Many astrobiologists believe that if we find living organisms on other planets in our solar system and elsewhere in the universe, they will be recognizable to us as life. They believe that the properties of carbon that allowed it to become the basis for all life on Earth are unique to that atom. The variety of types of chemical bonds that can be formed by carbon make it able to be the basis of complex chains of different molecules. No other atom seems to be able to do this in a similar way. Even silicon, which has the same number of valence electrons as carbon, cannot form the variety of molecules that carbon can. However, this does not mean that all life would necessarily be based on DNA and cells, as it is on Earth.
Water is another very likely requirement for life to arise. Any life which is based on molecules almost certainly requires some kind of liquid solvent to be able to move them around. Although chemical reactions can take place in gases and solids these are much less ideal than liquid. Gas phase reactions happen only with molecules that are volatile enough to be present in large quantities in a gas. Reactions can take place in solids, but occur very slowly. Both of these limitations make it much more likely for life to develop in liquid, as indeed it seems to have on Earth.
Water has many unique physical and chemical properties that make it well suited to support the complex chemistry required for life. Expanding when it freezes keeps oceans and lakes on Earth from freezing solid. Water can dissolve many substances easily and it also has a high heat capacity, which means it takes a lot of energy to cause water to change temperature. This property of water gives Earth its relatively moderate climate.
Water is also the second most common molecule in the universe (after H2). Other liquids exist naturally in the universe, but not in the sort of abundance water does. Most of these liquids don’t have many of the other key properties of water that make it so suitable as the basis for life.
Many astrobiologists believe that in order for life to arise and survive, it must be found on a planet or moon within the habitable zone of a star. The habitable zone refers to the region around the star in which liquid water can form and remain liquid. The size of the star is important as well. Stars that are much larger than the Sun have such short lifetimes, that it is unlikely that there would be enough time for any kind of life, particularly complex life, to develop.
The diagram below from Wikipedia compares the habitable zone of the Sun and a much smaller star, Gliese 581. The larger and more luminous a star, the farther away its planets must orbit to be in the habitable zone.
Planets in the habitable zone of small stars may still not be habitable because these planets are so close to their star, they are tidally locked. This means that the gravitational attraction that keeps them in orbit around the star has caused the planet to always have one face of the planet facing towards the star and the other facing away. This would most likely cause the side facing the star to be too hot for liquid water to exist, and the other side would be too cold.
Our Sun seems to be just the right size to allow life to develop. It is small enough to have a long lifetime, but large enough that a planet can exist in the habitable zone and maintain rapid rotation as it orbits.
Recent discoveries about some of Jupiter’s moons have caused some scientists to consider expanding the definition of the habitable zone. The strong gravitational pull caused by large planets and tidal interactions between orbiting moons may produce enough energy to heat the cores of these moons. Under certain circumstances, this energy might be enough to keep at least parts of a moon warm enough to support liquid water, even if the moon was too far away from the star to be in the habitable zone created by the star.
The Milky Way also has its own habitable zone. The center of the Milky Way is much more dense with stars than the outer regions. Nearby supernova explosions are much more frequent, and the radiation would sterilize any planets with life in that region. Stars very close to center of the galaxy would receive intense x-ray radiation from the supermassive black hole at the center of the galaxy, and life would be very unlikely to be able to develop in such an environment.
Stars further towards the edge of the Milky Way galaxy tend to be Population II stars. These very old stars have very few heavy elements, and so these stars would be less likely to have planets, and less likely to have the complex chemistry required for life.
In addition, we are fortunate that our star continues to remain in the habitable zone as it has done for billions of years. Many stars in the galaxy orbit with more eccentric orbits, so although they may cross the habitable zone from time to time, they probably do not remain long enough for life to arise and survive long term.
The green band on this image by NASA/Caltech shows the galactic habitable zome, which is often described as an annulus 4–10 kpc from the galactic center.
Solar System Clean-Up
Another key ingredient to the formation of life seems to be having a large planet - like Jupiter - in a planetary system. Because Jupiter is so much more massive than all the other planets, it attracts many asteroids, comets and other objects that travel within the Solar System. This is important because otherwise, some of these objects would end up crashing into Earth, and many did in the very early formation of the Solar system. Jupiter’s gravity, along with Earth’s atmosphere combine to protect the Earth from many impacts that would certainly have sterilized Earth many times.
Unlike on Venus and Mars, the crust of the Earth is constantly being recycled. This keeps the carbon dioxide levels in the atmosphere from getting too high or too low. If the levels become too high, (as they did on Venus) they act as a greenhouse gas and the planet becomes too hot. Liquid water evaporates and the surface of the planet dries up. If the levels become too low, the planet cools and an ice age begins. This has happened several times in Earth’s history, but each time, because of the motion of the plates and the continued recycling of the carbon in rocks, carbon was released into the atmosphere eventually raising the levels of carbon dioxide and allowing the planet to warm again. Without this carbon cycle, planets don’t seem to be able to maintain a climate balance appropriate to sustain life.