<p>I'll try to put time scales on each of these events.</p>

<ol>
<li><p><strong>The Sun as a red giant - 5 to 6 billion years</strong>$^1$</p>

<p>The Sun is currently on the <a href="https://en.wikipedia.org/wiki/Main_sequence" rel="nofollow noreferrer">main sequence</a>, which means that it's a "full grown" star - think of it as being middle-aged. It's been on the main sequence for around 4 to 5 billion years, and in 5 to 6 billion years, it will leave the main sequence.</p>

<p>Most yellow (i.e. <a href="https://en.wikipedia.org/wiki/G-type_main-sequence_star" rel="nofollow noreferrer">spectral class G-ish</a>) stars spend about 10 billion years on the main sequence. Then, they swell up as <a href="https://en.wikipedia.org/wiki/Red_giant" rel="nofollow noreferrer">red giants</a>. The Sun is just over halfway through the main sequence, so it has about 5 to 6 billion years left. After 5.4 billion years, it will grow. Here's an image showing how its radius (and other factors) will increase:</p>

<p><a href="https://upload.wikimedia.org/wikipedia/commons/6/6c/Solar_evolution_%28English%29.svg" rel="nofollow noreferrer"><img src="https://upload.wikimedia.org/wikipedia/commons/6/6c/Solar_evolution_%28English%29.svg" alt="Future of the Sun"></a></p>

<p>The Sun won't swallow the inner planets up right away - see how it takes a while for it to grow - but it will happen eventually. It will take another billion years to reach a radius 200 times that of its current size. It will enter the <a href="https://en.wikipedia.org/wiki/Asymptotic_giant_branch" rel="nofollow noreferrer">AGB phase</a>, where it will finally grow enough to swallow the inner planets. It's uncertain as to whether or not Earth will be destroyed, but it will be heated up so much that liquid water will disappear and the planet will be uninhabitable.</p>

<p><a href="http://science.nasa.gov/science-news/science-at-nasa/2012/25oct_friedplanets/" rel="nofollow noreferrer">NASA</a> has an article on a Sun-like star swallowing up one of its inner planets, just as the Sun may do. The original paper can be found <a href="http://arxiv.org/pdf/1206.4938.pdf" rel="nofollow noreferrer">here</a>.</p></li>
<li><p><strong>Supernova/nova - Never</strong></p>

<p>There are two types of supernovae: <a href="https://en.wikipedia.org/wiki/Type_II_supernova" rel="nofollow noreferrer">Type II</a> and Type I. Type II supernovae are characterized by hydrogen; Type I supernovae have no hydrogen. You've probably heard of Type II supernovae when you learned about supernovae, and they're the more well-known. They're the result of the collapse of the core of a massive star. However, the progenitor, has t be much larger than the Sun - usually at least 8 solar masses.</p>

<p>Type I supernovae have no hydrogen. There are two main classes: <a href="https://en.wikipedia.org/wiki/Type_Ia_supernova" rel="nofollow noreferrer">Type Ia</a> and Type Ib/c. The first category are the result of white dwarfs accreting mass from a companion star. The Sun has no binary companion, so even though it will become a white dwarf, it will never undergo this kind of supernova. <a href="https://en.wikipedia.org/wiki/Type_Ib_and_Ic_supernovae" rel="nofollow noreferrer">Type Ib/c</a> also result from core collapse, just like Type II. However, their progenitors are unstable, violent stars, typically <a href="https://en.wikipedia.org/wiki/Wolf%E2%80%93Rayet_star" rel="nofollow noreferrer">Wolf-Rayet stars</a>. Our Sun will fortunately never become one of those!</p>

<p>A <a href="https://en.wikipedia.org/wiki/Nova" rel="nofollow noreferrer">nova</a> is small compared to a supernova. It, like Type Ia supernovae, may be the result of a white dwarf accreting matter from a companion. Once again, though, the Sun has no companion star to accrete matter from.</p></li>
<li><p><strong>Magnetosphere loss - Never</strong></p>

<p>The Earth's <a href="https://en.wikipedia.org/wiki/Earth%27s_magnetic_field" rel="nofollow noreferrer">magnetic field</a> is very important, because it shields us from the solar wind. Without it, life on Earth probably couldn't exist! Its disappearance would not be good for us.</p>

<p>Fortunately, it doesn't appear that that will ever happen.</p>

<p>The Earth's magnetic field is believed to be caused by the movement of fluids in Earth's core. Wikipedia lists three conditions necessary for the existence of the magnetic field:</p>

<blockquote>
  <ol>
  <li><p>An electrically conductive fluid medium</p></li>
  <li><p>Kinetic energy provided by planetary rotation</p></li>
  <li><p>An internal energy source to drive convective motions within the fluid.</p></li>
  </ol>
</blockquote>

