Sigma Draconis

Astronomers are hoping to use NASA's 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 Sigma Draconis. 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 around 2015. In November 2005, astronomer Margaret Turnbull, who has been working on identifying the best stars for the TPF to target its observatories, wrote to inform us that: "[S]igma Drac[onis] is one of our best TPF-C targets, and the 4th easiest star in the universe to detect terrestrial planets" -- more below).

The Star

Sigma Draconis is a main-sequence orange-red dwarf star of spectral and luminosity type K0 V. The star has about 89 percent of Sol's mass (RECONS), 79 percent of its diameter (Pasinetti-Fracassini et al, 2001), and 39 percent of its luminosity. Although the star may only be about 3.3 billion years old, it appears to be less enriched than Sol in elements heavier than hydrogen ("metals") with only about 56 to 59 percent of Sol's abundance of iron (Cayrel de Strobel et al, 1991, page 33). It has the Catalog and New Suspected Variable designations of CSV 101868 and NSV 12176. Useful star designations and catalogue numbers for Sigma Draconis include: Sig Dra, 61 Dra, HR 7462, Gl 764, Hip 96100, HD 185144, BD+69 1053, SAO 18396, LHS 477, LTT 15713, and LFT 1486.

Hunt for Substellar Companions

Using the radial velocity technique pioneered by Geoffrey W. Marcy and R. Paul Butler, the Lick Planet Search for substellar companions has thus far failed to find a brown dwarf or large Jupiter- or Saturn-mass object in a "torch" orbit around Sigma Draconis (Cumming et al, 1999). The distance from Sigma Draconis where an Earth-type planet would be "comfortable" with liquid water can be crudely estimated as centered around 0.62 AU -- inside the orbital distance of Venus in the Solar System -- where a planet would have an orbital period of only about 199 days, or over half an Earth year. At such a distance to Sigma Draconis, astronomers are hoping to use NASA's 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 Sigma Draconis.

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 around 2015. In November 2005, astronomer Margaret Turnbull, who has been working on identifying the best stars for the TPF to target its observatories, wrote to inform us that: "[S]igma Drac[onis] is one of our best TPF-C targets, and the 4th easiest star in the universe to detect terrestrial planets. Due to its proximity, the angular habitable zone size is relatively large: from 68 to 150 milliarcseconds. And due to its low luminosity, an Earth-like planet would be about 2e-10 times as bright as the star at optical wavelengths. Believe it or not that is actually quite good for stars whose habitable zones are that large on the sky. Brighter stars, while their HZ's are large, swamp the light of their planets, which makes them difficult targets for TPF." (The TPF-C's top 100 target star list is now available (Robert A. Brown, forthcoming; in pdf).)