Astrobiologists (researchers who are studying the possibility of life on other worlds) are interested in Saturn’s moon Titan as a possible abode for life. But any indigenous life on Titan would have to be far different from any known life form on Earth, as the mean surface temperature is much colder (~93 K compared to 288 K on Earth), turning water into rock-solid ice. All life that we know of on Earth requires liquid water during at least part of its life cycle. Titan does have lakes of liquid methane on its surface, and it is conceivable that some form of alien life might have evolved there. While I’m skeptical about this myself, this hypothesis could be tested by returning to Titan with a boat and sampling the methane lakes. Jonathan Lunine at Cornell University has suggested just such a mission as part of NASA’s Discovery program, although it has yet to be selected for funding.
Definition of the habitable zone
For those astrobiologists like myself who view life more conventionally, the habitable zone around the Sun, or any star, is defined as the region where liquid water can be maintained on the planet’s surface. This does not necessarily preclude life on places such as Jupiter’s moon Europa, which is thought to have a subsurface liquid water ocean. But it’s an appropriate definition for planets around other stars. That’s because, in order to obtain evidence for life on such planets, that life needs to be present at the surface so that it can modify the planet’s atmosphere in a way that we could potentially detect. Earth’s atmosphere, of course, is filled with molecular oxygen, O2, most of which has been produced by photosynthesis. It also contains smaller amounts of methane, CH4, nearly all of which has been produced by methanogenic bacteria. These gases could potentially be detected spectroscopically by aliens studying the Earth remotely using a large space telescope orbiting their own planet. Similarly, if we build such a telescope ourselves, we can look for evidence of life on other Earth-like worlds. The planets of greatest interest will be those that lie in the habitable zone of their parent star.
Habitable zone limits
So, where exactly do the boundaries of the habitable zone lie? NASA’s Kepler Space Telescope Mission, which operated at full capacity for about 4 years (ending in Spring, 2013, when it lost a critical reaction wheel), has spawned a renewed interest in answering this question. Kepler’s goal is to determine the fraction of stars that have rocky planets within their habitable zones by using the transit method. A planet that passes in front of its star as it orbits around it will block out a small percentage of the star’s light when it passes between the star and the observer. Kepler is able to measure the small decrease in a star’s brightness, and it has successfully detected transits of Earth-sized planets orbiting within the habitable zones of their parent stars. About 10-20 percent of Sun-like stars appear to harbor such planets, and that percentage rises to 40-60 percent for stars much smaller than the Sun.
The conventional habitable zone, as defined by climate models, extends from about 0.95 AU to ~1.8 AU. (1 AU = 1 astronomical unit = mean Earth-Sun distance.) The Sun was about 30 percent less bright when it formed, 4.6 billion years ago, so the habitable zone was slightly closer in at that time (Fig. 1). Nevertheless, Earth lies comfortably within it, then and now, whereas Venus is inside the inner edge. That, we think, caused Venus to develop a runaway greenhouse effect, resulting in the dry, hot planet that we observe today. Mars is thought to be within the habitable zone today and near the outer edge back then. But Mars is uninhabitable today, most likely because it’s a small planet which has cooled internally. Thus, it is no longer volcanically active and cannot recycle carbon dioxide into its atmosphere. Mars also loses atmosphere to space because of its small size.
Titan orbits around Saturn, which itself orbits the Sun at a distance of 9.6 AU. So, according to the conventional definition of the habitable zone, Titan lies far outside of it. Some researchers, however, have proposed that the habitable zone may extend as far out as 10 AU for a particular type of hydrogen-rich planet. The conventional habitable zone is based on the assumption that CO2 and H2O are the only available greenhouse gases. Such gases contribute to the greenhouse effect which helps keep a planet’s surface warm. Molecular hydrogen, H2, turns out to be a surprisingly strong greenhouse gas, and so planets that capture large amounts of H2 from the surrounding stellar nebula as they form could remain warm and habitable at large distances from their parent stars. One needs an atmosphere that is at least 40 times thicker than Earth’s atmosphere, however, and which is composed primarily of H2. Titan’s own atmosphere is 1.5 times thicker than Earth’s (in terms of pressure) and is composed mostly of N2 and CH4. So, Titan itself is cold and very un-Earth-like.
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