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.
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)?