Tau Boötis Ab

τ Boötis Ab (τ Boo Ab / Tau Boötis Ab) is an extrasolar planet orbiting star τ Boötis A. The designation is often written as τ Boo b, but since τ Boo B is a red dwarf companion of τ Boo A, it is better to use the first mentioned designation. In 1999 the planet was dubbed as the Millennium Planet.

Discovered in 1996, the planet is one of the first extrasolar planets discovered. It was discovered by Paul Butler and Geoffrey Marcy (San Francisco Planet Search Project) using the highly successful radial velocity method. Since the star is visually bright and the planet is massive, it produces very strong signal which was quickly confirmed by Michel Mayor and Didier Queloz from data collected over 15 years. It was later confirmed also by the AFOE Planet Search Team.

The planet is rather massive, minimum mass being over four times that of Jupiter. It orbits the star in a so-called "torch orbit", distance from the star being less than one seventh that of Mercury from the Sun. One orbital revolution takes only 3 days 7.5 hours to complete. Because τ Boo is hotter and larger than the Sun, and planet's orbit is short, it is one of the hottest known planets temperature being about 1500 K. Although it has not been detected directly, it is certain that the planet is a gas giant.

As τ Boötis Ab is more massive than most known "hot Jupiters", it was speculated that it was originally a brown dwarf, a failed star, which had lost most of its atmosphere in the heat of the star. However, it seems very unlikely. Still, such a process has actually been detected on the famous transiting planet HD 209458 b.

In December, 1999 a group led by A. C. Cameron announced that they had detected reflected light from the planet. They calculated that the orbit of the planet has an inclination of 29° and thus the absolute mass of the planet would be about 8.5 times that of Jupiter. They also suggested that the planet is blue in color. Unfortunately, their observations could not be confirmed and were later proved to be spurious.

Since no reflected light has been detected, the planet must be rather dark for a gas giant, albedo being less than 0.39.

There are also some indications of another, more distant planet orbiting τ Boötis A. However, until the possible planet has completed one orbit it remains speculation.

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