Image in the header: Goldilocks and the three bears. Source: kidsshortmoralstories.com
In the story of Goldilocks (Goldilocks in English), the sweet beautiful child tries everything in the mountain house: the chairs, the plates of food, the beds. Papa bear’s things are too big, too hard, too warm. Mummy bear’s are too soft, too cold, etc. Luckily, the bear family has a baby, because his things are all perfect. The ‘goldilocks zone’ in astronomy is the zone around a star where everything is just right (where liquid water can exist permanently), and the ‘goldilocks planet’ has just the right properties to keep everything within well-livable limits.
If you have read Part 1 of this course in full, then I hope you have learnt that the Earth is a very special planet, and that there are lots of properties right in the Goldilocks range. To refresh your memory, I put below a list of the most important of those Goldilocks properties:
- Distance from the Sun in habitable zone (suitable temperature for liquid water) + near circular orbit around the Sun (stability in temperature throughout the year)
- Ideal ocean-to-land ratio – thermohaline circulation provides temperature buffers and heat distribution
- Ideal mass of the planet – atmosphere of right density (stability climate, stable carbon cycle and other element cycles)
- Air pressure/temperature/greenhouse gas ratio always good for liquid water and livable temperatures
- Oxygen content (origins of photosynthesis, balance reducing geosphere vs oxygen increase)
- Planetary core composition and rotation rate : permanent magnetic field
- Sufficient internal heat: plate tectonics & volcanism & magnetism
- Crust not too thin and not too thick: plate tectonics
- Long term: decreasing concentration of greenhouse gases, offsets ‘faint sun’
- Collision Theia: addition heavy elements (core, radioactiveve heat), addition water, right angle of incidence, right velocity, right mass
- Large moon stabilises obliquity (tilt of 23°, seasons and temperate climates)
- Jupiter lowers chance of major impact that could kill everything
- Distance to core galaxy: in ideal zone
- The Sun is neither too big nor too small: no tidal lock, and sufficiently long stability
- The Sun is a very stable third-generation star with correct chemical composition
So the Earth is so ideal that the chances become rather small that we would find an equally good, equally livable planet around other suns. Then again, Earth’s habitability has remained stable enough for 4 billion years. That was apparently necessary to give something as unlikely as complex multicellular life time to develop, and eventually even produce an intelligent species. Remember also that the age of multicellular complex life on Earth (‘the age of the animals’) lasts only 1 billion years, while planet Earth did exist for more than 10 billion years.
Rare Earth
25 years ago, Donald Brownlee and Peter Ward published their famous book ‘Rare Earth’. In it, they paint the Goldilocks picture I described above. Judging from all the knowledge we have of our Earth, life, and the other planets, they conclude that it is highly unlikely that on another planet all the properties are just right and this over billions of years. In other words, complex multicellular life they don’t actually expect to find. Not in several tens or hundreds of light years around us, or even perhaps nowhere at all in the Milky Way galaxy! Simple, bacteria-like life, on the other hand, might be possible. More so, the more we learn about the earliest life on Earth, the more we realise that the conditions to make it possible are very common in our universe. In other words, ‘simple’ life (like our prokaryotes) could well be very common around other stars around us. However, should we find even one such alien bacterial species during our lifetime, it would still be a hugely impressive milestone in our search for life.
Now, so many years later, Ward and Brownlee remain even more convinced of their theory than before. They even argue that ‘rare’ (English: rare) totally fails to adequately express how small the chances are that complex life (with ecosystems of multicellular organisms) would arise on another planet. Of course, not everyone agrees. At the end of part 2, we will discuss this further. But first, we will explore the essential properties of life on Earth, so that we understand even better what exactly happens on a ‘living planet’.