READING: Chapter 18
Astrobiology is a discipline that studies the nature and origin of life in the Universe. A discussion of astrobiology therefore usually begins with some definition of WHAT IS LIFE? I'm going to take a definition from the book "Life in the Universe" by Bennett, Shostak, & Jakosky. (Some of the below text is taken directly from that textbook.)
Defining life is a difficult task, and we must note a couple caveats here. There are exceptions to some of these rules for some living things. For example, a sterile animal like a mule is clearly a living organism, even though it can't reproduce its species. Viruses are also an interesting case, they can't reproduce in isolation, but they can reproduce once they have infected a host organism. Also, many non-living things meet several of these criteria, but clearly aren't alive. For example, a car is an ordered structure that utilizes energy, but obviously is not alive. A fire grows and develops, utilizes energy, and responds to its environment, but it is not alive.
HOW DID LIFE ON EARTH BEGIN?
(Option 1) The most basic building blocks of life are AMINO ACIDS and NUCLEOTIDE BASES. Experiments (for example, the Miller-Urey experiment in 1953) have shown that beginning with very simple primordial materials (water, methane, carbon dioxide, and ammonia), which all existed on the early Earth, adding energy (approximating the effect of lightning) leads to the formation of amino acids and nucleotide bases. Simple life forms fitting the definition above have never been produced in one of these experiments, but these experiments demonstrate that complex, organic molecules probably formed in this way on the early Earth.
(Option 2) An alternative option to life beginning on Earth has begun to be studied. The idea of "PANSPERMIA" is that simple life forms can apparently survive the harsh conditions of space, so perhaps life arrived on Earth via a comet or meteor impact that spread microbes from another location in the Solar System or a nearby star. There is an article by Seth Shostak that summarizes some recent research on this topic. Some researchers have even suggested that SARS may have originated on another body in our Galaxy and come to Earth from space!
IS THERE LIFE ANYWHERE ELSE IN OUR SOLAR SYSTEM?
We have found evidence of life in the most extreme environments on Earth (This article from 1997 gives a brief introduction to "extremophiles". This much more recent APOD discusses the possibility of life in a frozen lake in Antarctica, too.) This has led researchers to believe that simple life forms can probably exist in some harsh environments, like on parts of Mars, in the icy moon of Jupiter called Europa, and perhaps on Titan, a moon of Saturn.
Let's start with the Drake Equation. In the early 1960's, Frank Drake proposed a simple equation for estimating the NUMBER OF CIVILIZATIONS IN THE MILKY WAY PRODUCING DETECTABLE ELECTROMAGNETIC SIGNALS.
I'm actually going to use a slightly simpler version of the Drake equation, which again comes from the Bennett et al. book, "Life in the Universe".
The first thing we need to know is, how many habitable planets are there in the galaxy?
The HABITABLE ZONE is the area around a star where liquid water can exist. It is largest for A- and F-type stars, and gets smaller for G, K, and M stars. We do not think that O and B stars will survive long enough for life to develop on their planets. Here is a comparison of the habitable zones for different stars (Figure 18.8 from C&M):
We should probably limit this number a bit more. There is probably a "Galactic habitable zone", too. Stars in the bulge are probably less likely to have habitable planets, because of the intense high energy radiation from the Galactic Center. Frequent gravitational interactions with nearby stars in the high density bulge region are also likely to greatly influence conditions on planets surrounding these stars, preventing intelligent life from evolving there.
Like all of the terms in the Drake equation, we can argue about the number of habitable planets in the Galaxy. The most pessimistic view is that the Earth is somewhat unique, and other planetary systems are very different from ours. Certainly, the planets that have been discovered so far around other stars are not like the Earth. So perhaps Earth-like planets are rare? The more optimistic view is that all stars form planets, like our Solar System, about 10 planets per star is average and about 3 out of 10 may lie in the habitable zone. If this is the case, there might be hundreds of billions of Earth-like planets in the Galaxy.
