Open Questions: Extraterrestrial Life

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Introduction

Where are they?

What are they?

Can we talk?

Possible life in our Solar system

Search for extraterrestrial intelligence

Exobiology

Recommended references: Web sites

Recommended references: Magazine/journal articles

Recommended references: Books

Introduction

Life exists on Earth. We're pretty sure of that. Whether there is intelligent life on this planet is still an open question.

But let's assume, for the sake of discussion, that there are intelligent life forms on Earth. Once such life forms come to realize that the home planet is not unique and that there are at least some, and possibly very numerous other planets and possible habitats for life, then it's natural for them to ask three questions:

Does life exist elsewhere?
If so, does intelligent life exist elsewhere?
If there is intelligent life elsewhere, can we communicate with it?

What is "intelligent" life, anyway? As a working definition we might take it to mean a form of life that is capable of asking those questions. On this definition, then, there is "intelligent" life on Earth.

Another definition, however, would require of intelligent life the ability to communicate with other intelligent life. In this more restrictive sense (at least) the question of the existence of intelligent life on Earth is still open.

Where are they?

Let's not forget that our first question was whether any life at all exists elsewhere. It's easy to lose sight of that, because it may be easier to answer the following question as to whether intelligent life exists elsewhere. This is because, except for planets (and some of their satellites) in our own Solar System, it could be a rather long time before we are able to detect the existence of rudimentary life forms elsewhere. But intelligent life should be much easier to detect even at interstellar distances, even if we can't actually "communicate" with it.

Naturally, then, most of the focus has been on the question of the existence of extraterrestrial intelligent life.

Site indexes

Open Directory Project: Extraterrestrial Life: Categorized and annotated links. A version of this list is at Google, with entries sorted in "page rank" order. May also be found at Netscape.
Galaxy: Extraterrestrial Life: Categorized site directory. Entries usually include descriptive annotations. Has a subcategory for Fermi's Paradox.
Galaxy: SETI: Categorized site directory. Entries usually include descriptive annotations. More listings here.
Galaxy: Astrobiology: Categorized site directory. Entries usually include descriptive annotations.
SETI SuperStar Award Winning Websites: List of SETI Web sites that have been selected monthly since May 1999 for special merit by the SETI League.
Astrobiology: Related Links: Short list from the Astrobiology at NASA site, mostly to other NASA sites.

Sites with general resources

New Scientist Special Report on Astrobiology: Primarily offers links to many news articles from the past several years of New Scientist magazine. But there are also other features, including facts and figures, external links, frequently-asked questions, and a short bibliography.
Astrobiology Magazine: Contains a wealth of information on astrobiology and space science in general. This includes news stories, overview articles, image galleries, external links, and discussion forums. Some of the topics covered are origins of life, extrasolar planets, SETI, planetary science, extremophiles, and stellar evolution.
Yahoo News Full Coverage: Search for Extraterrestrial Life: Links to recent news stories from various sources. Also includes links to sites dealing with the search for extraterrestrial life.
The Astrobiology Web: Impressive site dedicated to information on life in the universe. Resources include news and external links. Maintained by the Discovery Channel.
BBCi: Space: Life: Good collection of general information on life in the universe, from the BBC. Topics include how life starts, the search for extraterrestrial life, and the seaarch for extraterrestrial intelligence.
SETI Institute: "The SETI Institute serves as an institutional home for scientific and educational projects relevant to the nature, distribution, and prevalence of life in the universe." Site contents include news, bibliography, and general SETI information.
SETI@home: Cooperative distributed computing project to assist in the search for extraterrestrial intelligence.
Sky and Telescope's SETI Section: Contains news and background articles on SETI by varions contributors.
Astrobiology at NASA: NASA site dealing with programs and news related to research on astrobiology. The site has many features, such as news, mission information, and external links.
NASA Astrobiology Institute: "NAI carries out collaborative research and education in astrobiology.... It supports investigation of these issues on Earth and serves as a portal to space for the scientific community." The site offers a newsletter, an extensive series of tutorial articles, an ask an astrobiologist feature, and some external links.
Genetics and Astrobiology: Good collection of resources on the topic -- external links, magazine articles, and books.
Life On Other Planets in the Solar System: Educational site that covers topics like extreme environments for life on Earth, origins of life, bacteria & archaea, and astrobiology. There are many external links in each topic area.
Life in the Universe: Good site with many resources, sponsored by several European scientific organizations.
Life in the Universe: Page describing involvement of the European Southern Observatory in extraterrestrial life research.
New Scientist: Astrobiology: Collection of New Scientist news stories on astrobiology.
SETI Central: Articles and news related to SETI from a variety of sources. A WebSeed site.
The Planetary Society: SETI Page: General news and information on SETI, with emphasis on projects sponsored by The Planetary Society. Has good history and overview articles. There is also information on the Society's SETI Optical Telescope.
Jodrell Bank Observatory SETI Research: Pages that describe the Observatory's SETI research. They include a history of SETI, common questions and answers, and an explanation of the Drake equation.
The SETI League: Web site of an "international grass-roots organization dedicated to privatizing the electromagnetic Search for Extra-Terrestrial Intelligence." The site includes general information on the organization and SETI. There is an archive of publications, such as newsletters, journals, press releases, and photos. The organization also selects other SETI sites for a special award.
Princeton Optical SETI Project: "The Optical SETI (OSETI) project at Princeton University, working in collaboration with Harvard University, searches nearby stars in an effort to detect brief, high-powered laser pulses that might signify the existence of extraterrestrial life in other solar systems."

