Looking up into the night sky we see thousands of stars, on a clear night we might see the vague smudge of a nebula, the most visible of which being the Orion Nebula which rises in the autumn night sky, but those worlds and the life which must exist in at least some of those distant points of light is a complete mystery to us.
We feel alone in the universe because we do not know any of our neighbours, so when we look out into the twinkling darkness, we see only mystery and the limits of our knowledge, and our isolation from the rest of the cosmos. But imagine if we could look at a distant point of light and we knew how many planets orbited that sun and even knew what kind of life forms populated those planets, and we even knew the name the most advanced creatures called themselves.
And so, with other stars, if we only knew what advanced life might exist out there, knew what their language sounded like, knew about their great works of art, even their food and music culture, imagine how much richer that night-sky would appear to us, and how comforting it would be to see our distant twinkling neighbours and wonder how they were doing. I always try to do that, to imagine the universe as full of life, which it surely must be.
In 1979, Isaac Asimov published the book Extra-terrestrial Civilisations where he systematically analyses the probability of life existing in our galaxy somewhat analogous to ours at a comparable level of civilisation. Of the 300 billion stars in our galaxy he implements a selection criteria such as the planet being within an optimal habitable or so-called Goldilocks zone, not too close to the star where there would be no chance for an atmosphere to form or where surface temperature would be too hot, nor too remote from the star where temperatures would be too cold to support life as we know it. He found that of the 300 billion stars there were probably only 530,000 possible worlds where an Earth-like technological civilisation might exist. This cautious figure was substantially increased in recent years with the work of NASA’s now retired Kepler telescope orbiting the sun, which detected exo-planets by measuring fluctuations in starlight caused by planetary occultations and now find that between as much as 15-30 percent of the stars in the Milky Way should have habitable worlds.
This habitable zone will of course, vary relative to the nature of the star in question, the nearest star to our solar-system for instance is the red dwarf: Proxima Centauri, which even though it is our closest neighbour is not actually visible to the naked eye due to its relatively low mass and low luminosity. A possible exo-planet, known as Proxima Centauri B, has been identified as orbiting Proxima Centauri within the star’s habitational zone at .05 Astronomical Units or 4.6 million miles, where 1 Astronomical Unit is the distance between the sun and the Earth. Mercury by comparison lies at .387 Astronomical Units or 36 million miles from the sun. So we can see that because of Proxima Centauri’s extremely low mass and low luminosity, in order to be within the star’s habitable zone the planet must lie seven times closer to the Proxima Centauri than Mercury is to our own sun. Compared to Mercury’s year of 88 days, the time it takes Proxima B to completes is orbit of the sun is a mere 11 days. The other unusual aspect of Proxima B is that it has no day, or rather because of its extreme proximity to its star, it is tidally locked which means one side will always face the sun while the other side will be in eternal darkness. In such an unusual situation it is likely that the only habitable areas might be those at the eternal sunset or sunrise point where the direct exposure to permanent solar radiation is somewhat reduced compared to the permanent full glare of infra-red convection oven conditions. However, before you start dreaming of what fantastic civilisations might rise and fall living by the perpetual gloomy red light of a permanent dusk and never venturing into the endless night which lies just beyond the horizon, in 2016 a solar flare which made the star 68 times brighter and visible to the naked eye was spotted by the Evryscope telescope array in Chile. It seems that such a powerful solar flare would instantly wipe out all life which had evolved until that point, and although the flare was the highest recorded it is likely that five super-flares occur every year and would thus, significantly reduce the probability of life existing in the company of such an inhospitable star.
This instability of red dwarf stars could be a major limiting factor in the search for life on other planets. Research indicates that 2 thirds of all red dwarf stars exhibit potentially destructive solar flare activity and since 75 percent of the stars in our galaxy are red-dwarves, then this analysis might reduce the somewhat optimistic appraisal of between 15 and 30 percent of all stars in the galaxy having habitable worlds and steer us more towards Asimov’s somewhat more cautious appraisal.
Earth itself is within this habitable zone while our closest neighbour, Mercury of course is much too hot and close to the sun to hold an atmosphere.
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