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 axionlike 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 $5M_{\oplus }\approx 1.5\times {10}^{-5}{M}_{\odot }$ and ${\mathrm{\Omega }}_{\mathrm{AS}}/{\mathrm{\Omega }}_{\mathrm{DM}}\simeq 1/10$. 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.

• Received 5 September 2023
• Accepted 18 October 2023

DOI:https://doi.org/10.1103/PhysRevD.108.103038

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Published by the American Physical Society