<p>The fluid in the Earth's core will never go away, and so long as the planet keeps rotating, that kinetic energy will always be there. So will the <a href="https://en.wikipedia.org/wiki/Coriolis_effect" rel="nofollow noreferrer">Coriolis effect</a>, which also helps circulate the fluid and control the magnetic field. The source of the energy that keeps the material hot is due in part to <a href="https://en.wikipedia.org/wiki/Tidal_force" rel="nofollow noreferrer">tidal forces</a> between the Earth and other bodies, such as the Moon and the Sun.</p></li>
<li><p><strong>Collision - Not for a long time</strong></p>

<p>The Earth has had one major collision throughout its life, and that spawned the moon. This idea is known as the <a href="https://en.wikipedia.org/wiki/Giant_impact_hypothesis" rel="nofollow noreferrer">giant impact hypothesis</a>. When the solar system - and Earth - was young, the Sun had a disk of small pre-planetary bodies, known as <a href="https://en.wikipedia.org/wiki/Protoplanet" rel="nofollow noreferrer">protoplanets</a>. There were more protoplanets than there are planets today, because many collided to form the planets.</p>

<p>The hypothesized protoplanet, named <a href="https://en.wikipedia.org/wiki/Theia_(planet)" rel="nofollow noreferrer">Theia</a>, is generally described as Mars-like. It didn't hit the Earth head-on, but rather hit it an a 45 degree angle. The result was that Theia was destroyed and some of Earth was ejected. Much of this material came back to Earth, but some stayed and formed the Moon.</p>

<p>There aren't any more protoplanets, but there are still some large asteroids. The biggest is <a href="https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)" rel="nofollow noreferrer">Ceres</a>, which is <em>tiny</em> compared to Theia. Here's a picture of Earth, the Moon and Ceres (bottom left):</p>

<p><a href="https://upload.wikimedia.org/wikipedia/commons/1/16/Ceres_Earth_Moon_Comparison.png" rel="nofollow noreferrer"><img src="https://upload.wikimedia.org/wikipedia/commons/1/16/Ceres_Earth_Moon_Comparison.png" alt="Earth, Moon and Ceres"></a></p>

<p>We're not likely to see another giant impact again. The other planets all have stable orbits, and the Moon's actually receding from Earth due to tidal forces. Sure, <a href="https://en.wikipedia.org/wiki/Impact_event#Frequency_and_risk" rel="nofollow noreferrer">asteroid impacts</a> are a possibility, but the possibility of a doomsday asteroid is tiny. Even a body like the one that killed the dinosaurs wouldn't extinguish <em>all</em> life. <a href="http://www.jpl.nasa.gov/asteroidwatch/" rel="nofollow noreferrer">The Jet Propulsion Laboratory</a> has information on the likelihood of such a catastrophic event (not a high one!).</p></li>
<li><p><strong>Gamma-ray burst - Probably never</strong></p>

<p>I wrote <a href="https://astronomy.stackexchange.com/questions/2585/how-likely-and-severe-is-the-threat-of-a-gamma-ray-burst-to-earth">an answer</a> on Astronomy about this a while ago. If I may, I'll quote from that answer:</p>

<blockquote>
  <p>Okay, back to the topic at hand. Wikipedia, of course, has a short tidbit on the frequency of gamma-ray bursts effecting Earth:</p>
  
  <blockquote>
    <p>Estimating the exact rate at which GRBs occur is difficult, but for a galaxy of approximately the same size as the Milky Way, the expected rate (for long-duration GRBs) is about one burst every 100,000 to 1,000,000 years. Only a small percentage of these would be beamed towards Earth. Estimates of rate of occurrence of short-duration GRBs are even more uncertain because of the unknown degree of collimation, but are probably comparable.</p>
  </blockquote>
  
  <p>'A small percentage' isn't too specific. <a href="http://www.bbc.com/news/science-environment-21082617" rel="nofollow noreferrer">The BBC article</a> that is cited in this passage says the following:</p>
  
  <blockquote>
    <p>Observations of deep space suggest that gamma ray-bursts are rare. They are thought to happen at the most every 10,000 years per galaxy, and at the least every million years per galaxy.</p>
  </blockquote>
  
  <p>However, the 'small percentage' is never elaborated on.</p>
</blockquote></li>
</ol>

<hr>

<p>$^1$<strong>An estimate</strong></p>

<p>Earth will become uninhabitable long before the Sun swallows it up - if it is swallowed up - because its surface temperature will be so high life cannot exist. I wanted to see if I could figure out when that would be.</p>

<p>You can approximate the temperature of a planet by using the formula for <a href="https://en.wikipedia.org/wiki/Effective_temperature" rel="nofollow noreferrer">effective temperature</a>:
$$T=\left(\frac{L(1-a)}{16 \pi \sigma D^2} \right)^{\frac{1}{4}}$$
where $L$ is the star's luminosity, $a$ is the planet's albedo $\sigma$ is the <a href="https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_constant" rel="nofollow noreferrer">Stefan-Boltzmann constant</a>, and $D$ is the distance from the planet to the star.</p>

