[LINK] "Alpha Centauri B: A Close Look at the Habitable Zone"

[rfmcdonald]

Paul Gilster's space-centered blog Centauri Dreams takes its name from Alpha Centauri, the nearest star to Earth. To the naked eye a single star visible only on Earth from the Southern Hemisphere, through the telescope it turns out that Alpha Centauri includes three stars, with two of these stars being pretty Sun-like stars in a stable orbit that could support planets with Earth-like environments. Gilster's most recent post examines the prospects for an Earth-like planet orbiting Alpha Centauri B, the dimmer and oranger of Alpha Centauri's two Sun-like stars. B has become a major target of astronomers since it's the less massive of the two Sun-like stars, and it would be easier to find an Earth-mass planet around B than around A.

What's the story? First, the background.

Centauri A and B, being high metallicity stars, are presumably prime candidates for circumstellar disks with a high solid material component, making the building blocks of planets readily available, and deepening the spectral lines for improved precision in radial velocity studies. Another useful factor for observations is that the binary is inclined by only 11 degrees with respect to our line of sight, an important fact because it means that any planets we discover through RV methods will yield a mass that is fairly accurate, assuming that the planets around these stars have formed in the same orbital plane. Without such knowledge, the mass figures from RV studies vary widely depending on assumptions about the target system’s inclination.

Studies on planet formation have shown that both Centauri A and B should be capable of forming terrestrial planets even when the perturbations caused by the binary companion are taken into account. Early studies on this question have found that the planetesimal disks seem to be stable out to about 3 AU of the parent stars, assuming a reasonable inclination of the disk relative to the binary plane, meaning something less than 60 degrees. More recent work by Thébault and colleagues has shown that the later stages of accretion may not be efficient because the binary companion can inhibit the growth of larger objects outside 0.75 AU (Cen A) and 0.5 AU (Cen B).

What does this mean? Most likely that the formation of gas giants is unlikely here (a finding that squares with previous radial velocity surveys), while if we can get past the problem of forming larger planetesimals referred to above, Earth-mass planets should be able to form in the habitable zone of Centauri B, assuming an eccentricity of no more than 0.3. A 2009 study I’m not familiar with by Michtchenko & Porto de Mello makes the case that any terrestrial planets that do form in Centauri B’s habitable zone should be dynamically stable despite perturbations from Centauri A under certain conditions of eccentricity and orbital inclination, but planets with inclinations to the orbital plane larger than about 35 percent should experience strong instability.

Next, comes the question of where B's habitable zone--the area of space where a world could conceivably support a moderate Earth-style climate, not too hot like Venus' nor too cold like Mars', orbiting around its sun.

Kasting and team used a model that assumed Earth-mass planets with similar atmospheric composition and found a habitable zone ranging from 0.5 to 0.9 AU, although this 1993 study did not include the perturbing influence of Centauri A. But [in a new paper astronomer Duncan] Forgan notes this with regard to the light reaching Centauri B planets:

If main sequence relations for the luminosity of each object are assumed, the insolation experienced by planets in the habitable zone of α Cen B due to α Cen A would be no more than a few percent of the total insolation of the α Cen AB system at the binary’s periastron, and around one tenth of a percent at apastron. This insolation can be diminished further by eclipses of α Cen A by α Cen B, the duration of which is estimated to be of order a few Earth days.

[. . .]

So what does Forgan find? It turns out that calculating the habitable zone of Centauri B’s inner and outer boundaries can be roughly correct if we leave Centauri A out of the picture — the dimensions of the habitable zone remain more or less the same. But adding Centauri A does create oscillations in the planet’s climate that happen when Centauri A is at its closest to Centauri B. The temperature variations caused by Centauri A are no more than several K, and could alter the fraction of habitable surface on planets at the habitable zone boundaries by about 3 percent, a figure made flexible depending on the size of oceans or planetary obliquity.

The paper goes on to note the possible effect on life (science fiction writers take note):

It is reasonable to speculate that if life were to exist on planets around α Cen B, that they may develop two circadian rhythms (cf Breus et al. 1995) corresponding to both the length of day around the primary, and the period of the secondary’s orbit (approx 70 years). Altering the available habitat by a few percent may also influence migration patterns and population evolution.

The impact of B on a planet orbiting A would be lesser, I'd think, since B is the dimmer star and would have proportionally less effect.