Surveys, overviews, tutorials

Extraterrestrial life: Article from Wikipedia. See also Astrobioloby, Xenobiology, SETI, Drake equation, Fermi paradox.
SETI Institute/Space.com articles: A series of articles about SETI and (sometimes distantly) related subjects, published weekly since Nopvember 2000 by the SETI Institute and Space.com. The most relevant articles provide excellent information on astrobiology and SETI..
Ask a High-Energy Astronomer: Astrobiology: Common questions, with answers, provided by NASA's Ask a High-Energy Astronomer service.
Frequently Asked Questions in Astronomy: Extraterrestrial Life: Questions and answers from the Usenet sci.astro newsgroup.
Ad Astra Magazine: Astrobiology: January/February 1999 issue of Ad Astra Magazine. Contains 10 articles unedited for size constrains of the printed edition.
Our Greatest Quest: Good July 2003 article by Martin Rees in New Scientist, discussing the possibility of the existence of intelligent extraterrestrial life.
An Inhabited Universe?: Overview by Seth Shostak.
Life beyond Earth: September 2003 feature produced by the Australian Broadcasting Corporation on the science of astrobiology.
Astronomy - SETI: Explanation of Drake's equation.
Searching For Aliens: July 2006 interview with Chris McKay of NASA on the subject of astrobiology and how to detect it.
Are We Alone?: Special feature story from ABC News.
The Search for Extraterrestrial Life: Part of the SEDS site. Contains overview information and external links.
The search for life on Mars: August 2004 article from News@nature.com that examines possible evidence -- the existence of methane in the atmosphere -- for life on Mars.
8 Worlds Where Life Might Exist: March 2006 article from Space.com. Seth Shostak discusses the most likely locations in our solar system for the existence of some form of life.
NASA Scientist Sees Possible Mat of Martian Microbes: August 2004 article from Space.com. Discusses controversial speculation of NASA scientist David McKay that a microbial mat may have been seen among photos returned by the Mars Rover mission.
Life on Mars Likely, Scientist Claims: August 2004 article from Space.com. Presents views of astrobiologist Gilbert Levin, which are partially based on current Mars Rover findings, but not widely accepted by other researchers.
The New Hunt for Life on Mars: March 2004 article from Space.com. Describes the current state of knowledge of factors affecting the possibility of life on Mars.
Mars Underground: The Harsh Reality of Life Below: March 2004 article from Space.com. Discusses the possibilities of life, similar to Terran extremophiles, living deep below the surface of Mars.
Mars Gullies Could Harbor Martian Biology: April 2003 article from Space.com. Discusses how the form of observed gullies on the surface of Mars may indicate the existence of conditions favorable to life.
Mars Water, Odd Surface Features Tied to Life: March 2003 article from Space.com. Discusses recent observations that indicate the possiblity of life on Mars.
Amid the Universe's Chaos, a Few Habitable Places: May 2002 article from Space.com. Explains the concept of the "galactic habitable zone", and why planets with intelligent life could be quite rare.
The Search for the Scum of the Universe: May 2002 article from Space.com. Discusses various problems with searching for planets that may support life, including the fact that very little is known about the origins of life on Earth.
The Race to Find Life: February 2002 article from Space.com. Describes different approaches to searching for extraterrestrial life.
Is There Life Beyond Earth?: January 2002 article from Space.com. Discusses the possibilities of extraterrestrial life, and how it may be very different from life on Earth.
Scientists Say Mars Viking Mission Found Life: July 2001 article from Space.com. Discusses controversial opinion of a few scientists that the 1976 Viking mission really did find evidence of life on Mars.
Life On Mars: Swimming Right Under the Surface?: July 2001 article from Space.com. Discusses the possibilities of simple forms of life on Mars.
Rethinking Viking: The Life on Mars Debate Rages On: July 2001 article from Space.com. Discusses ambiguous findings of the 1976 Viking mission and new missions planned to search for traces of life on Mars.
Mars or Europa: Where Does Life Exist?: March 2001 article from Space.com. Provides answers from three astrobiology experts to questions about the possibilities of finding other forms of life in the Solar System.
Looking for Aliens: July 2001 Scientific American In Depth article that assumes the most common forms of extraterrestrial life will be as simple as bacteria, but quite possibly harder to detect.
They Came from Outer Space: Real Aliens: Article by biologist Mia Molvray about what aliens might be like.
Searching for Life in Other Solar Systems: March 1998 article by Roger Angel and Neville J. Woolf in a special issue of Scientific American. Main focus is on development of a space-based interferometer for studying extrasolar planets.
The Search for Extraterrestrial Intelligence: Extensive January 1997 article from Scientific American online, by Carl Sagan and Frank Drake. "There can be little doubt that civilizations more advanced than the earth's exist elsewhere in the universe. The probabilities involved in locating one of them call for a substantial effort."
Life on Mars?: NASA site about the 1996 announcement of suspected evidence for life on Mars found in a meteorite that originated on Mars. (Subsequent investigation has not confirmed this as evidence of Martian life.)

