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"Your children will be born in a world of two suns. They will never know a sky without them. You can tell them that you remember when there was a pitch black sky with no bright star, and people feared the night."

Except, I am very much afraid Christopher's children will never be born.

So Jupiter becomes a small star. Let's assume nothing else changes in the solar system gravitationally and orbitally speaking. Jupiter is farther away from Earth than the Sun. Let's suppose its apparent luminosity (once ignited) is 15% that of the Sun; should be enough to cancel out any other star in the night sky. Fusion initiated and is maintained due to some alien technology.

Will the increased light and energy from the Jupiter-star destroy the Earth's biosphere and/or human civilization within a timescale of years?

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  • $\begingroup$ Which kind of star does Jupiter become? Will its mass change? If so, how much? $\endgroup$ – L.Dutch - Reinstate Monica Mar 27 '17 at 9:47
  • $\begingroup$ How would it become a sun and how would it get so high apparent luminosity? Mechanism of this process is really important and will determine answer. $\endgroup$ – Mołot Mar 27 '17 at 9:48
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    $\begingroup$ The smallest stars are about 13 Jupiter masses, those are brown dwarfs and they don't even really look like stars, they're kind of heavy planets that fuse only lithium. Real stars begin around 75 Jupiter masses (red dwarfs). To do what you say, Jupiter would need to be 75 times what it is today. That ain't happening. At 15% luminosity of our sun, well, that would require about 500 Jupiters. Long before it got bright, the problem would be orbital perturbations. If Jupiter put on that much mass, none of the other planets would be stable. $\endgroup$ – userLTK Mar 27 '17 at 9:53
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    $\begingroup$ The "Space Odyssey" series (2001, 2010, 2061, 3001) had this as a large plot point. Clarke did use super advanced machines to force Jupiter into fusion, but the lifetime of the Jupiter-star was very limited because of Jupiter's tiny mass. $\endgroup$ – Michael Richardson Mar 27 '17 at 12:55
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    $\begingroup$ I edited to question to clarify the handwavium/magic needed or Jupiter to begin fusion without changing mass. I also removed all the social effects and concentrated the question on just enviornmental effects. Nominating for re-opening. $\endgroup$ – kingledion Mar 27 '17 at 13:43
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Brightness without extra mass:

If Jupiter only starts emitting light, and doesn't become more massive (hadwavium alien technology), 15% of Sol's luminosity at a closest approach of (5.2-1) AU makes it 15%/(4.22) = 0.15*5.66…% = 0.85…% the apparent brightness of Sol. Given that anthropogenic climate change (currently about 1°C) has the same effect as 0.15% of solar luminosity, this will be Bad. Not wipe out all life bad, but definitely very bad — but humans are adaptable and would probably make do.

(Given humanity could fly to Jupiter without much difficulty in the 2001 and 2010 films, a massive sun-shield is not entirely implausible, which would mitigate the climate problems of two stars much more effectively than the climate-and-pollution problems of CO2, but I'm not sure if you want that from your question).

Brightness by becoming heavy enough to start fusion:

Now, what happens if Jupiter does get more massive, enough so to produce 15% Sol luminosity? I happen to have just written an orbital simulator for the story I'm writing to make sure the specifics of my plot device doesn't accidentally break the entire system.

Assuming 15% solar luminosity requires 500 Jupiter masses, as per comment from @userLTK, this is how it turned out:

At ten years, it's not looking too bad:

  • Mercury: 0.24677… AU
  • Venus: 0.67828… AU
  • Earth: 0.99387… AU
  • Mars: 1.8782… AU
  • Jupiter: 4.8489… AU
  • Saturn: 10.259… AU
  • Uranus: 14.408… AU
  • Neptune: 38.403… AU

The problems happen (in this run of the simulation) at about 53 years, where the Earth is now 1.1 AU from Sol, getting 83% of the warmth from Sol that it currently gets. But that's just a cold snap, it doesn't even last to the end of the year. Regardless, long enough for one or two more generations if perhaps not two or three.

  • 54 years, six months: 0.77… AU, 165.8…% Sol apparent luminosity
  • 60 years, three months: 0.715… AU (inside Venus' orbit), nearly doubles the apparent luminosity of Sol
  • 63y3m: 1.138… AU, 77% Sol brightness
  • 65y2m: 1.279… AU, 61% Sol brightness
  • 104y: 2.780… AU, 13% Sol brightness
  • 105y: 3.712… AU, 7.2% Sol brightness
  • 106y: 5.379… AU (further from Sol than from Jupiter), 3.5% Sol brightness

And, in the far future:

  • 200 years: 587.3… AU, Sol is a tiny speck of light almost like any other star, 0.00029% of its current apparent luminosity. The atmosphere of Earth has now condensed into a 12m thick layer of solid nitrogen, solid oxygen, solid everything. All structures more than 12m above sea level are now in hard vacuum.

(I stopped the simulation after 1000 years, by which point Earth is 0.6 light years away from Sol, further than all of the other planets).

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    $\begingroup$ For 15% of Sol's luminosity, and 4.2 AU from Earth, I calculate 0.15/(4.2**2) = .0085 = .85% apparent brightness, not the 5.66% you calculate. Your link indicates ~1.5 W/m$^2$ radiative forcing from CO$_2$. Solar insolation for the inhabited Earth is in the 150-300W/$^2$ range, so the .85% increase would be 1.2-2.5 W/m$^2$ of increase, or about the same as current radiative forcing. Since current CO$_2$ levels amount to less than 1C of temperature increase over pre-industrial, I disagree that another 1C of increase will be 'BAD' or that the poles will be tropical. $\endgroup$ – kingledion Mar 27 '17 at 13:36
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    $\begingroup$ Also, please link to your orbital simulator so we can fact check that too. For now, -1 because I don't think your math is correct. $\endgroup$ – kingledion Mar 27 '17 at 13:36
  • $\begingroup$ Ah, derp. Yes, I can see my mistakes now: 1) I did 1/4.2^2; 2) I think I'm right to use be using 1kW/m^2 for "light hitting Earth" rather than 150-300W/m^2 "light absorbed by Earth". The first point alone certainly stops it being tropical, but 3-6°C is still "really bad"… assuming I'm correctly using total rather than absorbed light intensity. $\endgroup$ – BenRW Mar 28 '17 at 14:11

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