Tau Boötis, Latinised from τ Boötis, is an F-type main-sequence star approximately 51 light-years away^[1] in the constellation of Boötes. It is a binary star system, with the secondary star being a red dwarf. In 1999, an extrasolar planet was detected orbiting the primary star. In December 2020, astronomers may have observed, for the first time, radio emissions from a planet beyond the Solar System. According to the researchers: "The signal is from the Tau Boötis system, which contains a binary star and an exoplanet. We make the case for an emission by the planet itself."^[8][9]

The system is a binary. The primary component is a yellow-white dwarf (spectral type F6V) and secondary is a dim red dwarf (spectral type M2).^[4] The system is relatively nearby, distance being about 51 light-years. The primary star should be easily visible to the unaided eye under dark skies.

The primary star, Tau Boötis A is a yellow-white dwarf. It is 35 percent more massive and 42 percent larger than the Sun and thus is somewhat brighter and hotter. It is about 1.3 billion years old, making it younger than the Sun as well.^[4] Since it is more massive than the Sun, its lifespan is shorter—less than 6 billion years. Tau Boötis is the first star apart from the Sun to be observed changing the polarity of its magnetic field.^[10] It is also listed as a suspected variable star. The magnetic activity cycle for this star shows a period of 122 days—much shorter than the solar cycle.^[11]

Tau Boötis B (with a capital B, as opposed to the planet) is a dim, 11 mag red dwarf with only about half the mass and radius of the Sun. It orbits the primary star at an average distance of about 220 AU (14 arcseconds) but comes as close as about 28 AU to the primary, giving its orbit a very high eccentricity of about 0.87. One orbit around the primary would take approximately 2400 years to complete.^[4]

In 1996 the planet Tau Boötis b was discovered orbiting the primary star by a team of astronomers led by R. Paul Butler.^[12] There are also some indications of another planet orbiting the star with a period of roughly 5,000 days; however, this could be due to an instrumental effect or a stellar magnetic activity cycle.^[13] Tau Boötis and its planet appear to be tidally locked to each other.^[14]

• List of exoplanets discovered before 2000 - Tau Boötis b