Criticism
- The concept of a habitable zone is criticized by Ian Stewart and Jack Cohen in their book Evolving the Alien, for two reasons: the first is that the hypothesis assumes alien life has the same requirements as terrestrial life; the second is that, even assuming this, other circumstances may result in suitable planets outside the "habitable zone". For instance, Jupiter's moon Europa is thought to have a subsurface ocean with an environment similar to the deep oceans of Earth. The existence of extremophiles (such as the tardigrades) on Earth makes life on Europa seem more plausible, despite the fact that Europa is not in the presumed CHZ. Astronomer Carl Sagan believed that life was also possible on the gas giants, such as Jupiter itself. A discovery of any form of life in such an environment would expose these hypothetical restrictions as too conservative. Life can evolve to tolerate extreme conditions when the relevant selection pressures dictate, and thus it is not necessary for them to be "just right".
- Differing levels of volcanic activity, lunar effects, planetary mass, and even radioactive decay may affect the radiation and heat levels acting on a planet to modify conditions supporting life. And while it is likely that Earth life could adapt to an environment like Europa's, it is far less likely for life to develop there in the first place, or to move there and adapt without advanced technology. Therefore, a planet that has moved away from a habitable zone is more likely to have life than one that has moved into it.
- Just as different levels of atmospheric pressure can affect water remaining in liquid state, the presence of dissolved compounds, such as salts or ammonia (a powerful antifreeze) can lower the freezing point of water. Earth's oceans are saline, which helps prevent them freezing. Greenhouse effects, solvents in water or a combination of these factors could potentially enable planets with certain conditions to sustain surface water. For example, briny water is proposed as an explanation for Seasonal flows on warm Martian slopes.
Read more about this topic: Habitable Zone
Famous quotes containing the word criticism:
“Parents sometimes feel that if they don’t criticize their child, their child will never learn. Criticism doesn’t make people want to change; it makes them defensive.”
—Laurence Steinberg (20th century)
“...I wasn’t at all prepared for the avalanche of criticism that overwhelmed me. You would have thought I had murdered someone, and perhaps I had, but only to give her successor a chance to live. It was a very sad business indeed to be made to feel that my success depended solely, or at least in large part, on a head of hair.”
—Mary Pickford (1893–1979)
“Unless criticism refuses to take itself quite so seriously or at least to permit its readers not to, it will inevitably continue to reflect the finicky canons of the genteel tradition and the depressing pieties of the Culture Religion of Modernism.”
—Leslie Fiedler (b. 1917)