Science Sunday: Are We Alone?
Astronomers outline methods for finding earth-like planets orbiting distant stars.There are two schools of thought about the possibility of life elsewhere in the universe. One school says that the universe is teeming with undiscovered beings. The other school says it isn’t.
This problem was outlined neatly in the 2003 book Lonely Planets by University of Colorado astrobiologist David Grinspoon:
“In 1943, physicist Enrico Fermi was having lunch with some colleagues at the Los Alamos National Laboratory, and the topic of alien life came up. Fermi, like most scientists with some kind of grasp on what a universe with 100,000,000,000,000,000,000,000 stars implies, was a firm believer in extraterrestrial life. But, he asked, if life and intelligence are likely to evolve on even a fraction of other planets, why aren’t the signs more obvious? Why haven’t we been visited of contacted? “Where are they?”
Artist’s concept of an Earth-like planet around another star. NASA JPL
It’s possible to argue either school of thought forcefully and convincingly. On the one hand, statistically speaking, there are so many stars, so many possible planets, so many building blocks to life wandering the universe that it stands to reason some hospitable planets developed intelligent life forms. Probably lots of planets did.
On the other hand, astronomer Michael Hart wrote in 1975, as reported again by Grinspoon:
“Hart showed that the stars can be reached using much less energy. If you allow for centuries, instead of decades, then the energy needs are much more modest. Traveling at one-hundredth the speed of light, you reach the nearest stars in a few centuries, without a fuel tank a billion times larger than your own vehicle. Assuming that interstellar colonies sometimes spawn their own colonies, a migrating species crosses the galaxy in only a few million years.
“Now, a few million years sounds like a long time to you an me. Hell, I get bored driving from Tucson to Phoenix. But it’s just a fraction of a percent of the 10-billion-year age of our galaxy. Thus Hart argued, the absence of evidence is the evidence of absence.”
In other words, because we haven’t seen any evidence of space aliens yet, we aren’t likely to.
The trouble with both hypotheses is the lack of information. There is only one data point for inferring intelligent life in the universe: Earth. Intelligent life unquestionably exists. But we’re the sum total of it so far.
Jupiter. Cassini Imaging Team & NASA/JPL/SSI
With so many stars in the universe, it’s hard to know where to look for life. The first planets outside our sun’s solar system were discovered in the mid-1990s. Extra-solar planets, as they’re called have been popping up like eggs at Easter ever since. About 200 have been identified. But because of the technological limits, these have all been giant planets, roughly the size of Jupiter. They are viewed as unlikely candidates for hosting life for one reason and another.
So the hunt for earth-like planets within the “habitable zone” is heating up. The habitable zone is a spherical area around a star, not too close and not too far away, in which liquid water can be maintained on the planet’s surface. The actual distance depends on the size of the star and other factors. The habitable zone in our solar system extends from about .95 to 1.37 astronomical units (AU). The earth is by definition one astronomical unit from the sun.
In a paper in this week’s Science, several authors, including John Raymer of the Center for Astrophysics and Space Astronomy at the University of Colorado, say:
“Recent advances in instrumentation and new missions are extending the search to planets the size of Earth but closer to their host stars.”
The methods for detecting distant planets are truly remarkable. For instance, astronomers measure the “radial velocity” of the star. This will surprise Copernicus, but planets do not, strictly speaking, orbit their star. They orbit the gravitational center of the system, known as the barycenter, which the star itself also slowly orbits. This is because the planets exercise a gravitational force on the star, causing it to “wobble.” The size of the wobble indicates the presence and potential size of the planets.
The reason that most planets detected so far are giants is because their radial velocity is relatively large. Jupiter, for instance, has a radial velocity of 12.4 meters per second, while earth’s is 0.08 meters per second. Current technology allows measurement as low as one meter per second, about the speed of a person walking. This is pretty good considering they are measuring an object 300 trillion miles away. But it isn’t quite good enough to detect an earth-sized planet.
The Science paper says that improved technology may be able to see earth-like planets orbiting stars whose orbits are “edge-on” to earth. This is a small percentage of the systems, but as the planets pass in front of the star, it’s light diminishes in a way that might be measurable. An earth-sized planet occults on 0.01 percent of the starlight, however, which is very hard to detect unless the star is small, close by, and faint.
“An alternative method for finding smaller planets exists in systems with a transiting giant planet … The transits of giant planets can be timed to a precision of a few tens of seconds with ground-based telescopes, and Earth-mass or smaller planets are detectable.”
The authors point out that any of these worlds discovered would probably not be like earth. But techniques to find earth-like planets would reduce the target stars for the search for life elsewhere, and could help answer the question: Are we alone?
Artist’s impression of Jupiter-sized extrasolar planet. ESA – C. Carreau.