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Cake day: March 17th, 2024

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  • The sun and Jupiter are pretty close in terms of density, and Jupiter would need to get at least an order of magnitude heavier to start fusion. I think it’s just a coincidence that the outward pressure of the sun’s fusion makes these numbers roughly line up.

    Thirteen Jupiters seems to be a commonly-given lower limit for fusion, so let’s go with that. To increase mass by thirteen times while maintaining density (and assuming the whole thing is a perfect sphere, which it obviously isn’t), Jupiter needs to increase its radius by a factor of about 2.35. This increases its equatorial radius to about 168,000 km, which does swallow up the three innermost moons, but leaves the four big ones alone


  • The IAU’s list of requirements to be a planet is:

    • Orbit a star
    • Be big enough that it becomes round
    • Clear the neighbourhood (meaning you’re way bigger than anything else in your orbit)

    The last one is the one that disqualifies Pluto. For comparison, Pluto is roughly 8% of the mass of the other stuff in its orbit (not including Neptune, given that their orbits cross), whereas Neptune is thousands of times more than the rest of its orbit. The closest non-planet to meeting this criterion is Ceres, which is roughly one third of the rest of its orbit (in the asteroid belt).

    Based on this list, I think Jupiter’s four biggest moons (Io, Europa, Ganymede, and Callisto) would make the cut.