Space Facts

Planet Earth

Our Home World

The Mercury-bound Messenger spacecraft snapped this image when it was 34,692 miles (55,831 kilometers) above Earth, August 2, 2005. The western half of South America is still in daylight. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Earth is planet number three of the Solar system. Named for no god or goddess, its name, instead, comes for another word for "dirt!" Oh, well. It's home.

The planet Earth is special because it supports life. The space age is still in its barest infancy, but we are on the verge of discovering other planets that might be like Earth. Already we know that star systems with planets are not rare. The variety of worlds out there is astounding. Finding other, Earth-like worlds is what this web site is all about — habitable planets potentially for humanity, if not for colonization or settlement, then for trade and commerce. The Earth is our model, though perhaps imperfect. It has something that all the other planets in the Solar system don't have — a thriving bounty of life in a variety of environments. It also has intelligent life — something we're hoping to find out there, as well. We can also hope they're friendly.

The atmosphere of Earth is moderately thick for a terrestrial world. The average temperature is compatible with liquid oceans. The average amount of sunlight is compatible with photosynthesis in plants. Earth's rotation creates a comfortable diurnal cycle — the days long enough to be productive, but not too long to scorch life by mid-afternoon, and the nights short enough not to become dangerously cold in most areas. And the percentage of ocean coverage is compatible with abundant rainfall in some areas, clearing the air of dangerous, high-silica dust, and giving nourishing water to thirsty plants. Our planet has its extremes, but life can be found there, too. The penguins in frigid Antarctica, the anaerobic bacteria living in the hot, volcanic vents at the bottom of the ocean, and the camels of the African desert, all attest to life's versatility.

Earth has had its ups and downs. Interplanetary "target practice" has wiped out much of life on our world on perhaps numerous occasions in what are called an "extinction level events." Other causes of extinction events might include sea level changes, flood basalt (volcanic) events, and the current extinction event caused by human activity (destructive rate of resource consumption including logging, pollution, and other effects). Earth has also experienced swings in climate change, super volcanic events and mega-tsunamis. Scientists are working on ways of detecting and perhaps ameliorating the effects of some of these events in the future. Human activity, however, is something that we all need to participate to improve. Our attitudes are key, and this requires greater self-awareness. As responsible citizens of this planet, it is up to us to make the necessary changes.

Our world is about 7,930 miles in diameter (12,756 kilometers — equatorial diameter), with an atmosphere that is roughly 60 miles thick (100 kilometers, though there is no hard boundary, and the gas can extend outward for thousands of miles). About 71% of the Earth's surface is covered with salt water in its oceans and seas. Our day is 24 hours long and year is 365.256 days long. The axis of Earth is tilted by about 23.5 degrees, giving rise to our seasons. It has an atmosphere that is 78% nitrogen, 21% oxygen, and about 1% argon, with traces of carbon dioxide, water vapor (varies with climate), and other gases. And our world has one natural satellite we call "the Moon."

Size comparison of terrestrial planets with the Moon (left to right): Mercury, Venus, Earth, Luna and Mars.		The first four planets of the Solar system are called "terrestrials" because of their rough similarity to Earth. The other type of planet is called "gas giant," because the bulk of each such planet is gaseous. The Solar System Facts table gives more comparisons between the planets.

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Could Earth Ever Stop Sustaining Life?

One possible future for our planet Earth — a scorched desert as the sun expands with old age, approximately 4–5 billion years in the future. Artwork by Carl Martin.

There are many things that could end life on Earth. Human pollution could poison our world to the point it cannot recover. There also remains the threat of nuclear annihilation — bombs destroying life in the immediate vicinity, radiation poisoning the rest of the world, and the effect of a "nuclear winter" reducing the ability of life to recover. We could also experience several super volcanic disasters, or collision with one or more large asteroids.

The only certain end is built in to the life of our sun. When Sol, our sun runs out of hydrogen fuel (hydrogen at the core, where the nuclear reactions take place), it will expand in size, first becoming a sub-giant star, then a red giant. This will occur in about 5 billion years (5.0 × 10⁹). Yet life may run into trouble a little sooner than that. Our sun, like all main sequence stars, has been gradually brightening throughout its life, perhaps as much as 10% per billion years. The next billion years could prove fatal to life on our planet as Earth becomes hotter, the oceans evaporate, and hydrogen from that water is "boiled off" the upper atmosphere.

But don't worry. We have plenty of time to clean up our act and build the necessary ships to find other Earth-like planets in our Solar neighborhood.

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