Life has yet to be found anywhere but on Earth. Over a hundred planets beyond our solar system have been identified, with more being found every day. None, so far, is suitable for life as we know it, although this could be a selection bias since only the largest planets will initially be seen. Perhaps life may be someday confirmed on Mars or elsewhere in the solar system, such as under the ice on Jupiter’s moon Europa. But such life will be at best rare and primitive. Certainly humans cannot live on Mars or in an ocean on Europa without extensive life support. Maybe one day the SETI project will hear signals from a civilization beyond Earth. But we are very unlikely ever to sit down together for tea and crumpets.
Much is made of human space flight. It is hyped as the search for new worlds akin to the explorations on Earth during the Age of Discovery. Space operas like Star Trek lead people to think that someday all we will have to do is hop in a spaceship and cross the galaxy at warp speed. Every planet we land on will have an atmosphere and other conditions sufficiently like Earth that we will be able to walk around without spacesuits. In this way, it is widely imagined, humanity will gradually populate the cosmos.
This is very unlikely ever to happen. The chance of humans finding new worlds to live on without extensive life support is very small. A spaceship moving at 11.1 kilometers per second, the escape velocity for Earth, would take 114,000 years to reach Proxima Centauri, the nearest star. That same spaceship would take 3 billion years to cross our galaxy.
Einstein’s special theory of relativity makes it possible in principle to reach any place in the universe in the lifetime of an astronaut onboard a spacecraft. The ship just has to go fast enough relative to Earth.
In order to reach the next nearest galaxy (excluding the Magellanic Clouds just outside the Milky Way), Andromeda, in ten years ship-time, the ship would have to travel very close to the speed of light. By the time the ship reached Andromeda, 2.3 million years would have elapsed on Earth. That is longer than the time Homo sapiens have been Homo sapiens. Any humans exploring the universe will effectively cut themselves off from Earth. Even if they traveled to Andromeda and back, aging only twenty years in the process, they would return to an Earth almost five million years in the future as measured on Earth clocks.
Notice that I have not used any technological limitations to argue that space flight to distant stars and galaxies is impossible. While a method for accelerating a spaceship to near the speed of light is beyond any technology we can currently imagine, I will not rule that out for future generations. People also speculate about traveling through wormholes, tunnels through space-time that act as shortcuts to other parts of the universe. I don’t know if that will ever prove possible, but I doubt it.
The amount of effort that is needed to provide life support for humans in space makes it clear that we did not evolve to live in space. Life on Earth evolved under the very special set of conditions that exist here. The Sun is a stable source of energy. The orbit of Earth is almost an exact circle, which keeps us from experiencing too great a variation in temperature. Planets in multiple star systems, such as Alpha Centauri, of which Proxima Centauri is a member, would not have such gravitational stability, making them unlikely candidates for life.
Earth’s distance from the Sun is such that temperatures on our world are very suitable for the evolution of life structures based on carbon chemistry. The materials on Earth-large amounts of water, an atmosphere containing oxygen, a surface containing carbon and other elements, have made life possible.
By a fortunate coincidence, the spectrum of radiation from the Sun is maximal in the same wavelength region where the transparency of Earth’s atmosphere is maximal. These conditions, and many more, provide the delicate balance needed for complex carbon structures to survive on Earth. No other planet in the solar system has such conditions, and it seems likely that future star trekkers will never find another planet in our galaxy where humans could live without substantial life-support systems.
Notice also that I am not denying the possibility of some kind of life elsewhere in the universe. Indeed, I regard that as very likely. Here I am talking about Earthly life, specifically humans.
The suggestion is frequently made that humanity might someday live in outer space, inside space stations orbiting Earth and other planets. However, even if these space stations duplicate all the conditions on Earth, they may not be able to deal with the cosmic rays from which we on Earth are shielded by the atmosphere. The same threat prohibits lengthy space travel. The types of Mars missions people dream about would expose astronauts to life-shortening radiation poisoning.
Perhaps future technologies will solve this problem, too. Maybe genetic engineering will make new kinds of humans, really a new species, suitable for space travel. And, of course, we can always send automatons. The fact remains that humans are not constructed to live unsupported anywhere but on this tiny blue speck in a vast universe. Our species is marooned in space, on spaceship Earth.