SE Astrobiologie:
Habitabilitätskriterien für (Exo-)planeten
Was ist Leben?
Johannes J. Leitner
Institut für Astronomie, Universität Wien
Trying to define classic life … I:
More than 350 different definitions of life can be found in literature.
Nevertheless, up to now there is no internationally accepted definition.
In general, but especially for the search for life and for the agendas of astrobiological research
projects (as FPL ExoLife) such a definition/w.h. would be necessary. Only with a definition/w.h.,
the outcome of search programmes in planetary habitats or of laboratory work can be verified.
Classification of life definitions (Tsokolov, 2009):
(1) Life is defined by terms, which are not exactly defined
for themselves (or in different disciplines as informatics,
physics, mathematics, biology, etc. in a different way).
(2) Combinations of characteristics of terrestrial life will be
designated as a life definition.
(3) Gradual evolution: from simple to complex chemical
reactions and further on to primitive (living) systems:
the arbitrary approach of “minimal life”.
Trying to define classic life … II:
(1) Life is defined by terms, which are not exactly defined for themselves:
Examples:
An organism maintains itself stationary at a fairly high level of orderliness (Schrödinger, 1944).
Life is a process that organizes matter to higher levels of complexity and maintains
that complexity (Orgel, 1973).
Life is defined as a material system that can acquire, store, process, and use information
to organize its activities (Dyson, 1999).
Life is an expected, collectively self-organized property of catalytic polymers
(Kaufffman, 1993).
What are the definitions of complexity, information, orderliness, etc.?
Tautologies:
Examples:
Life is an attribute of living systems (Nair, 2002).
Life is a way of capturing and using energy and materials (Starr and Taggart, 1992).
Trying to define classic life … III:
(2) Combinations of characteristics of terrestrial life will be designated as a life definition:
Life = acquiring energy + growth + adapation + reproduction (Webster, 1979).
Life = Individuality + reproduction + evolution (Korzeniewski, 2001).
Life = homeostatic metabolism + development + growth + reproduction (Abel, 2002).
Life = self-organization + self-replication + evolution + metabolism + concentrative
encapsulation (Arrhenius, 2002).
Life = thermodynamic disequilibrium + low-entropy state + information encoding and
transformation (Schulze-Makuch and Irwin, 2004).
Problems: not all characteristics are necessary,
not all are only characteristics of life,
most of them depend on the context,
etc.
Trying to define classic life … IV:
(3) Gradual evolution: from simple to complex chemical reactions and further to
primitive (living) systems: the arbitrary approach of “minimal life”:
minimal living system: primary chemical reactions as a consequence of a minimal set of
characteristics (A + B + C + D) (Luisi, 1998).
If a system shows only characteristics A + B + C, then it is not a living system.
But, if it shows A + B + C + D + E, then it is not a minimal system any more.
Problem: definition of A, B, C, D, E, etc. (depending on the authors)
e.g.: (Luisi, 1998)
A = self-sustaining
B = able to use energy gradients
C = semi-transparent cell walls
D = able to produce components itself
etc.
Trying to define classic life … V:
running evolution
A
B
C
S y s t e m s
D
E
w i t h
marker of minimal life
Additional problems:
B + C + D or A + B + E, etc.
F
t h e
G
H
I
J
K
L
M
c h a r a c t e r i s t i c s
Living
organisms/
systems
inanimate
processes
(3) Gradual evolution: from simple to complex chemical reactions and further to
primitive (living) systems: the arbitrary approach of “minimal life”:
Trying to define classic life … VI:
National Research Council (NRC), USA, 2007: working hypothesis:
 Water as solvent.
 Ability to use energy gradients (life is in a thermodynamical non-equilibrium
with its environment).
 Life on Earth consists of different molecules, which can pass various transformations
(a metabolism).
 These molecules, which are used by terrestrial life for the maintenance of the
metabolism, for the creation of structures (e.g. cells) , for energy balance,
for transfer of information and for the control of the chemical reactions
are a consequence of the chemical characteristics of:
carbon, hydrogen, nitrogen, oxygen, phosphor and sulphur.
 A system, which allows for evolution (not only reproduction), but as well as:
deselection of non-optimally suited organisms
adapation to chancing environmental conditions
interaction with other organisms.
What is denoted as exotic life I?
in general: any solvent
 Water as solvent.
 Ability to use energy gradients (life is in a thermodynamical non-equilibrium
with the environment.
 Life on Earth consists of different molecules, which can pass various transformations
(a metabolism).
on Earth islife for the maintenance of the
 These molecules, which are used Life
by terrestrial
interpreted
as an (e.g. cells) , for energy balance,
metabolism, for the creation
of structures
accommodation
to
for transfer of information
and for the control
of the chemical reactions
the physical
are a consequence of the chemical
characteristics of:
conditions on Earth!
carbon, hydrogen, nitrogen, oxygen, phosphor and sulphur.
various elements possbile
 A system, which allows for evolution (not only reproduction), but as well as:
deselection of non-optimally suited organisms
adapation to chancing environmental conditions
interaction with other organisms.
What is denoted as exotic life II?
Final working hypothesis:
(1) life is based on molecules which undergo chemical transformation
(a metabolism, which is working symbiotically or asymbiotically)
(2) life exploits a thermodynamic disequilibrium
(3) the macromolecules that enable the metabolisms, the
building of structures, energy management and transfer of information,
are a consequence of specific elements to form polymers with the ability
to bound functional groups (organic and/or inorganic)
(4) the molecules of life interact with a solvent to be soluble (or not)
or to react (or not) in a way that confers fitness of the life-form
(5) life has the ability to conserve its species
Fragen für die Diskussion:
2 Def.: Leben ist eine Eigenschaft von lebendigen Systemen.
Leben heißt sterben.
Sind Viren Lebewesen?
Ist ein Mann ein Lebewesen?
Sind Frauen über 70 Lebewesen?
Ist ein Maultier/Maulesel ein Lebewesen?
Ist Evolution ein Kriterium für Leben?
Ist Reproduktion/Replikation ein Kriterium für Leben?
Benötigen wir eine Def./Arbeitshypothese?
Wie können wir etwas finden, was wir nicht definieren können
 nach was sollen wir suchen?