Earth and its Neighbors

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Earth, our home and the '3rd 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

Axial Rotation (days)

Equatorial Radius (103 km)

Mass (relative
to Earth)

Density (kg/m3)


~240 Ma 25.38* 696.00 333x103 1409


57.91 87.97 59 2.42 0.05 5410


108.21 224.7 244.3 6.15 0.82 4990


149.6 365.26 1 6.38 1.00 5517


227.94 686.98 1.026 3.40 0.11 3940



778.34 4332.59 0.410* 71.40 317.89 1330


1427.01 10759.2 0.426 59.65 95.14 706


2869.60 30685 0.451 23.55 14.52 1700


4496.70 60190 0.625 22.40 17.46 2260


5900 91000 6.390 2.95 0.10 5500?

*at the equator, as the period varies with latitude


  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


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

bullet 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 0C and 100C, is so abundant on the Earth but a rarity elsewhere:


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: Plate Tectonics

Visit these sites to learn more about plate tectonics:


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 Earths surface are between 0C and 100C,


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.