The Red Sky Paradox

Introduction: 

Humans are scanning and searching the whole sky in order to find any evidence of alien life. But why finding alien life is so hard and next to impossible? Why can't we find any traces of alien life forms in our home (Milky Way) galaxy? Is there any common rule which defines the possibility of extraterrestrial life? All these questions strike in our minds whenever we talk about aliens. On one side there is a high probability of the existence of alien life in the universe and on another side, we do not have any evidence to prove such high probabilities of existence. This conflict between high probability and zero evidence is termed as Fermi Paradox. The Fermi Paradox soon became a subject for hundreds of books, thousands of lectures and millions of discussions. It still remained one of the most famous paradoxes. To answer this paradox, hundreds of theories emerged. One possible answer is that the possibility for two extremely intelligent civilizations to meet each other is very rare. Another one suggests that the aliens would be too shy to communicate with Earth or they might not be intelligent enough to communicate.

What is Red Sky Paradox?

Astronomer David Kipping proposed another paradox known as the Red Sky Paradox. David stated that the majority of stars in the universe are red dwarfs, especially M- dwarf stars. Then why life is developed on a planet revolving around a yellow star (The Sun)? M- dwarfs rule the universe, their mass lies between one-twelfth and one-half of that of sun-like stars. They even have a longer life span than the sun-like stars. Some of them are expected to live for more than 10 trillion years! That's about 1000 times the age of the current universe. The process of evolution of the living body is quite rapid in comparison with the evolution process of stars. So there is enough time for any planet revolving around the red dwarf to grow advanced civilizations through steps of evolution. Also, the M-dwarf has plenty of planets. Red Sky Paradox questions why there is no evidence of civilisation in the red dwarf-planet system? What are the odds of finding a civilization around an F, K or G type star instead of an M- type star? To answer this question, we first need to have a basic idea about spectral types of stars. 

Spectral types of Stars

Based on the splitting of the electromagnetic radiations from the star, they are classified into various types known as the stellar classification. Here stars are divided into different groups namely O, B, A, F, G, K and M, with O being the hottest and M coolest. Each class is further divided into ten different subclasses from 0 to 9 (0 being the hottest and 9 being the coolest). That is A9 type is the coolest star in the A-type stars. F types are hydrogen fusing stars with masses nearly equal (1.0 to 1.5 times) to that of the Sun. About 1 in 33 stars in the solar neighbourhood is an F-type. These stars are also termed, yellow-white dwarfs. 

G type, including our Sun, are the stars converting hydrogen into helium at its core and has about 0.9 to 1.1 solar masses. Our Sun is a G2 type star. About 8% of stars in the observable universe belong to this class. K type or the orange dwarfs are relatively cooler than the Sun. They make up around 12% of the stars in the solar neighbourhood. M-type or the red dwarfs are the most common stars. They make up around 76% of the solar neighbourhood. Most of the largest stars in the Milky Way belongs to this class. The Hertzsprung-Russell diagram shown below will give you a more clear idea about stellar classification. 

Why only F/G/K type stars and not M type?

As stated above, the life span of M type dwarf stars is very long. It is clear that the universe is not too old for M-type dwarf stars to exhaust their hydrogen supply. Stars having greater solar masses are grouped into A, B and O types. Their average life span is around 2 billion years, which is less than our Sun (10 billion years). Hence it clearly shows that A, B and O types do not live long enough to allow their planets to evolve advanced, intelligent life. Perhaps this is the reason why we do not find ourselves around the A, B or O type star. Also, these stars are extremely rare in the universe, less than 1% of total stars.

However, all these arguments can not be applied to M-type stars. They are less luminous than our Sun. These stars have almost all qualities for a civilisation to evolve. Then can't we find any life around these stars? The question itself is Red Sky Paradox. However, the probability of planets around different types of stars is not uniform. Hence one cannot answer this question with complete certainty. Using Baye's theorem and some other probability concepts, David derived an equation to find the probability of intelligent life around an F/G/K type or M type star. The equation is shown below.

Here Pr(I) is the probability of Intelligent life, n(G) is the number of F/G/K type stars and n(M) is the number of M type stars, ⋀(G) and ⋀(M) are the rates at which life emerges on the respective habitat and T(G) and T(M) are life period of intelligent civilization. Using this equation, David provided four possible answers to this paradox.

Resolution 1: An Unusual Outcome

If ⋀(G) and ⋀(M) are equal, that is the rate at which life emerges at both star types are equal then the probability for any intelligent life to be found around the FGK type is 1 in 100. Hence our existence around the Sun (g type star) is not normal. The evolution of life on planet Earth, revolving around the Sun can be considered a very rare event in the universe. This means that there are still chances for us to find alien life on planets orbiting the M-type stars. Hence resolution 1 suggests we should focus on M-type dwarf stars to find aliens. However, this result is not satisfying, yet not impossible. Till today we find no evidence of life on planets orbiting M-dwarfs. Hence, David stated another three resolutions to provide more satisfying answers.

Resolution 2: Inhibited life under a Red Sky

Here, David stated that the probability of emergence of life form around yellow dwarfs (like our Sun) is about 100 times that of around red dwarfs (M type). There are actually a lot of theoretical shreds of evidence to support this resolution. The M-type stars tend to have a very high frequency of solar flares which makes the planet orbiting it nearly inhabitable. Also, the planet system of M-type stars lacks Jupiter sized planets. Jupiter plays a vital role in protecting Earth from the asteroids. Imagine life on Earth without Jupiter. The absence of a Jupiter like planet will result in more frequent asteroid hits and life cannot evolve on such a planet.

Resolution 2 tells us not to focus on planets orbiting M-type stars. This is in complete contrast with what was stated in resolution 2. We can find both resolutions are completely opposite. The former suggests focusing on M-type while the latter suggests not to do that.

Resolution 3: A truncated window for Complex Life

This resolution states that Sun-like yellow stars provide a wider window or time period for life to evolve. The M-type certainly has a longer life period, but it is also possible that their evolution period might not provide enough time for life forms to grow. When a red dwarf is fusing hydrogen, it remains quite hot and brighter. During this stage, any form of life might not survive. David stated that this hypothesis can be proved by studying young M-type dwarf stars. Thus resolution 3 tells us to focus on younger M-type stars if we wish to find alien life.

Resolution 4: A Paucity of Pale Red Dot

In the last resolution, David stated that there is a possibility that M-dwarf stars do not have enough rocky planets orbiting them. Furthermore, we have studied only the larger M-type stars (M0 to M5), because they are brighter and easier to study. However, maximum M-type stars lie between M6 and M8. We have not studied these types enough because the smallest stars produce insufficient luminosities to be detected in large numbers.

We can say that just by studying M0 or M2 stars, we cannot create enough data which can be applied to the whole distribution of the M class. The majority of stars are between M6 and M8, hence they might have greater influence over the whole sample of the M class. In other words, planets orbiting the M6 or M8 star might have different structures than those orbiting larger stars of the same class. Thus resolution 4 tells us to focus more on M6 to M8 type stars for searching alien life.

Astronomer David Kipping while talking about the Red Sky Paradox

Conclusion:

David himself is not sure which one from the above 4 resolutions will work as a correct explanation for the paradox. It is possible that none of the above work. That's why it is a paradox. The possibility of the existence of life forms near the nebular areas, rogue stars, brown dwarfs and other evolved stars is neglected in this paradox. Also, the influence of the moon on the parent planet is also neglected for easier calculations. Many phenomenons were just assumed for easier calculations of probability. Even after all these assumptions and neglections, the Red Sky Paradox might work as a base theory for finding extraterrestrial life forms.