Criteria for Habitable Planets and Implications for Earth

The search for extraterrestrial life has always been one of our greatest fascinations with the universe, and there is no better time to be fascinated than now.  With the advent of newer technology, the search for habitable planets has recently been successful in identifying planets that could possibly harbor life, even if it may exist in the form of microbes.  The question of the definition of a “habitable” planet is fundamental in understanding the requirements for life to exist on Earth.  There is a frightening number of criteria that must be fulfilled in order for simple microbes to survive on a planet, and even more limiting criteria for intelligent life.  The magnetic fields, the direction of the orbit, the position of the planet and even the presence of a moon are all essential characteristics of a habitable planet like Earth.  What are the other criteria according to science?  How successful has this search been in actually finding an Earth-like planet?  And what can we do when we do find such a planet?

Since man had first learned of the existence of the stars, he has always wondered whether life exists somewhere out there in the vast universe.  Despite recent cuts of government funding of NASA, this fascination has not faltered with time.  The search for extraterrestrial life has been as intense as ever, and it all starts with finding “habitable” planets.  The definition of “habitable” is a mystery to most people, and in fact this definition is constantly changing.  Yet there is a number of “basic ingredients” required for any planet to harbor at least the smallest forms of life: microbes.  As simple as these criteria may be, however, they are strict and uncommon among planets in the universe.  As scientists turn their telescopes to the stars and the billions of planets among them, they keep these criteria in mind. The most fundamental criteria can be summarized in three categories: a source of energy, complex chemistry, and atmospheric protection from ultraviolet radiation.1 Using these criteria to narrow down which are the habitable planets is humanity’s first major step in finding extraterrestrial life out there.

A source of energy is primarily essential.1 For Earth, the Sun is the source energy that comes in the form of heat and light.  There are two reasons why energy is essential.  One is that, with the presence of sufficient heat, liquid water is able to exist.  Scientists have long believed that wherever in the galaxy one finds water, one finds life.  Of course, this need not always be the case.  Liquid methane has been suggested as a possible replacement for water because it is an organic molecule.  Nevertheless, energy is required to keep these materials at a liquid state, and so every planet needs a source of heat.  The second reason why energy is essential is that it is necessary for chemical reactions to occur.  All chemical reactions—whether making a molecule or breaking it—have an energy barrier known as an activation energy.  This barrier can be overcome only if the reaction occurs at a given temperature.  Without heat, there would be no reactions.  Without chemical reactions, life would be impossible.  On the other hand, a planet cannot be too close to this source of energy, or all molecules would degrade.  In fact, there is a thinly specified “Habitable Zone” in each solar system that a planet must be in based on its distance from the Sun.  In our case, Mars is too far and Venus is too close.  Because of their respective distances, neither liquid water nor organic chemical reactions can occur on these planets.

Life cannot form by throwing molecules like water and methane together.1 A complex chemistry is required; an abundant amount of molecules such as hydrogen, carbon, nitrogen, oxygen and halogens must be present.  This is not the case with many planets, so it is at this basic criterion that the search for habitable planets narrows.  If these atoms are present, they may independently join together and form larger molecules capable of carrying out individual processes within a bacterial cell.  For example, phosphorus is necessary to produce a molecule known as ATP, which provides the energy for most chemical reactions to occur in most living organisms on Earth.  As long as there are these fundamental atoms that make up organisms, it is possible for life to exist on a planet.

The third criterion that scientists look for in a planet is the presence of an atmosphere.  This atmosphere is necessary because every sun bombards all of its planets with solar radiation.  Ultraviolet radiation is harmful because it distorts the DNA of life forms, causing mutations.  Even though Earth has an atmosphere, most forms of life that are not microscopic have two sets of DNA to protect from mutation.  If a planet is not protected, it is as good as a base for life as a desert.  However, it is possible for life forms to be protected by some other means.  For example, on Saturn’s moon Titan, life may exist beneath the frozen surface of the moon, because the underground layer is at least protected from radiation.  Heat from geologic activity on this moon, scientists say, could provide energy.

Astrobiologists keep these criteria in mind, but it is difficult to determine how well a planet fits these.  After all, planets are absolutely minuscule in size to a star and are thus outshone by all the stars in the universe.  Nevertheless, when scientists do find a planet, they assess it with two indices: one is the Earth Similarity Index and the second is the Planetary Habitability Index.2 The Earth Similarity Index is simply based on how large the planet is compared to Earth and how it may be similar (in terms of position in the solar system, number of moons, etc.).  The Planetary Habitability Index uses the criteria mentioned already, as well as attempting to answer the question: is photosynthesis possible on this planet?  By being specific to these two indices, astrobiologists can narrow down from the trillions of planets out there to a select few.

Thus far, this time-consuming endeavor has churned out a few likely candidates, one of which happens to be in our solar system.4 That is Saturn’s moon Titan, which has a habitability index of 0.64.  Earth has a habitability index of 0.96, and by contrast, Mars has an index of only 0.59. Another is Gliese 581D, with an index of 0.43.  Yet another is Kepler 22-b, a planet 600 light-years away with a radius 2.4 times that of Earth, that has an index of about 0.43 as well.3

Based on these statistics, it seems that finding a habitable planet out there is a futile endeavor.  The criteria for housing any form of life are not numerous, yet very few planets seem to fit.  So far, the two most promising planets happen to be less habitable than Mars.  Yet scientists remain optimistic and open-minded,2 willing to accept that liquid methane could support life instead of water, or that a planet could be its own source of heat (like the thermal vents at the bottom of Earth’s ocean).  Perhaps one day we will find extraterrestrial microbes, or not.  Either way, it all begins with determining the possible planetary candidates.


  1. Pogge, Richard. “Lecture 46: Are We Alone? Life in the Universe.” Ohio State University. Last modified November 28, 2007.
  2. Boyle, Rebecca. “Q&A: How to Rate the Habitability of Other Planets.” Popular Science. Last modified November 23, 2011.
  3. Johnson, Michele. NASA’s Kepler Mission Confirms Its First Planet in Habitable Zone of Sun-like Star. NASA. Last modified December 5, 2011.
  4. Mone, Gregory. “Could one of these worlds be E.T.’s home?” Discover, March 2012.
  5. Image credit (public domain): MGS, MSSS, JPL, and NASA. “A Year of Extraterrestrial Fountains and Flows.” NASA. Last modified December 31, 2006.
  6. Image credit (public domain): NASA, JPL-Caltech, and C. Lisse (Johns Hopkins University Applied Physics Laboratory). “Birth of an Earth-like Planet.” NASA. Accessed June 20, 2012.

Sunny Parmar is a rising junior at Georgetown University. Follow The Triple Helix Online on Twitter and join us on Facebook.