What would happen if the amount of light reaching the Earth from the Sun were cut in half?

What would happen if the amount of light reaching the earth from the sun were cut in half?
 The position of the Habitable Zone, as a function of the mass of the star the planets orbit. Image credit:  Chester Harmon , CC A-SA 4.0

The position of the Habitable Zone, as a function of the mass of the star the planets orbit. Image credit: Chester Harmon, CC A-SA 4.0

We’ve tackled a very similar question to this before here at Astroquizzical; check out this post! In that post, we explored what would happen to the Earth if we could slice the Sun in half. And because cutting the Sun’s matter in half doesn’t translate to a slice in brightness of one half, it’s a pretty dramatic shift for our solar system.

However, if we don’t go quite as far with our solar slicing, but instead just drop the brightness of our sun by half, we’ve actually only removed 18% of the mass. This is still a relatively massive star, at 82% the mass of our Sun, but that’s enough to change the distance from the star where liquid water is stable.

As the mass and brightness of a star decreases, that zone of possible liquid water (usually known as the habitable zone) shrinks to a smaller and smaller shell around the star, but because we’re changing the star by a smaller amount this time compared to the earlier post, the habitable zone won’t shrink all the way down to Mercury’s orbit - it would sit closer to where Venus is now. The Earth’s orbit might still fall within the bounds of the habitable zone, but it’d be more in the position that Mars finds itself in now - much colder than Earth now, but able to sustain water under certain circumstances.

Our Sun won't be dropping in brightness anytime soon - on the contrast, as our Sun ages, it becomes slightly brighter, increasing in brightness by about 10 percent every billion years. As it does, the habitable zone around our star has been gradually expanding outwards, and at some point in the next billion years, the Earth will exit the habitable zone entirely.


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