  The anomalous orbits of trans-Neptunian objects can be accounted for by the
planet 9 hypothesis. One intriguing possibility is that planet 9 could be a
dilute axion star captured by the solar system, with the ratio of the axion
star to dark matter being approximately 1/10. Although dilute axion stars can
emit monochromatic signals through two-photon decay, the spontaneous decay
signal is too weak to be detected by radio telescopes. However, we find that
stimulated decay of the dilute axion star, which explains planet 9, can occur
by directing a radio beam with a power of 50MW into the star. The resulting
echo can be detected by terrestrial telescopes such as SKA, FAST, ngLOBO, and
LOFAR. Therefore, the dilute axion star can be distinguished from other planet
9 candidates, such as a primordial black hole or a free-floating planet
captured by the solar system.
