<p>For low-mass stars like the Sun, the habitable zone moves outwards, by something like 50% over the course of its main sequence lifetime.</p>
<p>Therefore, if part of your &quot;criteria for life&quot; is that a location must spend a long time (billions of years?) inside the habitable zone, then that might rule out locations presently in the inner part of the habitable zone of a young star. Conversely, something in the outer part of a habitable zone may have a long future of being habitable, but may not have become habitable until recently.</p>
<p>These considerations are less important for K and M-dwarfs who's habitable zones move very slowly. On the other hand they have slowly decaying rotation, magnetic fields and consequent magnetic activity that probably means you prefer the outer parts of any habitable zone.</p>
<p>In terms of atmospheres well that depends on how you define the habitable zone. If it is just based on an equilibrium temperature with no atmosphere, then yes, it is possible to make a planet warmer than that, but not to make it colder. Locations that are outside the &quot;habitable zone&quot; may in fact be habitable, given the right atmosphere.</p>

CC BY-SA 4.0
2023-04-23T08:02:13.480Z
2023-04-23T08:02:13.480Z
ProfRob
<p>I was thinking about superhabitable worlds a while ago, and I figured the best position regarding probability of life would be nearer the <em>outer</em> habitable zone, since it's easier to develop a greenhouse effect than to counteract heat (excluding a red-giant phase). Am I right? I'm no professional...</p>
<p>EDIT 23/APR/2023: Sorry, I meant the <em>current</em> position of exoplanets in the <em>current</em> habitable zone of a star, though the answers thus far have been enlightening!</p>

CC BY-SA 4.0
2023-04-22T23:42:06.507Z
By definintion the habitable zone is the zone which is habitable.  The edges are going to be "less habitable" ... so the middle of the habitable zone is best.
James K
CC BY-SA 4.0
2023-04-23T00:13:01.907Z
But as we only have sample size of one, we don't really know, so I think this is a speculative question.
James K
CC BY-SA 4.0
2023-04-23T00:19:45.343Z
I think best also could be qualified a bit here for clarity; is it the best as in it will support life for the longest? Are we considering the eventually late stages of a stars life? Because that would then shift it towards the outer edges so that it would stay in the zone as long as possible, or does perhaps best mean it would develop life the fastest? A little more context here I think would make this question easier to answer at least from the grounds of models
Justin T
CC BY-SA 4.0
2023-04-23T01:12:09.280Z
Thank-you, @JustinT. I'll edit accordingly.
Kazon
CC BY-SA 4.0
2023-04-23T02:46:58.317Z
It's always intrigued me as to why some people have thought that life in our solar system might have developed on Mars before transferring to Earth, via meteor impacts. Mars is farther from the Sun & is colder. Also, in the early days of the solar system, the Sun's energy output was less than what it is now, implying Mars would have been even colder than it is now.
Fred
CC BY-SA 4.0
2023-04-23T08:02:49.073Z
@Fred Mars had an atmosphere and was warmer and wetter in the past.
ProfRob
CC BY-SA 4.0
2023-04-23T10:42:04.217Z
a thicker atmosphere I should say.
ProfRob
<astrobiology><extra-terrestrial>
What part of a star's habitable zone has the best chance of developing life?
Kazon
# What part of a star's habitable zone has the best chance of developing life?

(Asked by: username_0 on 2023-04-22)

I was thinking about superhabitable worlds a while ago, and I figured the best position regarding probability of life would be nearer the *outer* habitable zone, since it's easier to develop a greenhouse effect than to counteract heat (excluding a red-giant phase). Am I right? I'm no professional...

EDIT 23/APR/2023: Sorry, I meant the *current* position of exoplanets in the *current* habitable zone of a star, though the answers thus far have been enlightening!

**Tags:** astrobiology, extra-terrestrial

> **Comments:**
> **username_1**: By definintion the habitable zone is the zone which is habitable.  The edges are going to be "less habitable" ... so the middle of the habitable zone is best.  
> **username_1**: But as we only have sample size of one, we don't really know, so I think this is a speculative question.  
> **username_2**: I think best also could be qualified a bit here for clarity; is it the best as in it will support life for the longest? Are we considering the eventually late stages of a stars life? Because that would then shift it towards the outer edges so that it would stay in the zone as long as possible, or does perhaps best mean it would develop life the fastest? A little more context here I think would make this question easier to answer at least from the grounds of models  
> **username_0**: Thank-you, @JustinT. I'll edit accordingly.  
> **username_3**: It's always intrigued me as to why some people have thought that life in our solar system might have developed on Mars before transferring to Earth, via meteor impacts. Mars is farther from the Sun & is colder. Also, in the early days of the solar system, the Sun's energy output was less than what it is now, implying Mars would have been even colder than it is now.  
> **username_4**: @username_3 Mars had an atmosphere and was warmer and wetter in the past.  
> **username_4**: a thicker atmosphere I should say.  

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## 1 answers

### Answer by username_4 on 2023-04-23. Score: 1

For low-mass stars like the Sun, the habitable zone moves outwards, by something like 50% over the course of its main sequence lifetime.

Therefore, if part of your "criteria for life" is that a location must spend a long time (billions of years?) inside the habitable zone, then that might rule out locations presently in the inner part of the habitable zone of a young star. Conversely, something in the outer part of a habitable zone may have a long future of being habitable, but may not have become habitable until recently.

These considerations are less important for K and M-dwarfs who's habitable zones move very slowly. On the other hand they have slowly decaying rotation, magnetic fields and consequent magnetic activity that probably means you prefer the outer parts of any habitable zone.

In terms of atmospheres well that depends on how you define the habitable zone. If it is just based on an equilibrium temperature with no atmosphere, then yes, it is possible to make a planet warmer than that, but not to make it colder. Locations that are outside the "habitable zone" may in fact be habitable, given the right atmosphere.


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