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Exobiologie
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Die Exobiologie, auch Astrobiologie oder Xenobiologie genannt (altgriechische Kunstworte: εξωβιολογία, αστροβιολογία, ξενοβιολογία, jeweils Komposita aus exo- (außer-, außen-), astro- (stern-), xeno- (fremd-, auswärtig-) und -biologia (-lebenskunde)), ist eine Protowissenschaft, welche die Möglichkeit der Entstehung und Existenz von außerirdischem Leben erforscht.

Die Bezeichnung Astrobiologie stammt von dem Astronomen Otto Struve und wurde 1995 von der NASA für ihr Astrobiologisches Institut übernommen. Der Begriff ist ursprünglich für den Zugang aus der Astronomie charakteristisch. Im englischsprachigen Bereich hat sich diese Bezeichnung aber seither weitgehend durchgesetzt. Die Bezeichnung Exobiologie wurde von dem Biologen Joshua Lederberg geprägt, und wird oft von einem biologischen Standpunkt aus benutzt. Die ESA benutzt diese Bezeichnung bevorzugt und auch die NASA hat nach wie vor einen „Exobiology branch“. Eine weitere Bezeichnung, Kosmobiologie, stammt von dem Physiker John Desmond Bernal, wird aber selten benutzt. Schließlich kennt die IAU die Bezeichnung Bioastronomie, die aber außerhalb der IAU nicht benutzt wird.[1][2][3]

Nach Untersteiner (2006) ist die Exobiologie jener interdisziplinäre Wissenschaftszweig, der den Ursprung, die Verbreitung und die Evolution des Lebens im Universum untersucht. Das NASA Astrobiology Institute (NAI) definiert Astrobiologie schlicht als „Das Studium des Lebenden Universums“. Damit schließt dieser Wissenschaftsbereich auch Fragen zur weiteren Evolution des irdischen Lebens und seiner möglichen Verbreitung im Universum mit ein.
Inhaltsverzeichnis
[Verbergen]

* 1 Arbeitsbereiche
* 2 Suche nach Leben
o 2.1 Wissenschaftsphilosophische Kritik und Probleme
o 2.2 Leben in unserem Sonnensystem
o 2.3 Leben in anderen Planetensystemen
o 2.4 Nachweis von außerirdischem Leben
* 3 Siehe auch
* 4 Literatur
o 4.1 Monographien und Einzelbeiträge
o 4.2 Periodika
o 4.3 Einzelnachweise
* 5 Weblinks

Arbeitsbereiche [Bearbeiten]

Die Exobiologie als interdisziplinäres Forschungsfeld umfasst verschiedenste Arbeitsgebiete[4]:

* Bildung der Sterne und Planetensysteme.
* Geologische und geochemische Entwicklung der Planeten und ihrer Atmosphären.
* Geochemie und der Ursprung des Lebens.
* Die Evolution des Lebens und die Entwicklung der Biodiversität.
* Suche nach Exoplaneten und ihre Erforschung.
* Leben im Weltraum und Erforschung der Planeten des Sonnensystems durch Raumsonden.

Ziel dieser Forschungen ist es, den Ursprung (chemische Evolution) und die Evolution des Lebens auf der Erde und im All zu untersuchen, herauszufinden ob, wie und welches Leben außerhalb der Erde existiert und die Zukunft des Lebens auf und außerhalb der Erde vorherzusagen.
Suche nach Leben [Bearbeiten]
Wissenschaftsphilosophische Kritik und Probleme [Bearbeiten]

Die Exobiologie zog sich zu Beginn heftige Kritik zu, z.B. als „Wissenschaft, die erst noch zeigen muss, dass ihr Forschungsobjekt existiert“ (George Gaylord Simpson), oder auch durch Otto Struve, der „die Zeit noch nicht reif“ für Astrobiologe fand. Allerdings ist es gerade in Physik und Astronomie eine erfolgreiche Tradition, teilweise jahrzehntelang Objekte zu erforschen, deren Existenz erst später erwiesen wurde.

Ein wirkliches Hauptproblem der Exobiologie ist allerdings, dass es keine allgemein anerkannte Definition von Leben gibt. Tatsächlich gibt es zwar zahllose Versuche, Leben zu definieren, aber es lässt sich zeigen, oft sogar durch Gegenbeispiele, dass keine davon vollständig oder auch nur befriedigend ist.[5] Ein mögliche Schlussfolgerung ist, dass eine feste Trennlinie zwischen „belebt“ und „unbelebt“ gar nicht existiert.[6]

Als Arbeitsdefinition wird in weiten Teilen der Exobiologie, vor allem wenn es um die direkte Suche innerhalb des Sonnensystems geht, daher von „Leben in der uns bekannten Form“ gesprochen.[7]

Ein weiteres ernstzunehmendes Problem ist die „Rare Earth“-Hypothese.[8][9] Sie besagt, dass das Leben auf der Erde nur durch eine außergewöhnliche und höchst unwahrscheinliche Kombination von Faktoren zustande gekommen ist, so dass eine Suche nach Leben außerhalb der Erde hoffnungslos sei. Allerdings leidet die Rare-Earth-Hypothese an methodischen Problemen, insbesondere daran, dass sie aus einer „danach“ Position genau die Umstände untersucht, die auf der Erde zu genau unserer Form von Leben geführt hat, und diese dann als allgemein notwendig zu fordern, ohne zu untersuchen ob andere Umstände nicht zu ähnlichen Ergebnissen führen können. Nach dieser Ansicht ist die Rare-Earth-Hypothese eine Fehlanwendung des anthropischen Prinzips, da sie den Auswahleffekt des Beobachters bezüglich sich selbsts nicht ausreichend berücksichtigt.

