Earth isn’t as obviously habitable as we think.
Throughout history, man has wanted to find other worlds with life, whether similar to ours or perhaps more science fiction-like. Nasa’s Kepler Mission is dedicated to doing just that: exploring a portion of our Milky Way to detect potentially habitable planets. Enough evidence has been gathered to categorise planets into three types of exoplanets: gas giants, hot-super-Earths and ice-giants. The ultimate challenge is to try and locate terrestrial planets similar to Earth. These can range from half to double the size of Earth, and they need to be far enough away from their sun so as not to boil, and close enough to not freeze over. These are the conditions for life: where liquid water can be found.
The Kepler Mission’s scientific objective is “to explore the structure and diversity of planetary systems”. This is achieved by surveying a large sample of stars to:
-Determine the abundance of terrestrial and larger planets in or near the habitable zone of a wide variety of stars;
-Determine the distribution of sizes and shapes of the orbits of these planets;
-Estimate how many planets there are in multiple-star systems;
-Determine the variety of orbit sizes and planet reflectivities, sizes, masses and densities of short-period giant planets;
-Identify additional members of each discovered planetary system using other techniques; and
-Determine the properties of those stars that harbor planetary systems.”
So the question arises, if we use our criteria of detecting habitable planets, how would they rank our planet Earth? A team at the University of Washington’s Virtual Planetary Laboratory, lead by Rory Barnes have published a paper stating Earth is not as habitable as we might think.
Barnes et al explain that the main factor is usually the comparison of “a planet’s semi-major axis to the location of its host star’s “habitable zone”, the shell around a star for which Earth-like planets can possess liquid surface water.” (http://arxiv.org/pdf/1509.08922v1.pdf) This means that the “longest” radius of the elliptical path of the planet around its sun is studied to see if it stays within the region where water will remain liquid rather than evaporate into gas or freeze.
They “propose a method to compare transiting planets for their potential to support life based on transit data, stellar properties and previously reported limits on planetary emitted flux.” The first prerequisite is that the planet is in radiative equilibrium, meaning that the incoming solar energy is the same amount as the planet emits. For such a planet the emitted flux (the energy passing through an area perpendicular to the radiation) increases as its orbit moves further away from being a perfect circle (ellipse), but decreases with the amount of reflection of solar energy back into space (albedo). This index basically compares the amount of energy reflected from the planet’s surface against how circular its orbit is.
Barnes et al call this comparison the “habitability index for transiting exoplanets”. It is in effect the probability of an exoplanet supporting liquid water; in other words, the habitability index calculates how much net energy the planet receives from its sun. If the energy is too much the planet will be like Venus, if the energy is too low, it will be icy like our outer planets.
Barnes et al applied this method to several planets they deemed interesting from Keplar’s discoveries, and concluded that “planets that receive between 60–90% of the Earth’s incident radiation, assuming circular orbits, are most likely to be habitable.” They also make some predictions for upcoming TESS and JWST missions.
The results they find for the Kepler selection of planets are quite surprising: several of them have larger values of the Habitability Index than Earth. They continue to explain that this does not make these planets more habitable than Earth. It does, however, mean that “an Earth twin orbiting a solar twin that is observed by Kepler would not have the highest probability of being habitable. The best candidates have incident radiation levels, assuming circular orbits, of 60–90% that of Earth’s.”
What are the implications of this? Does this mean their method needs refining? Does it require a factor they have perhaps not thought of as relevant? Or does it imply that life on Earth is not to be taken for granted? If the probability of life on Earth is not as likely as we thought, perhaps the conditions that create and sustain life need to be revised? This opens the door to some very interesting questions.
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