 Size comparison of Jupiter with Upsilon Andromedae b | |
| Discovery | |
|---|---|
| Discovered by | Marcyet al. |
| Discovery site | California and Carnegie Planet Search  United States |
| Discovery date | June 23, 1996 |
| Radial velocity | |
| Orbital characteristics | |
| Apastron | 0.0601 AU |
| Periastron | 0.0587 AU |
| 0.0594±0.0003 AU[1] | |
| Eccentricity | 0.012±0.005[1] |
| 4.617111±0.000014 d[1] (0.01264096 a; 110.8107 h) | |
| Inclination | 24±4[1] |
| 2,450,034.05±0.33[1] | |
| 44.11±25.56[1] | |
| Semi-amplitude | 70.51±0.37[1] |
| Star | Upsilon Andromedae A |
| Physical characteristics | |
| ~1.8[2]RJ | |
| Mass | 1.70+0.33 −0.24[1]MJ |
Upsilon Andromedae b (υ Andromedae b, abbreviatedUpsilon And b,υ And b), formally namedSaffar/ˈsæfɑːr/, is anextrasolar planet approximately 44light-years away from theSun in theconstellation ofAndromeda. The planet orbits its host star, theF-type main-sequence starUpsilon Andromedae A, approximately every five days. Discovered in June 1996 byGeoffrey Marcy andR. Paul Butler, it was one of the firsthot Jupiters to be discovered. It is also one of the first non-resolved planets to be detected directly. Upsilon Andromedae b is the innermost-known planet in itsplanetary system.
In July 2014 theInternational Astronomical Union launchedNameExoWorlds, a process for giving proper names to certain exoplanets and their host stars.[3] The process involved public nomination and voting for the new names.[4] In December 2015, the IAU announced the winning name was Saffar for this planet.[5] The winning name was submitted by the Vega Astronomy Club ofMorocco and honours the 11th-century astronomerIbn al-Saffar ofMuslim Spain.[6]
Upsilon Andromedae b was detected by the variations in its star'sradial velocity caused by the planet'sgravity. The variations were detected by making sensitive measurements of theDoppler shift of Upsilon Andromedae'sspectrum. The planet's existence was announced in January 1997, together with55 Cancri b and the planet orbitingTau Boötis.[7]
Like51 Pegasi b, the first extrasolar planet discovered around a normal star, Upsilon Andromedae b orbits very close to its star, closer thanMercury does to the Sun. The planet takes 4.617days to complete an orbit, with asemimajor axis of 0.0595AU.[8]
A limitation of the radial velocity method used to detect Upsilon Andromedae b is that only a lower limit on themass can be found. Thetrue mass may be much greater depending on theinclination of the orbit. A mass of1.70 MJ and an inclination of 24° were later found using high-resolutionspectroscopy.[1]
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Given the planet's high mass, it is likely that Upsilon Andromedae b is agas giant with nosolid surface.
TheSpitzer Space Telescope measured the planet'stemperature, and found that the difference between the two sides of Upsilon Andromedae b is about 1,400 degrees Celsius, ranging from minus 20 to 230 degrees to about 1,400 to 1,650 °C.[9] The temperature difference has led to speculation that Upsilon Andromedae b istidal 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 tochemical equilibrium, predicted Upsilon Andromedae b to have reflectiveclouds ofsilicates andiron in its upperatmosphere.[10] The cloud deck instead absorbs the star's radiation; between that and the hot, high-pressure gas surrounding the mantle, exists astratosphere of cooler gas.[11] The outer shell of dark, opaque, hot cloud is assumed to consist ofvanadium andtitanium oxides, but other compounds liketholins 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, onlywater vapor has been detected in this planet, whilecarbon monoxide andmethane are still under the detection limit.[1]
The planet is unlikely to have largemoons, sincetidal forces would either eject them from orbit or destroy them on short timescales compared to the age of the system.[12]
The planet (with51 Pegasi b) was deemed a candidate for direct imaging byPlanetpol.[13] 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 ageometric albedo of 0.35 in visible light.[14] In 2016–2017 the direct detection of the planetary thermal emission was claimed, but the detection was questioned in 2021.[15] Tidal heating models predict a similar mass for the planet.[2]
The planet orbits a (F-type)star named Titawin (Upsilon Andromedae A). The star has a mass of 1.27M☉ and a radius of around 1.48R☉. It has a temperature of 6,074K 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 ametallicity ([Fe/H]) of 0.09, or about 123% of the solar amount. Its luminosity (L☉) is 3.57 times that of the Sun.
The star'sapparent 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 increasedchromospheric 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 themagnetic fields of the planet and the star. The mechanism may be similar to that responsible for the activity ofRS Canum Venaticorum variable stars, or the interaction between Jupiter and its moonIo.[16]
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