Ellie Arroway (played by Jodie Foster) listens for alien signals in the movie “Contact.” (Credit: Warner Bros.)
Are we alone? Fifty-five years ago, astronomer Frank Drake came up with an equation that weighed the odds for aliens, and now two astronomers have tweaked the formula to come up with a slightly different spin.
Their bottom line? There’s an astronomically high chance that other civilizations have arisen elsewhere in the universe at some point in its 13.8 billion-year history.
The University of Washington’s Woody Sullivan and the University of Rochester’s Adam Frank published their assessment in the May issue of Astrobiology, and Frank is following up with an op-ed in Sunday’s New York Times.
“While we do not know if any advanced extraterrestrial civilizations currently exist in our galaxy, we now have enough information that they almost certainly existed at some point in cosmic history,” Frank writes.
He and Sullivan come to that conclusion by deconstructing the Drake Equation. The classic formula starts out with the rate of formation of stars in our Milky Way galaxy, then multiplies that by the fraction of those stars with planetary systems, the average number of habitable planets for each of those systems, the fraction of those planets that give rise to life, the fraction of those life-bearing planets that give rise to civilizations, the fraction of those civilizations that beam out evidence of their existence, and the average length of time during which they’re able to do the beaming. The result tells you how many alien civilizations in the Milky Way you think should be sending out signals.
Simple, right? You can use this interactive to plug in your own estimates and come up with a figure.
The problem is that the latter terms of the equation are squishy, because we just don’t know enough about life beyond Earth to hazard an educated guess. But thanks to the data from NASA’s planet-hunting Kepler space telescope, astronomers are getting a good handle on the first three terms: the number of stars, planetary production and the prevalence of habitable planets.
So Frank and Sullivan focus on what’s known about the big picture, and set aside the time factor.
The Drake Equation, top, combines several factors to come up with an estimate for the number of extraterrestrial civilizations in the Milky Way currently beaming out signals. The Frank-Sullivan Equation, bottom, eliminates the time factors and provides an estimate for the number of advanced cvilizations likely to have developed over the history of the observable universe. Click on the graphic for a larger version. (Credit: University of Rochester)
They start out with an estimate of 20 sextillion stars in the observable universe (2 x 1022). There appears to be at least one planet for every star (1.0). And about one-fifth of those planets appear to orbit in habitable zones (0.2). That gives you a really big number for the estimate of habitable planets in the universe: 4 sextillion, or 4 x 1021.
Then the astronomers add a twist to the equation. How low do you have to set the chances that a habitable planet gives rise to a signal-beaming civilization, in order to reduce what you get when you do the multiplication (planets times probability) to just one world, as in Earth? The number would have to be one chance in 4 sextillion, or 2.5 x 10-22.
“To me, this implies that other intelligent, technology-producing species very likely have evolved before us,” Frank said in a news release about the tweaked equation. “Think of it this way: Before our result you’d be considered a pessimist if you imagined the probability of evolving a civilization on a habitable planet were, say, one in a trillion. But even that guess, one chance in a trillion, implies that what has happened here on Earth with humanity has in fact happened about 10 billion other times over cosmic history!”
Or as Carl Sagan put it in “Contact,” the novel that was made into a movie starring Jodie Foster: “If it’s just us, seems like an awful waste of space.”
Sullivan pointed out that there’s still room to be pessimistic about the search for extraterrestrial intelligence.
“The universe is more than 13 billion years old,” he said. “That means that even if there have been a thousand civilizations in our own galaxy, if they live only as long as we have been around — roughly 10,000 years — then all of them are likely already extinct. And others won’t evolve until we are long gone. For us to have much chance of success in finding another ‘contemporary’ active technological civilization, on average they must last much longer than our present lifetime.”
On one level, calculating the odds for alien life is just a numbers game. But on a deeper level, it should get us thinking about what it takes to make a civilization sustainable for the long term — so that we can stick around long enough to make our own mark in the cosmos.
Yes, There Have Been Aliens
LAST month astronomers from the Kepler spacecraft team announced the discovery of 1,284 new planets, all orbiting stars outside our solar system. The total number of such “exoplanets” confirmed via Kepler and other methods now stands at more than 3,000.
