Mu Arae

JPL, CalTech, NASA


Larger illustration


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

The Star

Mu Arae is a yellow-orange dwarf star of spectral and luminosity type G3 V-IV. The star has about 108 +/- 5 percent of Sol's mass (exoplanets.org; Giradi et al, 2002); it may have a similar diameter (102 percent) as Sol (Johnson and Wright, 1983, page 689) with around 1.7 times its luminosity. Its relative brightness compared to Sol and relative chromospheric inactivity suggest that the star is more highly evolved and probably older than Sol, at around 6.41 billion years (Saffe et al, 2005), as the accumulation of helium ash in its core is moving the star out of the main sequence into subgiant status. The star may be 1.8 to 1.9 times as enriched than Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (McCarthy et al, 2004; and exoplanets.org). Useful catalogue numbers and designations for the star include: Mu Ara, HR 6585, Gl 691, Hip 86796, HD 160691, CD-51 11094, CP(D)-51 10535, SAO 244981, FK5 662, and LTT 7053.

Planetary System

As of August 29, 2006, astronomers have announced the discovery of four planetary candidates around Mu Arae.

NASA

Cassini-Huygens Mission
to Saturn and Titan


Larger image.


Planetary candidates "e" appears to
be a gas giant with at least half the mass of
Jupiter (shown here with Europa).

In August 2006, two teams of astronomers confirmed that they had discovered a fourth planet candidate "e" with at least half (52.19 percent) of Jupiter's mass (Gozdiewski et al, 2006; and Pepe et al, 2006). This planet has an average orbital distance of about 0.921 AUs, close to Earth's orbital distance from the Sun in the Solar System. It has a relatively circular orbit (e= 0.0666 ± 0.0122) which takes about 310.55 ± 0.83 days (85 percent of an Earth year) to complete.

JPL, NASA


Larger image.


Planetary candidate "d" may be rocky
or gaseous like Uranus and Neptune
with a similar diameter, but with a
temperature of around 1,500° C or
2,700° F (more from Planet Quest)

On August 25, 2004, a team of astronomers (including Nuno C. Santos, François Bouchy, Jean-Pierre Sivan, Michel Mayor, Francisco Pepe, Didier Queloz, Stephane Udry, Christophe Lovis, Sylvie Vauclair, Michael Bazot, Gaspare Lo Curto, Dominique Naef, Xavier Delfosse, Willy Benz, Christoph Mordasini, and Jean-Louis Bertaux) announced the discovery of a third planetary candidate "d" (also called "c" by those unaware of now confirmed "c" candidate) in a very hot but circular, inner orbit (e~0) (ESO press release; Observatoire de Genève page on HD 160691; and Nuno et al, forthcoming, in pdf or ps). The planet is estimated to have at least 10.5 (revised from 14.0) times the mass of Earth (Pepe et al, 2006). Located at only 0.09 AUs from the star (less than one fourth of Mercury's orbital distance in the Solar System), the planet competes an orbit in 9.64 (revised from 9.55 +/- 0.03) days and may have a surface temperature of 1,160° Fahrenheit (or 900° Kevin). The astronomers believe that under the most likely planetary developmental scenario of inner migration from around 3 AUs under the influence of outer giant planet "b" now at 1.5 AUs, this planet is likely to have an "essentially rocky core" with an atmosphere of five to 10 percent of its total mass (Nuno et al, forthcoming, in pdf or ps).

On December 11, 2000, a team of astronomers (including Chris G. Tinney, Hugh R. A. Jones, Alan J. Penny, Kevin Apps, R. Paul Butler, Geoffrey W. Marcy, Steven S. Vogt, and Gregory W. Henry) announced the discovery of a Jupiter-class planet around Mu Arae using highly sensitive radial-velocity methods (Butler et al, 2001; and (AAO press release). According to the latest radial velocity data, planet "b" has at least 1.67 +/- 0.11 times Jupiter's mass. It moves around Mu Arae at an average distance of only 1.50 +/- 0.02 AUs (a semi-major axis around Mars's orbital distance) in an elliptical orbit (e= 0.31, revised from 0.20 +/- 0.03) that takes around 1.79 years (654.5, revised from 645.5 +/- 3 days) to complete (McCarthy et al, 2004; and exoplanets.org).

NASA

Cassini-Huygens Mission
to Saturn and Titan


Larger image.


Planetary candidates "b" and "c" appear
to be gas giants somewhat larger than
Jupiter (shown here with Europa).

On June 13, 2002, astronomers announced the possible detection of a second giant planet of around 3.10 +/- 0.71 Jupiter-masses based on a residual trend in radial velocity measurements over more than 13 years, which was supported with better data by September 2004 (McCarthy et al, 2004). Planetary candidate "c" has a highly eccentric outer orbit (e~ 0.57 +/- 0.01) with a semi-major axis of 4.17 +/- 0.07AUs, which may take roughly 8.18 years (2986 +/- 30 days) to complete. It has a maximum separation of 0.43 arc-seconds (about 6.6 AUs) from Mu Arae, but this was appeared to have been insufficient for astronomers using the the Hubble space and Earth-based telescopes to take a direct image.

The orbit of an Earth-like planet (with liquid water) around Mu Arae may be centered around 1.3 AU -- between the orbital distances of Earth and Mars in the Solar System -- with an orbital period around 1.31 years (478 days). However, the eccentric orbit of planet b at an average orbital distance of around 1.5 AU would disrupt the orbit of an Earth-type planet around Mu Arae's water zone. (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)

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 Mu Arae. 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.