ASTR 1210 (O'Connell) Study Guide 23


Are we alone?
Are there billions of advanced lifeforms in the universe?

A. Perspective

The most powerful impact of astronomy on popular thinking in the last 150 years concerns something that it hasn't discovered yet and possibly never will: aliens.

The two questions listed at the top of the page frame the possibilities for the existence of other advanced species in the universe.

When you combine the ideas of life and the universe you reach a stunning conclusion no matter which way you argue:

Most astronomers subscribe to the latter view---that conditions on Earth are typical (or at least not uncommon)---which implies that extraterrestrial life is widespread and that there are many advanced lifeforms.

This is not just a modern point of view. It goes back to the ancient Greeks and was historically advocated by many other writers, e.g. Huygens, the discoverer of Saturn's moon Titan. See this translation of his book Cosmotheoros.

"What a wonderful and amazing Scheme have we here of the magnificent Vastness of the Universe! So many Suns, so many Earths..."
                                                    --- Christiaan Huygens (1698)

A revolution in prospects for astrobiology

Several remarkable discoveries lend credence to this picture:

  1. 1995+: The discovery of planetary systems around nearby stars (see Guide 11). We have discovered hundreds of other planetary systems in the last 18 years. Apparently, many stars like the Sun have planets. Despite the technical challenges, we have already identified several Earth-size candidate planets. Most astronomers are confident that Earth-like planets will be found soon in the "habitable zones" around other stars.

  2. 1996: Evidence suggesting the presence of fossil microorganisms on Mars (see Guide 17). This claim is highly controversial. But right or wrong, it has stimulated the development of vigorous research in the rapidly growing field of "Astrobiology."

  3. The discovery of water-rich environments on Mars (in the past) and under the surfaces of Europa and Enceladus. And the unique but promising hydrocarbon-rich surface of Titan.

These breakthroughs, coming after centuries of speculation, have changed the whole character of the study of exterrestrial life.

B. Life on Earth

Just as our "common-sense" perspective provides no clue to the scale of the real universe, our naive assumptions about the nature of life on Earth, as they existed at the beginning of the scientific age, have been shattered by the evidence:

350 million years of evolution on Earth: characteristic terrestrial lifeforms
from the Cambrian (500 Myr ago, left) to the Jurassic (150 Myr ago, right) periods

C. Evolution

The proliferation and diversification of lifeforms is produced by evolution through natural selection for better adapted types (Darwin, Wallace 1858)

Despite the raging "creationism," "intelligent design," and other controversies surrounding Darwinian evolution that get prominence in the media, biological evolution is as well established a basic fact of science as any other, e.g. that Earth is a planet or that the Sun is a star.

All the basic "predictions" of Darwinian evolution have been thoroughly confirmed in the last 150 years of biology, physics, astronomy, geophysics, and paleontology. The evidence is overwhelming.

The astronomical evidence for evolution of the universe and its contents over a period of 10-15 billion years is as strong as, but entirely independent of, the biological & paleontological evidence for evolution of life on Earth. See Study Guide 2 and links therein.

Controversies over the reality of evolution are confined to political, religious, & education circles.

Most anti-evolution arguments are conceptually medieval. You can disregard evolution only if you are prepared to disregard the rest of modern science and scientific thinking. Beware of those who urge you to do this.

D. Origin of Life on Earth?

Molecular evolution from simple, abundant, pre-organic chemicals

Alternative: panspermia (seeding of Earth from an external source, accidental or deliberate)

Size of the "habitable zone" for Earthlike planets surrounding four different types of stars;
"F" stars are more massive than the Sun, "K" and "M" are less massive

E. Life Elsewhere in Our Solar System

Are there plausible biospheres elsewhere in the Solar System?


Habitable Zone

Possible biospheres on the outer satellites: (Left) The icy surface of Europa (pseudocolor, Galileo mission); (Middle) The water vapor geysers of Enceladus (pseudocolor, Cassini mission); (Right) The hydrocarbon-rich surface of Titan, as viewed by the descending Huygens probe.


  1. Venus: no! High temperature, pressure, and corrosive atmosphere are sufficient to sterilize surface of all Earth-like life.

  2. Mars: plausible evidence for biosphere > 1 Byr ago with abundant water; SNC meteorites provide some evidence for microorganisms. Too cold and dry now for life? Absence of ozone in atmosphere allows damaging solar UV flux at surface.

