Habitability and Biology
What are the Properties of Life?It is possible that life on other planets could be very different from what we are accustomed to on Earth. But in order to look for life in the universe, or even in our own Solar System, researchers must have a set of criteria by which to define what life is. Based on our current level of understanding, the best approach is to look for characteristics or traits that are common among many different living organisms.
Biologists have identified at least six properties that are shared by all living organisms on Earth:
Many species have evolved to have unique body shapes and characteristics in order to adapt to unique situations in their environment. The hammer-shaped head of the hammerhead shark gives it superior vision and sensory perception. (Image credit: NOAA)
The Habitable ZoneSince the telescope was first invented in 1608, technological advances have made it possible for us to look at other planets in our solar system, and even far beyond to other galaxies. One of the primary motivations for exploring our solar system is to answer the question, "Are we alone?".
When considering what places in our Solar System might harbor life now or in the past, scientists must look at the conditions that make a planetary surface habitable. Most scientists agree that the presence of liquid water is the primary requirement for life. But can liquid water exist anywhere in the solar system?
Earth's distance from the Sun allows its surface to be within a precise temperature range that makes it possible for liquid water to exist. If Earth's temperature were much warmer, liquid water would evaporate or be lost to space; if it were colder, liquid water would freeze. The region of the Solar System where temperatures allow liquid water to exist on a planetary surface is called the habitable zone.
The habitable zone (shown in blue) of our solar system is the region around the Sun in which a planet could potentially have surface temperatures that would support liquid water. (Image credit: NASA)
Lastly, processes that lead to atmospheric loss play a key role in habitability. Most researchers agree that early Mars had a much thicker atmosphere, but it gradually lost its atmosphere over billions of years, which was directly related to the planet's dramatic cooling. In addition, many scientists believe that plate tectonics are a requirement for habitability, because they drive the carbon cycle. This cycle is a complex series of processes where much of the Earth's carbon is exchanged between the crust, oceans, and atmosphere. Earth is the only planet currently known to have active plate tectonics.
The inner boundary of our solar system's current habitable zone lies somewhere between the orbits of Earth and Venus. Optimistic estimates state that the outer boundary lies just beyond Mars, while more conservative estimates say that it lies between Earth and Mars. In theory, if Mars were larger and had retained its thick atmosphere, it could have a habitable surface today. Therefore, the outer boundary of the habitable zone could lie beyond the orbit of Mars.
But what about a planet or moon that lies outside the habitable zone? Is it possible for life to exist there? It is important to remember that the habitable zone, by definition, is the region where a planet could potentially have surface temperatures that would support liquid water, but a more accurate name for this zone might be the "zone of liquid water," as scientists now believe that habitability can occur outside this zone in certain conditions. For example, subsurface groundwater, which may still exist on Mars today, could harbor life, as well as subsurface oceans like the one believed to exist on Jupiter's moon Europa. Recent images taken by NASA's Cassini Spacecraft show what may be liquid water reservoirs that erupt in Yellowstone-like geysers on Saturn's moon Enceladus.
THE DRAKE EQUATION
Shortly after the invention of radio communication, scientists began to ponder the possibility that if other civilizations in our galaxy also used radio, we might be able to detect their presence by listening for their radio signals. Experiments related to the Search for Extraterrestrial Intelligence (SETI) involve tuning receivers to listen for simple, deliberate signals at certain frequencies. But is there really anything to hear out there?
In 1961, astronomer Frank Drake came up with an equation that outlined the factors to be considered in calculating the number of civilizations in our galaxy from which we could potentially get a signal. His equation is written as:
Number of civilizations =
NHP x flife x fciv x fnow
NHP = the number of habitable planets in the galaxy;
flife = the fraction of these habitable planets that actually have life;
fciv = the fraction of the life-bearing planets where a civilization capable of interstellar communication has existed at some time; and
fnow = the fraction of these civilization-bearing planets that have a civilization now.
Although the Drake Equation cannot currently give us a precise number, because we don't know the values that need to be plugged into each of its terms, it is still important because it outlines the variables that determine the odds of our receiving a signal from elsewhere in our solar system or galaxy.