Why is the Earth's size not odd?

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Discussion Overview

The discussion revolves around the question of why Earth is considered "odd" in size compared to its neighboring planets, particularly in the context of rocky planets in the inner solar system. Participants explore various theories regarding planetary formation, size determination, and the implications of Earth's size for the potential for life.

Discussion Character

  • Debate/contested
  • Exploratory
  • Conceptual clarification

Main Points Raised

  • Some participants argue that Earth's size is odd because it is larger than its neighboring rocky planets, though it is not as large as gas giants, which are not considered "real solid planets."
  • Others counter that Earth's size is only marginally larger than Venus, suggesting that this is not odd.
  • One participant mentions that Earth's size was influenced by a collision with a Mars-sized body, contributing to its iron core and the formation of the Moon.
  • There is speculation about the role of chance in determining planetary sizes, with some suggesting that collisions and accumulation of material play a significant role.
  • Some participants propose that Earth's size may be close to the limit for stability during formation, referencing tidal forces and the Roche limit.
  • One viewpoint introduces the Anthropic Principle, suggesting that larger planets may be more conducive to the development of life due to their ability to retain heavier elements in their atmospheres.
  • Another participant discusses the necessity of a certain size for Earth to maintain liquid water and a stable atmosphere, which are essential for life.
  • There is a suggestion that if Earth were the smallest planet, the perception of its size would be different, indicating that the notion of "oddness" is subjective.

Areas of Agreement / Disagreement

Participants express differing views on whether Earth's size is odd, with some asserting it is not odd due to its relative size to Venus, while others maintain that its status as the largest rocky planet is indeed peculiar. The discussion remains unresolved with multiple competing perspectives on the implications of Earth's size.

Contextual Notes

Participants reference various factors influencing planetary size, including collisions, material accumulation, and atmospheric retention, but these points are not universally accepted or agreed upon. The discussion includes speculative elements regarding the conditions necessary for life and the implications of size on habitability.

AtomicJoe
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It is bigger than all it's neighbours, which is odd.
Obviously it is not as big as the gas giants but they are not real solid planets.


[PLAIN]http://blrdb.com/images/BlrDb_Astronomy_Planet_Sizes.gif
 
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I don't accept your premise: it is only 5% bigger than Venus, so that's not odd.
 
Well, Earth was a bit smaller until something the size of Mars made a glancing collision, donating its iron core, and their crustal debris congealed as our Moon...
 
russ_watters said:
I don't accept your premise: it is only 5% bigger than Venus, so that's not odd.

That is the diameter, the volume is 15% bigger, but nonetheless it is curious we are the biggest small planet.


What determined our size? And indeed the sizes of the other planets?

Somebody must know.
 
AtomicJoe said:
That is the diameter, the volume is 15% bigger, but nonetheless it is curious we are the biggest small planet.


What determined our size? And indeed the sizes of the other planets?

Somebody must know.
Chance determined the size. Things collide. Things collect together. Things get destroyed.

http://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System
 
AtomicJoe said:
It is bigger than all it's neighbours, which is odd.
Obviously it is not as big as the gas giants but they are not real solid planets.[PLAIN]http://blrdb.com/images/BlrDb_Astronomy_Planet_Sizes.gif[/QUOTE]

The Earth may be close to the limit a rocky body can be and still have been stable during its formation in terms of the tidal forces in the inner solar system. Look up the Roche limit.
 
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AtomicJoe said:
It is bigger than all it's neighbours, which is odd.
Obviously it is not as big as the gas giants but they are not real solid planets.


[PLAIN]http://blrdb.com/images/BlrDb_Astronomy_Planet_Sizes.gif[/QUOTE]

In the context of the universe, it's pretty damn small.
 
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One may perhaps even speculate that there may be a touch of Anthropic Principle [1] at work here. For instance, it may conceivably be that it is more rare for life to evolve on a smaller planet that a large planet because smaller planets are less able to hold on to the heavier elements in an atmosphere. Or one could speculate that during solar system formation planets that form in the habitable zone [2] have a tendency to be "larger" rather than "smaller" through some mechanism.

[1] http://en.wikipedia.org/wiki/Anthropic_principle
[2] http://en.wikipedia.org/wiki/Habitable_zone
 
  • #10
As an above user said, a large Mars-sized body crashed into the Earth during its early development.

Also, being made of iron, naturally, gravity attracts the lighter elements.
 
  • #11
jduster said:
As an above user said, a large Mars-sized body crashed into the Earth during its early development.

Also, being made of iron, naturally, gravity attracts the lighter elements.

Gravity affects all elements almost equally. Until you get to a large fraction of the Earth's mass, the acceleration due to gravity on the surface is still about 9.8 m/s^2.
 
  • #12
What Filip Larsen mentioned may be interpreted as a form of natural selection. The Earth is the size it is because if it was much smaller, it could not allow us to come into existence.

Earth organisms need liquid water to metabolize and grow in, and water will not be liquid below its triple-point pressure. So there must be something creating that pressure, and on the Earth, it's the atmosphere. If the Earth was much smaller and less massive, its escape velocity would be much less, and its atmosphere would escape into outer space.
 
  • #13
^ which is what happened with the hydrogen and helium in our atmosphere during the early Earth
 
  • #14
I suspect that if Earth was the smallest planet in the solar system, you would be saying:

"It is smaller than all it's neighbours, which is odd.
Obviously it is not as small as the moons but they are not real planets."

There is nothing odd about the Earth's size. Or at least, we cannot say anything at this time with regards to it's oddness.
 

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