[extropy-chat] Most star systems are single

[Amara Graps]

amara
>>  Most star systems are single.
>>
>>  Charles Lada made this conclusion based on the Milky Way's most common
>>  stars: red dwarfs (spectral class M), which comprise about 85% of the
>>  total number of stars... Amara

spike
>If Lada is correct, it's the best news I have heard since 20 October 2005.
>My unsophisticated BOTECs suggest that single stars would be more likely to
>have their angular momentum being carried in orbiting planets and dust.
>That is far more interesting than having the rocky stuff end up falling into
>the star because of the influence of a companion.
>
>Come to think of it, in any double system, not only is there less space for
>stable orbits, the habitability of any possible planets would presumably be
>reduced by variation of radiation from the two (or more) stars.  I would
>think one steady star would be more friendly to evolution of life than two.


There are four issues with M-stars' suitability as hosts for habitable
planetary systems [1]. The best thing is to not assume that life would
emerge similarly to ours.

1) The 'habitable zone' is close in and narrow : 0.2 to 0.5 A.U. where
the A.U. is the distance to the M-class main sequence star.

2) Synchronous rotation is likely for a planet in the habitable zone of
an M star because of the proximity of the planet to the star and the
consequent large tidal distortion. Such conditions _could_ hinder the
emergence of life because the longtime dark side would act as a cold
trap, but the author of [1] says it is not necessarily a problem since
there could be greenhouse gases at the right temperature and pressure
preventing an atmospheric collapse.

3) The small proportion of the star's output at visible wavelengths
means that any lifeform would need to adapt to light where the maximum
of radiant output is at about 1.2 microns.

4) M dwarf variability; they flare more frequently than the Sun, several
times a day. The frequency declines on a timescale of the order of 1000 Ma
so life might emerge later in the M stars life when its variability
becomes less. Also an M star's "starspots" (sunspots) are
proportionately much larger than those no the Sun, the largest can cause
a decrease of a few tens of percent in luminosity for up to several
months.

Amara


[1] F. W. Jones, _Life in the Solar System and Beyond_, Springer (2004).
-- 

Amara Graps, PhD      www.amara.com
Istituto di Fisica dello Spazio Interplanetario (IFSI), Roma, ITALIA
Associate Research Scientist, Planetary Science Institute, Tucson