The habitable zone

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 

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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. 

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 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. 

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... 

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 discovery of evidence of liquid water-ammonia eutectics on Titan provides a context for the potential for polar fluids outside what is normally regarded as the habitable zone. The stay of the Cassini-Huygens mission on the surface of Titan was brief, but this moon of Saturn is the locale that is most likely to support exotic life. The committee believes that it is important to consider whether the planned missions to the solar system should be reordered to permit returning to Titan earlier than now scheduled. 

Figure 1 The habitable zone (Kasting et al, 1993). Too close to the Sun, a planet s surface is too hot to be habitable too far, it is too cold. Early in the history of the solar system, the Sun was faint and the habitable zone was relatively close 4.5 Ga later, with a brighter Sun, planets formerly habitable are now too hot, and the habitable zone has shifted out. Note that boundaries can shift. By changing its albedo and by altering the greenhouse gas content of the air, the planet can significantly widen the hounds of the hahitahle zone (Lovelock, 1979, 1988).

The Earth lies in the habitable zone, the right distance from its star (Kasting et al. 1993). But the Sun s output both varies and is steadily brightening. How wide is the allowable range of... 

Reductive metabolism captures free energy ultimately produced by the fission of uranium, thorium, and potassium-40 in the earth s mantle, but makes no use of the richer free energy stress from solar fusion reactions, other than exploiting liquid water as a solvent in the habitable zone. Photosynthesis captures this independent fusion energy source, but appears to have become accessible only with the molecular complexity of modem cells. It therefore evolved to be self-supporting by artificially... 

First, the Earth occupies the "habitable zone" of the solar system. This means that it lies at a distance from the sun which, over the past 4.5 Ga, as the sun has warmed, has consistently maintained a surface temperature conducive to life (Kasting et al. 1993 Kasting Catling, 2003). 

Spacecraft missions are being planned by both NASA (the Terrestrial Planet Finder) mid ESA (Darwin) to search for Earth-like planets around other stars and to take spectra of their atmospheres and surfaces 44, 45). These missions are designed to detect planets located in the habitable zones (HZ) of other stars. The... 

In this Chapter we discuss how we can read a rocky planet s spectral fingerprint and characterize if it is potentially habitable. In Sect. 5.1 we explore the Earth as seen as an exoplanet, in Sect. 5.2 focus on low resolution biosignatures in the spectrum of an Earth-like planet, in Sect. 5.3 set the focus on the first set of measurements to characterize a potentially habitable planet. In Sect. 5.4 we discuss the concept of the Habitable Zone, Sect. 5.5 discusses the influence of the host star on the detectable features. Sect. 5.6 explores the spectral evolution of Earth through geological time, Sect. 5.7 the detectability of surface features like the vegetation red edge and Sect. 5.8 summarizes the chapter. 

The ability to associate a physical surface temperature to the spectrum relies on the existence and identification of spectral windows probing the surface. For an Earth-like planet there are some atmospheric windows that can be used in most of the cases, especially between 8 and 11 pm as seen in Fig. 5.1. Such identification is not trivial for non Earth-analogue atmospheres. Note that this window would however become opaque at high H2O partial pressure (e.g. the inner part of the Habitable Zone (HZ) where a lot of water is vaporized) and at high CO2 pressure (e.g. a very young Earth or the outer part of the HZ). 

The Habitable Zone is a concept only defined to detect life as we know it, remotely, and should be termed the Remotely detectable Habitable Zone, but the common use of Habitable Zone (HZ) is frequent. It is by no means meant as the exclusive zone where fife around a star could exist. The definition of the HZ was driven by the possibility to remotely detect life as we know it. It is tied to the region around a star where water could be liquid on the surface of an Earth-analogue planet. This criterion is sometimes justified by liquid water being in our current state of knowledge, an essential part in the origins of life (see Chaps. 6, 7 and 8 for a detailed discussion). Subsurface water could provide similar conditions for life even though different energy sources would be needed than on the surface of a planet, but several options are available. 

Pierrehumbert R, Gaidos E (2011) Hydrogen greenhouse planets beyond the habitable zone. Astrophys J 734 13L... 

Kaltenegger L, Sasselov D (2011) Exploring the habitable zone for Kepler planetary candidates. Astrophys J 736 L25... 

Wilhams DM, Pollard D (2002) Earth-like worlds on eccentric orbits excursions beyond the habitable zone. Int J Astrobiol 1 61... 

The concept of habitability in astrobiology is associated with the idea that life can exist in environments where life is found on Earth. A rough definition of the habitability zone that has been selected by astronomers in their search for... 

Let us consider stars of different types and ask the question whether there exists a habitable zone around them or not. In this context often the term circumstellar habitable zone or ecosphere is used. Obviously, the Earth is located in the center of the HZ of the solar system. Let d denote the mean radius of the habitable zone in AU, L the bolometric luminosity of a star and L the bolometric luminosity of the Sun, then ... 

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