern science fiction and the idea that aliens might exist. In 1938, director Orson Welles directed and narrated an adaptation of H.G. Wells book for radio. The radio dramatization told a similar tale in a series of news bulletins and eyewitness accounts that added to the story’s realism. Despite repeated warnings about the fictional nature of the broadcast, some listeners believed that a small town in New Jersey was in the midst of an alien invasion—which the press in turn had a field day with. In this photo, Welles meets with reporters to explain that no one connected with the broadcast knew it would cause panic. Images courtesy of Acme News Photos/Wikimedia.
Investigating Life on Other Worlds
So—can humans live anywhere else in our solar system?
Only the Moon, Mars, and perhaps some of the larger moons of the giant planets—Jupiter, Saturn, Uranus, and Neptune—might support a long-term colony. The giant planets themselves have no hard surfaces to walk on. It is also impossible to survive on many of their moons.
However, according to a NASA study, along with our Moon and Mars, humans might make long term visits to only two of the gas giants’ larger moons: Jupiter’s Callisto, and Saturn’s Titan. Jupiter’s and Saturn’s other moons are affected by the gas giants’ strong radiation fields, and the smaller moons have such low gravity that an explorer’s muscles would weaken over time.
That leaves us with Mercury and Venus. Mercury is too close to the Sun to support human life. Its surface temperature averages 800°F (430°C) and is bathed by solar radiation. Venus is even hotter than Mercury. The average surface temperature is hot enough to melt lead. The pressure of the atmosphere is crushing—90 times what we experience on Earth!
So is there any possibility of life on other planets? Finding the answer is a large part of what drives space exploration. One thing we know is that planets change. Earth today looks nothing like it did over four billion years ago when life began.
The movement of Earth’s surface by plate tectonics has created the continents we see today. The atmosphere has changed from mostly carbon dioxide to mostly nitrogen and oxygen. Even the saltiness of the oceans has varied over time. These changes have wiped out clues to how life began.
This map shows continental movement from 250 million years ago to today. Image courtesy of USGS.
Conductive material at Earth’s core generates the magnetic field that surrounds our planet. The magnetic field protects us from the Sun’s solar wind, which would blow away our atmosphere. Some of the solar particles are deflected harmlessly toward the poles, where they form auroras—like the well-known aurora borealis, or northern lights.
These characteristics which make Earth unique, or some combination of them, may be essential for creating life and for sustaining it long enough to become intelligent. Some of the earliest evidence of life on Earth are stromatolites—mounds with fossil material created by primitive organisms. These organisms consumed carbon dioxide and released oxygen as waste, building up oxygen in Earth’s atmosphere over billions of years. Life that thrived on oxygen evolved.
Despite the fact that we have not yet found life outside of Earth, scientists continue to search for it—in our Museum, NASA centers, and other laboratories worldwide. We’ve also tried to reach out to other life … out there. One of the first things humans sent into space was a metal record, similar to a phonograph. Voyager 1 and 2, which launched from Earth in 1977, both carried these records, along with the needle needed to read it, which contained information about Earth and life on it, including sounds and images of life. Who knows if someone will one day hear it?
The markings on the Voyager records cover, seen here on this duplicate record cover, were intended to inform advanced alien beings how to decode the information on the record. Image courtesy of the Smithsonian Institution.