Alone in the Milky Way


By John Gribbin J. グリビン
English 日本語 日本語
Astronomers have now found thousands of planets orbiting other stars in the Milky Way, and 100 billion more stars in the galaxy presumably host planets of their own. Given the sheer number of worlds out there, scientists find it easy to hope that some of them must be harboring sentient beings. After all, could Earth really be unique among so many planets?  太陽以外の恒星を周回する系外惑星がこれまでに天の川銀河で数千個見つかっており,さらに同銀河内で1000億個の星が惑星を擁しているとみられている。それだけの膨大な惑星があるのだから,その一部が知覚力のある生き物を宿していると期待しても無理はないだろう。それほど多くの惑星のなかで生物のいる惑星が実際に地球だけということが,ありうるだろうか?
It could. Optimism about the possibilities of intelligent extraterrestrial life ignores what we know about how humans came to exist. We are here because of a long chain of implausible coincidences—many, many, many things had to go right to result in the situation in which we find ourselves. This chain is so implausible, in fact, that there is good reason to conclude that humans most likely are the only technological civilization in the galaxy. (Let us leave aside the other countless galaxies in the cosmos because, as the saying has it, “in an infinite universe, anything is possible.”)  いや,ありうる。地球外知的生命体の可能性に関する楽観論は,人類がどのように出現するに至ったかについて私たちが知っている事柄を無視している。私たち人間がいまここにいるのは,およそありそうにない偶然が長く連鎖した結果だ。私たちがいるこの状況が生じるには,非常に多くの事柄が適切に起こる必要があった。そうした連鎖が起こる可能性は実に低いと考えられるので,人類が銀河系で唯一の技術文明であると結論できる(ここで宇宙に存在する数え切れない他の銀河を考えるのはやめておこう。「無限の宇宙では何でも可能」になってしまうから)。
The coincidences begin with the manufacture of heavy elements, which include everything heavier than hydrogen and helium. The first stars were born in clouds of these two lightest elements, the residue of the big bang, more than 13 billion years ago. They cannot have had planets, because there was nothing to make planets from—no carbon, oxygen, silicon, iron or any other metals (with cavalier disregard for chemical subtleties, astronomers call all elements heavier than hydrogen and helium metals).
Metals are created inside stars and spread through space when stars throw off material as they die, sometimes in spectacular supernova explosions. This material enriches interstellar clouds, so each successive generation of stars made from the clouds will have a greater metallicity than the one before it. When the sun came into being about 4.5 billion years ago, this enrichment had been going on for billions of years in our galactic neighborhood. Even so, the sun contains roughly 71 percent hydrogen, 27 percent helium and just 2 percent metals. Its composition mirrors that of the cloud that made the solar system, so the rocky planets, including Earth, formed from only that tiny amount of elemental construction material. Stars older than the sun have even fewer metals and, correspondingly, less chance of making rocky, Earth-like planets (giant gaseous planets, such as Jupiter, are easier to form but not as likely to host life). This means that even if we are not the only technological civilization in the galaxy, we must be one of the first.  金属は恒星の内部で作られ,それらの星が一生を終えて死ぬときに宇宙空間に広がる(華々しい超新星爆発によってまき散らされるのが一例)。この物質が星間雲に加わるので,そこから生まれた後の世代の恒星ほど金属量が多くなる。