GRB vs Death Star: Could Gamma Rays Destroy Planets?

[Daniel Petka]

A gamma ray burst radiates an enormous amount of energy.
Could it blow up planets like, let's say a Death Star can if they were close enough? I can imagine that, since grbs are able do destroy an atmosphere while being light years away.

 

[mfb]

An Earth-like atmosphere has a negligible mass compared to a planet, and "messing up the chemistry" is different from "completely removing the atmosphere".

GRB emit something like 10⁴⁴ J in a cone of a few degrees width, something like 1/3000 of the full solid angle. Values from here. Inside the cone, the intensity corresponds to an isotropic emission of 3*10⁴⁷ J.

Earth has a gravitational binding energy of 2*10³² J.

A naive application of the inverse square law shows the planet doesn't receive enough energy to be disintegrated if it is more than 820 AU away. This value is much smaller for heavier planets (90 AU for Jupiter) and larger for smaller planets.

A planet in the system of the exploding star could receive enough energy. There are two caveats, however:
- the energy is emitted in a narrow beam aligned with the stellar rotation axis. In most systems, this is outside the plane where the planets orbit. The intense main beam will miss the planets.
- The assumption that all the energy hitting the planet goes into disassembling it is unrealistic. You would get a scenario like the last one here, where you quickly get a plasma layer and then most energy is absorbed by a surface layer of plasma.

 

[Daniel Petka]

Thanks