The Instigator
Con (against)
1 Points
The Contender
Pro (for)
5 Points

Is complex extraterrestrial life common?

Do you like this debate?NoYes+0
Add this debate to Google Add this debate to Delicious Add this debate to FaceBook Add this debate to Digg  
Post Voting Period
The voting period for this debate has ended.
after 1 vote the winner is...
Voting Style: Open Point System: 7 Point
Started: 4/1/2015 Category: Science
Updated: 3 years ago Status: Post Voting Period
Viewed: 674 times Debate No: 72741
Debate Rounds (4)
Comments (0)
Votes (1)




This will be a debate on if extraterrestrial life is common in the universe. If anybody thinks it is, then by all means take me on. I will be waiting to challenge.

I'm not going to go into all those fancy rules which many other debators do because I like a more open envoronment rather than all those strict rules such as First for acceptance only for example. I'm only going to make one rule with this and that is in my next paragraph.

So competitor, whoever you are, do you want to go first or second? If first then write away but no more arguments on the last round. If you choose to go second, don't use the first round, you use the last round. Just doing this to make it fair, we both get three turns this way.


I accept.
Debate Round No. 1


So, you choose to go second. That's fine, blow me away with the last comment :D

So let's get started. I believe quite strongly in the Rare Earth hypothesis which goes into the many different variables which go into decideing if multicellular lifeforms can develop on that planet.

In my beliefs there are many different factors which decide the likelyness. If any of these are missing, it would be MUCH more unlikely for complex life to develop.

Firstly we will start with the obvious ones such as the planet's orbit. If the planet has an orbit outside of the host star's habitible zone, since complex life requires much less extreme temperatures than simple lifeforms such as bacteria, then there will be a very slim chance of complex life developing on that such planet.

The planet will need to be in the correct area of the galaxy as well. It needs to be close enough to the galactic core for there to be enough metals to allow planets to form around the star but not too close to the centre as closer to the centre as there is much more radiation which could be very damaging to complex lifeforms and preventing their formation. (Metal means any element except from hydrogen and helium in astronomy terms); Our sun is in a "galactic habitable zone" which balances these two outcomes.

Next the planet needs to orbit the right kind of star. Any main sequence star which is too massive such as spectral types O B A and early F stars will exhaust their hydrogen too quickly for complex extraterrestrial life to develop and when stars use up all their hydrogen they start to expand into red giants which can vapourise any planets which are close to them. (When our Sun will die, it will expand to Earth's orbit during its Red Giant phase) If a star is not massive enough, the habitible zone of the star will be close to the star. For a red dwarf star, this can be closer than a tenth of the distance from the sun to the earth. At such close distance, the planet will likely become tidally locked to the star, this means that the planet will always keep the same side facing the parent star. Our moon is tidally locked to the Earth with the same face constantly facing us. For a planet, this means that one side is in constant daylight, the other is a permanent night time. Complex life forming here will likely be much harder than on a Sun like star. Furthermore with the star, it will need to be metal rich so globular clusters, which are made up of Population II stars which are metal poor and tightly packed together are out of the picture.

There is more variables than these however. Some of the more obscure ones are as follows.

A large gas giant orbiting outside the earthlike planet to prevent more asteroid bombardment such as Jupiter which keeps the Asteroid Belt in line.

A large moon to keep the orbital tilt of the planet under control. Without the moon, our planet would be more like Mars which wobbles on its axis)

There are many others but I will be waiting for your input on this so far.


Voidness forfeited this round.
Debate Round No. 2


And I win lol :D


No, you don't :D


Before proceeding, I would like to apologize to my audience for my forfeiture [I won't go into the exact reasons why I forfeited]. I urge the audience to award Con the conduct point.

I will divide this round into two sections. First, I shall attack Neg's arguments for negating the resolution. Then, I shall provide reasons of my own for affirming the resolution.

---Con's case---

Con argues that complex extraterrestrial life is uncommon, through attempting to affirm the Rare Earth hypothesis. Given that this is his only avenue of argumentation, were I to succeed in refuting the hypothesis, the audience has no reasons to vote Con in "arguments". Let's begin.

