Astrobiology

I

Introduction

Astrobiology or Exobiology, study of the origin, evolution, distribution, and future of life in the universe, including life on Earth. The term exobiology may be used interchangeably with the term astrobiology. Some scientists, however, restrict exobiology to mean only the study of how life might exist beyond Earth (extraterrestrial life). Exobiology is accepted as a study area within astrobiology in both approaches.

Astrobiologists investigate how the formation of stars and solar systems led to the existence of planets suitable for life and how life originated on Earth and perhaps elsewhere. Astrobiologists also explore which factors have influenced biological evolution in the past and in the present, or may influence evolution in the future. The understanding of these events shapes the study of how life arises and evolves in the universe.

Astrobiology brings together a wide range of scientific fields within space sciences, planetary sciences, Earth sciences, chemistry, and life sciences, including astronomy, microbiology, molecular biology, ecology, and paleontology. The term astrobiology comes from the Greek word astron (“star”) as in astronomy, combined with biology, the scientific study of life; exobiology comes from the Greek prefix exo- (“outside”), referring to a perspective on life that includes the possibility of life beyond Earth.

II

The Probability of Life in the Galaxy

Earth is the only planet that we know harbors life. Nonetheless, the basic chemicals and processes needed for life appear to be widespread in our Milky Way Galaxy and in the universe beyond. A significant number of scientists think that some form of life beyond Earth is possible, even probable. Astrobiologists can use their knowledge about life on Earth to guide their search for extraterrestrial life.

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A

Conditions for Life

Life elsewhere in the universe might also form near a star like our sun. The Sun is an average star, bright and hot enough to warm the inner planets but calm and cool enough that Earth is relatively safe from some forms of destructive radiation. Most importantly, our sun has been stable for billions of years. Life would also benefit from a planet like Earth, large enough to provide the gravitational force to hold an atmosphere. The atmosphere protects the surface against radiation and rapid temperature changes and holds elements that may be important to sustaining life. The atmosphere also allows water to exist in liquid form on the surface.

The combination of a suitable star and planet might be vital to the formation of life. Scientists use the term “habitable zone” to describe regions around a star where suitable planets might enjoy Earthlike conditions of temperature or radiation exposure. For a star that is cooler and smaller than the Sun, the habitable zone would be closer in than in our solar system while a star hotter than the Sun might have a habitable zone farther out than in our solar system. The recent discovery of ocean waters deep beneath the frozen surface of Europa, a moon of Jupiter, suggests that other habitable zones are possible, even when a star is not at a convenient distance.

B

Drake Equation

Detecting life on planets outside our solar system presents many challenges. If a planet with Earthlike temperatures was detected at a distance of many light-years, chemicals in its atmosphere such as oxygen, methane, or water vapor could be possible indicators of biological activity or at least of conditions where life might exist. In some ways, however, an intelligent, communicating civilization might be much easier to detect than primitive life. Intelligent life might have the technology to produce signals such as radio waves that could be much more powerful than even natural light from a star.

To calculate the likelihood that intelligent life could be detected elsewhere in the galaxy, American astronomer Frank Drake developed an equation for the number of communicating civilizations that might exist. This equation is called the Drake equation and is represented by N = R* f_p n_e f_l f_i f_c L. N is the number of communicating civilizations in the Milky Way galaxy. R* is the rate of formation of suitable stars, f_p is the fraction of those stars that have planets, n_e is the average number of suitable planets around a star, f_l is the fraction of those planets that develop life, f_i is the fraction of those planets with intelligent life, f_c is the fraction of such planets with a technological civilization that communicates, and L is the average lifetime of such a civilization. The only term for which astrobiologists currently have a good estimate is R*, although recent success in detecting planets around other stars suggests that the value of f_p is greater than one-half. Astrobiologists need to learn about the galaxy and life on the Earth (and perhaps elsewhere in the solar system) to come up with appropriate estimates for the other terms in the Drake equation. One particular Drake-equation factor, f_l (the fraction of suitable planets that develop life), depends on how life originates.

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