The possibility that Earth is not the only inhabited place in the Galaxy is something that fascinates many people. But is there any reason to believe there is life on other planets? In 1961, Frank Drake proposed to analyze this issue in a more scientific way. While we have made advances in this study, after half a century and thousand of millions of dollars spent we have no idea if we're close or not to an answer.
When speaking of that possibility the first thing that comes to the mind of many is that the Universe is very large. There are over 400 thousand million stars in our galaxy alone. And if there is just us, then it would be a huge waste of space.
In the '50s, with the invention of Radio Astronomy, we acquired the ability of interstellar communication. And immediately, attempts to listen if there yonder is someone broadcasting their presence began.
The pioneer in this field, Frank Drake, started to wonder whether there are grounds for optimism. Thus he considered some factors on which the success of his project depended. He concluded that they are basically seven. He wrote those in the form of an equation (now known as the "Drake Equation"), although we can look at it as a series of questions, some with known answers and others that we may never be able to answer. It served as inspiration for many independent investigations.
HOW MANY STARS LIKE THE SUN ARISE IN OUR GALAXY?
By using star census, we now know that the answer is one per year. The stars come out of large clouds of gas and dust in space, and that process continues to this day. Most of them are not like the Sun: they are much hotter, or colder, or live for a short time, or have very different chemical composition.
WHAT PERCENTAGE OF THESE STARS HAVE PLANETS?
Today we know that at least 7 %. Around some new stars disks of dust are visible, and even rings, that are thought to be a prelude to the formation of planets. Since 1995, by analyzing perturbations in the light of certain stars it was possible to detect dark bodies revolving around these. They are the size of Jupiter or Saturn (much larger than Earth), but new equipment, like the Kepler Space Telescope, can detect smaller planets.
WHAT IS THE NUMBER OF WORLDS WITH "SUITABLE" CONDITIONS, IN EACH PLANETARY SYSTEM?
We do not know. In our Solar System, Mars is another planet which is thought may have life. It's a desert now, but there are signs that in the distant past there were rivers up there, and even lakes or seas. Maybe somewhere, maybe underground, it still has oases that can accommodate living beings, no matter if microscopic. Those oases are what robotic missions, usually launched every two years, are looking for.
Also Europa, a satellite of Jupiter, could have liquid water. It is covered by a thick layer of water ice, so if there is sufficient underground heat, it could have liquid oceans, kilometers below the surface. It has been proposed to send space probes with special radars to test this theory.
A third candidate is Titan, a moon of Saturn. It is the only satellite in the Solar System with a dense atmosphere; its chemical composition is similar to what Earth was believed to have at the time when life arose here. The unmanned Cassini-Huygens mission, the most expensive in History, is studying this possibility. However, the temperatures are very low, although signs of geysers were detected on Enceladus, a neighboring satellite.
Also many comets and meteorites have complex chemicals that are favorable to life, and great clouds in interstellar space have them as well. But in most parts of the Galaxy the substances that are present are too simple.
Another factor is the orbits. Most of the planets discovered around other stars have highly elliptical orbits, so that their weather would be highly variable or unstable.
The presence of atmospheres in extra-solar planets has been verified, and by the next decade it is expected to build adequate space telescopes to search for planets with similar physical characteristics to those of the Earth.
WHAT PERCENTAGE OF THOSE "SUITABLE" PLANETS ACTUALLY DO HARBOR LIFE?
We do not know. Life on Earth came forth quickly after the environmental conditions were good. And it may also have arisen on Mars, for at that time it was very similar to our planet. There was even a controversy in 1997 about a Martian meteorite with purported fossilized bacteria. The feasibility of putting rockets on Mars to bring more reliable samples is being studied at this moment.
If oceans are confirmed on Europa, special probes to penetrate the ice could be sent. On Earth, there are living beings who live in extreme temperature conditions; as well as at great ocean depths where sunlight never reaches (hence there is no photosynthesis): even there, ecosystems do flourish, thanks to volcanic heat.
On the other hand, it is speculated that the Earth is unique only due to the fact that the Moon is so large that its gravity affects our planet significantly, perhaps decisively, in terms of its stability.
In a couple of decades we might have space telescopes capable enough to analyze the light of Earth-like planets for signs of chlorophyll or methane, products of living things.
WHAT PERCENTAGE OF THOSE PLANETS WITH LIFE ACTUALLY DO HARBOR INTELLIGENCE?
We do not know. Our definition of intelligence is basically: use of tools, communication capabilities, and abstract thinking (such as art and philosophy). But of all known living species, only one has these features. In fact, since life began here around 3800 million years ago, 50 thousand million different species have passed, but a clever one emerged only recently.
We know that evolution is influenced by catastrophic extinctions, such as that of the dinosaurs. Without it we might not be here. But if in a planet disasters are too frequent there would not be enough time for intelligence to develop.
WHAT PERCENTAGE OF THOSE PLANETS WITH INTELLIGENT BEINGS DEVELOP DETECTABLE TECHNOLOGIES?
We do not know. We consider detectable technologies to be Astronautics and Astronomy. The latter is easier to apply at interstellar distances.
Since the pioneering work of Frank Drake, dozens of projects have listened the depths of space, but only once (in 1974, with an encore a few years ago) we have sent signals. If "they" also only listen without speaking, a contact will be difficult.
HOW LONG THESE INTELLIGENT BEINGS REMAIN WITH INTERSTELLAR COMMUNICATION CAPABILITIES?
We do not know. Pollution brought by our Technology could soon make Earth uninhabitable. And there's always the risk of a Nuclear Holocaust.
On the other hand, Astronautics has the ability to turn us into a multi-planetary species. That would make the Earth no longer necessary for Life. In that case, the limit is only our genetic evolution into another species, maybe within a million years in the future. But possibly, the Homo sapiens' legacy will survive in these new descendants... forever.
With all the efforts mentioned, we have discovered the first two answers. Within a few years, or decades, or centuries, we will find the following ones. But we will get the last three answers only when we do find extraterrestrial intelligence. If it is never discovered, in all probability we will never know them. That's one of the flaws of the Drake equation. Therefore, Frank Drake himself adds: "No theoretical study can deduce the abundance of intelligent, technology-using, life in the universe. That abundance can be established only through the actual observation of such life."
If you wish to share this article with others, you can establish an Internet link, but you cannot copy any part of this page. Copyright © 2002-2013. Reproduction prohibited. All rights reserved.
Based on a lecture given at USP, on 2 February 2002. Originally published in ABC Color, on 23 April 2006. A slightly retouched version of this article, joined to many other related articles from this website making a compilation titled "Do you believe in UFOs?", is available for sale in electronic book format at http://www.amazon.com/dp/B00GF0REFI. Photomosaic: The Great Orion Nebula (M42) is a picture book of star birth, from the massive young stars giving shape to the nebula, to the pillars of dense gas which would be the birthplace of stars being born at this very moment. The bright central region is home to the four chunkier stars of this giant cloud of gas and dust in space. This group is called "The Trapezium", because all four are arranged in a trapezoidal form. Ultraviolet light released by these stars is digging a huge bubble in the nebula and disrupting the growth of hundreds of smaller stars. Near the Trapezium stars, there are located other stars young enough to have disks of material surrounding them, although this is a little difficult to see clearly in this picture. These disks are called "protoplanetary disks" because they are the raw material from which planetary systems arise. Credit: NASA / ESA / Massimo Robberto (Space Telescope Science Institute) and the Hubble Space Telescope Orion Treasury Project Team.