Sibylla

Poul Anderson, "The Alien Enemy" IN Anderson, The Queen Of Air And Darkness And Other Stories (London, 1977), pp. 69-85.

Sibylla is a humanly colonized planet:

33.25 light years from Sol;
50% more diameter than Earth;
a third more gravity;
pressure gradient steeper;
lowlands too hot;
highlands lacking air;
metal-poor;
low density;
slow rotation;
very low magnetic field;
strong radiation background.

These are not just facts. They are adding up to something. And we have not been told who the alien enemy is yet.