Auch weitere Anwendungen des anthropischen Prinzips, um zu Aussagen zur Häufigkeit von Leben, vor allem von intelligentem Leben im Universum, zu kommen, erscheinen bei näherer Betrachtung oft als problematisch.[1] obwohl die Diskussion hier noch lange nicht abgeschlossen ist.[10]
Leben in unserem Sonnensystem [Bearbeiten]

Theoretisch könnte auch außerhalb der Erde auf anderen Planeten des Sonnensystems Leben existieren. So geht die exobiologische Abteilung der NASA davon aus, dass auf den Planeten Venus und Mars sowie auf einigen größeren Monden, wie denen des Jupiters - vor allem Europa. aber auch Ganymed und Kallisto - Leben existieren kann oder konnte. Eine besondere Stellung nimmt der Saturnmond Titan ein, auf dem unter einer dichten Atmosphäre aus Stickstoff und Methan Bedingungen herrschen könnten, die denen der Ur-Erde ähneln. Die relativ lebensfreundlichsten Bedingungen außerhalb der Erde scheint nach derzeitigem Kenntnisstand allerdings der nur 500 km große Saturnmond Enceladus zu bieten.[11]

Um die Grenzen möglichen Lebens bzw. lebenstragender Umgebungen zu ermitteln, untersucht man auf der Erde extreme Umgebungen (Vulkane, Tiefsee, luftleere Räume, chemische Belastungen, Antarktis) und vergleicht diese mit den Bedingungen, die auf Planeten wie dem Mars vorherrschen.

Weiter untersuchen Exobiologen Meteoriten auf Versteinerungen. Dabei wurden seit 1990 Spuren in vom Mars stammenden Meteoriten gefunden, die als Anzeichen einzelligen Lebens (vergleichbar Bakterien) interpretiert wurden. Diese Interpretation ist jedoch umstritten und nicht allgemein anerkannt.

Sowohl beim innersten Planet Merkur als auch bei den weit außen liegenden Eiswelten ab Uranus wird die Möglichkeit für Leben faktisch ausgeschlossen. Auf Merkur sind die Tag- und Nachttemperaturen (und damit auch die Schwankungen) zu extrem (-180 °C bis 460 °C), auf den weit äußeren Planeten ist die Temperatur dauerhaft zu tief (unter -190 °C), um Leben entstehen zu lassen.
Leben in anderen Planetensystemen [Bearbeiten]

Leben, so wie wir es kennen, kann sich in einem Planetensystem nur in der Ökosphäre des jeweiligen Sterns entwickeln. Die Ökosphäre ist jener Teil der kosmischen Umgebung, in der auf Planeten oder Monden flüssiges Wasser bestehen kann, welches die Entstehung und das Überleben zumindest einfacher Organismen ermöglicht. Um die Ökosphäre eines Sterns beurteilen zu können, ist es wichtig, zu wissen, welcher Spektralklasse er angehört. Als Spektralklasse bezeichnet man ein System der Harvard-Klassifikation nach der alle Sterne nach ihrer Oberflächentemperatur und Leuchtkraft eingruppiert werden. Das System besteht aus 7 Grundklassen, die mit den Buchstaben O, B, A, F, G, K und M bezeichnet werden. Darüber hinaus enthält die heute in der Astronomie allgemein angewandte MK-Klassifikation auch Leuchtkraftklassen, die mit den römischen Ziffern I, II, III, IV und V bezeichnet werden. I steht dabei für Überriese, II für Heller Riese, III für Normaler Riese, IV für Unterriese und V für einen Hauptreihenstern. Unsere Sonne ist nach dieser Klassifikation ein Stern der Klasse G2V. Die Ökosphäre erstreckt sich bei Klasse G Sternen in einem Bereich von 0,6 bis 1,6 AE. Für eine ausreichend stabile Ökosphäre, d.h. mit nur geringen Änderungen über mehrere Milliarden Jahre hinweg, kommen nur Sterne der Spektralklassen F-M und der Leuchtkraftklasse V in Betracht.

Es gibt auch Überlegungen zu sehr exotischen Lebensformen, die nicht auf Kohlenstoff basieren (Kohlenstoffchauvinismus), planetare Ausmaße annehmen (eine Biosphäre als „ein“ Lebewesen) oder gar im interplanetaren und interstellaren Raum leben. Diese Überlegungen werden aber meist dem Bereich der Science Fiction zugeordnet.
Nachweis von außerirdischem Leben [Bearbeiten]

Die Exobiologie versucht auf drei Arten Leben außerhalb der Erde nachzuweisen.

1. Direkte Suche innerhalb des Sonnensystems, und möglicherweise Transport einer Probe zur Erde, durch Raumsonden
2. Suche nach der biologischen Signatur von Leben auf Exoplaneten durch astronomische Beobachtungsmethoden. Es gibt mehrere indirekte Methoden, um auf Leben schließen zu können. So wird angenommen, dass bestimmte Molekülverbindungen nur durch Leben dauerhaft erzeugt werden können. Würde man also zum Beispiel im Lichtspektrum (Absorptionsspektrum) eines fernen Planeten solche Moleküle finden, wäre das ein starkes Indiz für Leben.
3. Suche nach Zeichen außerirdischer Technologie. Eindeutig für Leben sprächen z.B. Signale außerirdischer Zivilisationen, wie man sie mit dem SETI-Projekt[12] zu finden versucht. Bisher ist diesem Projekt allerdings noch kein Erfolg beschieden, auch wenn angenommen wird, dass es hunderte Zivilisationen allein in unserer Galaxie (Milchstraße) geben könnte. Diese Zahl wurde mit Hilfe der Drake-Gleichung ermittelt und unterliegt starken Schwankungen je nach den getroffenen Annahmen.

