Alpha Centauri Dreams

WHEN a supernova explosion generated a shockwave that stirred a cloud of gas and dust into forming the Sun and its planets five billion years ago, it may also have instigated the creation of the Alpha Centauri system. Because, at 4.3 light years, this star system is the nearest to the Sun and represents our best hope of finding a habitable planet with the minimum of effort. One of the two stars of Alpha (for Alpha is a double system) has an almost uncanny resemblance to the Sun. It is the same spectral type, the same colour, the same temperature and almost the same diameter. The other star, Alpha Centauri B, is also very similar although slightly redder and hence cooler.

Moreover both these stars share with the Sun the unlikely ratio of one atom of iron to every 31,260 atoms of hydrogen, a ratio that no other star is known to possess. It may be that in seeking to travel to it, we will not be exploring an ``alien'' solar system at all, but one that is in some respects part of our own. Alpha Centauri A and Alpha Centauri B make up a stable pair. They never approach each other more closely than eleven Earth‑Sun distances, less than the distance that Uranus is from the Sun. Therefore planets around either star would not be disturbed by the gravitational field of the other. Both stars therefore stand a very good chance of having planets, perhaps roughly corresponding to the Sun's inner worlds of Mercury, Venus, Earth and Mars. And if these exist, a world may well circle each star that is neither too hot nor too cold. But there may be some bad news. Although the two stars are far enough apart to allow each to have a planet in the right place to be habitable, such a world might lack the right environment. In short, it may be without water. For the two stars will nevertheless be too close together to allow either to have a cloud of comets that would, over many ages, have given oceans and an atmosphere to worlds of the right temperature. And a waterless planet can have neither atmosphere nor life. But happily there is another possibility. It has been known since 1915 that the Alpha Centauri system has a third member, a brown dwarf star known as Proxima Centauri (so called because it is literally the Sun's closest neighbour.) Proxima is one‑sixth of a light year from the other two stars, completing its orbit around them once every million years. The presence of this distant companion would have a most remarkable effect. Over billions of years it would have collected a great swathe of comets. These, periodically, would have rained down on the worlds circling the two inner suns and given them oceans. Water would give them atmospheres and plant life and make them habitable. This, at any rate, is one explanation of why the Earth is so fortunate. If in turn Proxima Centauri is truly a member of the Alpha Centauri system and not some wandering star that is unconnected with it, then there is every reason for our descendants to expect welcoming worlds almost on our doorstep.