<p>The current mean temperature of Earth is <a href="https://en.wikipedia.org/wiki/Earth#Atmosphere" rel="nofollow noreferrer">about 15 degrees Celsius</a> (288 K). However, this is the average <em>surface temperature</em>. Fortunately, the equation given for surface temperature is also of the form
$$T=C L^{\frac{1}{4}}$$
where $C$ is a constant. This means that
$$T^4 \propto L$$
The boiling point of water is 373 K, which is 1.295 times the current surface temperature. If we plug this in,
$$L= \propto (1.295)^{\frac{1}{4}} \approx. 2.81$$
So the luminosity of the Sun would have to be about 2.81 times the current luminosity of the Sun. This value isn't on the graph I gave at the top, though it appears to be about 7 billion years from now. <a href="https://en.wikipedia.org/wiki/Timeline_of_the_far_future" rel="nofollow noreferrer">Wikipedia</a> gives this point as only 1 billion years from now, because of a greenhouse effect. So we've only got 1 billion years to live!</p>

CC BY-SA 3.0
2015-01-15T22:35:09.600Z
It seems your figure on the rising luminosity of the sun suggests a much sooner time.  As the luminosity rises, the temperature of the Earth will rise.  How much it takes to render the planet uninhabitable is hard to assess. Good answer.
Ross Millikan
CC BY-SA 3.0
2015-01-15T22:36:45.583Z
@RossMillikan The Earth will absolutely get a lot hotter a lot sooner. I wasn't able to get a reliable figure, though, so I didn't put a definitive date. You *could* use [effective temperature](https://en.wikipedia.org/wiki/Effective_temperature), but you'd have to figure out a mean global albedo, which is tough, if not impossible.
HDE 226868
CC BY-SA 3.0
2015-01-15T22:50:27.143Z
Yes, I thought about raising global average to 373K from the current 300K or so to boil water.  My back of the envelope said that needed a doubling of the sun's luminosity, so that easy out wasn't available.  I'm not sure what temperature to use.  Do we mean uninhabitable for unprotected mammals or for thermophilic bacteria?
Ross Millikan
CC BY-SA 3.0
2015-01-15T22:52:31.703Z
@RossMillikan I'd take it all the way and kill the thermophiles, but that's kind of hard to do, given that there are many places where they could hide and stay cool. Plus, scientists don't know as much about them as they'd like.
HDE 226868
CC BY-SA 3.0
2015-01-16T05:16:57.627Z
3: I've read that the tidal forces make up only a small part of the heating energy, and the Earth is cooling. I didn't find well accepted figures for when the core will be frozen solid, or at least frozen enough to severely impact its magnetic field.
vsz
CC BY-SA 3.0
2015-01-16T08:11:09.410Z
[Related to your point 3](http://earthscience.stackexchange.com/q/2523/725)
Gimelist
CC BY-SA 3.0
2015-06-16T05:14:51.040Z
There's a 1% chance that Mercury's orbit is [destabilized](http://www.space.com/6824-long-shot-planet-hit-earth-distant-future.html) by Venus in 3.3 billion years, and then in turn a bit more substantial risk that it will collide with Earth, throw another planet at Earth, or move Earth away from the habitable zone.
LocalFluff
CC BY-SA 3.0
2015-01-15T22:00:50.827Z
2015-02-24T03:41:37.933Z
2017-04-13T12:59:06.653Z
HDE 226868
<p>According to a recent Scientific American article, in about a half-billion years the Sun will have expanded enough and the Earth's orbit decayed enough that Earth will no longer be in the "habitable zone".  (Earth is already on the inner margin of that zone.)</p>

<p>(Of course, humans will destroy things far sooner than that.)</p>

<p><strong>BTW:</strong> For folks interested in this it's worth it to look up that article: <a href="http://www.scientificamerican.com/article/planets-more-habitable-than-earth-may-be-common-in-our-galaxy/" rel="nofollow">Planets More Habitable Than Earth May Be Common in Our Galaxy</a> -- published in the last month or two.  I just skimmed it -- didn't read in any detail, but it discussed the criteria of a habitable planet.</p>

<p>Also relevant: <a href="http://www.scientificamerican.com/article/earths-days-are-numbered/" rel="nofollow">Earth's Days Are Numbered</a></p>

CC BY-SA 3.0
2015-01-16T17:53:43.743Z
Destroy or prevent... it can go both ways.
vsz
CC BY-SA 3.0
2015-06-19T07:35:09.120Z
Why is the Earth's orbit decaying?  I've not heard that, nor could I find a good source to back it up just now.   I agree with you on the expanding sun.
userLTK
CC BY-SA 3.0
2015-06-20T01:05:31.273Z
@userLTK - "Tidal forces" will eventually cause any planet's orbit to decay, unless it can siphon energy off another body (causing that body's orbit/rotation to decay).
Hot Licks
CC BY-SA 3.0
2015-06-20T05:01:27.540Z
I know it's just Wiki, but I don't think that's true. - source:  https://en.wikipedia.org/wiki/Tidal_acceleration   Also, this.   It's hard to predict because the other planets play a role.   https://en.wikipedia.org/wiki/Future_of_the_Earth  "The incremental effect of gravitational perturbations between the planets causes the inner Solar System as a whole to behave chaotically over long time periods."   If the Earth and Sun were alone in the solar system as a 2 body system, the earth would gradually move away, not towards the sun.
userLTK
CC BY-SA 3.0
2015-06-20T12:37:25.083Z
Tidal forces are complex.  If you have one body orbiting another larger one in isolation the two will "want" to "sync" such that the rotation rate of the large body matches the orbital rate of the small body -- this is happening between Earth and the Moon.  But with the Earth and the Sun there are also the other planets involved, and they are mostly moving slower and will tend to slow down Earth's orbit.
Hot Licks
CC BY-SA 3.0
2015-01-16T17:21:57.883Z
2015-01-16T21:32:22.140Z
2015-01-16T21:32:22.140Z
Hot Licks
<p>You've missed a number of calamities:</p>