Siehe auch [Bearbeiten]

* ALH 84001 (Meteorit)
* Exo-Soziologie
* Fermi-Paradoxon
* Panspermie
* Planetary protection in der engl. Wikipedia
* Von den Bewohnern der Gestirne

Literatur [Bearbeiten]
Monographien und Einzelbeiträge [Bearbeiten]

* Hansjürg Geiger: Auf der Suche nach Leben im Weltall. Wie Leben entsteht und wo man es finden kann, Kosmos Verlag, Stuttgart 2005, ISBN 3-440-10504-0
* Fred Adams: Leben im Universum, Deutscher Taschenbuch Verlag, München 2006, ISBN 3-423-34282-X
* Christian de Duve: Aus Staub geboren - Leben als kosmische Zwangsläufigkeit, Rowohlt Taschenbuch Verlag, Hamburg 1997, ISBN 3-499-60160-5
* L. Billings et al.: The Astrobiology Primer: An Outline of General Knowledge - Version 1, 2006. Astrobiology, Vol.6, 2006. S. 735 ff. PDF-Version
* I. Gilmour, M.A. Sephton: An introduction to astrobiology., Cambridge Univ. Press, Cambridge 2004, ISBN 0-521-83736-7
* G. Horneck, P. Rettberg: Complete course in astrobiology. Wiley-VCH, Weinheim 2007, ISBN 978-3-527-40660-9
* G. Horneck: Astrobiology - the quest for the conditions of life. Springer, Berlin 2002, ISBN 3-540-42101-7
* P. Ehrenfreund: Astrobiology - future perspectives Kluwer Academic, Dordrecht 2004, ISBN 1-4020-2304-9
* T. Penz: Habitable planets in the universe - an interdisciplinary approach regarding the origin and distribution of life. Dipl.-Arb., Univ. Graz 2005
* D. Beste: Leben im All. Spektrum der Wissenschaft Dossier 2002,3, Spektrum-d.-Wiss.-Verl., Heidelberg 2002, ISBN 3-936278-14-8, http://www.spektrum.de/artikel/849269
* W.T. Sullivan, J.A. Baross: Planets and life - the emerging science of astrobiology. Cambridge Univ. Press, Cambridge 2007, ISBN 978-0-521-53102-3
* Pascual Jordan: Diskussionsbemerkungen zur exobiologischen Hypothese. Steiner, Wiesbaden 1971
* Stuart Clark: Life on other worlds and how to find it. Springer, London 2000, ISBN 1-85233-097-X
* Michael D. Papagiannis: Strategies for the search for life in the universe. Reidel, Dordrecht 1980, ISBN 90-277-1181-X
* Mario Livio: Astrophysics of life.Cambridge Univ. Press, Cambridge 2005, ISBN 0-521-82490-7
* Andrew M. Shaw: Astrochemistry - from astronomy to astrobiology. Wiley & Sons, Chichester 2006, ISBN 0-470-09136-3
* Claude R. Canizares: Evaluating the biological potential in samples returned from planetary satellites and small solar system bodies - framework for decision making. National Academy Pr., Washington 1998, ISBN 0-309-06136-9 online
* Hubert Untersteiner: "Exobiologie - Wissenschaft vom Leben im All." edition nove 2006, ISBN 3-902546-42-5, ISBN 978-3-902546-42-5

Periodika [Bearbeiten]

* Astrobiology, Mary Ann Liebert Inc., ISSN 1531-1074 (Print) 1557-8070 (Online)
* International Journal of Astrobiology, Cambridge Journals, ISSN 1473-5504 (Print) 1475-3006 (Online)
* Origins of Life and Evolution of Biospheres, Springer-Verlag, ISSN 0169-6149 (Print) 1573-0875 (Online)

Einzelnachweise [Bearbeiten]

1. ↑ a b Astrobiology: The Study of the Living Universe. In: Annual Review of Astronomy & Astrophysics. 43, 2005, S. 31-74 (Online).
2. ↑ What is Bioastronomy? The International Astronomical Union's Commission 51
3. ↑ Division III Commission 51 Bio-Astronomy@ iau.org (abgerufen am 4. Februar 2010)
4. ↑ L. Billings et al.: The Astrobiology Primer: An Outline of General Knowledge-Version 1, 2006. In: Astrobiology. 6, 2006, S. 735-813, arXiv:astro-ph/0610926 (Online, abgerufen am 18. August 2009).
5. ↑ S. A. Tsokolov: Why Is the Definition of Life So Elusive? Epistemological Considerations. In: Astrobiology. 9, 2009, S. 401-412 (Online).
6. ↑ Annila, Annila: Why did life emerge?. In: International Journal of Astrobiology. 7, 2008, S. 293-300 (Online).
7. ↑ Margaret S. Race, Richard O. Randolph: The need for operating guidelines and a decision making framework applicable to the discovery of non-intelligent extraterrestrial life. Advances in Space Research, Volume 30, Number 6, 2002 , S. 1583-1591 Abstract
8. ↑ Übersetzt „seltene Erde“, allerdings hat diese Ausdruck eine spezielle, völlig andere Bedeutung in der Chemie
9. ↑ Ward, Brownlee: Rare earth : why complex life is uncommon in the universe. Copernicus, New York 2000, ISBN 0387987010 (Online).
10. ↑ M. Ćirković: On the First Anthropic Argument in Astrobiology. In: Earth, Moon, and Planets. 91, 2002, S. 243-254, arXiv:astro-ph/0306185 (Online).
11. ↑ Parkinson, Mao-Chang, Yung,. Kirschivnk; Habitability of Enceladus: Planetary Conditions for Life.
12. ↑ J. Tarter: The Search for Extraterrestrial Intelligence (SETI). In: Annual Review of Astronomy and Astrophysics. 39, 2001, S. 511-548 (Online).

