# What effect would a supernova have on an orbiting planet?

(Not sure if this is a better fit for worldbuilding.SE...) Suppose we have an earth-like planet orbiting comfortably far from a red giant of ~10 solar masses. If the star explodes in a supernova, what is the order of magnitude of the effect on the planet? Would the planet be irradiated, or would some outer layers of material be stripped off, or would it be fractured into pieces, or would it turn to dust or elementary particles?

How far away would a planet need to be to conceivably "survive" (the event itself, not necessarily the thrown-into-deep-space part)?

• You'd probably need "far from" in the first paragraph defined more precisely to even stand a chance of an answer. You'd also need to define "survive" more precisely. Does it mean e.g. not a mouse was harmed or does it mean a planet shaped object still existed afterwards, albeit a red hot charred ruin ? – StephenG Aug 1 '18 at 3:27
• It should be noted that red giants do not undergo supernovae, but rather evolve into white dwarfs; you would need a much more massive star for this to happen. Also, Worldbuilding mod here - I don't think it makes sense to send this over to us; we're not a "What-if" site. – HDE 226868 Aug 1 '18 at 3:34
• @StephenG The intended meaning of "far from" is in the goldilocks zone for the star, such as it is. I'm really going for an order of magnitude estimate so it doesn't have to be exact. As for "survive" I am interested in multiple senses - both the "red hot charred ruin" sense (i.e. there is still a planet with most of its mass remaining) and the "big aurora" sense. – Mario Carneiro Aug 1 '18 at 4:59
• @HDE226868 I may be getting the terms wrong, but I am referring to a star in the low end of the mass limit for stars that undergo supernovae at the end of their lives, in its final state before supernova. Is this a supergiant then? What is its goldilocks orbital radius? – Mario Carneiro Aug 1 '18 at 5:20
• Goldilocks zone for a massive star might be a long way out. I'm wondering how much the initial SN mass loss (eg gamma rays, neutrinos) would affect the planet's orbit, and how quickly. Would the orbit simply expand in radius (in which case the "blast" will still hit the planet within days/weeks), or will the planet fly off into interstellar space (and the shock wave take months/years to catch it)? – Chappo Oct 10 '18 at 22:17

Hans Bethe coined the FOE for supernova: ten to the Fifty One Ergs, or $10^{44}$ Joules as the order of magnitude for the energy output. Let's spread that out over the Earth's (rounded--numerically) orbital radius of $R=100$ million km:

$$D = \frac E {4\pi R^2} = 8 \times 10^{20}\ \mathrm{J/m^2}=200,000\ \mathrm{MT/m^2}$$

Or 200 kilotons of TNT per square millimeter. Atmosphere and oceans are gone, certainly the surface vaporizes, but how deep?

An internet search suggests vaporizing a cubic meter of quartz with heat capacity $\alpha = 1$J/K/g (with specific gravity 3) requires 9 GJ to heat it by 2750K, so the depth it can go is:

$$d = D/ (9\ GJ) = 88\mathrm{million\ km}$$

So the planet is gone. I think even a 1000 times further out the crust is vaporized. Run the numbers and see what you think.

• One nitpick: I assume that the orbital radius of an earthlike planet in the goldilocks zone around a red giant would be rather larger than 1 AU, indeed that may actually be inside the surface of the sun, I'm not sure. Still I doubt it would make a significant impact on these numbers. – Mario Carneiro Aug 1 '18 at 4:56
• What happens to the orbital mechanics of the planet given all this? I assume it is not just heated but also pushed away from the sun. Will it hold together? Also, isn't a lot of that energy carried by neutrino emission, which presumably will not contribute much momentum to the planet? – Mario Carneiro Aug 1 '18 at 5:09
• @MarioCarneiro As JEB explained above, a supernova explosion for a planet is the same as a nuclear bomb explosion right in front of your face. The planet would be instantly vaporized, pushed away at near the speed of light, and spread into a cosmic dust. – safesphere Aug 1 '18 at 5:33
• @MarioCarneiro I said at 1000AU, energy density is down by a 1000-squared = 1 million (so a huge nuke per square meter, not square mm)--enough to vaporize the top 88 km of crust (give or take). It is difficult to grasp the power of a supernova, it's ....astronomical. – JEB Aug 1 '18 at 14:24