Let's say that the Sun suddenly almost completely shuts down and the Earth is now able to only receive an amount of sunlight equal to that received by the Moon during a full-moon night.

(Assuming day/night cycles remain the same : only change would be the reduced sunlight during the day)

What would eventually happen to all life on Earth ?

Which species would be able to survive this in the long-term (if any) ?

  • 1
    $\begingroup$ Eart will freeze solid. The only surviving life will be around deep ocean thermal vents, and possibly some lithophile microorganisms. $\endgroup$
    – AlexP
    Mar 9 '20 at 12:42
  • $\begingroup$ The Simpsons explored this: "Every plant and tree will die. Owls will deafen us with incessant hooting. The town sundial will be useless" $\endgroup$
    – user535733
    Mar 9 '20 at 14:02
  • $\begingroup$ @user535733 pls note that my question is slightly different and it's about reduced sunlight and not about sun going completely dark $\endgroup$ Mar 9 '20 at 14:42
  • $\begingroup$ Yes, your question seems hair-splittingly different,,,but the answers are what matter, and they will mostly be the same.. $\endgroup$
    – user535733
    Mar 9 '20 at 14:44
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    $\begingroup$ @nullPointer There is no practical difference between a >99.99% reduction in energy input and a 100% reduction in energy input. $\endgroup$
    – gerrit
    Mar 9 '20 at 16:08

The temperature of the planet would dramatically drop, and as such no photosynthetic life form would be able to survive. Also the water cycle will stop, because most if not all of the water will freeze.

This would lead to the collapse of most of the food chains, with resulting mass extinctions.

Only life organisms thriving around volcanic spots, where they could get more favorable temperatures, would continue almost undisturbed.

  • 1
    $\begingroup$ Not to mention that without plants, the atmosphere would be depleted of oxygen after a 1000 years $\endgroup$
    – Lucas
    Mar 9 '20 at 12:36
  • $\begingroup$ Not to mention that all the oceans will freeze solid, maybe with the exception of some pockets around deep sea vents. $\endgroup$
    – AlexP
    Mar 9 '20 at 12:42

The Earth would be so much colder nothing could survive

Using apparent magnitude as a rough guide:

Example: Sun and Moon

What is the ratio in brightness between the Sun and the full Moon?

The apparent magnitude of the Sun is −26.74 (brighter), and the mean magnitude of the full moon is −12.74 (dimmer).

Difference in magnitude:


Brightness factor:

$v_{b}=10^{0.4x}=10^{0.4\times 14.00}\approx 398\,107.$

The Sun appears about 400,000 times brighter than the full moon

So if the Sun is now transfering 400,000 times less light it also transfers 400,000 times less heat.

From here we see:

The bottom line is that, of the total heat reaching the surface of the Earth of (1.8+0.0000058) = 1.8000058 watts/cm^2, only 0.0000058/1.8 = 0.0003% is contributed by the Earth's internal heat. This, of course, will dominate everything else if the Sun were to magically vanish!

So of the heat reaching the Earth almost 100% of this is from the sun. Lets take an average of $14^{o}C$ or $287^{o}K$ - dividing this by 400,000 (less than $1^{o}K$) and its so cold even the hardy Tardigrade won't survive past a few minutes.

  • 4
    $\begingroup$ If the output of the sun were to drop, we can expect that the black body temperature dropped. If that is the case, the spectral content of received radiation from the sun would shift. With a downward temperature shift, one can expect that the infrared radiation bands would have a much higher percentage of the total energy received. Consequently, one must define in what manner the sun shifted it's output radiation. If in the visible spectra, there could still be substantial thermal output received by the earth. I disagree with your assessment of "heat" reduction being linear to illuminance. $\endgroup$
    – mongo
    Mar 9 '20 at 13:24

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