Weblinks [Bearbeiten]

* NASA Astrobiology (englisch)
* NASA Ames Exobiology Branch (englisch)
* Centre for Astrobiology, Cardiff University
* European Astrobiology Network Association
* International Space University Summer Session Programm 2002-A Discovery Sourcebook for Astrobiology; Final Report, PDF 13 MB, abgerufen am 28. August 2009
* R. Weinberger: Sind wir allein im Universum? (PDF-Datei; 65 kB)
* L. Hauser: Außerirdisches Leben: Herausforderung für die Theologie? pdf online
* Lochfraß im Marsgestein - Forscher finden mögliche Spuren von Bakterien in einem Meteoriten vom Mars
* ORF-Artikel zu möglichen Leben auf der Venus

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What is astrobiology is explained under the following link:
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Astrobiology
From Wikipedia, the free encyclopedia
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For the journal, see Astrobiology (journal).
Nucleic acids may not be the only biomolecules in the Universe capable of coding for life.[1]

Astrobiology (other terms have been exobiology, exopaleontology, bioastronomy and xenobiology) is the study of the origin, evolution, distribution, and future of life in the universe. This interdisciplinary field encompasses the search for habitable environments in our Solar System and habitable planets outside our Solar System, the search for evidence of prebiotic chemistry, life on Mars and other bodies in our Solar System, laboratory and field research into the origins and early evolution of life on Earth, and studies of the potential for life to adapt to challenges on Earth and in outer space.[2]

Astrobiology makes use of physics, chemistry, astronomy, biology, molecular biology, ecology, planetary science, geography and geology to investigate the possibility of life on other worlds and help recognize biospheres that might be quite different from the Earth's.[3][4] However, astrobiology concerns itself with an interpretation of existing scientific data; given more detailed and reliable data from other parts of the Universe, the roots of astrobiology itself —biology, physics, chemistry— may have their theoretical bases challenged. Much speculation is entertained in the field to give context, and astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories.
Contents
[hide]

* 1 Overview
* 2 Methodology
o 2.1 Narrowing the task
o 2.2 Elements of astrobiology
+ 2.2.1 Astronomy
+ 2.2.2 Biology
+ 2.2.3 Astrogeology
* 3 Life in the Solar System
* 4 Criticisms
* 5 Research outcomes
* 6 Rare Earth Hypothesis
* 7 See also
* 8 References
* 9 Bibliography
* 10 External links

Overview
It is not known whether life elsewhere in the Universe would utilize cell structures like those found on Earth. (Chloroplasts within plant cells shown here.)[5]
The Martian meteorite ALH84001 shows microscopic formations that may have been created by life.

The etymology of astrobiology comes from Greek ἄστρον, astron, "constellation, star"; βίος, bios, "life"; and -λογία, -logia, study. Although astrobiology is an emerging field and still a developing subject, the question of whether life exists elsewhere in the universe is a verifiable hypothesis and thus a valid line of scientific inquiry. David Grinspoon, a planetary scientist, calls astrobiology a field of natural philosophy, grounding speculation on the unknown, in known scientific theory.[6][7] Though once considered outside the mainstream of scientific inquiry, astrobiology has become a formalized field of study. NASA funded its first astrobiology project in 1959 and established an astrobiology program in 1960.[2][8] NASA’s Viking missions to Mars, launched in 1976, included three biology experiments designed to look for possible signs of life. In 1971, NASA funded the Search for Extra-Terrestrial Intelligence (SETI) to survey the sky to detect the existence of transmissions from a civilization on a distant planet. The Mars Pathfinder lander in 1997 carried a scientific payload intended for exopaleontology in the hopes of finding microbial fossils entombed in the rocks.[9]

In the 21st century, astrobiology is a focus of a growing number of NASA and European Space Agency Solar System exploration missions. The first European workshop on astrobiology took place in May 2001 in Italy,[10] and the outcome was the Aurora programme.[11] Currently, NASA hosts the NASA Astrobiology Institute and a growing number of universities in the United States (e.g., University of Arizona, Penn State University, and University of Washington), Britain (e.g., The University of Glamorgan[12]), Canada, Ireland, and Australia (e.g., The University of New South Wales[13]) now offer graduate degree programs in astrobiology.

A particular focus of current astrobiology research is the search for life on Mars due to its proximity to Earth and geological history. There is a growing body of evidence to suggest that Mars has previously had a considerable amount of water on its surface, water being considered to be an essential precursor to the development of carbon-based life.[14]

Missions specifically designed to search for life include the Viking program and Beagle 2 probes, both directed to Mars. The Viking results were inconclusive,[15] and Beagle 2 failed to transmit from the surface and is assumed to have crashed.[16] A future mission with a strong astrobiology role would have been the Jupiter Icy Moons Orbiter, designed to study the frozen moons of Jupiter—some of which may have liquid water—had it not been cancelled. Recently, the Phoenix lander probed the environment for past and present planetary habitability of microbial life on Mars, and to research the history of water there.

In 2011, NASA plans to launch the Mars Science Laboratory rover which will continue the search for past or present life on Mars using a variety of scientific instruments. The European Space Agency has been developing the ExoMars astrobiology rover, which is to be launched on 2018.

