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Having done a bit of research lately, I learnt that hot Jupiters are often victim to having their atmospheres gradually stripped away by their parent stars. That got me thinking:
Assuming a planet of roughly Jupiter's mass and composition, and a parent star similar to the Sun, how much of the planet's mass would be lost over the lifetime of the planet?
Would it depend on the distance from the star?
From Wikipedia (source)
If the atmosphere of a hot Jupiter is stripped away via hydrodynamic escape, its core may become a chthonian planet. The amount of gas removed from the outermost layers depends on the planet's size, the gases forming the envelope, the orbital distance from the star, and the star's luminosity. In a typical system, a gas giant orbiting at 0.02 AU around its parent star loses 5–7% of its mass during its lifetime, but orbiting closer than 0.015 AU can mean evaporation of a substantially larger fraction of the planet's mass.[28] No such objects have been found yet and they are still hypothetical.
Zendejas et al. discussed mass loss of planets around low-mass stars, but we can adapt their basic tools to our situation. They found that a planet with a radius $R_p$ orbiting at a distance $D$ from a star with a stellar wind of density $\rho$ and speed $v$ will lose mass at a rate $$\dot{M}_p\approx2\pi R_p^2\alpha\rho v$$ or $$\dot{M}_p\approx\left(\frac{R_p}{D}\right)^2\frac{\dot{M}_*\alpha}{2}$$ with $\dot{M}_*$ the mass-loss rate of the star and $\alpha$ an efficiency parameter. In the worst-case scenario, the winds remove the entirety of the star's mass (impossible, in reality), and if we set $\alpha=1$, then for a $1M_{\odot}$ star, a Jupiter-mass planet orbiting at $D=0.02\text{ AU}$ (as Ash considered) would lose 29% of its mass (where the mass lost is $\dot{M}_p\Delta t$, so $\dot{M}_*\Delta t=M_{\odot}$, with $\Delta t$ the star's lifetime). In reality, $\alpha$ should be closer to $1/3$, at least for terrestrial planets, and if we assume that the star loses more like half of its mass (still quite high), we find that it would lose 4.8% of its mass.
This is in close agreement with the numbers Ash cites. Note that $\dot{M_p}$ is proportional to the square of the radius of the planet and the inverse square of its orbital radius. At $D=0.01\text{ AU}$, we find that the planet should lose 19% of its total mass; at $D=0.1\text{ AU}$, we get 0.2%. A typical hot Jupiter lies within those two extremes, so that gives us some rough bounds.
