Exoplanets are planets outside our solar system, first recorded evidence of them dates back to 1917, while the first confirmed detection was in 1992.
Exoplanets can be detected by various method, the most common of which are the Transit and Doppler Spectroscopy methods. But these two methods rely heavily on the star the exoplanet is orbiting, hence, they are limited to exoplanets that are close to their star.
Our target is to locate habitable planets, in which life may originate or be transferred to, either by a life-carrying asteroid or interstellar future travelers. There are billions of planets out there, and it’s not possible to analyze all of them in our progress of search. This is why we are focusing our search on planets that are most likely to be habitable, by extrapolating the conditions on earth. i.e., we are looking for Earth-sized planets, orbiting a solar analog star.
Solar analog stars are stars that are similar to our sun, for example, the star must have a temperature that is no more or less by 500 degrees Kelvin than the temperature of the sun (5278K-6278K). The star must also be rich in heavy elements, which support the formation of planets around the star. However, none of the planets orbiting the star should be too close to it, because that would irritate the star, making it too active.
There is a countless number of such stars in the universe, however, there is one more requirement, for one or more of the planets orbiting the star to be in the habitable zone. The habitable zone is the range of orbits around the star in which the temperature of the planet would support liquid water. If the planet is too close, the water might evaporate, if the planet is too far, the water might freeze. And yet again, this is not the full picture, we need to take atmosphere into account. Since atmosphere provides pressure, it can manipulate the temperature of the planet and the possible state of water on it.
Taking the star characteristics, the planet’s size, and the habitable zone into account, there is approximately 11 billion planets that satisfy our assumed conditions of life. Now you might be wondering, if there are so many habitable planets, how come we haven’t found life yet? Well, this is known as the Fermi paradox. There are some possible answers to this question, some of which are:
1- Intelligent life is extremely rare.
2- The span of life is very short, or limited by some unknown factor.
3- Life does exist, but is very hard to find evidence of it.
The first possible answer is examined through evolutionary biology.
The second possible answer is more of a philosophical one, arguing that intelligent life is inherently self-destructive.
The third possible answer, is the one we will examine in this article.
In order to study the difficulty of observing life, some scientists have been examining what earth would look like from far away. This study not only help answer one of the previous questions, but yet another important question, how easy would an alien life detect us?
Two articles, titled “Earth as an exoplanet” and “Earth as an Exoplanet: II. Earth's Time-Variable Thermal Emission and its Atmospheric Seasonality of Bio-Indicators” have examined the detection of bio-signatures and bio-indicators of earth from far away. To summarize their results, they found that visible and near infrared examinations could detect the presence of liquid water and an atmosphere in chemical disequilibrium, while Radio examinations could detect features of intelligent life. However, some examinations' results varied by season and viewing geometry, such as the thermal emission spectra of earth and detections of Oxygen and methane. What this essentially means, is that for the confirmation of life on a planet, a large number of studies must be done, across different periods of time, and different locations. This suggests that we may not be ready for such an observation yet. The researchers also encourage the study of clouds and their effect on the study of a planet’s habitability.
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