Upsilon Andromedae 2

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

As Upsilon Andromedae has become one of the top 100 target stars for NASA's planned Terrestrial Planet Finder (TPF), images of this star and its position relative to the Milky Way in Earth's night sky are now available from the TPF-C team.

JPL, CalTech, NASA


Larger illustration


Astronomers have identified
Upsilon Andromedae A as a
prime target for the Terrestrial
Planet Finder (TPF), now
planned for launch between
2014 and 2020.

Upsilon Andromedae A

Star A is a yellowish main sequence dwarf star of spectral and luminosity type F8 V, with 1.3 times the mass of Sol, 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.

© 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.

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).

The planet appears to be tidally locked to its 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.

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). Planetary candidate "c" have at least 1.9 times the mass of Jupiter. 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). Unfortunately, the inclination of its orbit to Earth's line of sight is unknown.

© 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.

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 astrometric analysis suggest that planet d may have 10 times the mass of Jupiter with an inclination of 155.5° from Earth's line of sight (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).

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).

In order for an Earth-type rocky planet to have liquid surface water, its orbital distance around Upsilon Andromedae A may have to be 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. However, the presence of planet b at its average orbital distance of 2.5 AU should have disrupted the development of an Earth-type planet in the water zone.

Astronomers are hoping 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 currently planned, the TPF will include two complementary observatory groups: a visible-light coronagraph to launch around 2014; and a "formation-flying" infrared interferometer to launch before 2020, while Darwin will launch a flotilla of three mid-infrared telescopes and a fourth communications hub beginning in 2015. (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)

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 has at least three large planets (Lowrance et al, 2002).