The International Astronomical Union regularly organizes major international conferences through its Commission 51: Bioastronomy. Commission 51 - Bioastronomy: Search for Extraterrestrial Life was established by the IAU in 1982 and is now hosted by the Institute of Astronomy at the University of Hawai'i.
Methodology
Narrowing the task
Main article: Planetary habitability

When looking for life on other planets, some simplifying assumptions are useful to reduce the size of the task of the astrobiologist. One is to assume that the vast majority of life forms in our galaxy are based on carbon chemistries, as are all life forms on Earth.[17] While it is possible that non-carbon-based life exists, carbon is well known for the unusually wide variety of molecules that can be formed around it.
This planetary habitability chart shows where life might exist on extrasolar planets based on our own Solar System and life on Earth.

The presence of liquid water is a useful assumption, as it is a common molecule and provides an excellent environment for the formation of complicated carbon-based molecules that could eventually lead to the emergence of life.[18] Some researchers posit environments of ammonia, or more likely water-ammonia mixtures.[19] These environments are considered suitable for carbon or noncarbon-based life, while opening more temperature ranges (and thus worlds) for life.

A third assumption is to focus on sun-like stars. This comes from the idea of planetary habitability.[20] Very big stars have relatively short lifetimes, meaning that life would not likely have time to evolve on planets orbiting them. Very small stars provide so little heat and warmth that only planets in very close orbits around them would not be frozen solid, and in such close orbits these planets would be tidally "locked" to the star.[21] Without a thick atmosphere, one side of the planet would be perpetually baked and the other perpetually frozen. In 2005, the question was brought back to the attention of the scientific community, as the long lifetimes of red dwarfs could allow some biology on planets with thick atmospheres. This is significant, as red dwarfs are extremely common. (See Habitability of red dwarf systems).

It is estimated that 10% of the stars in our galaxy are sun-like; There are about a thousand such stars within 100 light-years of our Sun. These stars would be useful primary targets for interstellar listening. Since Earth is the only planet known to harbor life, there is no evident way to know if any of the simplifying assumptions are correct.
Elements of astrobiology
Astronomy
Main article: Astronomy
Artist's impression of the extrasolar planet OGLE-2005-BLG-390Lb orbiting its star 20,000 light-years from Earth; this planet was discovered with gravitational microlensing.
The NASA Kepler mission, successfully launched in March 2009, searches for extrasolar planets

Most astronomy-related astrobiological research falls into the category of extrasolar planet (exoplanet) detection, the hypothesis being that if life arose on Earth, then it could also arise on other planets with similar characteristics. To that end, a number of instruments designed to detect Earth-like exoplanets are under development, most notably NASA's Terrestrial Planet Finder (TPF) and ESA's Darwin programs. Additionally, NASA has launched the Kepler mission in March 2009, and the French Space Agency has launched the COROT space mission in 2006.[22][23] There are also several less ambitious ground-based efforts underway. (See exoplanet).

The goal of these missions is not only to detect Earth-sized planets, but also to directly detect light from the planet so that it may be studied spectroscopically. By examining planetary spectra, it would be possible to determine the basic composition of an extrasolar planet's atmosphere and/or surface; given this knowledge, it may be possible to assess the likelihood of life being found on that planet. A NASA research group, the Virtual Planet Laboratory,[24] is using computer modeling to generate a wide variety of virtual planets to see what they would look like if viewed by TPF or Darwin. It is hoped that once these missions come online, their spectra can be cross-checked with these virtual planetary spectra for features that might indicate the presence of life. The photometry temporal variability of extrasolar planets may also provide clues to their surface and atmospheric properties.

An estimate for the number of planets with intelligent extraterrestrial life can be gleaned from the Drake equation, essentially an equation expressing the probability of intelligent life as the product of factors such as the fraction of planets that might be habitable and the fraction of planets on which life might arise:[25]: N = R^{*} ~ \times ~ f_{p} ~ \times ~ n_{e} ~ \times ~ f_{l} ~ \times ~ f_{i} ~ \times ~ f_{c} ~ \times ~ L
where, N = The number of communicative civilizations, R* = The rate of formation of suitable stars (stars such as our Sun), fp = The fraction of those stars with planets. (Current evidence indicates that planetary systems may be common for stars like the Sun), ne = The number of Earth-like worlds per planetary system, fl = The fraction of those Earth-like planets where life actually develops, fi = The fraction of life sites where intelligence develops, fc = The fraction of communicative planets (those on which electromagnetic communications technology develops), L = The "lifetime" of communicating civilizations.

However, whilst the rationale behind the equation is sound, it is unlikely that the equation will be constrained to reasonable error limits any time soon. The first term, Number of Stars, is generally constrained within a few orders of magnitude. The second and third terms, Stars with Planets and Planets with Habitable Conditions, are being evaluated for the sun's neighborhood. Another associated topic is the Fermi paradox, which suggests that if intelligent life is common in the Universe, then there should be obvious signs of it. This is the purpose of projects like SETI, which tries to detect signs of radio transmissions from intelligent extraterrestrial civilizations.

Another active research area in astrobiology is Planetary system formation. It has been suggested that the peculiarities of our Solar System (for example, the presence of Jupiter as a protective shield[26]) may have greatly increased the probability of intelligent life arising on our planet.[27][28] No firm conclusions have been reached so far.
Biology
Main articles: Biology and Extremophile
Hydrothermal vents are able to support extremophile bacteria on Earth and may also support life in other parts of the cosmos.