<ul>
<li><p>Everything dies because all plant life dies from ever decreasing levels of CO<sub>2</sub> in the atmosphere.<br>
This is projected to happen within less than a billion years.</p></li>
<li><p>Everything dies because the oceans boil away and the Earth becomes Venus 2.0.<br>
This is projected to happen within one or two billion years due to increased luminosity from our Sun.</p></li>
<li><p>Everything dies because plate tectonics stops.<br>
This is projected to happen shortly after the oceans boil away.</p></li>
</ul>

<p>These inevitable events will all happen well before our Sun becomes a red giant (which is another inevitable event). All of the other calamities presented in the opening post are only possibilities. Unless something intervenes to move the Earth away from the Sun and add CO<sub>2</sub> to the atmosphere, the end of life as we know it is an inevitability that will happen long before the Sun turns into a red giant. That something that staves off the end of life as we know it a billion or so years from now  will be the doing of our children's children's ... children, if humanity ever lives that long.</p>

CC BY-SA 3.0
2015-01-18T16:18:03.873Z
Why is plate tectonics dependent on oceans?
Paŭlo Ebermann
CC BY-SA 3.0
2015-01-18T18:25:13.260Z
Why is life dependant on plate tectonics?
Dronz
CC BY-SA 3.0
2015-01-18T19:57:44.100Z
@PaŭloEbermann - Volatiles (e.g., hydrogen and oxygen) in rock change how rock melts and makes rock weaker. Subduction takes hydrated rock down toward the mantle. Without water, you get a stagnant lid (e.g., Venus, Mars, Mercury, and the Moon) instead of plate tectonics.
David Hammen
CC BY-SA 3.0
2015-01-18T20:02:57.693Z
@Dronz - See http://www.astrobio.net/news-exclusive/plate-tectonics-could-be-essential-for-life/ and http://www.dailygalaxy.com/my_weblog/2014/05/without-plate-tectonics-life-on-earth-might-never-have-gained-a-foothold-harvard-smithsonian-center-.html
David Hammen
CC BY-SA 3.0
2015-01-16T18:47:08.320Z
2015-01-16T18:47:08.320Z
David Hammen
<p>There are quite a few things which can make an otherwise habitable planet uninhabitable, and some of these will eventually happen, sooner or later:</p>

<ol>
<li>engulfed by its star when it becomes a red giant</li>
<li>destroyed by the nova or supernova of its star</li>
<li>loses its magnetosphere and the solar wind blows its atmosphere away</li>
<li>collides with a large enough object</li>
<li>gamma ray burst</li>
<li>maybe other astronomical phenomena?</li>
</ol>

<p>some of these will inevitably happen to Earth in the future, but which one is to be expected to happen sooner? By uninhabitable I mean uninhabitable to currently known multicellular life.</p>

<p>For the sake of this question, assume that humans don't invent technology in the future which would stop this from happening, and also no extraterrestrial visitors, supernatural forces or similar effects intervene. Actually, let's assume that humans magically vanish today, just to not get distracted by the effects of human technology.</p>

