<p>I was reading about red giants and came across this statement:</p>
<blockquote>
<p>Some research suggests that, during the evolution of a 1 M☉ star along the red-giant branch, it could harbor a habitable zone for several billion years at 2 astronomical units (AU) out to around 100 million years at 9 AU out, giving perhaps enough time for life to develop on a suitable world.</p>
</blockquote>
<p>Red giants have luminosities of <span class="math-container">$\sim 3000 L_\odot$</span>. According to the inverse square law, the habitable zone must be <span class="math-container">$1^{+1.5}_{-0.2} \cdot \sqrt{3000} \approx 54.772^{+82.158}_{-10.954} \text{AU}$</span> away from the star. If the star is exhibiting variability, its luminosity may change by about/over <span class="math-container">$50\%$</span>. This will make the habitable zone vary by a factor of <span class="math-container">$2\sqrt{0.5}=\sqrt2 \approx 1.414$</span>. So do any stable orbits (in terms of habitability) exist for planets orbiting such a red giant star, assuming the variability timescale is within the orbital period of such a planet?</p>

CC BY-SA 4.0
2021-04-21T05:40:20.610Z
At what timescale are they variable?
planetmaker
CC BY-SA 4.0
2021-04-21T06:35:25.207Z
How do you define habitabity?
ProfRob
CC BY-SA 4.0
2021-04-28T22:57:25.007Z
@ProfRob By habitability, I mean the circumstances where humans can colonize the planet and survive without major additions (i.e. spacesuits)
WarpPrime
CC BY-SA 4.0
2021-04-28T23:10:00.637Z
Then I don't understand what information you require beyond what you have already put in your question. A planet can orbit in a stable way at 54 au from a star. You seem to be asking about some sort of timescale though that you haven't defined in your question or in your comment.
ProfRob
<star><planet><habitable-zone><red-giant><variable-star>
What is the theoretical maximum variability a pulsating red giant can have such that a habitable planet can stay habitable for long periods of time?
WarpPrime
# What is the theoretical maximum variability a pulsating red giant can have such that a habitable planet can stay habitable for long periods of time?

(Asked by: username_0 on 2021-04-20)

I was reading about red giants and came across this statement:

> Some research suggests that, during the evolution of a 1 M☉ star along the red-giant branch, it could harbor a habitable zone for several billion years at 2 astronomical units (AU) out to around 100 million years at 9 AU out, giving perhaps enough time for life to develop on a suitable world.

Red giants have luminosities of $\sim 3000 L\_\odot$. According to the inverse square law, the habitable zone must be $1^{+1.5}\_{-0.2} \cdot \sqrt{3000} \approx 54.772^{+82.158}\_{-10.954} \text{AU}$ away from the star. If the star is exhibiting variability, its luminosity may change by about/over $50\%$. This will make the habitable zone vary by a factor of $2\sqrt{0.5}=\sqrt2 \approx 1.414$. So do any stable orbits (in terms of habitability) exist for planets orbiting such a red giant star, assuming the variability timescale is within the orbital period of such a planet?

**Tags:** star, planet, habitable-zone, red-giant, variable-star
> **Comments:**
> **username_1**: At what timescale are they variable?  
> **username_2**: How do you define habitabity?  
> **username_0**: @ProfRob By habitability, I mean the circumstances where humans can colonize the planet and survive without major additions (i.e. spacesuits)  
> **username_2**: Then I don't understand what information you require beyond what you have already put in your question. A planet can orbit in a stable way at 54 au from a star. You seem to be asking about some sort of timescale though that you haven't defined in your question or in your comment.  

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## 0 answers

