Upsilon Andromedae 2

© James B. Kaler, UIUC -- more information
(Photo from Stars, Planet Project, and Upsilon Andromedae; used with permission)

Due in part to the discovery of planetary companions, Upsilon Andromedae A was identified as one of the top 100 target stars for NASA's proposed Terrestrial Planet Finder (TPF). NASA, however, has indefinitely postposed the TPF project until policy priorities are favorable and funding is available.

JPL, CalTech, NASA


Larger illustration


Astronomers have identified
Upsilon Andromedae A as a
prime target for the Terrestrial
Planet Finder (TPF), now
indefinitely delayed.

In 1997, astronomers announced the discovery of a Jupiter-like planet "b" around the Sun-like primary star (Butler and Marcy, 1997 -- details below), with indications of two additional larger planets "c" and "d" that were later confirmed. On May 21, 2002, another team of astronomers announced the discovery of a stellar companion B in a wide orbit (Lowrance et al, 2002 -- details below). On October 12, 2006, astronomers using NASA's Spitzer Space Telescope announced the discovery that the innermost planet b has hot spot on its tidally locked dayside surface atmosphere (NASA press release -- more below). In May 2010, astronomers announced a 30-degree difference in orbital inclination between planets "c" and "d" and indications of a fourth, outermost planet "e" (NASA news release; and McArthur et al, 2010 -- more below).

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	Orbital Distance (a=AUs)	Orbital Period (P=years)	Orbital Eccentricity (e)	Orbital Inclination (i=degrees)	Mass (Earths)	Diameter (Earths)	Density (Earths)	Surface Gravity (Earths)	Metallicity (Solar)
Upsilon Andromedae A	0.0	...	...	...	432,000	174	...	...	1.32
Planet "b"	0.06	0.013	0.012	125.5?	>219	13	...	...	...
Planet "c"	0.83	0.66	0.28	125.5?	4,440	...	...	...	...
Inner H.Z. Edge?	1.5	1.6	0	125.5?	...	...	...	...	...
Outer H.Z. Edge?	2.9	4.4	0	125.5?	...	...	...	...	...
Planet "d"	2.5	3.5	0.27	155.5?	3,260	...	...	...	...

Upsilon Andromedae A

Star A is a yellowish main sequence dwarf star of spectral and luminosity type F8 V, with 1.31 times the mass of Sol (McArthur et al, 2010), 1.6 times its diameter, and 3.4 times its luminosity. The star may be more (132 percent) enriched than as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (see Ups And b at exoplanets.org). It may be around five billion years old. Useful catalogue numbers and designations for the star include: ups And, 50 And, HR 458*, Gl 61, Hip 7513, HD 9826, BD+40 332, SAO 37362, FK5 1045, and LTT 10561.

Ann Feild, STScI, ESA, NASA



Larger and jumbo illustrations.



The orbits of Star A's planetary system
appears to have suffered from a major
disturbance since the formation of its
planets, when compared with planetary
orbits found in the Solar System (more).

Habitable Zone around Star A

According to one type of model used in calculations for the NASA Star and Exoplanet Database, the habitable zone orbits of an Earth-like planet (with liquid water) around Upsilon Andromedae A may range between 1.5 to 2.9 AUs -- between the orbital distances of Mars and the Main Asteroid Belt in the Solar System. Assuming that the mass of Star A is 1.31 Solar-masses, then such orbital periods could range from around 1.47 years (566 days) to 4.41 years (1,610 days). Alternatively, the habitable zone orbits have also be calculated to lie between 1.80 to 3.5 AUs (Jones and Sleep, 2003) -- between the orbital distances of Mars and the Main Asteroid Belt in the Solar System -- with an orbital period of two to several Earth years. In any case, the presence of planet b at its average orbital distance of 2.5 AU could have disrupted the development of an Earth-mass planet in the water zone. Astronomers are hoping that they will eventually be able to use NASA's Terrestrial Planet Finder (TPF) and the ESA's Darwin planned groups of observatories to search for a rocky inner planet in the so-called "habitable zone" (HZ) around Upsilon Andromedae A. As previously proposed, the TPF would include two complementary observatory groups, a visible-light coronagraph and and a "formation-flying" infrared interferometer, while Darwin would launch a flotilla of three mid-infrared telescopes and a fourth communications hub.

As of October 19, 2010, three planets have been detected around Star A, and the potential presence a fourth outermost planet "e" is suggested in preliminary data.

© John Whatmough -- larger image
(Artwork from Extrasolar Visions, used with permission)

View of of tidally locked, cold side of planetary candidate b
with ice clouds on dark side, as imagined by Whatmough.

Planet "b" - In 1996, a team of astronomers (including Eric Williams, Heather M. Hauser, and Phil Shirts) led by Geoffrey W. Marcy and R. Paul Butler announced the discovery of a Jupiter-class planet around Upsilon Andromedae (ups And) A using highly sensitive radial-velocity methods (Butler and Marcy, 1997. Planet b has at least 69 percent of Jupiter's mass (see Upsilon Andromedae at exoplanets.org). It moves around ups And at an average distance of only 0.06 AUs (a semi-major axis well within Mercury's orbital distance) in a slightly elliptical orbit (e=0.012) that takes 4.6 days to complete. Assuming a Jupiter-like composition, its radius is projected to be about 1.2 times that of Jupiter, enlarged relative to Jupiter because of greater absorbed stellar radiation in its inner orbit. Unfortunately, the inclination of its orbit to Earth's line of sight is unknown.

Robert Hurt, SSC, Caltech, JPL, NASA




Larger illustration.




In 2006, outer atmospheric temperature
variations on planet b that are indicative
of weather patterns were detected by
infrared observations (more).

