What would life on Earth be like if the Sun was replaced by two blue giants?

Blue supergiants are massive. Really, really massive - up to dozens of times the mass of the Sun. They are also extremely hot, large and luminous. They live much shorter lives that stars like the Sun, and may die spectacularly. All of this means that they aren't great stars to host habitable planets.

The luminosity of the Sun is on the order of $10^{26}$ watts - an incredible amount (think of that compared to a lightbulb)! But that's nothing compared to a blue supergiant. Rigel A has a luminosity 120,000 times that! Let's see if we can replicate the Sun's effects by comparing the stellar flux density. The flux $F$ from a star at a radius $r$ is $$F=\frac{L}{4 \pi r^2}$$ Setting the flux of the Sun equal to that of Rigel, we find that $$F_{\text{Sun}}=F_{\text{Rigel}}$$ $$\frac{L_{\text{Sun}}}{4 \pi r_{\text{Earth}}^2}=\frac{L_{\text{Rigel}}}{4 \pi r^2}$$ Doing some cancellations, and writing Rigel's luminosity in terms of the Sun's luminosity, $$r^2=\frac{120,000 L_{\text{Sun}}}{L_{\text{Rigel}}}r_{\text{Earth}}^2$$ $$r=346r_{\text{Earth}}$$ So Earth would have to be 346 AU away from Rigel to receive the same flux as it does from the Sun. Put it in a binary system with two Rigel-equivalents and that figure is multiplied by $\sqrt{2}$, becoming roughly 490 AU. That's well into the Kuiper Belt - right in the area of Planet Nine.

A binary system of blue supergiants is UW Canis Majoris, with spectral types O7.5 and O9.7. Their combined luminosity is approximately 260,000 solar luminosities, and they orbit close together, likely at about 0.16 AU. Therefore, their circumstellar habitable zone should be comparable to that of Rigel's; you won't be able to orbit closer than about 350 AU and still have a habitable planet.

Put Earth around the central blue supergiants in the system and things get interesting. It's going to receive 120,000 times the flux that it receives from the Sun, so it will be hot. To put that in perspective, the Earth would have to be about 0.00288 AU from the Sun to receive that kind of heat. I would expect temperatures to be - well, probably many hundreds of degrees, no matter what scale you use. Life on the surface is out of the question.

Any life will have to be belowground. Even the extremophiles will be feeling pretty warm. There won't be any water, so subterranean life is our only option. I doubt that anything bigger than small bacteria would have a shot. Even something like a worm would have a whelk's chance in a supernova. Oh, and this assumes that such a small planet could form around these stars (I wouldn't bet on it) and that life could develop quickly enough.

posted almost 10 years ago

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