Pi3 Orionis 2?

"Starting at the north end, the "Pi's" are numbered Pi-1 through Pi-6 Orionis (though have nothing physically to do with each other). In the middle lies the brightest, mid-third magnitude (3.19) Tabit (an Arabic word probably meaning "the Endurer"), Pi-3 Orionis, a star that brilliantly illustrates the confusion often surrounding star names. An earlier list of Pi-1, 2, 3, and 4 (numbered in the traditional fashion, west to east), was later and oddly changed to the current north-to-south Pi-3, 2, 4, 1, so Pi-3 was once Pi-1. Worse, the name "Thabit" (same meaning), which was most likely applied to Upsilon Orionis (well to the southeast and below Orion's Belt), was in more recent times somehow changed to Tabit, and then re-applied to the current Pi-3. The westernmost of the Pi-stars, Pi-3 is also (consistently) Flamsteed's number 1."

Pi3 Orionis came to within 15 light-years of Sol about 210,000 years ago. The star was identified one of the top 10 target stars for NASA's indefinitely postponed Terrestrial Planet Finder (James Kasting, 2010, page 352).

JPL, CalTech, NASA


Larger illustration


Astronomers have identified Pi3
Orionis as a Top 10 target for the
Terrestrial Planet Finder (TPF),
now indefinitely postponed.

Pi3 Orionis

This star is a white-yellow main sequence dwarf star of spectral and luminosity type F6 V, with almost 1.3 times Sol's mass (NASA Star and Exoplanet Database; and David F. Gray, 1992), 1.2 5o 1.3 times its diameter (von Belle and von Braun, 2009, HD 30652 in Table 4, page 7; Perrin and Karoji, 1987; NASA Star and Exoplanet Database; and Kenneth R. Lang, 1980), and over 2.6 times of its bolometric luminosity (NASA Star and Exoplanet Database; and Kenneth R. Lang, 1980). It may be anywhere from 20 to 151 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (NASA Star and Exoplanet Database; and Cayrel de Strobel et al, 1991, page 10). With a rotational period of 3.9 days, Pi3 Orionis A may be around 1.4 to 2.2 billion years old (Lawler et al, 2009, HD 30652 in Table 1, page 24; Holmberg et al, 2009; and James B. Kaler, web page on Tabit).

An unconfirmed, Delta Scuti-type variable, the star has been designated as a New Suspected Variable (NSV) 1731. Star A has a probable optical companion (Abt and Levy, 1976, page 20) but may also have a spectroscopic companion that speckle interferometry has not been able to resolve (Hartkopf and McAlister, 1984). Useful star catalogue numbers for the star include: Pi3 Ori, 1 Ori, HR 1543*, Gl 178, Hip 22449, HD 30652, BD+06 762, SAO 112106, FK5 1134, and LTT 11517.

Pi3 Orionis B?

According to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 1543, its visual companion may be optical.

Hunt for Substellar Companions

The distance from star A where an Earth-type planet would be "comfortable" with liquid water is centered around 1.7 AU -- beyond the orbital distance of Mars in the Solar System. 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 Pi3 Orionis. As originally planned but indefinitely postponed, the TPF would include two complementary observatory groups: a visible-light coronagraph; and a "formation-flying" infrared interferometer, while Darwin would include a flotilla of three mid-infrared telescopes and a fourth communications hub beginning in 2015.