Upsilon Andromedae d
|Discovered by||Butler, Marcy et al.|
|Discovery site||California and Carnegie|
|Discovery date||April 15, 1999|
|Inclination||23.758 ± 1.316|
|4.073 ± 3.301|
|2,450,059 ± 3.495|
|252.991 ± 1.311|
|Semi-amplitude||68.14 ± 0.45|
|Star||Upsilon Andromedae A|
|Temperature||218 K (−55 °C; −67 °F)|
Upsilon Andromedae d (υ Andromedae d, abbreviated Upsilon And d, υ And d), formally named Majriti //, is a super-Jupiter exoplanet orbiting within the habitable zone of the Sun-like star Upsilon Andromedae A, approximately 44 light-years (13.5 parsecs, or nearly 4.163×1014 km) away from Earth in the constellation of Andromeda. Its discovery made it the first multiplanetary system to be discovered around a main sequence star, and the first such system known in a multiple star system. The exoplanet was found by using the radial velocity method, where periodic Doppler shifts of spectral lines of the host star suggest an orbiting object.
In July 2014 the International Astronomical Union launched a process for giving proper names to certain exoplanets and their host stars. The process involved public nomination and voting for the new names. In December 2015, the IAU announced the winning name was Majriti for this planet. The winning name was submitted by the Vega Astronomy Club of Morocco, honoring the 10th century scientist Maslama al-Majriti.
Mass, radius and temperature
Upsilon Andromedae d is a super-Jupiter, an exoplanet that has a radius and mass larger than that of the planet Jupiter. It has a temperature of 218 K (−55 °C; −67 °F). It has a mass of 10.25 MJ and a likely radius of around 1.02 RJ based on its mass.
The planet orbits a (F-type) star named Upsilon Andromedae A. The star has a mass of 1.27 M☉ and a radius of around 1.48 R☉. It has a temperature of 6074 K and is 3.12 billion years old. In comparison, the Sun is about 4.6 billion years old and has a temperature of 5778 K. The star is slightly metal-rich, with a metallicity ([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'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 d orbits its star nearly every 3.5 years (about 1,276 days) in an eccentric orbit, more eccentric than that of any of the known planets in the Solar System. To explain the planet's orbital eccentricity, some have proposed a close encounter with a now-lost outer planet of Upsilon Andromedae A. The encounter would have moved planet "d" into an eccentric orbit closer to the star and ejected the outer planet.
Upsilon Andromedae d lies in the habitable zone of Upsilon Andromedae A as defined both by the ability for an Earthlike world to retain liquid water at its surface and based on the amount of ultraviolet radiation received from the star.
For a stable orbit the ratio between the moon's orbital period Ps around its primary and that of the primary around its star Pp must be < 1/9, e.g. if a planet takes 90 days to orbit its star, the maximum stable orbit for a moon of that planet is less than 10 days. Simulations suggest that a moon with an orbital period less than about 45 to 60 days will remain safely bound to a massive giant planet or brown dwarf that orbits 1 AU from a Sun-like star. In the case of Upsilon Andromedae d, the orbital period would have to be no greater than 120 days (around 4 months) in order to have a stable orbit.
Tidal effects could also allow the moon to sustain plate tectonics, which would cause volcanic activity to regulate the moon's temperature and create a geodynamo effect which would give the satellite a strong magnetic field.
To support an Earth-like atmosphere for about 4.6 billion years (the age of the Earth), the moon would have to have a Mars-like density and at least a mass of 0.07 M⊕. One way to decrease loss from sputtering is for the moon to have a strong magnetic field that can deflect stellar wind and radiation belts. NASA's Galileo's measurements hints large moons can have magnetic fields; it found that Jupiter's moon Ganymede has its own magnetosphere, even though its mass is only 0.025 M⊕.
Discovery and further studies
Like the majority of known extrasolar planets, Upsilon Andromedae d was detected by measuring variations in its star's radial velocity as a result of the planet's gravity. This was done by making precise measurements of the Doppler shift of the spectrum of Upsilon Andromedae A. At the time of discovery, Upsilon Andromedae A was already known to host one extrasolar planet, the hot Jupiter Upsilon Andromedae b; however, by 1999, it was clear that the inner planet could not explain the velocity curve.
In 1999, astronomers at both San Francisco State University and the Harvard-Smithsonian Center for Astrophysics independently concluded that a three-planet model best fit the data. The two new planets were designated Upsilon Andromedae c and Upsilon Andromedae d.
Preliminary astrometric measurements suggest the orbit of Upsilon Andromedae d may be inclined at 155.5° to the plane of the sky. However, these measurements were later proved useful only for upper limits; worthless for HD 192263 b and probably 55 Cancri c, and contradict even the inner planet u And b's inclination of >30°. The mutual inclination between c and d meanwhile is 29.9 degrees. The true inclination of Upsilon Andromedae d was determined as 23.8° after combined results were measured from the Hubble Space Telescope and radial velocity measurements.
When it was discovered, a limitation of the radial velocity method used to detect Upsilon Andromedae d is that the orbital inclination is unknown, and only a lower limit on the planet's mass can be obtained, which was estimated to be about 4.1 times as massive as Jupiter. However, by combining radial velocity measurements from ground-based telescopes with astrometric data from the Hubble Space Telescope, astronomers have determined the orbital inclination as well as the actual mass of the planet, which is about 10.25 times the mass of Jupiter.
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