Earth and its Neighbors

Earth and its Neighbors
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					Updated on 05/05/2015

Earth, our home and the '3^rd Rock from the Sun', is also called the 'Lonely Planet' because, to our knowledge as yet, earth is the only planet with the evidence of life. It is called the 'Blue Planet' because of the abundance of water on Earth's surface.
	Click on this image of solar system on the left to visit the BBC pages on Solar System or browse the NASA Planetary Photojournal by clicking on the image below.

	Orbital Radius (10³ km)	Sideral Period (days)	Axial Rotation (days)	Equatorial Radius (10³ km)	Mass (relative to Earth)	Density (kg/m³)
Sun	···	~240 Ma	25.38*	696.00	333x10³	1409
Mercury	57.91	87.97	59	2.42	0.05	5410
Venus	108.21	224.7	244.3†	6.15	0.82	4990
Earth	149.6	365.26	1	6.38	1.00	5517
Mars	227.94	686.98	1.026	3.40	0.11	3940
Asteroids	···	···	···	···	···	···
Jupiter	778.34	4332.59	0.410*	71.40	317.89	1330
Saturn	1427.01	10759.2	0.426	59.65	95.14	706
Uranus	2869.60	30685	0.451†	23.55	14.52	1700
Neptune	4496.70	60190	0.625	22.40	17.46	2260
Pluto	5900	91000	6.390	2.95	0.10	5500?
*at the equator, as the period varies with latitude					† retrograde

Earth and the other terrestrial or earth-like planets (Mercury, Venus, Mars, Asteroids and perhaps Pluto) are the Solar system's smaller but heavier planets. They are called 'rocky' because of the abundance of silica (the common constituent of most of earth's minerals and rocks) in their composition.

As the following graph of abundance of different elements shows, hydrogen and oxygen are available aplenty over the entire solar system.

Compositionally, three groups of elements form the major constituents of Solar System:

	the gaseous elements H and He (e.g., Sun, Jupiter and Saturn),
	the ice-forming elements C, N and O that occur as solid NH₃ (ammonia), CH₄ (methane) and H₂O (ice) (e.g., Uranus and Neptune), and

Members of the Solar System	Dominant Chemistry
Sun, Jupiter, Saturn	Hydrogen, Helium
Uranus, Neptune	Carbon, Nitrogen, Oxygen
Mercury, Venus, Earth (and Moon), Mars, Asteroids, Pluto	Iron, Silicon, Magnesium, Potassium, Aluminum, Calcium, Sodium

the rock-forming elements Mg, Fe and Si (e.g., the terrestrial planets —
Mercury, Venus, Earth, Mars — and the asteroids and Moon.

Two reasons explain why water, which should occur all over the Solar System wherever the temperatures are between 0ºC and 100ºC, is so abundant on the Earth but a rarity elsewhere:

	hydrological cycle, and
	plate tectonics.

Of these, hydrological cycle is the continuous recycling of water between oceans, atmosphere and land. As the run-off from land would eventually fill up the ocean basins and level the land, hydrological cycle carries the seeds of its own destruction because the resulting smoothening of the surface eventually translates into the drying up of the Earth.

Browse these sites to learn about the

hydrological cycle:

	The Hydrologic Cycle: online meteorology guide
	NASA's Observatorium -- The Hydrologic Cycle
	Hydrological cycle at the UNESCO web-site

Plate tectonics, on the other hand, involves the creation of new surface area, in the form of ocean basins, so compensating for the surface area lost in folded mountain belts and deep sea trenches. This explains why the ocean floor is made up of basalt, a volcanic rock, and ascribes the changing geography of land and oceans over geological time, seen in these images on the right for the past 250 Ma for instance, to relative angular motions of the lithospheric plates (lithosphere is the earth's rigid outermost shell).

Not surprisingly, therefore, the boundaries of these plates are essentially characterized by seismicity or earthquake activity.

	USGS: Understanding plate motions [This Dynamic Earth]
	PBS: A Science Odyssey: You Try It: Plate Tectonics
	UCMP: P late Tectonics

Visit these sites to learn more about plate tectonics:

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Obviously, the water on Earth would have long disappeared had plate tectonics not existed to continually create the ocean basins that hydrological cycle would then fill up. Earth remains the water planet because

	temperatures over most of the Earth’s surface are between 0ºC and 100ºC,
	the temperature gradient in the troposphere is steep enough to allow the precipitation of atmospheric moisture,
	the hydrological cycle has been perennially present, and
	plate tectonism has occurred throughout the earth's history, ever since the oceans evolved ~4 Ga ago.