PHY111 Our Evolving Universe:
Lecture Summaries

The habitable zone around a star is defined as the range of distances from the star at which an Earth-like planet could have liquid water on its surface. This image, from Wikimedia Commons, shows the planets of the solar system and the class M main-sequence star Gliese 581 superimposed on the calculated habitable zone. The Gliese 581 system is the most promising so far discovered in terms of perhaps hosting habitable planets, though Gl 581g (the best prospect) is an unconfirmed recent discovery and may not exist.

Summary of Lecture 14 – Planets and Life

1. Although searches to date have only been sensitive to massive planets, there are techniques that could detect Earth-sized planets around nearby stars:
   • transit observations – monitoring a star's brightness very precisely to detect the small dip when a planet passes in front of it:
     • the number of planets observed to transit is growing rapidly as techniques improve;
     • the Kepler mission hopes to detect 50-640 terrestrial planets around Sun-like stars (50 if they are similar to Earth, several hundred if most are somewhat larger than Earth);
     • will require spectrscopic follow-up to eliminate misidentifications (brown dwarfs or grazing binary-star eclipses misinterpreted as planets would give much larger Doppler shifts in the parent star's spectrum, because they are more massive objects)
   • direct observation requires very high resolution to separate the faint planet from the much brighter star:
     • space-based interferometer, in which a number of small telescopes are combined to produce a "virtual telescope" with a resolution corresponding to the size of the whole array;
       • design concepts Darwin (ESA) and Terrestrial Planet Finder (NASA) appear feasible but have not been funded
     • proposed extremely large telescopes (ground-based instruments with 30-100 m diameter) work better for giant planets but might be able to image some rocky planets in favourable circumstances;
2. Life on Earth is carbon-based and uses water as a solvent:
   • no credible alternative to carbon – it is chemically the most suitable basis for complex chemistry, and is also very common in the cosmos (4th after H, He, O)l
   • water is also exceptionally suitable – very good solvent, very abundant, expands when freezes (so oceans do not freeze solid in winter)
   • hence most studies of extraterrestrial life assume carbon and water based
3. The history of life on Earth suggests that life might develop "easily" on suitable planets:
   • fossils of bacteria are found in the oldest rocks capable of preserving fossils (dating from 3.5 billion years ago);
   • however earliest multicellular organisms date from only ~550 million years ago –
   • difficult issue may not be life, but complex (multicellular) life.
4. Life elsewhere in the solar system is unlikely:
   • Venus is much too hot, owing to runaway greenhouse effect;
   • Mars is much smaller than Earth, and its lower gravity has led to the loss of most of its atmosphere;
     • however, much evidence from Martian land-forms that Mars had liquid water earlier in its history;
     • life might have developed then, so worth searching for fossils or even remaining living organisms (e.g. underground);
     • discovery of life on Mars, assuming independent origin, very important in supporting theory that life evolves "easily";
   • Europa, a moon of Jupiter, would be much too cold, but it is close enough to Jupiter to be heated by tidal stresses, and there is evidence that it may be covered with a liquid water ocean beneath its water-ice crust – just possible that life might have developed here:
     • several other icy satellites also show evidence for subsurface oceans (e.g. Enceladus) – may be quite common;
     • however no way to tell whether life has developed without sending probe with very large drill;
     • therefore not helpful when looking for life outside solar system (though, if discovered, extremely convincing evidence for easy evolution of life).
5. Requirements for suitable stars for life-bearing planets:
   • not so low in heavy elements that planets are unlikely to form (rules out very old stars made from un-enriched gas);
   • lifetime long enough to allow complex life to evolve (not O, B or A stars);
   • stable star in stable system (probably rules out close binaries).
6. Requirements for suitable planet for detectable life:
   • rocky planet, not gas giant;
   • at appropriate distance from star for liquid water on surface (in habitable zone);
   • preferably not so close to star that it will become tidally locked, so that one hemisphere always faces star (rules out low-mass class M stars).
7. Detection of (not necessarily complex) life on an extrasolar planet could be achieved by spectroscopy:
   • oxygen is highly reactive and is maintained at high atmospheric concentration on Earth only by biological activity (photosynthesis);
   • ozone (O₃) has strong spectral features in infra-red, where studies would probably be done to minimise glare from star;
   • detection of ozone would be strong evidence of life.
8. Probability of intelligent life is often addressed via the Drake equation:
   • N = R_*×f_planets×n_E×f_life×f_int×f_tech×L, where
   • N is the number of technological civilisations in the Galaxy, which is what we want to estimate;
   • R_* is the rate of formation of suitable stars
     • we already have a good estimate of star formation in the Galaxy, and a fair idea of what sort of stars would be "suitable"
   • f_planets is the fraction of such stars that have planets
     • we have a lower limit on this already (there could be more planetary systems that we can't yet detect) and will soon have a good estimate (from Kepler)
   • n_E is the number of Earth-like planets per planetary system
     • we don't know this yet, because we can't detect Earth-sized planets, but Kepler will give us a first estimate
   • f_life is the fraction of Earth-like planets on which life evolves
     • we don't know this yet – we could get a good estimate quite soon (if a future Mars rover discovers evidence of past or surviving Martian life), or it could take decades (if there is no evidence of life elsewhere in the solar system, and we have to develop the capacity to do spectroscopy on extrasolar Earth-like planets), but it is certainly possible to acquire observational evidence on which to make an estimate
   • f_int is the fraction of life-bearing planets on which an intelligent species evolves
     • we don't know this, and there seems no obvious way to estimate it (unless you argue that the absence of another species on Earth with intelligence comparable to ours indicates that the probability of evolving intelligence is low)
   • f_tech is the fraction of intelligent species that develop technology adequate to support interstellar communication
     • we don't know this, and the only way to determine it would be to detect a signal!
   • L is the average lifetime of such civilisations
     • the only information we have on this is that our own civilisation reached this stage of technology about 60 years ago, and hasn't yet collapsed!
   • Conclusion: although within a couple of decades we could have a good estimate of the number of life-bearing planets in the Milky Way, there seems to be no clear way to work out the number of these which support alien civilisations – depending on the numbers you guess for the last three factors, the number could be quite large (thousands) or exactly 1 (us!)

Web links

• The PowerPoint file for this lecture.
• A good source for the history of life on Earth is the website of the University of California at Berkeley's Museum of Palaeontology, source of some of my fossil pictures.
• Nick Strobel has a chapter on "life beyond the Earth", though it only goes into detail on the Drake equation and SETI; Gene Smith doesn't cover this at all.
• SolStation.com has a good summary of the concept of habitable zones, rather biased towards the capacities of NASA's Kepler mission. The simulated spectrum showing ozone came from the Darwin homepage (no longer active, sadly – the link is to an archived copy).
• Professional SETI sites include The SETI Institute, The Planetary Society and SETI@home.
• The images of Europa and Venus were from Galileo and Magellan respectively; the image of Mars was taken at opposition by the HST.
• A short self test for this lecture.

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