Potential Habitable Worlds
Potential habitable exoplanets are those extrasolar planets (planets around other stars) that might be able to support any form of life, from simple life (microorganisms) to complex life (plants and animals). Millions of years ago Earth was only able to sustain microbial life, today it can also support plants and animals and a more diverse biosphere. A planet that is potentially habitable does not mean that it is necessarily habited, only that it has some requirements that are known necessary for life as we known it, such as the right temperature, liquid water, and atmospheric gases (i.e. carbon dioxide, oxygen).
Water is just a chemical compound that is necessary for life but not necessarily indicates the presence of life. Water is also very abundant in the universe. In fact, planets that are formed farther from a star tend to have a lot more water in their surface and subsurface, but usually as ice. Earth is special because it has that water as a liquid, a requirement for life, but it has little water by comparison, a "thin" layer over its surface that we call oceans. The average depth of terrestrial oceans is 4 km. Europa, a satellite of Jupiter, has oceans below a thick layer of ice with a total depth nearly 100 Km. That is about 6 time more ocean water than on Earth even that the satellite is much smaller (one quarter of Earth's radius).
Too little or too much water is also not good for life. Terrestrial deserts are characterized by water limitation and they usually look devoid of life in the most extremes cases. Microbial life is present even in such conditions but the absent of plants and animals make them look without of life. The same is true for many parts of the oceans. There is plenty of water there but some deep regions look like terrestrial deserts, without visible life. Even the surface waters usually look like life deserts. The water looks clear and you don't see a lot of life on it like algae and fishes. Even on these cases microbial life is always present but now the limitation of nutrients is the factor controlling the abundance of life. Most of the complex life on oceans prefer the shallow coastal zones. Life on water is also more evident in lakes and rivers.
We expect that extraterrestrial life in exoplanets, if any, is most probably microbial life. Even at close range they are not evident but we know that abundant microbial life can impact the atmosphere of a planet, and that is something we can measure. For example, oxygen is a very reactive gas and tends to get trapped by rocks in a diverse mixture of compounds. The increase of oxygen on Earth's atmosphere was due to microbial life. Today half of the oxygen produced by life is by oceanic phytoplankton, a form of microbial life (the other half is by plants). More complex life such as animals and vegetation is a second possibility. Intelligent life should be much rarer. Only in the last 7 to 5 million years we have had an intelligent species on Earth. The fact is that 99.999% of Earth history have been a planet just habited by microbial life, animals, and plants, and without intelligent life.
If we randomly put on stars around us 1,000 Earth replicas but at different evolutionary stages, we will find about 130 without life, 740 with only microbial life, 130 with plants and animals, and only one of those with intelligent life. These statistics are not bad but they also assume that we will be able to recognize those habited. We will be lucky to even identify the presence of life in any of those due to our current observational limitations of exoplanets, something that is more limited now by budget rather than technology. So far, out of the over 2,000 exoplanets that have been detected, we have not even found a single Earth replica, but we are getting closer.
The 123 Steps for Identifying Potential Habitable Worlds
Recognizing habitable worlds around other stars is a challenging process. Extrasolar planets, also known as exoplanets, are very far away and most of them are only known by their effect on their parent star. They make their stars wobble in space (radial velocity oscillations) or periodically block their light (transits). Only a few have been directly imaged, those large and hot enough. Even with these observational limitations scientists are starting to identify potential habitable candidates within the over 2,000 exoplanets that have been detected so far.
A habitable world is one that seems capable of supporting life, as we know it, but not necessarily implies that it is habited. The process of identifying habitable worlds can be divided in three steps, as fallows:
Step 1: Physical Indicators: Basic Stellar and Planetary Properties.
Habitable world are first recognized by their orbital position with respect to their parent star and some basic planetary properties such as mass and radius. Only exoplanets with the right size and inside the stellar habitable zone, or the right distance from their star to support liquid water, are considered potentially habitable. Small exoplanets will not be able to hold an atmosphere and much larger ones will have very high surface pressures that will even make any water solid, independently of temperature.
Step 2: Chemical Indicators: Atmospheric Chemistry
Habitable worlds are also recognized by the composition of their atmosphere. A planet that has the right temperatures for liquid water does not necessarily mean that it has water. The light from the atmosphere of the planet is analyzed in search for the presence of water and other gases required by life such as oxygen, carbon dioxide, methane, and nitrogen. The presence of these gases is not a guarantee for the existence of life but only strong indicators of habitability.
Step3: Biological Indicators: Biosignatures
Habitable or habited worlds are finally confirmed by the observation of strong indicators for the presence of life. Life that is abundant and globally distributed will have and impact on the chemistry of the atmosphere and how light is absorbed at their surface. For example, any vegetation will absorb light in some particular energies that can be recognized. The presence and abundance of oxygen and methane, two usually relatively short-lived gases in planetary atmospheres, are also good indicators of biological process that are replenishing them.
These steps are necessary to confirm that any extrasolar worlds are in fact habitable by any terrestrial life or even habited. Unfortunately, scientists are only able now to barely complete the first step and in some very special cases the second. Future ground and orbital observatories are necessary to go through all steps and therefore confirm their habitability or habitation.Additional Information about Habitable Worlds