Recommended references: Magazine/journal articles

The Color of Plants on Other Worlds
Nancy Y. Kiang
Scientific American, April 2008
Did Life Come from Another World?
David Warmflash; Benjamin Weiss
Scientific American, November 2005
Life on Mars?
Carl Zimmer
Smithsonian, May 2005, pp. 76-83: Identifying the earliest signs of life on Earth has been difficult enough, with some purported discoveries being seriously challenged. But researchers are hard at work figuring out how to search for evidence of life on Mars.
[Article in PDF format]
Using Lasers to Detect E. T.
Steve Nadis
Astronomy, September 2002, pp. 44-49: There's no good reason an advanced extraterrestrial civilization would use radio frequencies to signal its presence instead of infrared or visible light. Within just the last few years searches have begun for such signals -- which might be generated by laser pulses that could be (briefly) a million times brighter than a star.
No Place Like Zone
Mark A. Garlick
Astronomy, August 2002, pp. 44-51: There may be regions of a galaxy which favor the formation of planets likely to develop advanced forms of life, due to factors such as availability of a sufficent quantity of metallic elements. But the theory remains controversial and may reflect overly "Earth-centric" notions of conditions required for life.
Refuges for Life in a Hostile Universe
Guillermo Gonzalez; Donald Brownlee; Peter D. Ward
Scientific American, October 2001, pp. 60-67: It has long been recognized that planets capable of supporting life -- especially relatively advanced life -- require certain favorable conditions within their solar system. It has recently become apparent that a "good" location within the galaxy is also important.
Did Life on Earth Come from Mars?
Robert Irion
Discover, August 2001, pp. 64-69: The chance that some form of life could tarvel on meteorites between planets of the Solar system is larger than one might suppose. If such exchange of living organisms between planets is not uncommon, it could help preserve life in a planetary system from catastrophes occuring on the "home" planet.
Where Are They?
Ian Crawford
Scientific American, July 2000, pp. 38-43: If advanced civilizations exist elsewhere in the galaxy, they should develop the ability so spread throughout the galaxy in only a few million years. As Enrico Fermi first noted, this means we should have seen more evidence of such civilizations than we have.
Is There Life Elsewhere in the Universe?
Jill C. Tarter; Christopher F. Chyba
Scientific American, December 1999, pp. 118-123: The dedicated search for signs of extraterrestrial life and intelligence has barely begun. But the search, by a variety of means, is due for rapid acceleration.
Are We Alone?
David J. Eicher
Astronomy, November 1999, pp. 60-63: Several researchers who have studied the issues involved in SETI discuss opinions, both optimistic and pessimistic, on the likelihood of discovering extraterrestrial intelligence.
Who's Out There?
Jeff Greenwald
Discover, April 1999, pp. 64-70: New techniques and technology continue to be deployed in the search for extraterrestial intelligence. One of the latest involves using home computers to process data.
Diving into Europa's Ocean
Michael C. Milstein
Astronomy, October 1997, pp. 38-43: Jupiter's satellite Europa seems to have a very thick surface of water ice. Beneath this may be an ocean of liquid water that could harbor life.
Looking for Life in All the Wrong Places
Will Hively
Discover, May 1997, pp. 76-85: Various terrestrial life forms live in extremely inhospitable environments such as Antarctica, Chile's Atacama desert, and Siberian permafrost. These environments are not so different from those found elsewhere in the Solar system.
Water World
Kathy A. Svitil
Discover, May 1997, pp. 86-88: Jupiter's satellite Europa seems to be covered by a thick shell of water ice, under which may lie oceans of liquid water. Life could have developed there given adequate sources of heat, such as submarine volcanoes.