Next: What fraction of habitable planets develop life?
There are both pessimistic (close to 0%) and optimistic (close to 100%) views here, too. Our only guidance is to consider the Earth. Experiments by astrobiologists seem to suggest that life originated on Earth somewhat easily. That is, if you were to create a planet just like Earth somewhere else in the Galaxy, we assume that simple life would develop there, too. So let us assume that this number is closer to 100% instead of 0%.
Next: What fraction of life bearing planets gave rise to intelligent civilization, capable of communication at some time in its history?
This is another question where you can pessimistically assume that the conditions on Earth are unique, and we are the only planet in the Galaxy to develop civilization. However, most of the planets in our Galaxy are older than ours, so if civilizations just need time to evolve, it is possible that some planets have developed several different civilizations over their lifetimes.
Finally: What fraction of planets with civilizations have them at the present time (taking into account the light travel time)?
This last factor determines how many civilizations we can possibly try to listen for at the present time. What determines this fraction is how long we expect an intelligent civilization capable of communicating with us will remain on its planet. If an intelligent civilization lasts 1 billion years once it is born on a planet, than the chance of us being able to detect one other civilization is 1 billion years / lifetime of Galaxy = 10%. If civilizations only last 1 million years, then it is 1 million years / lifetime of Galaxy .01%. On this question, we have to decide how long we think our civilization on Earth will last. Taking into account possible natural causes (giant meteor impacts, for example) as well as human causes (nuclear war) we might pessimistically think our civilization may only last a short time longer.
To summarize:
If the number of habitable planets is high enough (100 billion) then, even if the three fractions are all small (1/1000 planets develop life, 1/1000 planets that develop life give rise to intelligent civilization, 1/1000 civilizations are alive now) then the total number of civilizations we might contact is 10!
Based on optimistic calculations like this one (or even more optimistic numbers that suggest that there might be 1,000 civilizations in the Galaxy today), scientists started listening for extraterrestrial civilizations in the 1960s. In this article, the SETI League briefly lists all of the experiments that have searched for Extraterrestrial Intelligence over the years. In class, we will talk about a few of these.
The distances between stars are so large, that actually travelling between them to visit other civilizations is impractical. For example, given our current technology, if we launched a satellite towards the nearest star, it would take 50,000 years to arrive. The only practical way for us to communicate with other civilizations or vice versa, is to use light, since the speed of light is the fastest way to traverse interstellar distances. If a civilization near Alpha Centauri beamed a message to us using light, it would only take 4 years for it to arrive. The next question is then, what type of light should we be looking for?
Well, we assume that other intelligent civilizations have similar scientific understanding as us. So astronomers believe that other civilizations would beam a message to us at a frequency where we would be most likely to be looking. We think that extraterrestrial civilizations might broadcast a signal at a wavelength close to the 21cm wavelength emission line of interstellar atomic hydrogen.
In 1960, Frank Drake started PROJECT OZMA, which used the 85 foot radio telescope in Green Bank, WV to search for radio beacons from ET civlizations. He used his time to stare at two nearby, sun-like stars, but found nothing. In 1974, scientists broadcast a signal towards a globular cluster in our Galaxy just in case any civilizations in that part of the sky are listening for signals from Earth. In 1977, the Ohio State University "Big Ear" telescope detected a strong signal, called the "Wow Signal", that was a good candidate for a real ET signal. Unfortunately, many follow up studies have failed to detect another signal from the same part of the sky. Today, several radio SETI searches continue. Astronomers are using the 300 meter Arecibo radio telescope in Puerto Rico, the 140 foot diameter radio telescope in Green Bank, WV, and the 64 meter Parkes radio telescope in Australia to search for signals. Some searches target specific, sun-like stars, while others scan the sky at random to search for signals coming from anywhere in the sky. Some of the largest of these endeavors require enormous amounts of computer power to process the data to search for real signals. You may have heard of SETI@home, a project that lets people around the world donate their computer time to the search for SETI signals.