889 planets outside the solar system (in 694 planetary systems, including 133 multiplanetary systems), orbiting various kinds of star, had been identified as of May 22 2013.
Almost all were within our home galaxy the Milky Way, but there had been a small number of possible detections of extragalactic planets.
In the Milky Way, it is thought that there is at least one, on average, orbiting each star. There are 200-400 billion stars in the galaxy. There may be many (perhaps 100,000 times) more rogue planets than stars.
Astronomers at the Harvard-Smithsonian Center for Astrophysics reported in January 2013 that there are “at least 17 billion” Earth-sized exoplanets in the Milky Way. The nearest known exoplanet, Alpha Centauri Bb, 4.37 light years away, is Earth-sized, but outside the habitable zone of Alpha Centauri B.
The discovery of extrasolar planets has intensified interest in the search for extraterrestrial life. Data from the Habitable Exoplanets Catalog suggested that, of the 859 exoplanets which had been confirmed as of January 3 2013, nine were potentially habitable. Some could have habitable extrasolar moons.
Alan Boss of the Carnegie Institution of Science and Duncan Forgan of Edinburgh University suggest that, although there is still no evidence of extraterrestrial life, there could be thousands of civilisations in our galaxy. Toynbee identified twenty-three in the history of the Earth.
When life originates, it may spread among habitable planets by natural or directed panspermia.
Besides exoplanets, exocomets, comets beyond our solar system, have been detected and may be common in the Milky Way.
If the biosphere were to cease to be any longer a possible habitat for life, mankind, so far as we know, would suffer the fate of extinction that would then overtake every other form of life. Moreover, the nearest potential biosphere to ours (if any other, besides ours, is to be found anywhere in the physical cosmos) may be hundreds of millions of light-years distant from our planet. In our generation, a few human beings have been landed on the surface of our planet’s moon and, after a brief stay there, they have been brought back again to Earth still alive in almost every case. [That should be every case! Bad OUP editing.] This has been a magnificent feat of science applied to technology, but it has been a still more notable feat of sociality, considering that, so far, human beings have been far less successful in managing their relations with each other than they have been in mastering the non-human part of Nature. This feat has taught us some lessons which are of practical importance for estimating our prospects and choosing our policy on Earth.
The Moon is much closer to the Earth than is any other star; it is our planet’s satellite. Yet to land a few men on the Moon for a few hours has required the precisely co-ordinated and enthusiastically co-operative work of several hundreds of thousands of human beings. It has also required a vast expenditure of material resources and a considerable draft on the courage and ability that are mankind’s rarest and most precious assets. Even if the Moon were to prove to be as rich in resources for human life as the Americas, the exploitation of these resources would not be remunerative economically. A permanent colonization of the Moon by earthlings would be impracticable. Human bodies have a physical structure that enables them to withstand, without feeling the strain, the particular gravitational pull of the Earth’s mass and the particular pressure of the Earth’s envelope of air. They need food in the form of other organic substances, either vegetable or animal. All these features and necessities of human life were present in the Americas for those Europeans who reached the Americas by crossing the Atlantic in the tenth century of the Christian Era from Scandinavia and in the fifteenth century from Spain. Their meeting with other human beings who had anticipated the Europeans in reaching and occupying the Americas was evidence that these other parts of the Earth’s dry land were habitable.
The Moon is not habitable for any form of life. The only lunar matter that could be a resource for human beings would be inanimate matter that has never been even temporarily organic. In order to be made useful, this lunar matter would have to be transported from the Moon to the Earth by human beings camping and working on the Moon under the handicap of extremely trying conditions. This would not pay, as it did pay to convey tobacco from America to Europe and to cultivate in Europe and in Asia other plants – for instance maize and potatoes – that had been domesticated in America by the Europeans’ predecessors who had reached America from the opposite side.
Though neither the Moon nor the Earth’s sister planets, which are far more remote from the Earth than the Moon is, are habitable for inhabitants of our biosphere, it is conceivable that some other sun than ours, perhaps a sun in some other galaxy, might have a planet that would be habitable for us; but, even if we could locate another habitable planet, it would hardly be feasible for travellers from our biosphere to reach it. Suppose we were to discover how to steer the course without being attracted, en route, into one of the burning fiery furnaces of the innumerable suns that are on the move through space; the journey might take a hundred years. [That assumes a travelling speed of about a fifth of the speed of light to Alpha Centauri Bb.] We should therefore have to devise a spaceship on board of which the passengers could beget children who would be able to live on board and beget children and grandchildren there in their turn, before the conveyance could land and disembark the third or fourth generation. And, even if this arriving and landing generation could count on finding breathable air and drinkable water and edible food and tolerable air pressure and gravitational pull in this hypothetical replica of our biosphere, the conveyance (a modernized Noah’s Ark) in which they had made the voyage from one habitable biosphere to another would have had to be stocked with rations of air and food and drink that would keep successive generations on board alive for a century. It seems most unlikely that this fabulous voyage will ever really be made.
Thus our present knowledge and experience point to the conclusion that the habitat of the denizens of the biosphere on the face of the planet Earth is going to continue to be confined to this capsule within which life, in the form known to us, has made its appearance. Though it is possible that other biospheres, habitable for denizens of our biosphere, may exist, it is so improbable that we could ever reach and colonize any of them that the possibility cannot reasonably be taken into account. This fantasy is, in fact, Utopian.
If we do conclude that our present biosphere, which has been our only habitat so far, is also the only physical habitat that we are ever likely to have, this conclusion will admonish us to concentrate our thoughts and efforts on this biosphere: to survey its history, to forecast its prospects, and to do everything that human action can do to ensure that this – which, for us, is the – biosphere shall remain habitable until it is made uninhabitable eventually by cosmic forces beyond human control.
Mankind and Mother Earth, OUP, 1976, posthumous