CC BY-SA 3.0
2015-01-15T23:20:19.027Z
There's actually a neat article on Wikipedia about [the far future](http://en.wikipedia.org/wiki/Timeline_of_the_far_future) and it lists a few potential life-destroying events.
Cat
CC BY-SA 3.0
2015-01-16T10:16:52.720Z
Good that you made the humans vanish, otherwise the answer would surely be humans.
PlasmaHH
CC BY-SA 3.0
2015-01-16T15:21:30.367Z
A bit related to this question, you can also check: http://worldbuilding.stackexchange.com/questions/8758/can-planets-die-of-old-age
arkaia
CC BY-SA 3.0
2015-01-16T21:42:29.670Z
By the Copenhagen Interpretation, without any observers none of these things would ever happen.
Jim
CC BY-SA 3.0
2015-01-16T22:01:53.120Z
@Jim : No, that's not what the Copenhagen Interpretation is about.
vsz
CC BY-SA 3.0
2015-01-16T22:59:44.703Z
@Jim An observer does not have to be a being like a human; it can be something as mundane as another particle.
HDE 226868
CC BY-SA 3.0
2015-01-17T03:27:32.033Z
Ok so you're excluding (man-made) climate change? and all other self-induced factors? (nuclear war, etc.) This all sounds hopelessly academic considering the pace of climate change.
smci
CC BY-SA 3.0
2015-01-17T20:12:51.877Z
Are you sure you want to exclude end of life caused by humans? While we're not there yet, we (or AI singularity we create) may well have means to make the Earth inhabitable soon...
hyde
CC BY-SA 3.0
2015-01-17T23:26:33.497Z
@smci: No one is expecting climate change to eradicate life altogether (see the ["current risk" section](https://en.wikipedia.org/wiki/Runaway_climate_change#Current_risk) of the Runaway climate change wikipedia article). Plenty of individual species, sure, but not all "currently known multicellular life".
naught101
CC BY-SA 3.0
2015-01-18T12:44:19.470Z
Then change the question wording from 'uninhabitable' to 'uninhabitable by any currently known multicellular life, not just mankind'. Or move that to the first sentence. As stands the question asks something different. As to your analogy, it would be more like insisting answers only reference pogo-sticking from SF-NY and cannot reference mechanized transport.
smci
CC BY-SA 3.0
2015-01-19T01:03:16.487Z
Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackexchange.com/rooms/20346/discussion-on-question-by-vsz-what-would-be-the-first-thing-which-will-render-th).
hichris123
CC BY-SA 3.0
2015-01-19T06:11:01.273Z
@hichris123 : in this case move the rest also to chat. You moved all of my answers, but left two of the comments by smci here. This will just confuse future readers.
vsz
CC BY-SA 3.0
2015-02-25T14:55:10.390Z
Sooner is strongly related to the chance of it happening. For that reason, Phil Plait has probably answered your question in [Death from the Skies!: These Are The Ways The World Will End](http://en.wikipedia.org/wiki/Death_from_the_Skies!)
Jan Doggen
CC BY-SA 3.0
2015-06-16T05:26:08.770Z
Another kind of disaster: The carbon cycle will remove most carbon from the surface in 0.5 bn years. It won't exterminate, but surface life is slowly starving itself through its role in the carbon cycle. I heard this in a talk by someone from the Deep Carbon Observatory. I never heard about it in any other context. Maybe someone here knows about it?
LocalFluff
<geophysics><hypothetical>
What would be the first thing which will render the Earth uninhabitable?
vsz
# What would be the first thing which will render the Earth uninhabitable?

(Asked by: username_0 on 2015-01-15)

There are quite a few things which can make an otherwise habitable planet uninhabitable, and some of these will eventually happen, sooner or later:

1. engulfed by its star when it becomes a red giant
2. destroyed by the nova or supernova of its star
3. loses its magnetosphere and the solar wind blows its atmosphere away
4. collides with a large enough object
5. gamma ray burst
6. maybe other astronomical phenomena?

some of these will inevitably happen to Earth in the future, but which one is to be expected to happen sooner? By uninhabitable I mean uninhabitable to currently known multicellular life.

For the sake of this question, assume that humans don't invent technology in the future which would stop this from happening, and also no extraterrestrial visitors, supernatural forces or similar effects intervene. Actually, let's assume that humans magically vanish today, just to not get distracted by the effects of human technology.

**Tags:** geophysics, hypothetical

> **Comments:**
> **username_1**: There's actually a neat article on Wikipedia about [the far future](http://en.wikipedia.org/wiki/Timeline_of_the_far_future) and it lists a few potential life-destroying events.  
> **username_2**: Good that you made the humans vanish, otherwise the answer would surely be humans.  
> **username_3**: A bit related to this question, you can also check: http://worldbuilding.stackexchange.com/questions/8758/can-planets-die-of-old-age  
> **username_4**: By the Copenhagen Interpretation, without any observers none of these things would ever happen.  
> **username_0**: @username_4 : No, that's not what the Copenhagen Interpretation is about.  
> **username_5**: @username_4 An observer does not have to be a being like a human; it can be something as mundane as another particle.  
> **username_6**: Ok so you're excluding (man-made) climate change? and all other self-induced factors? (nuclear war, etc.) This all sounds hopelessly academic considering the pace of climate change.  
> **username_7**: Are you sure you want to exclude end of life caused by humans? While we're not there yet, we (or AI singularity we create) may well have means to make the Earth inhabitable soon...  
> **username_8**: @username_6: No one is expecting climate change to eradicate life altogether (see the ["current risk" section](https://en.wikipedia.org/wiki/Runaway_climate_change#Current_risk) of the Runaway climate change wikipedia article). Plenty of individual species, sure, but not all "currently known multicellular life".  
> **username_6**: Then change the question wording from 'uninhabitable' to 'uninhabitable by any currently known multicellular life, not just mankind'. Or move that to the first sentence. As stands the question asks something different. As to your analogy, it would be more like insisting answers only reference pogo-sticking from SF-NY and cannot reference mechanized transport.  
> **username_9**: Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackexchange.com/rooms/20346/discussion-on-question-by-vsz-what-would-be-the-first-thing-which-will-render-th).  
> **username_0**: @username_9 : in this case move the rest also to chat. You moved all of my answers, but left two of the comments by username_6 here. This will just confuse future readers.  
> **username_10**: Sooner is strongly related to the chance of it happening. For that reason, Phil Plait has probably answered your question in [Death from the Skies!: These Are The Ways The World Will End](http://en.wikipedia.org/wiki/Death_from_the_Skies!)  
> **username_11**: Another kind of disaster: The carbon cycle will remove most carbon from the surface in 0.5 bn years. It won't exterminate, but surface life is slowly starving itself through its role in the carbon cycle. I heard this in a talk by someone from the Deep Carbon Observatory. I never heard about it in any other context. Maybe someone here knows about it?  

