EAS-134 Is Earth Unique?
Habitable Zones of the Universe
Chapter 2
1. Hostile Places
Even before we start, a number of locations must be ruled out for life as we know it
- empty space (between planets, stars and galaxies)
- the interior of stars (but perhaps not moons or planets)
- frigid gas clouds (in galaxies)
- the (non-existent) surfaces of gaseous planets
We speculate instead on the zones where we might find similar planets to Earth. Note that an astronaut who is properly equipped could live in many hostile environments (even deep space), but this is not the same as requiring a habitat for the initiation and development of life.
2. Distance from a Star
- habitable zones (HB) exist at certain distances from a star, depending on its type and age; we live by a G2 star (Figure 2.1)
- requirement is that water is neither permanently frozen, nor exceeds it boiling point (and cannot exist as a liquid)
- imagine sitting around a campfire on a cold night – too near and one would burn too far and one would freeze
- Venus is typical of the inner edge (too hot for a liquid ocean), Mars is typical of one possibility for the outer edge (frozen to a good distance below it surface). Alternatively the outer edge may be the freezing point of CO2.
- some calculations place limits of where Earth must be (Hart, 1978) – no more than 1% further or 5% closer than now!
3. The Carbon Cycle
- HZ extended by feedback effect of CO2
- CO2 is a greenhouse gas, absorbs infrared radiation from Earth’s surface adding 40ºC to temperature
- physical and chemical weathering of rock removes CO2 from atmosphere, thus cooling atmosphere
- as Earth cools, weathering decreases, thus removing less CO2
4. Different HZs
- animal HZ: a restricted HZ in which water is liquid and temperatures < 50ºC
- human HZ: a more restricted HZ in which mankind can grow food crops for survival
- microbial HZ: where primitive (microbial) life may survive, e.g. perhaps the liquid ocean under the surface of Europa
- temporal HZ: defined by certain time limits in history of a star (e.g. 5-8 By from formation);. long time required for development of complex organisms
5. Planets ejected from HZ
- gravitational interactions between planets can cause orbital instabilities, and planets could be ejected (loss of Saturn)
- planets would retain interior heat (for a while)
- a moon orbiting an ejected planet might be a habitat, particularly if heated tidally
6. Other Stellar Systems
- stars get brighter as they age, so radius of HZ increases
- stars more massive than the sun age more rapidly, so HZ is shorter; also radiate more UV that is dangerous to organisms (breaks organic molecular bonds), also UV reduces atmosphere by excessive heating
- less massive stars comprise 95% of all stars (e.g. M stars), are less luminous (HZ smaller), so planets have to be closer; danger of gravitational locking (e.g. Moon, Mercury) that might freeze atmosphere
7. Multiple Star Systems
- if a binary system has 2 close stars, planets may orbit both,
- a system has widely spaced stars, planets made orbit only one of the pair
- need stable orbits (more difficult)
- more problems with stellar aging
- open clusters not good candidates (too young)
- globular clusters ( up to 10 000) stars may occupy small space (e.g. 20 Ly), danger of excessive radiation, supernovae, and too old to have abundant heavy elements
- single stars are the best (no gravitational effects)
8. HZs in Galaxies
- our galaxy is spiral; we are on inside edge of one arm 25KLy from center (diameter 80KLy)
- too close to center is bad (unstable), too far is bad (low heavy elements)
- neutron stars particularly dangerous due to hard radiation up to 30 Ly away
- elliptical galaxies are old, and heavy elements is low
9. Time Constraints
- Big Bang (~14 By) did not produce heavy elements, only supernovae can
- radioactive isotopes may be essential to warm planetary interiors, not present in older galaxies
- huge number of galaxies found, especially by Hubble Telescope, many irregular ones far back in time
- galaxies are clumped into vast regions, may affect habitability
- all planets eventually will become uninhabitable due to evolution of central star (still 4 By yet for Sol!), heat will boil oceans, so all life seems to be ephemeral
