Pedagoguery

Recently, the Kepler satellite has demonstrated that quite a few planets exist out there. However, we are still not able to detect and confirm the real goal of exoplanet searches – finding an Earth-like world. Proposed missions like the Terrestrial Planet Finder would provide such a means, by directly imaging such planets and being able to take spectra of their atmospheres to determine their composition. However, we need to know what to look for in order to properly interpret the results, since the Earth itself has changed considerably during its 4.5 billion year history. Looking back at Earth's past can help provide insight into what to look for.

The earliest of Earth's geologic eras is called the Hadean. It lasted from Earth's initial formation 4.56 billion years ago to about 3.8 billion years ago. This was a turbulent time in Earth's history, marked by near constant bombardment from interplanetary debris, not least of which was the impact of a Mars-sized object that resulted in the creation of our moon. During this time, Earth's atmosphere was composed of nitrogen, carbon monoxide, and water vapor. There was a high degree of volcanism, and thus the Earth was more intrinsically hot than it is now, despite the fact that the luminosity of the sun was only about 70% of what it is today. The Earth gradually cooled to the point where the first oceans appeared about 4.2 billion years ago. This would have resulted in increased albedo (reflectivity) and thus made the Earth brighter at visible wavelengths. There is some evidence that at about 3.9 billion years ago, Jupiter and Saturn passed through a 2:1 orbital resonance, resulting in a disruption of the orbits of comets and asteroids, triggering what is referred to as the Late Heavy Bombardment.

The next era, the Archean, starts with the advent of primitive life 3.8 billion years ago, and ending at 2.5 billion years ago. Early life would have used molecular hydrogen for energy, which would have been rapidly depleted from the atmosphere and oceans. As a result of the loss of this easy energy supply, early photosynthesis developed. This type of photosynthesis used molecular hydrogen, hydrogen sulphide, and ferrous (Fe⁺²) iron to produce carbohydrates. Other types of micro-organisms produced methane, a powerful greenhouse gas. This helped keep the Earth warm during this time when the sun's luminosity was only 75% to 85% of its current value. The Earth's atmosphere was composed of nitrogen, carbon dioxide, methane, and water vapor. As the percentage of methane increased to about 10% the concentration of carbon dioxide, an organic haze developed, much like that which we see currently on Titan. Toward the end of this era (2.7 billion years ago), a new type of photosynthesis developed, which yielded oxygen as a byproduct. This changed everything, since oxygen can deliver considerably more energy than the mechanisms in place at the time. However, for organisms that were not used to dealing with such an energetic gas, it was highly toxic. Initially, concentrations were kept down by reactions with rocks and dissolved minerals such as Fe⁺².

The Protozoic Era begins about 2.5 billion years ago when mineral reactions were unable to keep the oxygen concentration in the atmosphere low. A particular trigger was when the concentration in the upper atmosphere grew enough to provide an ozone layer, blocking ultraviolet radiation that was causing the oxygen and methane to react. This period is called the “Great Oxidation” and it significantly reduced the greenhouse effect in the atmosphere. In a way, that was good, since the sun was continuing to heat up, attaining between 85% and 95% of its modern luminosity. Oxygen concentrations would range from 1% to 50% of modern values. However, sometimes, the greenhouse effect was reduced too low. At least three times in the geologic record, we can identify that the Earth went through a “Snowball Earth” phase, when ice caps extended all the way to the equator. During these times, the Earth's atmosphere was composed of primarily nitrogen and carbon dioxide, with the levels of carbon dioxide increasing through vulcanism until enough of a greenhouse effect was established to start melting the ice at the equator. When that happened, a runaway greenhouse effect was initiated to top the Earth into a warm phase

Starting about 600 million years ago, life dominates. The sun's luminosity gradually climbs from 95% of its modern value to what we see today. The atmosphere is dominated by nitrogen and oxygen, with measurable amounts of water vapor. Concentrations of methane gradually decline. Distant spectra might catch the signature of chlorophyl or similar photosynthesizing molecules as plant life takes a greater hold over the land. Other planets would be unlikely to develop chlorophyl, but we can estimate the plant signatures for different types of stars and look for those. Detecting intelligent life at a development level similar to our own would be considerably more difficult.

In the future, the sun's luminosity increases by about 10% every billion years. We estimate that in about 1.5 billion years from now, the oceans would evaporate, triggering a runaway greenhouse effect that would render Earth very similar to Venus, with an atmosphere composed almost entirely of nitrogen and carbon dioxide.

Next time, some of the Earth-like exoplanets that we have already found.