This represents a revolution in planetary knowledge. A decade or so ago the discovery of even a single new exoplanet was big news. Not anymore. Improvements in astronomical observation technology have moved us from retail to wholesale planet discovery. We now know, for example, that every star in the sky likely hosts at least one planet.
But planets are only the beginning of the story. What everyone wants to know is whether any of these worlds has aliens living on it. Does our newfound knowledge of planets bring us any closer to answering that question?
A little bit, actually, yes. In a paper published in the May issue of the journal Astrobiology, the astronomer Woodruff Sullivan and I show that while we do not know if any advanced extraterrestrial civilizations currently exist in our galaxy, we now have enough information to conclude that they almost certainly existed at some point in cosmic history.
Among scientists, the probability of the existence of an alien society with which we might make contact is discussed in terms of something called the Drake equation. In 1961, the National Academy of Sciences asked the astronomer Frank Drake to host a scientific meeting on the possibilities of “interstellar communication.” Since the odds of contact with alien life depended on how many advanced extraterrestrial civilizations existed in the galaxy, Drake identified seven factors on which that number would depend, and incorporated them into an equation.
The first factor was the number of stars born each year. The second was the fraction of stars that had planets. After that came the number of planets per star that traveled in orbits in the right locations for life to form (assuming life requires liquid water). The next factor was the fraction of such planets where life actually got started. Then came factors for the fraction of life-bearing planets on which intelligence and advanced civilizations (meaning radio signal-emitting) evolved. The final factor was the average lifetime of a technological civilization.
Drake’s equation was not like Einstein’s E=mc2. It was not a statement of a universal law. It was a mechanism for fostering organized discussion, a way of understanding what we needed to know to answer the question about alien civilizations. In 1961, only the first factor — the number of stars born each year — was understood. And that level of ignorance remained until very recently.
That’s why discussions of extraterrestrial civilizations, no matter how learned, have historically boiled down to mere expressions of hope or pessimism. What, for example, is the fraction of planets that form life? Optimists might marshal sophisticated molecular biological models to argue for a large fraction. Pessimists then cite their own scientific data to argue for a fraction closer to 0. But with only one example of a life-bearing planet (ours), it’s hard to know who is right.
Or consider the average lifetime of a civilization. Humans have been using radio technology for only about 100 years. How much longer will our civilization last? A thousand more years? A hundred thousand more? Ten million more? If the average lifetime for a civilization is short, the galaxy is likely to be unpopulated most of the time. Once again, however, with only one example to draw from, it’s back to a battle between pessimists and optimists.
But our new planetary knowledge has removed some of the uncertainty from this debate. Three of the seven terms in Drake’s equation are now known. We know the number of stars born each year. We know that the percentage of stars hosting planets is about 100. And we also know that about 20 to 25 percent of those planets are in the right place for life to form. This puts us in a position, for the first time, to say something definitive about extraterrestrial civilizations — if we ask the right question.
In our recent paper, Professor Sullivan and I did this by shifting the focus of Drake’s equation. Instead of asking how many civilizations currently exist, we asked what the probability is that ours is the only technological civilization that has ever appeared. By asking this question, we could bypass the factor about the average lifetime of a civilization. This left us with only three unknown factors, which we combined into one “biotechnical” probability: the likelihood of the creation of life, intelligent life and technological capacity.
You might assume this probability is low, and thus the chances remain small that another technological civilization arose. But what our calculation revealed is that even if this probability is assumed to be extremely low, the odds that we are not the first technological civilization are actually high. Specifically, unless the probability for evolving a civilization on a habitable-zone planet is less than one in 10 billion trillion, then we are not the first.
To give some context for that figure: In previous discussions of the Drake equation, a probability for civilizations to form of one in 10 billion per planet was considered highly pessimistic. According to our finding, even if you grant that level of pessimism, a trillion civilizations still would have appeared over the course of cosmic history.
In other words, given what we now know about the number and orbital positions of the galaxy’s planets, the degree of pessimism required to doubt the existence, at some point in time, of an advanced extraterrestrial civilization borders on the irrational.
In science an important step forward can be finding a question that can be answered with the data at hand. Our paper did just this. As for the big question — whether any other civilizations currently exist — we may have to wait a long while for relevant data. But we should not underestimate how far we have come in a short time.
Quelle: The New York Times