An Astrometric Planetary Companion Candidate to the M9 Dwarf TVLM 513-46546

Astrometric observations of the M9 dwarf TVLM 513-46546 taken with the VLBA reveal an astrometric signature consistent with a period of 221 ± 5 days. The orbital fit implies that the companion has a mass m_p = 0.35-0.42 M_J, a circular orbit (e ≃ 0), a semimajor axis a = 0.28-0.31 au, and an inclination angle i = 71°-88°. The detected companion, TVLM 513b, is one of the few giant-mass planets found associated with ultracool dwarfs. The presence of a Saturn-like planet on a circular orbit 0.3 au from a 0.06-0.08 M_⊙ star represents a challenge to planet formation theory. This is the first astrometric detection of a planet at radio wavelengths.