Livable Zone

In Astronomy, a livable zone ( ZH ) is an area of space where the conditions are favorable to the appearance of the life. There are two types of areas which can be jointly favorable, one on the level of a Planetary system, the other on the level of a Galaxie.

The Planet S and the the moons located in these areas are possible candidates for Habitabilité of a planet and thus able to lodge a extraterrestrial Vie.

The astronomers think that the life is possible in a “livable zone circumstellaire” located inside a planetary system and in a “galactic livable zone” (although research on this last point hardly begins).

Livable zone circumstellaire

The livable zone circumstellaire (or écosphère ) is a theoretical sphere surrounding a star and where the temperature on the surface of planets orbiting would allow there the appearance of Eau liquid. Many thinks that liquid water is vital because of its role in the biochemical reactions . In 1959, the physicists Philip Morrison and Giuseppe Cocconi described the zone in a research of SETI. In 1961, Frank Drake popularized this idea in its equation.

One can calculate the distance from this zone since star according to the size and the luminosity of star thanks to the following equation:

$d_ {WITH the} = \ sqrt {L_ {star} /L_ {Sun}}$

where

d_ {WITH the} \,

is the ray of the astronomical ZH in units S,

L_ {star} \,

is the Luminosité of star, and

L_ {sun} \,

is the luminosity of the Sun.

For example a star with 25% of the luminosity of the Sun will have a centered ZH with approximately 0,50 UA, and a star with twice the luminosity of the Sun will have a centered ZH with 1,41 UA. It is a consequence of the Loi of the square reverses luminous intensity. The “center” of the ZH is defined as the distance that the exoplanète should have of its star to have an average temperature comparable with that of the Earth by supposing (with many of other things) which it has a similar atmosphere in composition and of density.

As during their evolution the stars become more brilliant and more heats, the ZH moves away logically from star. To optimize the potential of development of the life, a planet should be located on an orbit which remains more the possible for a long time in the ZH.

Galactic livable zone (ZHG)

To shelter the life, a planetary system must be enough close to the center of the galaxy to have sufficient heavy elements which support the formation of telluric planets and of the atoms necessary to the life, such as the Fer in the Hémoglobine or the Cuivre in the Hémocyanine and the Iode in the gland Thyroïde.

But this system will have to be enough far from the galactic center to avoid dangers such as:

of star rubbings causing of orbital instabilities or the rains of Comet S and Asteroid S;
radiations of Supernovas S;
large the black holes of the galactic center.

Studies also showed that areas where the heavy elements abound seem to be likely more to have giant planets very orbiting close to their star, which would destroy or eject out of the system of planets of the terrestrial type. For all these reasons, it is very difficult to determine the ZH of a galaxy.

In our galaxy, the Milky Way, the ZHG would be located at 25.000 light-years (8 kilo Parsec S) of the galactic center, containing old stars from 4 to 8 billion years. As the galaxies have different compositions, the ZHG can be more or less large, not to even exist whole.

Criticisms

The concept is criticized by Ian Stewart and Jack Cohen in their book Evolving the Alien (evolution of extraterrestrial), in two points:

first is that this assumption supposes that the extraterrestrial life has the same needs for development as the terrestrial life, which is reducing according to their point of view;
second is that even by accepting this first point, certain circumstances would make livable of planets located out of the ZH, such as for example for one of the satellites of Jupiter, Europe, that one suspect to shelter an ocean under an ice-barrier of several kilometers.

See too

English references

Charles H. Lineweaver, Yeshe Finner and Brad K. Gibson, The Galactic Livable Zone and the Old Distribution off Complex Life in the Milky Way, Science, January 2004, volume 303, n°5654, pages 59-62.

External English bonds

The Encylcopedia off Astrobiology, Astronomy and Spaceflight
" Stars and Livable Planets" At SolStation

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