Pi Mensae

NASA

Pi Mensae is a yellow-orange star
like our Sun, Sol. (See a Digitized
Sky Survey image of Pi Mensae
from the Nearby Stars Database.)

System Summary

Pi Mensae is located about 59.4 light-years from Sol. It lies in the central part of (5:37:9.9-80:28:8.8, ICRS 2000.0) of Constellation Mensa, the Table Mountain -- south of Gamma Mensae, west of Alpha and Theta Chamaeleonis, and southeast of Alpha Hydri. According to the Yale Bright Star Catalogue's notes entry for HR 2022, Pi Mensae is a member of the 61 Cygni stellar moving group. On October 15, 2000, astronomers announced the discovery of a Jupiter-like planet around this Sun-like star (press release and exoplanets.org -- details below). (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.) As Pi Mensae 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 Pi
Mensae as a prime target for the
Terrestrial Planet Finder (TPF),
now planned for launch between
2014 and 2020.

The Star

Pi Mensae is a yellow-orange main-sequence dwarf star of spectral and luminosity type G1-3 V, that has been classed as orange as G3 and as bright as IV. The star has about 1.10 times Sol's mass (exoplanets.org; and Santos et al, 2002), 1.52 times its diameter (Johnson and Wright, 1983, page 659), and 1.45 times its visual luminosity. Given its relative brightness, Pi Mensae appears to be more highly evolved than Sol. It also may be 1.23 times as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Santos et al, 2002). Useful catalogue numbers and designations for the star include: Pi Men, HR 2022*, GJ 9189, Hip 26394, HD 39091, CD-80 195, CP(D)-80 161, SAO 258421, Wo 9189, LHS 208, LTT 2359, LFT 429, and LPM 219.

Planet "b"

NASA

Cassini-Huygens Mission
to Saturn and Titan

Larger image.

Planetary candidate "b" appears to be a
gas giant that is 10 times more massive
than Jupiter (shown here with Europa).

On October 15, 2000, a team of astronomers (including Hugh R. A. Jones, R. Paul Butler, Chris G. Tinney, Geoffrey W. Marcy, Alan J. Penny, Chris McCarthy, Brad Carter, and Dimitri Pourbaix) announced the discovery of a Jupiter-class planet around Pi Mensae using highly sensitive radial-velocity methods (press release and exoplanets.org). According to the latest radial velocity data, planet "b" has at least 10.41 times Jupiter's mass (and, depending on its inclination, could actually exceed 13 Jupiter-masses and fuse deuterium as a brown dwarf). It moves around Pi Mensae at an average distance of 3.50 AUs (a semi-major axis within the the orbital distance of the Main Asteroid Belt in the Solar System) in a highly elliptical orbit (e=0.64) that takes almost 2,270 days or about 6.2 years to complete (exoplanets.org; and Jones et al, 2001).

The orbit of an Earth-like planet (with liquid water) around Pi Mensae may be centered around 1.2 AU -- between the orbital distances of Earth and Mars in the Solar System -- with an orbital period around 460 days (around 1.3 years). However, the eccentric orbit of superplanet b at an average orbital distance of around 3.5 AU would disrupt the orbit of an Earth-type planet around Pi Mensae'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 Nu2 Lupi. 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.

Closest Neighbors

The following table includes all star systems known to be located within 10 light-years (ly), plus more bright stars within 10 to 20 ly, of Pi Mensae.

Star System	Spectra & Luminosity	Distance (light-years)
Alpha Chamaeleonis	F4-6 IV-III	9.9
L 57-44	M V	10.0
* plus bright stars *		. . .
GJ 3021 / CD-80 9	G6 V	13
I Carinae AB	F0-3 IV F2 IV	17
CP(D)-65 475	K1 V-IIIp	18

Other Information

The late John Whatmough created illustrated web pages on this system in Extrasolar Visions.

Up-to-date technical summaries on these stars can be found at: Jean Schneiders's Extrasolar Planets Encyclopaedia; the Astronomiches Rechen-Institut at Heidelberg's ARICNS, and the Nearby Stars Database. Additional information may be available at Roger Wilcox's Internet Stellar Database.

Located in a South Polar region of the sky that lacks bright stars above 5th magnitude and other interesting objects, Constellation Mensa was named by the Abbé [Abbot] Nicholas Louis de La Caille (1713-1762). The original name, Mons Mensa, after the table mountain near de Lacaille's observatory in Cape Town, South Africa, was shortened when the International Astronomical Union (IAU) defined the 88 constellations that exist today. For more information about the stars and objects in this constellation, go to Christine Kronberg's Mensa. For another illustration, see David Haworth's Mensa.

For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.