The Nacreous Oughts

20 May 2014


The star now known as GJ 687, but in older lists of nearby stars BD+68°946 (also AOe 17415-6, Vyssotsky 322, & Ci[ncinnati 18h] 2354), in Draco, appears to have a Neptune-sized planet near the outer edge of its habitable zone. I am calling this one Cendrillon, for no very good reason, except that i've been reading multi-cultural versions of this fairy tale in some of my English tutoring. It is one of the closest stars to Earth, a M3.5 dwarf of either 0.21 or 0.413 times the mass of the Sun. At parallax "0.22084 & visual magnitude 9.15, its luminosity is 0.003907010002. For that spectral type, i would expect BC= -1.97, T= 3200 K, M= 0.32 (which gives a radius of 0.5), whereas the temperature given in the first, exoplanet discovery, source as 3413 K, corresponds to M0, BC= -1.12 & 0.50 times the solar mass (R= 0.3). The visual luminosity corresponds to a typical M1 star BC= -1.35 & 3350 K; its mass would be 0.45. They give the radius as 0.4183, & the best fit i can find is BC= -1.645 T= 3295 K (e.g. type M2.25--note that the B-V given as +1.5 in one of the sources is close to M2 standard) which gives a bolometric luminosity of 0.017776429 & a derived radius 0.413111619. (Not a bad fit for the larger mass.) These i will use.

If the stellar mass is 0.413 & the planet's period is 38.14 days, its semi-major axis works out to 0.165134804. The tidal effect is 15.145, so it is definitely trapped rotation. The blackbody temperature is 251 K, not much cooler than Earth's 279 K; trying some combinations of albedo, 0.19-0.36-0.51, with greenhouse effect 20-40-60, give average surface temperatures of -15, -8, -3. In short, this will be more "habitable" at the East Pole than anywhere near the terminator. There's a catch, though. Its mass is given as 18.394; thus there will be a strong tendency to hold molecular hydrogen at these temperatures (e.g. for any planet density greater than 0.336 times the Earth), & i expect this planet to have an ammonia-based biosphere, if any.

In Our Neighbor Stars (2012), Thomas William Hamilton writes: "The mass is 21% of the Sun's mass, but it has a much higher percentage of elements higher than helium than the Sun does [in Wikipedia i find: Fe/H +0.11; here +0.05]. This is generally taken to mean that the star is significantly younger than the Sun. ...It is a prolific producer of X-rays (also suggestive of youthfulness)..." Solstation has: "close binary? LHS 450" but no separate entry. Old books (ca. 1969) cite a dark companion, either 8-26 times the mass of Jupiter (1977), or 10-60. This is clearly another ghost planet, like those of Barnard's Star & 61 Cygni. Long ago i calculated Ci 2354 as a binary (period 26 years) with very high eccentricity (0.90). A separate, Earthlike planet close in is barely possible, but it does experience some wild gyrations of temperature. I wonder if everyone has forgotten this model?

(image via reneaigner on deviant art)


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