The Innermost stable circular orbit (often called the ISCO) is the smallest orbit in which a test particle can stably orbit a massive object in general relativity. The location of the ISCO, the ISCO-radius (
r
i
s
c
o
), depends on the angular momentum (spin) of the central object. For a non-spinning object, where the gravitational field can be expressed with the Schwarzschild metric, the ISCO is located at,
r
i
s
c
o
=
6
G
M
c
2
.
As the angular momentum of the central object increases,
r
i
s
c
o
decreases. Even for a non-spinning object, the ISCO radius is only three times the Schwarzschild radius, suggesting that only black holes have innermost stable circular orbits outside of their surfaces.
