<p>When the Sun becomes a red supergiant (more accurately, an <a href="https://en.wikipedia.org/wiki/Asymptotic_giant_branch">asymptotic giant branch star</a>), it will have <em>less</em> mass than it does now. That is because mass is lost by an increasingly massive stellar wind as the Sun approaches the end of its life. As a result, the orbital radii of the planets would <em>increase</em>. The surface temperature of this future AGB Sun would be <em>lower</em> than the temperature of the Sun, but its luminosity would be much larger.</p>

<p>However, even if the Earth manged to avoid being engulfed in the outer envelope of the evolved Sun (which is still uncertain), the amount of energy received per square metre at the Earth's surface would be vastly increased over the 1.4 kW per square metre it receives from the Sun now. So your conclusion that life (as we know it) would be fried is quite correct.</p>

<p>The <a href="https://en.wikipedia.org/wiki/Circumstellar_habitable_zone">Goldilocks zone</a> for the possible occurrence of life, uses the criterion of liquid water as the basis for allowing life to evolve. As a star becomes more luminous (and an AGB Sun would probably be about 100 times as luminous as the present Sun), the Goldilocks zone moves outwards roughly proportional to $\sqrt{L}$, so about a factor of 10 further away.</p>

<p>The problem then with allowing life to evolve on another planet (e.g. Mars after its orbit has expanded a bit, or Jupiter) is that (i) you need the ingredients to be there already, or some way for them to be delivered, but more importantly (ii) there is little time for life to evolve, because the Goldilocks zone position evolves really rapidly towards the end of the Sun's life as the AGB Sun's luminosity increases - the timescales would be measured in millions rather than billions of years. Furthermore, the luminosity of AGB stars can be highly variable as they undergo big pulsations, which may also be a problem. (iii) After the (comparatively brief) AGB phase has ended, you would end up with a very hot ($T \simeq 10^{5}$ K) white dwarf remnant that might effectively sterilise the whole system with ultraviolet light.</p>

<p>I don't think we know enough about how life gets started to say that these problems rule out your scenario; but they perhaps make it less likely. </p>

CC BY-SA 3.0
2016-01-19T02:52:58.373Z
P.S. Life on Earth will probably be fried way before this. If models are correct, the Earth will be so hot in about a billion years that the oceans will evaporate. Hello Mars!!
Jack R. Woods
CC BY-SA 3.0
2016-01-14T12:45:02.163Z
2016-01-14T12:45:02.163Z
ProfRob
<p>Let's talk about the time when our sun becomes a supergiant. We know that our solar system may get destroyed or it may get affected due the large mass of the supergiant and also due to the large temperature. But somehow let us consider nothing happened to our solar system by luck. It's obvious that life won't be possible on earth anymore(due to the extreme heat), but is there a possibility that, the events that took place during the formation of Earth, may take place in the far off planets? Can life formation take place in some other planet just like it happened in Earth??</p>

CC BY-SA 3.0
2016-01-14T21:15:47.257Z
The Sun will never become a supergiant. It will become a red giant, which is much different. Additionally, the mass of the Sun *will not increase*.
HDE 226868
<earth><distances><earth-like-planet>
Sun as our Supergiant
user10379
# Sun as our Supergiant

(Asked by: username_0 on 2016-01-14)

Let's talk about the time when our sun becomes a supergiant. We know that our solar system may get destroyed or it may get affected due the large mass of the supergiant and also due to the large temperature. But somehow let us consider nothing happened to our solar system by luck. It's obvious that life won't be possible on earth anymore(due to the extreme heat), but is there a possibility that, the events that took place during the formation of Earth, may take place in the far off planets? Can life formation take place in some other planet just like it happened in Earth??

**Tags:** earth, distances, earth-like-planet

> **Comments:**
> **username_1**: The Sun will never become a supergiant. It will become a red giant, which is much different. Additionally, the mass of the Sun *will not increase*.  

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## 1 answers

### Answer by username_2 on 2016-01-14. Score: 6 | Accepted answer

When the Sun becomes a red supergiant (more accurately, an [asymptotic giant branch star](https://en.wikipedia.org/wiki/Asymptotic_giant_branch)), it will have *less* mass than it does now. That is because mass is lost by an increasingly massive stellar wind as the Sun approaches the end of its life. As a result, the orbital radii of the planets would *increase*. The surface temperature of this future AGB Sun would be *lower* than the temperature of the Sun, but its luminosity would be much larger.

However, even if the Earth manged to avoid being engulfed in the outer envelope of the evolved Sun (which is still uncertain), the amount of energy received per square metre at the Earth's surface would be vastly increased over the 1.4 kW per square metre it receives from the Sun now. So your conclusion that life (as we know it) would be fried is quite correct.

The [Goldilocks zone](https://en.wikipedia.org/wiki/Circumstellar_habitable_zone) for the possible occurrence of life, uses the criterion of liquid water as the basis for allowing life to evolve. As a star becomes more luminous (and an AGB Sun would probably be about 100 times as luminous as the present Sun), the Goldilocks zone moves outwards roughly proportional to $\sqrt{L}$, so about a factor of 10 further away.

The problem then with allowing life to evolve on another planet (e.g. Mars after its orbit has expanded a bit, or Jupiter) is that (i) you need the ingredients to be there already, or some way for them to be delivered, but more importantly (ii) there is little time for life to evolve, because the Goldilocks zone position evolves really rapidly towards the end of the Sun's life as the AGB Sun's luminosity increases - the timescales would be measured in millions rather than billions of years. Furthermore, the luminosity of AGB stars can be highly variable as they undergo big pulsations, which may also be a problem. (iii) After the (comparatively brief) AGB phase has ended, you would end up with a very hot ($T \simeq 10^{5}$ K) white dwarf remnant that might effectively sterilise the whole system with ultraviolet light.

I don't think we know enough about how life gets started to say that these problems rule out your scenario; but they perhaps make it less likely.

> **Comments:**
> **username_3**: P.S. Life on Earth will probably be fried way before this. If models are correct, the Earth will be so hot in about a billion years that the oceans will evaporate. Hello Mars!!  

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