A New Age in Space Exploration?
What does the future hold for the exploration of outer space?
Even if there is life elsewhere in our solar system, few — if any — scientists expect it to be "advanced." But what about the more remote parts of the cosmos?
In the years since 1995, a new field of science has opened up — the study of other families of planets, in orbit around distant stars. What are the prospects of life on some of these?
Few experts were surprised that these planets existed. Astronomers already knew that other stars formed as our Sun did — from a slowly spinning interstellar cloud that contracted into a disc.
The dusty gas in these other discs could have agglomerated into planets — just as happened around the new-born Sun.
But until the 1990s, there were no techniques sensitive enough actually to disclose any of these faraway planets.
A hundred other stars like the Sun are already known to have at least one planet. Almost every month more are being discovered. Those planets found so far, orbiting solar-type stars, are all roughly the size of Jupiter or Saturn — the giants of our own solar system.
To observe Earth-like planets will require very large telescope arrays in space. NASA's "Origins" program is focused on the origin of the universe, of planets and of life. One of its key projects will be an array of telescopes in space. Europeans are planning a similar project, called "Darwin."
We were all, when young, taught the layout of our own solar system — the sizes of the nine major planets and how they move in orbit around the Sun. But 20 years from now, we will be able to tell our grandchildren far more interesting things on a starry night.
Nearby stars will no longer just be dwindling dots in the sky. We will think of them as the Suns of other solar systems. We will know the orbits of each star's retinue of planets — and even some topographic details of the bigger planets.
NASA’s Terrestrial Planet Finder and its European counterpart should discover many such planets, but only as faint points of light. Nonetheless, much can be learnt about them even without a detailed picture.
Viewed from, say, 50 light years away — the distance of a nearby star — Earth would be, in Carl Sagan's phrase, a "pale blue dot," seeming very close to a star (our Sun) that outshines it by a factor of many billions.
The shade of blue would be slightly different, depending on whether the Pacific Ocean or the Eurasian land mass was facing us. Even if we can't resolve detail on other planets’ surfaces, we can therefore infer whether they are spinning the length of their "day" — and even their gross topography and climate.
We will be especially interested in possible "twins" of our Earth, planets the same size as ours, orbiting other Sun-like stars — and with temperate climates where water neither boils nor stays frozen.
By analyzing such a planet's faint light, we could infer what gases existed in its atmosphere. If ozone existed, this would imply that it was rich in oxygen, as our Earth's atmosphere is. This would indicate a biosphere. Our own atmosphere did not start out that way, but was transformed by primitive bacteria in its early history.
But an actual image of such a planet — one that can be displayed on the wall-sized screens that will by then have replaced posters as room decorations — will surely have even more impact than the classic pictures of our own planet viewed from space.
How far away will we have to search to find another biosphere? Does life start on every planet in the right temperature range, where there is water, along with other elements such as carbon? At present, such questions are open.
As often in science, lack of evidence leads to polarized and often dogmatic opinions. But agnosticism is really the only rational attitude while we know so little about how life began, how varied its forms and habitats could be — and what evolutionary paths it might take.
Could some of these planets, orbiting other stars, harbor life forms far more exotic than even optimists might expect on Mars or Europa — even something that could be called intelligent?
To firm up the odds, we need a clearer understanding of just how special Earth's physical environment had to be in order to permit the prolonged selection process that led to the higher animal forms on Earth.
The greatest uncertainties lie in the province of biology, not astronomy. First, how did life begin? I think that there is a real chance of progress here, so that we will know whether it is a "fluke," or whether it is nearly inevitable in the land of initial "soup" expected on a young planet.
But there is a second question: Even if simple life exists, what are the odds against it evolving into something that we would recognize as intelligent? This is likely to prove far more intractable. Even if primitive life were common, the emergence of "advanced" life may not be.
Even if simple life existed on many planets around nearby stars, complex biospheres like Earth's could be rare. There could be some key hurdle in evolution that is hard to surmount.
Perhaps it is the transition to multicellular life. The fact that simple life on Earth seems to have emerged quite quickly, whereas even the most basic multicellular organisms took nearly three billion years, suggests that there may be severe barriers to the emergence of any complex life.
Or the biggest hurdle could come later. Even in a complex biosphere, the emergence of human-level intelligence isn't guaranteed.
If, for instance, the dinosaurs hadn't been wiped out, the chain of mammalian evolution that led to Homo sapiens may have been foreclosed. And we cannot predict whether another species would have taken our role.
Some evolutionists regard the emergence of intelligence as a contingency, even an unlikely one. Others dissent from this line. Perhaps more ominously, there could be a crucial hurdle at our own present evolutionary stage — the stage when intelligent life starts to develop technology.
If so, the future development of life depends on whether humans survive this phase. This does not mean that Earth has to avoid a disaster — only that before this happens, some human beings or advanced artifacts will have spread beyond their home planet.
Searches for life will justifiably focus on Earth-like planets orbiting long-lived stars. But science fiction authors remind us that there are more exotic alternatives.
Perhaps life can flourish even on a planet flung into the frozen darkness of interstellar space, whose main warmth comes from internal radioactivity (the process that heats Earth's core).
There could be diffuse living structures, freely floating in interstellar clouds: Such entities would live — and, if intelligent, think — in slow motion, but nonetheless may come into their own in the long-range future.
No life would survive on a planet whose central Sun-like star became a giant and blew off its outer layers.
Such considerations remind us of the transience of inhabited worlds — and also that any seemingly artificial signal could come from superintelligent (though not necessarily conscious) computers, created by a race of alien beings that had long since died out.