---

## 3 answers

### Answer by username_5 on 2015-01-15. Score: 23

I'll try to put time scales on each of these events.

1. **The Sun as a red giant - 5 to 6 billion years**$^1$

   The Sun is currently on the [main sequence](https://en.wikipedia.org/wiki/Main_sequence), which means that it's a "full grown" star - think of it as being middle-aged. It's been on the main sequence for around 4 to 5 billion years, and in 5 to 6 billion years, it will leave the main sequence.

   Most yellow (i.e. [spectral class G-ish](https://en.wikipedia.org/wiki/G-type_main-sequence_star)) stars spend about 10 billion years on the main sequence. Then, they swell up as [red giants](https://en.wikipedia.org/wiki/Red_giant). The Sun is just over halfway through the main sequence, so it has about 5 to 6 billion years left. After 5.4 billion years, it will grow. Here's an image showing how its radius (and other factors) will increase:

   [![Future of the Sun](https://upload.wikimedia.org/wikipedia/commons/6/6c/Solar_evolution_%28English%29.svg)](https://upload.wikimedia.org/wikipedia/commons/6/6c/Solar_evolution_%28English%29.svg)

   The Sun won't swallow the inner planets up right away - see how it takes a while for it to grow - but it will happen eventually. It will take another billion years to reach a radius 200 times that of its current size. It will enter the [AGB phase](https://en.wikipedia.org/wiki/Asymptotic_giant_branch), where it will finally grow enough to swallow the inner planets. It's uncertain as to whether or not Earth will be destroyed, but it will be heated up so much that liquid water will disappear and the planet will be uninhabitable.

   [NASA](http://science.nasa.gov/science-news/science-at-nasa/2012/25oct_friedplanets/) has an article on a Sun-like star swallowing up one of its inner planets, just as the Sun may do. The original paper can be found [here](http://arxiv.org/pdf/1206.4938.pdf).
2. **Supernova/nova - Never**

   There are two types of supernovae: [Type II](https://en.wikipedia.org/wiki/Type_II_supernova) and Type I. Type II supernovae are characterized by hydrogen; Type I supernovae have no hydrogen. You've probably heard of Type II supernovae when you learned about supernovae, and they're the more well-known. They're the result of the collapse of the core of a massive star. However, the progenitor, has t be much larger than the Sun - usually at least 8 solar masses.

   Type I supernovae have no hydrogen. There are two main classes: [Type Ia](https://en.wikipedia.org/wiki/Type_Ia_supernova) and Type Ib/c. The first category are the result of white dwarfs accreting mass from a companion star. The Sun has no binary companion, so even though it will become a white dwarf, it will never undergo this kind of supernova. [Type Ib/c](https://en.wikipedia.org/wiki/Type_Ib_and_Ic_supernovae) also result from core collapse, just like Type II. However, their progenitors are unstable, violent stars, typically [Wolf-Rayet stars](https://en.wikipedia.org/wiki/Wolf%E2%80%93Rayet_star). Our Sun will fortunately never become one of those!

   A [nova](https://en.wikipedia.org/wiki/Nova) is small compared to a supernova. It, like Type Ia supernovae, may be the result of a white dwarf accreting matter from a companion. Once again, though, the Sun has no companion star to accrete matter from.
3. **Magnetosphere loss - Never**

   The Earth's [magnetic field](https://en.wikipedia.org/wiki/Earth%27s_magnetic_field) is very important, because it shields us from the solar wind. Without it, life on Earth probably couldn't exist! Its disappearance would not be good for us.

   Fortunately, it doesn't appear that that will ever happen.

   The Earth's magnetic field is believed to be caused by the movement of fluids in Earth's core. Wikipedia lists three conditions necessary for the existence of the magnetic field:

   > 1. An electrically conductive fluid medium
   > 2. Kinetic energy provided by planetary rotation
   > 3. An internal energy source to drive convective motions within the fluid.

