Besides Earth, does life exist someplace else? Astrobiology is the science that tries to answer that question. As far as we know, life only exists on Earth. Nonetheless, astrobiologists ask questions and form theories about life's origin, evolution, and location in the vast universe. Biology, chemistry, and many other areas of science offer information about both Earth's life and possible extraterrestrial life—life existing outside the limits of the Earth.

Hundreds of planets orbit nearby stars. Some of those planets may have temperatures similar to those on Earth. Life on Earth began in the oceans. These planets may also have water, the cradle of life.

Biologists continue to make new discoveries about extremophiles. These are forms of life that grow and reproduce in very harsh and extreme conditions here on Earth. The existence of extremophiles gives astrobiologists a good reason to consider the requirements for extraterrestrial life in our solar system and on the planets that orbit nearby stars.

Long ago Mars had large oceans. Today Mars has many places with buried ice. Habitats suitable for extraterrestrial extremophiles may exist under the Martian ice. Meteorite ALH84001 is a piece of Mars that was sent to Earth when a large meteorite smashed into the Red Planet. Scientists have found structures in ALH84001 that may be the fossils of Martian life.

Europa is one of Jupiter's many moons. Europa has a huge ice-covered ocean. Could the water below Europa's ice contain some type of extraterrestrial extremophiles? Ceres is the largest asteroid. It, too, is covered with ice. Is Ceres another one of the many places astrobiologists need to look at closely? Spacecraft have discovered ice on the Earth's Moon. Could extraterrestrial life exist beneath our Moon's surface? These are some of the many exciting questions astrobiologists want to answer.

除了地球之外，別的地方是否也存在著生命？「天體生物學」是一門試圖回答這個問題的科學。目前據人類所知，只有地球有生命的存在。儘管如此，天體生物學家還是提出問題，構成了生命起源論、生命演化論，還有在浩瀚宇宙中生命之定位的理論。生物學、化學等諸多領域的科學，也都對地球上的生命和可能存 在的外星生命提供了資料。外星生命是指存在於地球之外的生命。

數百個行星繞著附近的恆星運行，其中有些行星的溫度可能和地球相似。地球上的生命最早出現於海洋，而這些行星也可能有水的存在，而水是生命的搖籃。

生物學家對於「極端微生物」持續有新的發現，這種生物可以在地球上非常惡劣和極端條件的環境下生長與繁殖。由於「極端微生物」的存在，值得天體科學家去思考的是，在太陽系裡以及繞著周圍恆星運行的行星裡，外星生命生存所需的條件。

火星在很久以前曾經存在過大片的海洋。如今，火星上有許多地方都埋藏著冰。火星冰層下有可能存在著適合外星「極端微生物」的棲息地。編號「ALH84001」的火星隕石，是一塊大隕星撞上火星這個紅色星球後，飛來地球上的一塊隕石。科學家們在ALH84001 中所發現的構造，有可能就是火星生命的化石。

「歐羅巴」是木星眾多衛星中的一顆。歐羅巴有一片被冰層覆蓋的大海洋，在冰層底下的水，有可能存在著某種外星「極端微生物」嗎？最大的小行星「穀神星」也被冰層覆蓋著，它是否也是一個值得天體生物學家們仔細研究的地方？太空船已經在地球的衛星——「月球」上發現了冰，月表下有可能就存在著外星生命嗎？這些都是令人興奮，也是天體生物學家想要解答的問題。