Extremophiles (organisms able to survive in extreme environments) are a core research element for astrobiologists. Such organisms include biota able to survive kilometers below the ocean's surface near hydrothermal vents and microbes that thrive in highly acidic environments.[29]

Until the 1970s, life was believed to be entirely dependent on energy from the Sun. Plants on Earth's surface capture energy from sunlight to photosynthesize sugars from carbon dioxide and water, releasing oxygen in the process, and are then eaten by oxygen-respiring animals, passing their energy up the food chain. Even life in the ocean depths, where sunlight cannot reach, was believed to obtain its nourishment either from consuming organic detritus rained down from the surface waters or from eating animals that did.[30] A world's ability to support life was thought to depend on its access to sunlight. However, in 1977, during an exploratory dive to the Galapagos Rift in the deep-sea exploration submersible Alvin, scientists discovered colonies of giant tube worms, clams, crustaceans, mussels, and other assorted creatures clustered around undersea volcanic features known as black smokers.[30] These creatures thrive despite having no access to sunlight, and it was soon discovered that they comprise an entirely independent food chain. Instead of plants, the basis for this food chain is a form of bacterium that derives its energy from oxidization of reactive chemicals, such as hydrogen or hydrogen sulfide, that bubble up from the Earth's interior. This chemosynthesis revolutionized the study of biology by revealing that life need not be sun-dependent; it only requires water and an energy gradient in order to exist. It is now known that extremophiles thrive in ice, boiling water, acid, the water core of nuclear reactors, salt crystals, toxic waste and in a range of other extreme habitats that were previously thought to be inhospitable for life.[31] It opened up a new avenue in astrobiology by massively expanding the number of possible extraterrestrial habitats. Characterization of these organisms—their environments and their evolutionary pathways—is considered a crucial component to understanding how life might evolve elsewhere in the Universe. Some organisms able to withstand exposure to the vacuum and radiation of space include the lichen fungi Rhizocarpon geographicum and Xanthoria elegans,[32] the bacterium Bacillus safensis,[33] Deinococcus radiodurans,[33] Bacillus subtilis,[33] yeast Saccharomyces cerevisiae,[33] seeds from Arabidopsis thaliana ('mouse-ear cress'),[33] as well as the invertebrate animal Tardigrade.[33]

The origin of life, distinct from the evolution of life, is another ongoing field of research. Oparin and Haldane postulated that the conditions on the early Earth were conducive to the formation of organic compounds from inorganic elements and thus to the formation of many of the chemicals common to all forms of life we see today. The study of this process, known as prebiotic chemistry, has made some progress, but it is still unclear whether or not life could have formed in such a manner on Earth. The alternative theory of panspermia is that the first elements of life may have formed on another planet with even more favorable conditions (or even in interstellar space, asteroids, etc.) and then have been carried over to Earth by a variety of means. See Origin of life. Jupiter's moon, Europa, is now considered to be the most likely location for extant extraterrestrial life in the Solar System.[31][34][35][36][37][38]
Astrogeology
Main article: Geology of solar terrestrial planets

Astrogeology is a planetary science discipline concerned with the geology of the celestial bodies such as the planets and their moons, asteroids, comets, and meteorites. The information gathered by this discipline allows the measure of a planet's or a natural satellite's potential to develop and sustain life, or planetary habitability.

An additional discipline of astrogeology is geochemistry, which involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks and soils, the cycles of matter and energy and their interaction with the hydrosphere and the atmosphere of the planet. Specializations include cosmochemistry, biochemistry and organic geochemistry.

The fossil record provides the oldest known evidence for life on Earth.[39] By examining this evidence, paleontologists are able to understand better the types of organisms that arose on the early Earth. Some regions on Earth, such as the Pilbara in Western Australia and the McMurdo Dry Valleys of Antarctica, are also considered to be geological analogs to regions of Mars, and as such, might be able to provide clues on how to search for past life on Mars.
Life in the Solar System
See also: Life on Mars, Life on Europa, and Life on Titan
Europa, due to the ocean that exists under its icy surface, might host some form of microbial life.

People have long speculated about the possibility of life in settings other than Earth, however, speculation on the nature of life elsewhere often has paid little heed to constraints imposed by the nature of biochemistry.[40] The likelihood that life throughout the universe is probably carbon-based is encouraged by the fact that carbon is one of the most abundant of the higher elements. Only two of the natural atoms, carbon and silicon, are known to serve as the backbones of molecules sufficiently large to carry biological information. As the structural basis for life, one of carbon's important features is that unlike silicon it can readily engage in the formation of chemical bonds with many other atoms, thereby allowing for the chemical versatility required to conduct the reactions of biological metabolism and propagation. The various organic functional groups, composed of hydrogen, oxygen, nitrogen, phosphorus, sulfur, and a host of metals, such as iron, magnesium, and zinc, provide the enormous diversity of chemical reactions necessarily catalyzed by a living organism. Silicon, in contrast, interacts with only a few other atoms, and the large silicon molecules are monotonous compared with the combinatorial universe of organic macromolecules.[40] Indeed, it seems likely that the basic building blocks of life anywhere will be similar to our own, in the generality if not in the detail.[40] Although terrestrial life and life that might arise independently of Earth are expected to use many similar, if not identical, building blocks, they also are expected to have some biochemical qualities that are unique.

