How many Earths?

Is the Earth Special?

Dave Watham, director of an MSc in geoscience and petroleum geology at Royal Holloway, London.

BRLSI 5^th May 2016

Notes by William Gaskell

One question we must ask is how many worlds are there? As the Earth must be very odd. Probably as rare as a world made of gold with diamond dust rings. There is a likelihood that such worlds exist given the size of the universe and our Earth must be that rare, so we assume there would be at least a couple of Earths.

We typically only see the hard rocks in nature as the softer rocks erode much more quickly. An example of this would be a granite rocky outcrop in the hills overlooking a river valley. Similarly, this affects what we see in the night sky, it is much easier to see a bright star such as Betelgeuse or object such as the Orion Nebula and Orion’s Belt than it is to see a dimmer start which may be more similar to our Sun. This is the Anthropic Principle – we only see things that are compatible with our life.

Earth’s features:

Plate tectonics are rare

Magnetic fields are rare – Mars doesn’t have one.

Life needs time for intelligence to arse so only planets with an early start in life become observers.

Water and life – geological evidence that it’s been on Earth for billions of years.

Venus is evidence that climate stability is not inevitable – there was water on Venues and we can tell from the hydrogen-deuterium mix we now see present on the planet but the toxic clouds would have removed the water from the planet.

Carl Sagan thought because the Sun has gotten warmer over time Earth should be 100 ˚C warmer now than in the past – Earth was previously a frozen planet. Solar warming has been reversed by biological cooling, the Gaia Hypothesis. But also seems like Goldilocks hypothesis that the Earth is suited perfectly to our evolution.

The Moon is outside of our biosphere. Tidal forces from the Moon are three times more powerful than tides from the Sun. Removing the Moon would create an instability in the Earth by affecting the rate at which the Earth’s axis precesses. There is perhaps a resonance in the Earth’s axis with other planets in the solar system such as the axis of the rings of Saturn but that may just be a red herring. The length of the Earth day gets longer as the Moon recedes from the Earth. The bulge from tidal effects of the Moon was much greater in the past as the Moon was closer to the Earth which made the Earth’s axis precess faster.

We find the conditions of the Earth-Moon system of lunar mass vs Earth’s rotational angular momentum is in the Goldilocks zone between instability and  a region where there would be much colder poles on Earth leading to carbon dioxide clouds forming bringing about runaway cooling and snowball Earth forming.

If the Moon’s orbit was tidally locked with the Sun then the Moon could quite quickly fall back to Earth.

We assume our example, Earth, is more likely to be typical than atypical we can then assume a situation halfway between Anthropic and Copernican principles.

anthropic principle
anˈθrɒpɪk/noun
the cosmological principle that theories of the universe are constrained by the necessity to allow human existence.
-Google 12:35 07/05/2016
The Copernican Principle
The scientist Nicolaus Copernicus recognized that Earth is not in any particularly privileged position in the solar system. This elegant fact can be extended to encompass a powerful idea, known as the Copernican Principle, that we are not in a special or favorable place of any sort. By looking at the world through the eyes of this principle, we can remove certain blinders and preconceptions about ourselves and re-examine our relationship with the universe.
https://www.edge.org/response-detail/10277 12:38 07/05/2016