  The anomalous orbits of Trans-Neptunian Objects (TNOs) can be explained by
the Planet 9 hypothesis. We propose that the Planet 9 can be an axion star.
Axion stars are gravitational bound clusters condensed by QCD axions or
axion-like particles (ALPs), which we call axions for brevity. We find that the
probability of capturing an axion star by the solar system is the same order of
magnitude as the probability of capturing a free floating planet (FFP), and
even higher for the case of axion star, with axion star mass
$5M_\oplus\approx1.5\times10^{-5}M_\odot$ and
$\Omega_{\rm{AS}}/\Omega_{\rm{DM}}\simeq 1/10$. Although axion star can emit
monochromatic signals through two-photon decay, we find that the frequency of
decay photon is either not within the frequency range of the radio telescope,
or the decay signal is too weak to be detected. Therefore, if Planet 9 is
composed by an axion star, it may be difficult to distinguish it from an
isolated primordial black hole by spontaneous decay of axion.