Thought on where in the Solar System life might occur was limited historically by the belief that life relies ultimately on light and warmth from the Sun and, therefore, is restricted to the surfaces of planets.[40] The three most likely candidates for life in the Solar System are the planet Mars, the Jovian moon Europa, and Saturn's moon Titan.[41][42][43][44][45] This speculation is primarily based on the fact that (in the cases of Mars and Europa) the planetary bodies may have liquid water, a molecule essential for life as we know it, for its use as a solvent in cells.[14] Water on Mars is found in its polar ice caps, and newly carved gullies recently observed on Mars suggest that liquid water may exist, at least transiently, on the planet's surface,[46][47] and possibly in subsurface environments such as hydrothermal springs as well. At the Martian low temperatures and low pressure, liquid water is likely to be highly saline.[48] As for Europa, liquid water likely exists beneath the moon's icy outer crust.[35][41][42] This water may be warmed to a liquid state by volcanic vents on the ocean floor (an especially intriguing theory considering the various types of extremophiles that live near Earth's volcanic vents), but the primary source of heat is probably tidal heating.[49]

Another planetary body that could potentially sustain extraterrestrial life is Saturn's largest moon, Titan.[45] Titan has been described as having conditions similar to those of early Earth.[50] On its surface, scientists have discovered the first liquid lakes outside of Earth, but they seem to be composed of ethane and/or methane, not water.[51] After Cassini data was studied, it was reported on March 2008 that Titan may also have an underground ocean composed of liquid water and ammonia.[52] Additionally, Saturn's moon Enceladus may have an ocean below its icy surface.[53]
Criticisms

The systematic search for possible life outside of Earth is a valid multidisciplinary scientific endeavor.[54] The University of Glamorgan, UK, started just such a degree in 2006,[12] and the American government funds the NASA Astrobiology Institute. However, characterization of non-Earth life is unsettled; hypotheses and predictions as to its existence and origin vary widely, but at the present, the development of theories to inform and support the exploratory search for life may be considered astrobiology's most concrete practical application.

Biologist Jack Cohen and mathematician Ian Stewart, amongst others, consider "xenobiology" separate from astrobiology. Cohen and Stewart stipulate that astrobiology is the search for Earth-like life outside of our solar system and say that xenobiologists are concerned with the possibilities open to us once we consider that life need not be carbon-based or oxygen-breathing, so long as it has the defining characteristics of life. (See carbon chauvinism).


Research outcomes
Asteroid(s) may have transported life to Earth.

As of 2010, no proof of extraterrestrial life has been identified. Examination of the ALH 84001 meteorite, which was recovered in Antarctica in 1984 and believed to have originated from Mars, is thought by David McKay, Chief Scientist for Astrobiology at NASA's Johnson Space Center, as well as other scientists, to contain microfossils of extraterrestrial origin; this interpretation is controversial.[55][56][57][58]

Methane

In 2004, the spectral signature of methane was detected in the Martian atmosphere by both Earth-based telescopes as well as by the Mars Express probe. Because of solar radiation and cosmic radiation, methane is predicted to disappear from the Martian atmosphere within several years, so the gas must be actively replenished in order to maintain the present concentration.[59][60] The Mars Science Laboratory rover will perform precision measurements of oxygen and carbon isotope ratios in carbon dioxide (CO2) and methane (CH4) in the atmosphere of Mars in order to distinguish between a geochemical and a biological origin.[61][62][63]

Planetary systems

It is possible that some planets, like the gas giant Jupiter in our solar system, may have moons with solid surfaces or liquid oceans that are more hospitable. Most of the planets so far discovered outside our solar system are hot gas giants thought to be inhospitable to life, so it is not yet known whether our solar system, with a warm, rocky, metal-rich inner planet such as Earth, is of an aberrant composition. Improved detection methods and increased observing time will undoubtedly discover more planetary systems, and possibly some more like ours. For example, NASA's Kepler Mission seeks to discover Earth-sized planets around other stars by measuring minute changes in the star's light curve as the planet passes between the star and the spacecraft. Progress in infrared astronomy and submillimeter astronomy has revealed the constituents of other star systems. Infrared searches have detected belts of dust and asteroids around distant stars, underpinning the formation of planets.

Planetary habitability

Efforts to answer questions such as the abundance of potentially habitable planets in habitable zones and chemical precursors have had much success. Numerous extrasolar planets have been detected using the wobble method and transit method, showing that planets around other stars are more numerous than previously postulated. The first Earth-like extrasolar planet to be discovered within its star's habitable zone is Gliese 581 c, which was found using radial velocity.[64]

Research into the environmental limits of life and the workings of extreme ecosystems is also ongoing, enabling researchers to predict what planetary environments might be most likely to harbor life. Missions such as the Phoenix lander, Mars Science Laboratory, ExoMars to Mars, the Cassini probe to Saturn's moon Titan, and the "Ice Clipper" mission to Jupiter's moon Europa hope to further explore the possibilities of life on other planets in our solar system.
Rare Earth Hypothesis
Main article: Rare Earth hypothesis

This hypothesis states that based on astrobiological findings, multi-cellular life forms found on earth may actually be more of a rarity than scientists initially assumed. It provides a possible answer to the Fermi paradox which suggests,"If extraterrestrial aliens are common, why aren't they obvious?" It is apparently in opposition to the principle of mediocrity, assumed by famed astronomers Frank Drake, Carl Sagan, and others. The Principle of Mediocrity suggests that life on Earth is not exceptional, but rather that life is more than likely to be found on innumerable other worlds.