(a) The Rare Earth hypothesis uses some form of post-hoc reasoning in evaluating the criteria necessary for a planet to sustain complex life. Post-hoc reasoning is a logically fallacious form of reasoning, when one proclaims that "since event Y followed event X, event Y must have been caused by event X." ( For instance, the rooster crows immediately before sunrise, therefore the rooster causes the sun to rise.

Looking at the criteria necessary for a planet to sustain complex life, as the Rare Earth hypothesis proclaims, it is clear that many, if not all of them rely on some form of post-hoc reasoning that is based on the factors necessary for the rise of complex life on Earth. The astronomer David Darling even proclaims that the Rare Earth hypothesis is mearly an account of some soft, of how complex life arised on Earth (! To expand on the point pertaining to Rare Earth using post-hoc reasoning, I'm not suggesting that because of this, all of the variables suggested are unreasonable. The variable that claims that a planet must be in a habitable zone, for instance, seems highly reasonable. However, other variables such as "a large gas giant" being needed, or "a large moon" being needed seem highly unimaginative with regard to what forms of complex life that could possibly exist, given that they only take into account complex life on Earth. Yet, it is possible that on another planet, life would, through evolution, be able to adapt to such variables, or be exempt from them altogether.

(b) Secondly, several variables are highly disputable, and are by no means, settled science. To use an example of a variable that has been scientifically disproved altogether, Ward and Brownlee [the founders of the Rare Earth hypothesis], on page 217 of their seminal, aptly titled book "Rare Earth" claim that "there is irrefutable evidence that oxygen is a necessary ingredient for [animal] life". However, some recently discovered multicellular organisms, such as the Spinoloricus Cinzia, rely on hydrogen, rather than oxygen.

Other variables, such as the role of Jupiter, are scientifically dubious, to say the least: "A study by Horner & Jones (2008) using computer simulation found that while the total effect on all orbital bodies within the Solar System is unclear, Jupiter has caused more impacts on Earth than it has prevented." (

--My case--

I shall provide a short, succint case:

First of all, exoplanet research and discovery has almost been limited to just those within the Milky Way. Even within the milky way, the exoplanets that have been discovered are minimal - we have only discovered about 1900 exoplanets. It is thus no suprise at all that we have not discovered any complex extraterrestrial life as of yet. However, by extrapolating from the already discovered exoplanets, as well as other data eg. the amount of stars in the Milky Way, it could be the case, according to a study published in PNAS, that there could be 11 billion planets located in a habitable zone. ( It also ought to be noted that the Milky Way is just one of 100 or 200 billion galaxies that exist. ( As such, it is likely that the Universe is teeming with so many planets located in a habitable zone, that even if most of the variables in the Rare Earth hypothesis were to be true [many are probably not] there would still be a significant amount of complex extraterrestrial life existing.

The likelihood is even higher, when facing recent developments in the origin of life. Astrobiologist and physicist Paul Davies, in his book the Eerie Silence, describes how recent OOL research developments have swung "the pendulum the other way [from being highly unlikely to likely]" with regards to the probability of life originating on a habitable planet. [Page 42, the Eerie Silence] Christian de Duve, claims that it is a "cosmic imperative" that life would on potentially habitable planets. (


In this round, I have refuted Con's arguments against the resolution, and has also shown that even in the face of many variables of the Rare Earth hypothesis being true, the sheer size of the universe, and the sheer number of planets existing, likely renders complex extraterrestrial life common. The resolution is affirmed.
Debate Round No. 3


Cazaam forfeited this round.


Vote for Pro.
Debate Round No. 4
No comments have been posted on this debate.
1 votes has been placed for this debate.
Vote Placed by kman100 3 years ago
Agreed with before the debate:Vote Checkmark--0 points
Agreed with after the debate:--Vote Checkmark0 points
Who had better conduct:Vote Checkmark--1 point
Had better spelling and grammar:--Vote Checkmark1 point
Made more convincing arguments:-Vote Checkmark-3 points
Used the most reliable sources:-Vote Checkmark-2 points
Total points awarded:15 
Reasons for voting decision: Pro successfully pointed out that Cons case relied on the assumption that all complex life has to be similar to the complex life that we know. Pro was able to provide examples of life that is dissimilar to most life (completely anaerobic bacteria) and made it seem probable that life could arise in unearthlike conditions. Con gave no reply to this, convincing me to give pro "most convincing arguments" Source vote goes to pro because pro had sources while con didn't, and conduct goes to con at pros urging.