Habitable Zones

The start of NASA s Kepler mission is planned for February 2009 and has goals similar to those of the COROT project, though rather more ambitiousiit is intended to determine the percentage of terrestrial and larger planets there are in or near the habitable zone of a wide variety of stars, and also to determine the distribution of sizes and shapes of the orbits of these planets. 

It is assumed that the greatest part of our solar system, and indeed of the Milky Way, is hostile to life. The term habitable zone (Franck et al., 2002) takes into account... 

Can we assume that there is a habitable zone somewhere in our galaxy This seems feasible astronomers divide the Milky Way into four regions, which cannot always be exactly separated ... 

Australian astronomers have attempted to characterise the habitable zone of the Milky Way more exactly. The models which they used were based on the following assumptions for complex life in our galaxy ... 

The group identified a circular belt between 7 and 9 kpc1 from the centre of the galaxy. This zone (Fig. 11.6) consists of a population of stars which developed between 8 and 4 billion years ago it contains about 10% of all the stars in the galaxy, and around 57% of the stars in the habitable zone are older than our sun (Lineweaver et al., 2004). 

When, many, many million years in the future, our sun expands in its Anal phase to become a red giant, the habitable zone of our solar system will shift by 1-2 AU, to the region where Triton, Pluto/Charon and the Kuiper Belt are found. This zone is referred to as the delayed gratification habitable zone . All the heavenly bodies in this zone contain water and organic material, so that chemical and molecular... 

AU from the Sun, just outside the orbit of Mars, whereas the inner boundary is determined by the runaway-greenhouse effect as observed on Venus. If the surface temperature were too hot, above 373 K, this would vaporise all water on the surface of the planet. The inner boundary is around 0.85 AU so the habitable zone spans 0.85-1.7 AU for our Sun (Figure 7.7) but the current habitable zone spans 0.85 - 1.3 AU (t — 0) in Figure 7.7. The habitable zone was much larger when the Sun s luminosity was greater, and narrower when the luminosity was smaller. 

Calculate the distance spanned by the habitable zone around the Sun at its current luminosity of 3.8 x 1026 W. Using Equation 7.5 for the flux at a distance corresponding to 273 K, the freezing point of water is given by ... 

Repeating the calculation for the inner end of the habitable zone with Te = 373 K, the boiling point of water gives d = 0.92 AU. 

There are many sources of error with understanding the evolution of the Sun s luminosity but it raises the question of a continually habitable zone around a... 

Current extrasolar planets are all much larger than the Earth. The total count at present (9 September 2005) is 168 found in 144 planetary systems, of which 18 contain multiple planets. The first to be discovered was 51-Pegasi in the constellation of Pegasus by the radial velocity method. It is about 0.45 Mjupiter and has an orbital period around the star of about 4.5 days. Of the 168 planets found so far only nine are present within a habitable zone around their star. The survey of the star catalogue for planets has only just started and we have found a large number of planets very quickly - solar systems, at least, are not special. 

The temperature of the planet within a habitable zone controls the rate of chemical reactions, the availability of chemicals and the phase in which the chemicals will be found. These have profound consequences for prebiotic chemistry and the origins of life. 

The radiative heading of a planet from the local start to produce a surface temperature on the planet that will allow liquid water. From these calculations, estimates of global warming contributions can be made. A continually habitable zone allows for the variations in stellar flux as it evolves... 

Flabitable and continually habitable zone Variation of temperature and pressure with height Steady-state atmospheric networks leading to Chapman layers... 

Calculate the effective surface temperature of an Earth-like planet with the same diameter, albedo and orbital radius around Aldeberan in Taurus, which has a surface temperature of 7200 K. Is the planet within the habitable zone of this star ... 

Calculate the extent of the habitable zone around the star Aldeberan. 

Continually habitable zone A region around a star where a planet can maintain a surface... 

Habitable zone A region around a star that maintains the surface temperature of a radia-tively heated planet above the melting point of water. 

Fig. 4.1. Planetary habitability zones, a Old view, b New vie Kargel (2004) with permission...

Vogel G (1999) Expanding the habitable zone. Science 286 7071 Vreeland RH, Rosenzweig WD, Powers DW (2000) Isolation of a 250 million-year-old halotolerant bacterium from a primary salt crystal. Nature 407 897-900... 

Earth may be just one of many models of planets that can evolve complex life. We do not know whether it is even practical or logical to assume that planets that exist outside our perception of a habitable zone could harbor life, particularly life that we know nothing about. Our practical search for extraterrestrial life is focused on water-rich planets and moons because of the possibility that they can support Earth-like life. That does not preclude... 

Many specific properties of water are cited to make the case that water is an ideal biosolvent uniquely suited to support life Frozen water floats. Water is an excellent solvent for salts. Water is liquid over a broad range of temperature. Indeed, the concept of a habitable zone, a region around a star where life is presumed to be possible, largely posits a region in which a planet s surface or subsurface might support liquid water. One such planet in a habitable zone could export life to other suitable environments via meteoroids. 

For Saturn, Uranus, and Neptune, the habitable zone is broader relative to the planetary radius. On Saturn, the temperature is about 300 K when dihydrogen becomes supercritical. On Uranus and Neptune, the temperature when dihydrogen becomes supercritical is only 160 K, a temperature at which organic molecules are stable. 

The atmospheres of these planets convect, of course. To survive on Jupiter, any hypothetical life based on molecules containing carbon-carbon covalent bonds would have to avoid being moved by convection to positions in the atmosphere where they are not stable. This is, of course, not impossible. Even on Earth, life in the oceans must avoid being moved by convection from its particular habitable zone. Sagan and Salpeter presented a detailed discussion of what might be necessary for a floater to remain stable in the jovian atmosphere.25... 

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