The Anthropic Principle states that fundamental laws of the universe work specifically in a way that life would be possible. The Anthropic Principle supports the Rare Earth Hypothesis by arguing the overall elements that are needed to support life on earth are so fine-tuned that it is nigh impossible for another just like it to exist by random chance (note that these terms are used by scientists in a different way from the vernacular conception of them). However, Stephen Jay Gould compared the claim that the universe is fine-tuned for the benefit of our kind of life to saying that sausages were made long and narrow so that they could fit into modern hot dog buns, or saying that ships had been invented to house barnacles. [65][66]
See also

* Astrosciences
* Aurelia and Blue Moon
* Colonization of the Moon
* Extraterrestrial life
* Extremophile
* Fermi paradox
* Gravitational biology
* Hypothetical types of biochemistry
* Life on Mars
* NASA Astrobiology Institute



* Origin of life
* Panspermia
* Planetary habitability
* Planetary Science
* Publications in astrobiology
* Purple Earth Hypothesis
* SETI
* Shadow biosphere
* Terraforming
* Xenolinguistics

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Bibliography

* The International Journal of Astrobiology, published by Cambridge University Press, is the forum for practitioners in this interdisciplinary field.
* Astrobiology, published by Mary Ann Liebert, Inc., is a peer-reviewed journal that explores the origins of life, evolution, distribution, and destiny in the universe.
* Dick, Steven J.; James Strick (2005). The Living Universe: NASA and the Development of Astrobiology. Piscataway, NJ: Rutgers University Press. ISBN 0813537339.
* Grinspoon, David (2003). Lonely planets. The natural philosophy of alien life. New York: ECCO. ISBN 0060185406.
* Jakosky, Bruce M. (2006). Science, Society, and the Search for Life in the Universe. Tucson: University of Arizona Press. ISBN 0816526133.
* Lunine, Jonathan I. (2005). Astrobiology. A Multidisciplinary Approach. San Francisco: Pearson Addison-Wesley. ISBN 0805380426.
* Gilmour, Iain; Mark A. Sephton (2004). An introduction to astrobiology. Cambridge: Cambridge Univ. Press. ISBN 0-521-83736-7.
* Ward, Peter; Brownlee, Donald (2000). Rare Earth: Why Complex Life is Uncommon in the Universe. New York: Copernicus. ISBN 0387987010.

External links
Search Wikiversity At Wikiversity you can learn more and teach others about Astrobiology at:
The Department of Astrobiology

* International Astronomical Union Commission 51: Bioastronomy, official website
* Astrobiology Instant Expert on New Scientist
* Australian Centre for Astrobiology
* The Astrobiology Web
* Astrobiology Magazine
* NASA Astrobiology Institute
* Podcast Interview with NAI's Director Dr. Carl Pilcher
* European Astrobiology Network Association (EANA)
* Possible Connections Between Interstellar Chemistry and the Origin of Life on the Earth
* Scientists Find Clues That Life Began in Deep Space - NASA Astrobiology Institute
* Stars and Habitable Planets
* NASA-Macquarie University Pilbara Education Project
* Conditions for Life Everywhere
* Snaiad, a world-building project with creatures designed with evolutionary biology in mind.
* Astrobiology Lecture Course Network (a.y. 2005–2006)
* Astrobio.eu

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Astrobiology

Biochemistry · Cosmology · Ecology · Evolutionary biology · ExoMars · Extremophiles · Life on Mars · Microbiology · Origin of life · Paleontology · Planetary habitability · Planetary science · Solar system formation · Terrestrial Planet Finder
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Major subfields of biology
Anatomy · Astrobiology · Biochemistry · Biomechanics · Biophysics · Bioinformatics · Biostatistics · Botany · Cell biology · Chronobiology · Conservation biology · Developmental biology · Ecology · Epidemiology · Evolutionary biology · Genetics · Genomics · Histology · Human biology · Immunology · Marine biology · Mathematical biology · Microbiology · Molecular biology · Neuroscience · Nutrition · Origin of life · Paleontology · Parasitology · Pathology · Pharmacology · Physiology · Quantum biology · Systems biology · α-Taxonomy · Toxicology · Zoology
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General subfields within astronomy and astrophysics

Cosmology · Extragalactic astronomy · Galactic astronomy · Stellar astronomy · Planetary science · Exoplanetology · Astrogeology · Astrometry · Astrochemistry · Astrobiology · Astrodynamics
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Extraterrestrial life
Events and objects
ALH84001 · Close encounter · Murchison meteorite · Radio source SHGb02+14a · Shergotty meteorite · Wow! signal
Extraterrestrial bodies
In the Solar System

Europa · Mars · Titan · Enceladus
Extrasolar

Earth analog · Gliese 581 c · Gliese 581 d · Gliese 581 e
Communication
Allen Telescope Array · Arecibo Observatory · Bracewell probe · Communication with Extraterrestrial Intelligence · Darwin space mission · ExoMars · Lincos (language) · Pioneer plaque · Project Cyclops · Project Ozma · Project Phoenix · SERENDIP · SETI · SETI@home · Active SETI
Subjects
Astrobiology · Astroecology · Biosignature · Brookings Report · Catalog of Nearby Habitable Systems · Exopolitics · Exotheology · Extraterrestrials in fiction · Extraterrestrial liquid water · Habitable zone · Habitability of red dwarf systems · Hypothetical types of biochemistry · Interstellar communication · Noogenesis · Planetary habitability · Planetary protection · San Marino Scale · Shermer's Last Law · Silencium universi · Xenoarchaeology · Xenolinguistics
Theories
Ancient astronauts · Aurelia and Blue Moon · Back-contamination · Cosmic pluralism · Drake equation · Fermi paradox · Great Filter · Kardashev scale · Mediocrity principle · Neocatastrophism · Panspermia · Rare Earth hypothesis · Sentience quotient · Zoo hypothesis
Missions
Mars sample return mission · Mars Astrobiology Explorer-Cacher
Museums & exhibitions
The Science of Aliens

Retrieved from "http://en.wikipedia.org/wiki/Astrobiology"
Categories: Astrobiology | Extraterrestrial life
Hidden categories: Articles containing Ancient Greek language text
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