ASTR 1210 (O'Connell) Study Guide 23


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

From the history of science, it's obvious that human beings have a deep urge to understand nature and the universe around them. We are the first species on our planet to have the desire and capacity to confront questions on a cosmic scale. The first recorded serious explorations were made over 2500 years ago, and since then we have invested tremendous effort into investigating basic questions about the mechanisms of nature and the place of humans in the universe.

Science has made great progress by insisting on strict standards of evidence. And, after 500 years, we now have a very good, if still preliminary, understanding of many aspects of the universe. The length of time it took to get meaningful answers indicates how difficult it is to obtain reliable knowledge on this scale.

One of the biggest unanswered questions concerns the presence of other, non-terrestrial lifeforms in the universe. In fact, the most powerful impact of astronomy on popular thinking concerns something that it hasn't discovered yet and possibly never will: aliens.

So, in this lecture we examine the questions of whether there is life elsewhere in the cosmos, where it might be, and what forms it might take.

A. Perspective

The two questions 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.

A revolution in prospects for astrobiology

B. Life on Earth

Just as our everyday "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, and sometimes state legislatures, 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?

As just noted, we do not have a good understanding of how life originated on Earth. Given what we do know, however, it is plausible that the chain of life began at the molecular level with molecular evolution from simple, abundant, pre-organic chemicals.

An alternative to a terrestrial origin for life is panspermia: the seeding of Earth from an external source, accidental or deliberate.

The "habitable zones" for Earth-like planets surrounding four different types of stars are shown in light blue;
"F" stars are more massive and hotter than the Sun, "K" and "M" are less massive and cooler.
The volume of the habitable zone increases for hotter stars.

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! The high temperature, pressure, and corrosive atmosphere are sufficient to sterilize the surface of all Earth-like life.

  2. Mars: There is plausible evidence for a biosphere > 1 Gyr ago with abundant water; SNC meteorites provide some evidence for microorganisms. Too cold and dry now for life? The absence of ozone in the present-day atmosphere allows damaging solar UV flux to penetrate to the 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. Ganymede, Jupiter's largest moon, has just recently been shown to contain a subsurface ocean.

  5. Titan (S) has an extraordinary hydrocarbon-rich atmosphere and surface, which is a possible biosphere. The Cassini mission 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, Ganymede, 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 a coating of organic molecules, could act as "portable reservoirs" of organisms; however, temperatures are normally very low.

The existence of advanced lifeforms anywhere else in our Solar System would astonish most scientists but is not completely impossible. On the other hand, primitive lifeforms are certainly possible in several settings, but remote detection is unlikely. We must search "in situ," which implies complicated and costly space missions.

F. Intelligent Life Elsewhere


The Drake Equation

Interstellar migration/exploration:

The Fermi "Paradox"

SETI = "Search for Extraterrestrial Intelligence"

G. Alien Artifacts?

Most people assume that if spacefaring aliens are active today in the vicinity of the Earth, or had arrived anytime over the last few 100 million years, there should be concrete evidence of that visitation -- artifacts of extraterrestrial intelligence. In fact, you can find many gigabytes on the Internet devoted to claims that there is "good" evidence for visitations. But, so far, these have not stood up under scrutiny.

Here are two prominent examples of a supposed alien presence in the Solar System:

Astronomers have also considered the possibility of detecting alien artifacts other than deliberate signals beyond the Solar System. For instance, an advanced civilization might have created a mega-structure to capture a large fraction of the energy output of their host star. Such a structure is called a Dyson Sphere, and its presence could be indicated by an unusual near-infrared flux excess not typical of normal stars. Deliberate searches among databases from infrared sky surveys have not yet turned up any viable candidates.

However, in 2015, a fascinating candidate for a Dyson-like structure emerged from the Kepler Mission exoplanet survey:

Evidence of alien technology, whether in the form of a communication signal or an artifact, would now be called a technosignature, and a 2018 NASA conference reviewed the status of work in this area.

H. The Recognition Chasm

There is a much more fundamental problem in communicating with alien civilizations than their distance from us. It presents major obstacles to even recognizing them in the first place.

Reading for this lecture:

Web Links:

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Last modified July 2019 by rwo

Text copyright © 1998-2019 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.