Tau Bootis

Tau Bootis b

Artist's impression of Tau Bootis b.

The location of Tau Bootis

The sky location of Tau Bootis.

Tau Bootis is a nearby binary star system in the constellation Boötes. It consists of an F star primary (a bit bigger and brighter than the Sun), Tau Bootis A, and a red dwarf companion, Tau Bootis B. The red dwarf moves around the primary in an extraordinarily elongated path (eccentricity = 0.91), taking about 2,000 years to complete each circuit at an average distance of 245 astronomical units (37 billion kilometers, or 23 billion miles).


In 1996, a massive planet was found in orbit around the primary at less than one eighth the distance of Mercury from the Sun, making it one of the most extreme examples yet found of an epistellar jovian or "hot Jupiter". It remains one of the nearest exoplanets known.


The atmosphere of Tau Bootis b

In 2012, Tau Bootis b was in the news again when two independent teams of researchers published results on the planet's atmosphere.[1,2] In the past, it's only been possible to learn something about an exoplanet's atmosphere if the planet passes directly in front of – in other words, transits – its central star as seen from Earth. When this happens the gases in the planet's atmosphere leave spectral 'fingerprints' in the light from the star which allow the makeup of the atmosphere to be figured out. But now astronomers have extracted information about the atmosphere of Tau Bootis b using sensitive equipment and 15 years of measurements of the faint reflected glow of light from the planet itself. Not only this, they've measured the slight changes in wavelength of the spectral lines of carbon monoxide in the planet's atmosphere as Tau Bootis b travels around its orbit to get a better estimate of the planet's mass and its orbital characteristics.


Host star : Tau Bootis A
Apparent magnitude 4.50
Spectral type IV
Distance 50.84 light-years (15.60 pc)
Temperature 6,340 K (6,070°C)
Luminosity (Sun = 1) 3.4
Mass (Sun = 1) 1.3
Position R.A. 13h 47m 15.7s, Dec. +17° 27' 25"
Other designations 4 Bootis, GJ 527, HR 5185, BD +18°2782, HD 120136, SAO 100706, HIP 67275, GC 18637


Mass (Jupiter = 1) about 6
Semimajor axis 0.05 AU (7.5 million km, 4.7 million miles)
Orbital period 3.31 days
Eccentricity 0.023±0.015
Year of discovery 1996
Discoverers Butler et al, Berkeley Planet Search
Method of discovery radial velocity



1. Matteo Brogi, Ignas A. G. Snellen, Remco J. de Kok, Simon Albrecht, Jayne Birkby & Ernst J. W. de Mooij. "The signature of orbital motion from the dayside of the planet Tau Bootis b." Nature 486, 502–504, (28 June 2012), doi:10.1038/nature11161 arxiv: http://arxiv.org/abs/1206.6109


2. Florian Rodler, Mercedes Lopez-Morales, Ignasi Ribas. "Weighing The Non-Transiting Hot Jupiter Tau BOO b." 27 Jun 2012