  3. Jupiter, Saturn atmospheres? Results from the Galileo probe (1995), which sampled the outermost layers of Jupiter's atmosphere, were not promising but don't exclude a biosphere.

  4. Europa (J) and Enceladus (S) each have evidence for a liquid water reservoir/ocean lying beneath the visible crust of ice. These are probably the most promising sites for bio-exploration after Mars.

  5. Titan's (S) extraordinary hydrocarbon-rich atmosphere is a possible biosphere. The Cassini mission has demonstrated the presence of liquid hydrocarbons (methane, ethane) on Titan, and some scientists think "methanogenic" lifeforms might exist there. Most believe the very low temperatures (-180 degrees C) would preclude living organisms on the surface. As in the case of Europa and Enceladus, however, deep reservoirs with more favorable temperatures are possible. Here is a prospectus for exobiology on Titan.

  6. Comet nuclei: these icy bodies, often with an apparent organic molecule coating, could act as "portable reservoirs" of organisms; however, temperatures are normally very low.

Overall: primitive lifeforms are possible in several settings, but remote detection is unlikely. We must search "in situ."

F. Intelligent Life Elsewhere


This is a fascinating but virtually 100% speculative subject. There is a paucity of facts, understanding, and imagination, and this leads to a wealth of conjecture and controversy.

The Drake Equation

The Drake Equation, named after astronomer Frank Drake, was a first attempt to estimate the number of advanced technical civilizations in our Galaxy capable of undertaking interstellar communication. Here are the elements in the Drake estimate based on our current understanding:

  1. There are 100 billion stars in our Galaxy

  2. Assume 0.001% - 5% of the stars have Earth-like planets.

      Support? Recent detection of extra-solar planets. Even though most of these are Jupiter-class, rather than Earth-size, most astronomers expect that the fraction of stars with Earth-like planets will prove to be of order 5% or larger. This is at the high end of the range considered plausible over the last 50 years.

  3. Assume all develop life leading to advanced civilizations (i.e. assume Earth is average)

      "Earth is average" is the Copernican assumption, which has proved so successful in studies of the structure of our inorganic universe. However, we have very little intuition here, and some biologists would argue that the chances of developing technological species are small.

  4. Assume the communication phase lasts 10,000 years.

      Note: we have only recently entered this phase. It has been 80 years since we developed commercial radio stations that could be detected over interplanetary distances. Our artificially generated EM radiation is the most definitive marker of advanced lifeforms on this planet. Human EM signals are now propagating out through the Galaxy, with the most distant signals being about 80 light years away.

  5. Combine all above. ===> 10 - 10,000 communicating civilizations in our Galaxy

  6. ===> Distance to nearest: 10,000 - 1000 light years

The quantities entering the Drake calculation under items (3) and (4) are highly uncertain and controversial. But the point is that no one can presently exclude the possibility that a large number (10,000!) of advanced civilizations currently reside in our Galaxy. The number is large because the Galaxy is huge.

Interstellar migration/exploration:

The estimated separation between advanced civilizations, 1000-10000 LY, is obviously a large distance. But it could be traversed with foreseeable technology on cosmically short time scales. (Remember that 10 million years is a "short time" in the cosmic context!) Even at propagation velocities far below the speed of light, stable civilizations could rapidly explore the Galaxy.

"Where are they?"

SETI = "Search for Extraterrestrial Intelligence"

The best alternative to interstellar travel to assay the prevalence of life in the nearby universe would be EM communication, probably in the radio or optical EM bands

Several passive listening, radio search programs are active. Most sophisticated are those operated by the SETI Institute, once a well-financed NASA program but killed by Congressional scepticism and now running on private contributions.

G. UFO'S, Alien Artifacts

Some people, most not active scientists, claim that there is actually good evidence for spacefaring aliens in our solar system. Naturally, there is much more space on the Internet devoted to these claims than to more sober, evidence-regarding work:

H. The Recognition Chasm

There is a much more fundamental problem in communicating with alien civilizations (or even recognizing them in the first place) than their distance from us.

Reading for this lecture:

Web Links:

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Last modified December 2013 by rwo

Text copyright © 1998-2013 Robert W. O'Connell. All rights reserved. Movie poster captured from the Internet Movie Data Base. Habitable zone drawing copyright © Brooks/Cole-Thomson. Starship painting by Don Davis. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.