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# Measurement of Probability

## Express ratio of favorable and possible outcomes as fractions, decimals, and percents.

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Odds of Life

Credit: KFDL
Source: http://www.flickr.com/photos/38261286@N04/4336744915

The Universe is full of planets. So far, we only know of one that supports life: Earth. Scientists are searching for life on other planets. Will they ever find another candidate?

#### Rocky, With a Chance of Life

When scientists look for extraterrestrial life, they focus on Earth-like planets. They're looking for planets made of rock, not gas. Astronomers believe that a planet must be within a certain distance from a star in order to be capable of supporting life, in a region known circumstellar habitable zone. Since planets in this zone are supposed to be not too hot and not too cold for life, it is also referred to as the "Goldilocks zone," after the famous fairy tale. Another requirement for planetary candidates is that their suns must be relatively calm. Overactive stars emit too much radiation, which would kill developing life. Any potentially habitable planets would also need to have atmospheres containing the chemicals necessary for life.

Credit: NASA and JPL-Caltech
Source: http://commons.wikimedia.org/wiki/File:Planets_Under_a_Red_Sun.jpg

Scientists have developed equations to calculate the probability that we'll find life on other planets. One, known as the Drake equation, calculates the probability that humankind will ever find another planet in our galaxy with intelligent life capable of sending messages into space. The equation was developed by astronomer Frank Drake in 1961. He originally concluded that the number of such civilizations in the Milky Way galaxy probably ranged between 1,000 and 100,000,000. However, depending on the assumptions you make (what numbers you plug in), the equation can yield estimates of anywhere from nearly 0 to 36.4 million civilizations.

In 2013, astronomer Sara Seager developed a modified version of the Drake equation, called the Seager equation, which calculates the probability that we'll find any life at all in the next 10 years. According to her calculations, the equation predicts that out of all the Earth-like planets we can detect, 2 may support life. However, the sort of life the Seager equation predicts won't come with spaceships. Instead, scientists will simply be looking for the presence of certain gases known to be produced by life—such as oxygen, which may be evidence that photosynthesis is occurring. A planet covered in bacteria or algae would satisfy those odds, and researchers expect to find one such world soon.

#### Explore More

Watch the videos below to find out more about the Goldilocks zone and the prospects of alien life.

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