On October 12, 2006, astronomers using NASA's Spitzer Space Telescope announced the discovery that the innermost planet b has hot spot on its tidally locked dayside surface atmosphere (NASA press release). In the first measurements of the day and night temperatures of an extra-Solar planet, infrared observations revealed that the Jupiter-class gas giant circling very close to Upsilon Andromedae A stays as hot as fire on one side while potentially remaining as cold as ice on the other. The temperature difference between the day and night sides is indicative of energy flows in the planet's atmosphere, as transmitted by "weather."

Bradley Hansen, UCLA,
Caltech, JPL, NASA





Larger illustration.





Because planet b
is tidally locked
and the outer
atmosphere is so
efficient at
re-radiating heat,
its "nightside" stays
dark and cold (top),
unlike bands of
even temperature on
a Jupiter-like planet
(shown below -- more).

The planet appears to be moving in a tidally locked, synchronous orbit around its host star, where it rotates slowly enough that the same side always faces the star (similar to the way that the same side of Earth's tidally locked Moon always faces the Earth and hides its "dark side"). Since planet b is gaseous, however, its outer atmosphere is probably circulating much faster than its interior. Hence, the observed temperature difference between the two sides of the planet is extreme -- about 1,400 degrees Celsius (2,550 degrees Fahrenheit). Such a large temperature difference indicates that the planet's atmosphere absorbs and re-radiates starlight so quickly that the gas circling around it in the outer atmosphere cools off quickly --- unlike Jupiter, which appears to have a relatively even temperature within planetary bands of atmospheric circulation.

© John Whatmough -- larger image
(Artwork from Extrasolar Visions, used with permission)

View of a ringed planetary candidate "c" (blue from Raleigh
scattering) with Mars-like moon, as imagined by Whatmough.

Planet "c" - A residual drift in the radial velocity data over a decade suggest the presence of even larger planets in outer orbits (Butler et al, 1999). After 11 years of observations, two additional outer planets were discovered with two other teams of astronomers at the Harvard-Smithsonian Center for Astrophysics and the High Altitude Observatory using the Whipple Observatory (1999 press release). Based on astrometic as well as high-cadence radial velocity measurements, planetary candidate "c" appears to have around 14.0 +2.3/-5.3 times the mass of Jupiter (McArthur et al, 2010). It lies at an average orbital distance of about 0.83 AUs from ups And (between the average orbital distances of Venus and Earth in the Solar System). However, its eccentric orbit (e=0.28) brings it as close as 0.60 AUs but as far as 1.06 AUs from ups And, taking around 242 days to complete (see Upsilon Andromedae at exoplanets.org).

© John Whatmough -- larger image
(Artwork from Extrasolar Visions, used with permission)

View of planet d from its cloud deck with rings, moons, star-like
planets b and c, and ups And, as imagined by Whatmough.

Planet "d" - On average, planetary candidate "d" lies about 2.5 AUs from ups And, just within the middle orbital distance of the Main Asteroid Belt of the Solar System. However, its eccentric orbit (e=0.27) brings it as close as 1.8 AUs but as far as 3.2 AUs from ups And, taking around 3.5 years to complete. It has at least 3.75 times Jupiter's mass, but subsequent astrometic as well as high-cadence radial velocity measurements suggest that planet d may have 10.25 +0.7/-3.3 times the mass of Jupiter with an inclination of 155.5° from Earth's line of sight (McArthur et al, 2010); Han et al, 2000; and Mazeh et al, 1999).

© Christoph Kulmann -- larger image


Artwork from Exoplaneten.de
(used with permission).


A Mars-like moon of planetary candidate d,
as imagined by Kulmann (more).

Water Emissions - In September of 2002, a team of astronomers (including Cristiano Cosmovici of the Institute for Cosmic and Planetary Science) announced at the Second European Workshop on Exo/Astrobiology that they had detected water "maser" emissions from three of 17 star systems suspected of hosting planets, including Upsilon Andromedae, using the 32-meter Medicina radio telescope near Bologna. These microwave emissions could be generated from water molecules in a planet's atmosphere when they are excited by the infrared light of its host star. In an interview with New Scientist magazine, Astronomer Hugh Jones (Liverpool John Moores University) noted that the water signals could be coming from the host star rather than from a planetary atmosphere, but that additional telescopic observation should be able to pinpoint the exact source of the signal. Astronomer Geoff Marcy (University of California at Berkeley) added that he would not expect water maser emissions from the planets to be strong enough to be detected from Earth, but noted: "It wouldn't be the first time a surprising result came from extra-Solar planets." On September 21, 27, and 28, 2002, however, P. Kondratko and J. Lovell were unable to confirm the detection of water-maser emissions with the 70-meter NASA Deep Space Network antenna (+ 400-MHz Smithsonian and 16-MHz Australia Telescope correlation spectrometers) near Canberra (More discussion at Extrasolar Planets Encyclopaedia).

Upsilon Andromedae B

NASA -- larger image


Upsilon Andromedae B is a dim red dwarf star, like
Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.

On May 21, 2002, a team of astronomers (Patrick J. Lowrance, J. Davy Kirkpatrick, and Charles A. Beichman) announced that Upsilon Andromedae has a dim stellar companion B that shares the same common proper motion as Star A. Upsilon Andromedae B currently is separated from Star A by around 750 AUs. It appears to be a main sequence red dwarf star of spectral and luminosity type M4.5 V. Because of its small mass and great distance from the primary (Star A), Upsilon Andromedae B appears to have a negligible effect on the radial velocity measurements used to determine that Star A has at least three large planets (Lowrance et al, 2002). In 2010, astronomers announced that parallaxes were used to successfully confirm that Star B is a stellar companion Upsilon Andromedae A (McArthur et al, 2010)..

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