On the planet Blob there is an unusual phenomenon. It is known to emanate from the planet's core but 2018-level scientists have no understanding of it.

The phenomenon

Gravity isn't constant. It varies between 1g (Earth units) and 0.5g. The variation is sinusoidal at a frequency of 1hz.

The inhabitants have evolved with this phenomenon and so are quite comfortable. In fact they are kangaroo-like and jump in synchrony with the change in gravity to get a boost.


The planet is Earth-like in size and distribution of water etc. at least as much as that is possible given the odd gravity.

Scientists have no way of drilling into the centre of the planet to find the cause. They just have to accept it as a mystery. Maybe there's a weird black-holey thing in there.


The weird gravity changing thing is definitely in there and is itself indestructible. Would such a planet even be able to hold together on the surface? Would it melt or blow apart? Would life have even the smallest chance of developing (say microscopically)?


2 Answers 2


Such a planet could form, though it would likely be a hot inhospitable volcanic planet for the same reason life can never evolve there: conservation of energy.

What you describe violates the conservation of energy laws. As your Kangaroo creatures noticed, it is possible to get free energy with a spring and a lever.

This means your planet must be constantly pumping energy into the system. Lots of it! Probably a wormhole to some exotic binary black hole pattern or something. Regardless, going to generate a lot of heat in the form of rocks contracting against eachother.

  • 2
    $\begingroup$ Your conservation of energy argument is correct: if that planet ever existed, a simple lever would lift a heavy weight with some energy, and then generate double that energy when the weigh goes down with double gravity. In other words, it is an over-unity or a perpetual motion machine. That is impossible. Even if this resulted from tidal forces, it would still subside with time as the planet's orbit becomes less eccentric. $\endgroup$ Commented Nov 24, 2018 at 18:33

If we consider that in the famous formula $ F = m \cdot a$ we are now applying an acceleration variable over time, meaning we are investigating the effect of $ F(t) = m \cdot a(t)$.

To all effects we are applying a variable load. This means that, on the long term, most if not all materials will fail and crack due to fatigue.

After that, we have to consider that the constant squeezing on all the bodies is actually pumping energy into them. We know that supplied energy ends up in heat, therefore it is reasonable to think that this planet will be producing heat via this mechanism.

To be able to host life, this planet should be at the outer range of the Goldilocks zone, so that the heat generated via its weird feature can compensate the low stellar energy supply.

  • $\begingroup$ Probably a lot of volcanoes too $\endgroup$
    – Andon
    Commented Nov 24, 2018 at 19:42

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