Upsilon Andromedae b

Extrasolar planet in the Andromeda constellation

Upsilon Andromedae b was detected by the variations in its star's radial velocity caused by the planet's gravity. The variations were detected by making sensitive measurements of the Doppler shift of Upsilon Andromedae's spectrum. The planet's existence was announced in January 1997, together with 55 Cancri b and the planet orbiting Tau Boötis. Like 51 Pegasi b, the first extrasolar planet discovered around a normal star, Upsilon Andromedae b orbits very close to its star, closer than Mercury does to the Sun. The planet takes 4.617 days to complete an orbit, with a semimajor axis of 0.0595 AU. A limitation of the radial velocity method used to detect Upsilon Andromedae b is that only a lower limit on the mass can be found. The true mass may be much greater depending on the inclination of the orbit. A mass of and an inclination of 24° were later found using high resolution spectroscopy. The temperature difference has led to speculation that Upsilon Andromedae b is tidal locked with the same side always facing Upsilon Andromedae A.

David Sudarsky had, on the assumption that the planet is similar to Jupiter in composition and that its environment is close to chemical equilibrium, predicted Upsilon Andromedae b to have reflective clouds of silicates and iron in its upper atmosphere. The cloud deck instead absorbs the star's radiation; between that and the hot, high pressure gas surrounding the mantle, exists a stratosphere of cooler gas. The outer shell of dark, opaque, hot cloud is assumed to consist of vanadium and titanium oxides, but other compounds like tholins cannot be ruled out yet. The chemical elements in the atmosphere can be studied by finding their absorption lines in the thermal spectrum of the planet; given typical planet temperatures, the spectrum has its peak at infrared wavelengths. So far, only water vapor has been detected in this planet, while carbon monoxide and methane are still under the detection limit. The planet (with 51 Pegasi b) was deemed a candidate for direct imaging by Planetpol. Preliminary results from polarimetric studies indicate that the planet has a predominately blue color, is 1.36 times as large and 0.74 times as massive as Jupiter, meaning that the mean density is 0.36g/cm3. It has a geometric albedo of 0.35 in visible light. In 2016–2017 the direct detection of the planetary thermal emission was claimed, but the detection was questioned in 2021. Tidal heating models predict a similar mass for the planet.

The planet orbits a (F type) star named Titawin (Upsilon Andromedae A). The star has a mass of 1.27 and a radius of around 1.48 It has a temperature of 6,074 K and is 3.12 billion years old. In comparison, the Sun is about 4.6 billion years old. The star is slightly metal rich, with a metallicity ([Fe/H]) of 0.09, or about 123% of the solar amount. Its luminosity is 3.57 times that of the Sun. The star's apparent magnitude, or how bright it appears from Earth's perspective, is 4.09. Therefore, Upsilon Andromedae can be seen with the naked eye.

Upsilon Andromedae b appears to be responsible for increased chromospheric activity on its parent star. Observations suggest that there is a "hot spot" on the star around 169 degrees away from the sub planetary point. This may be the result of interactions between the magnetic fields of the planet and the star. The mechanism may be similar to that responsible for the activity of RS Canum Venaticorum variable stars, or the interaction between Jupiter and its moon Io.