   The fluid in the Earth's core will never go away, and so long as the planet keeps rotating, that kinetic energy will always be there. So will the [Coriolis effect](https://en.wikipedia.org/wiki/Coriolis_effect), which also helps circulate the fluid and control the magnetic field. The source of the energy that keeps the material hot is due in part to [tidal forces](https://en.wikipedia.org/wiki/Tidal_force) between the Earth and other bodies, such as the Moon and the Sun.
4. **Collision - Not for a long time**

   The Earth has had one major collision throughout its life, and that spawned the moon. This idea is known as the [giant impact hypothesis](https://en.wikipedia.org/wiki/Giant_impact_hypothesis). When the solar system - and Earth - was young, the Sun had a disk of small pre-planetary bodies, known as [protoplanets](https://en.wikipedia.org/wiki/Protoplanet). There were more protoplanets than there are planets today, because many collided to form the planets.

   The hypothesized protoplanet, named [Theia](https://en.wikipedia.org/wiki/Theia_(planet)), is generally described as Mars-like. It didn't hit the Earth head-on, but rather hit it an a 45 degree angle. The result was that Theia was destroyed and some of Earth was ejected. Much of this material came back to Earth, but some stayed and formed the Moon.

   There aren't any more protoplanets, but there are still some large asteroids. The biggest is [Ceres](https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)), which is *tiny* compared to Theia. Here's a picture of Earth, the Moon and Ceres (bottom left):

   [![Earth, Moon and Ceres](https://upload.wikimedia.org/wikipedia/commons/1/16/Ceres_Earth_Moon_Comparison.png)](https://upload.wikimedia.org/wikipedia/commons/1/16/Ceres_Earth_Moon_Comparison.png)

   We're not likely to see another giant impact again. The other planets all have stable orbits, and the Moon's actually receding from Earth due to tidal forces. Sure, [asteroid impacts](https://en.wikipedia.org/wiki/Impact_event#Frequency_and_risk) are a possibility, but the possibility of a doomsday asteroid is tiny. Even a body like the one that killed the dinosaurs wouldn't extinguish *all* life. [The Jet Propulsion Laboratory](http://www.jpl.nasa.gov/asteroidwatch/) has information on the likelihood of such a catastrophic event (not a high one!).
5. **Gamma-ray burst - Probably never**

   I wrote [an answer](https://astronomy.stackexchange.com/questions/2585/how-likely-and-severe-is-the-threat-of-a-gamma-ray-burst-to-earth) on Astronomy about this a while ago. If I may, I'll quote from that answer:

   > Okay, back to the topic at hand. Wikipedia, of course, has a short tidbit on the frequency of gamma-ray bursts effecting Earth:
   >
   > > Estimating the exact rate at which GRBs occur is difficult, but for a galaxy of approximately the same size as the Milky Way, the expected rate (for long-duration GRBs) is about one burst every 100,000 to 1,000,000 years. Only a small percentage of these would be beamed towards Earth. Estimates of rate of occurrence of short-duration GRBs are even more uncertain because of the unknown degree of collimation, but are probably comparable.
   >
   > 'A small percentage' isn't too specific. [The BBC article](http://www.bbc.com/news/science-environment-21082617) that is cited in this passage says the following:
   >
   > > Observations of deep space suggest that gamma ray-bursts are rare. They are thought to happen at the most every 10,000 years per galaxy, and at the least every million years per galaxy.
   >
   > However, the 'small percentage' is never elaborated on.

---

$^1$**An estimate**

Earth will become uninhabitable long before the Sun swallows it up - if it is swallowed up - because its surface temperature will be so high life cannot exist. I wanted to see if I could figure out when that would be.

You can approximate the temperature of a planet by using the formula for [effective temperature](https://en.wikipedia.org/wiki/Effective_temperature):
$$T=\left(\frac{L(1-a)}{16 \pi \sigma D^2} \right)^{\frac{1}{4}}$$
where $L$ is the star's luminosity, $a$ is the planet's albedo $\sigma$ is the [Stefan-Boltzmann constant](https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_constant), and $D$ is the distance from the planet to the star.

The current mean temperature of Earth is [about 15 degrees Celsius](https://en.wikipedia.org/wiki/Earth#Atmosphere) (288 K). However, this is the average *surface temperature*. Fortunately, the equation given for surface temperature is also of the form
$$T=C L^{\frac{1}{4}}$$
where $C$ is a constant. This means that
$$T^4 \propto L$$
The boiling point of water is 373 K, which is 1.295 times the current surface temperature. If we plug this in,
$$L= \propto (1.295)^{\frac{1}{4}} \approx. 2.81$$
So the luminosity of the Sun would have to be about 2.81 times the current luminosity of the Sun. This value isn't on the graph I gave at the top, though it appears to be about 7 billion years from now. [Wikipedia](https://en.wikipedia.org/wiki/Timeline_of_the_far_future) gives this point as only 1 billion years from now, because of a greenhouse effect. So we've only got 1 billion years to live!