Recommended references: Books

Stephen Webb – Where Is Everybody? Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life
Copernicus Books, 2002: The title alludes to Fermi's question – if intelligent extraterrestrial life exists in our galaxy, why haven't they shown up yet in an obvious way? Webb considers 50 possible answers in three different categories. Along the way, he explains a lot of exobiology. Just so you know what you're getting with this book, Webb's own conclusion is that the conditions for life on Earth are so special that there are no intelligent extraterrestrials.
Peter D. Ward, Donald Brownlee -- The Life and Death of Planet Earth: How the New Science of Astrobioloty Charts the Ultimate Fate of Our World
Henry Holt and Company, 2002: It's possible to read this book as a sensationalistic account of all the terrible things that can (and will) happen to wipe out life on Earth eventually. But the real interest of it lies in what can be said about the limits put on the lifespan of any habitable planet by natural and inevitable catastrophes.
David Darling -- Life Everywhere: The Maverick Science of Astrobiology
Basic Bookis, 2001: The author covers, in a relativly short space, most of the important aspects of astrobiology. Important topics include how to define life, how life could have arisen, what conditions are necessary for life, and what is presently known from astronomy about places where life could exist. A key question is whether life is common or rare in the universe.
Monica Grady -- Astrobiology
Smithsonian Institution Press, 2001: This introduction is short and sweet. Fewer than 100 pages, with numerous pictures and diagrams. There's not much space to provide more than an outline of its major topics: the possible origins of life, life on Earth (especially extremophiles), other possible life habitats in the Solar System, and methods of searching for extraterrestrial life. But that job is done well.
Peter D. Ward; Donald Brownlee -- Rare Earth: Why Complex Life is Uncommon in the Universe
Springer-Verlag, 2000: Unlike many earlier investigations of the possibility of extraterrestrial life, the present volume goes into great detail to explain the many reasons why advanced (specifically, animal) life forms may not be a very common occurrence in the universe. Recent findings emphasize that simple forms of life such as bacteria developed very quickly on Earth and are capable of living in very hostile environments. But, on the other hand, it may be that a large number of rather unlikely circumstances are necessary for the eventual evolution of animal life. The circunstances that may be necessary for a planet like Earth to allow for the emergence of animals include a very precise orbital distance from the Sun, the operation of plate tectonics, the possession of a moon which is an appreciable fraction of the mass of the planet, and the existence in the solar system of a massive planet like Jupiter in a regular, circular orbit.

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