> **Comments:**
> **username_12**: It seems your figure on the rising luminosity of the sun suggests a much sooner time.  As the luminosity rises, the temperature of the Earth will rise.  How much it takes to render the planet uninhabitable is hard to assess. Good answer.  
> **username_5**: @username_12 The Earth will absolutely get a lot hotter a lot sooner. I wasn't able to get a reliable figure, though, so I didn't put a definitive date. You *could* use [effective temperature](https://en.wikipedia.org/wiki/Effective_temperature), but you'd have to figure out a mean global albedo, which is tough, if not impossible.  
> **username_12**: Yes, I thought about raising global average to 373K from the current 300K or so to boil water.  My back of the envelope said that needed a doubling of the sun's luminosity, so that easy out wasn't available.  I'm not sure what temperature to use.  Do we mean uninhabitable for unprotected mammals or for thermophilic bacteria?  
> **username_5**: @username_12 I'd take it all the way and kill the thermophiles, but that's kind of hard to do, given that there are many places where they could hide and stay cool. Plus, scientists don't know as much about them as they'd like.  
> **username_0**: 3: I've read that the tidal forces make up only a small part of the heating energy, and the Earth is cooling. I didn't find well accepted figures for when the core will be frozen solid, or at least frozen enough to severely impact its magnetic field.  
> **username_13**: [Related to your point 3](http://earthscience.stackexchange.com/q/2523/725)  
> **username_11**: There's a 1% chance that Mercury's orbit is [destabilized](http://www.space.com/6824-long-shot-planet-hit-earth-distant-future.html) by Venus in 3.3 billion years, and then in turn a bit more substantial risk that it will collide with Earth, throw another planet at Earth, or move Earth away from the habitable zone.  

---

### Answer by username_16 on 2015-01-16. Score: 8

You've missed a number of calamities:

* Everything dies because all plant life dies from ever decreasing levels of CO2 in the atmosphere.  
  This is projected to happen within less than a billion years.
* Everything dies because the oceans boil away and the Earth becomes Venus 2.0.  
  This is projected to happen within one or two billion years due to increased luminosity from our Sun.
* Everything dies because plate tectonics stops.  
  This is projected to happen shortly after the oceans boil away.

These inevitable events will all happen well before our Sun becomes a red giant (which is another inevitable event). All of the other calamities presented in the opening post are only possibilities. Unless something intervenes to move the Earth away from the Sun and add CO2 to the atmosphere, the end of life as we know it is an inevitability that will happen long before the Sun turns into a red giant. That something that staves off the end of life as we know it a billion or so years from now will be the doing of our children's children's ... children, if humanity ever lives that long.

> **Comments:**
> **username_17**: Why is plate tectonics dependent on oceans?  
> **username_18**: Why is life dependant on plate tectonics?  
> **username_16**: @username_17 - Volatiles (e.g., hydrogen and oxygen) in rock change how rock melts and makes rock weaker. Subduction takes hydrated rock down toward the mantle. Without water, you get a stagnant lid (e.g., Venus, Mars, Mercury, and the Moon) instead of plate tectonics.  
> **username_16**: @username_18 - See http://www.astrobio.net/news-exclusive/plate-tectonics-could-be-essential-for-life/ and http://www.dailygalaxy.com/my_weblog/2014/05/without-plate-tectonics-life-on-earth-might-never-have-gained-a-foothold-harvard-smithsonian-center-.html  

---

### Answer by username_14 on 2015-01-16. Score: 6

According to a recent Scientific American article, in about a half-billion years the Sun will have expanded enough and the Earth's orbit decayed enough that Earth will no longer be in the "habitable zone". (Earth is already on the inner margin of that zone.)

(Of course, humans will destroy things far sooner than that.)

**BTW:** For folks interested in this it's worth it to look up that article: [Planets More Habitable Than Earth May Be Common in Our Galaxy](http://www.scientificamerican.com/article/planets-more-habitable-than-earth-may-be-common-in-our-galaxy/) -- published in the last month or two. I just skimmed it -- didn't read in any detail, but it discussed the criteria of a habitable planet.

Also relevant: [Earth's Days Are Numbered](http://www.scientificamerican.com/article/earths-days-are-numbered/)

> **Comments:**
> **username_0**: Destroy or prevent... it can go both ways.  
> **username_15**: Why is the Earth's orbit decaying?  I've not heard that, nor could I find a good source to back it up just now.   I agree with you on the expanding sun.  
> **username_14**: @username_15 - "Tidal forces" will eventually cause any planet's orbit to decay, unless it can siphon energy off another body (causing that body's orbit/rotation to decay).  
> **username_15**: I know it's just Wiki, but I don't think that's true. - source:  https://en.wikipedia.org/wiki/Tidal_acceleration   Also, this.   It's hard to predict because the other planets play a role.   https://en.wikipedia.org/wiki/Future_of_the_Earth  "The incremental effect of gravitational perturbations between the planets causes the inner Solar System as a whole to behave chaotically over long time periods."   If the Earth and Sun were alone in the solar system as a 2 body system, the earth would gradually move away, not towards the sun.  
> **username_14**: Tidal forces are complex.  If you have one body orbiting another larger one in isolation the two will "want" to "sync" such that the rotation rate of the large body matches the orbital rate of the small body -- this is happening between Earth and the Moon.  But with the Earth and the Sun there are also the other planets involved, and they are mostly moving slower and will tend to slow down Earth's orbit.  

---

