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# Sampling methods

## Collecting representative samples

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Are We Alone?

Credit: NASA
Source: http://commons.wikimedia.org/wiki/File:Polycyclic_Aromatic_Hydrocarbons_In_Space.jpg

Are we alone in the Universe? Could there be life on other planets? Or is Earth the only planet that has living beings? These are questions astronomers and physicists ask themselves every day and answering them isn't easy.

#### Why It Matters

NASA and similar space agencies around the world spend millions of dollars and hundreds of hours each year developing technology to journey to other planets, take pictures of other solar systems, and try to make contact with other life that may exist in the Universe. But what if Earth is the only planet that has living beings—should we continue to search for other life?

In 1961, astronomer Frank Drake developed an argument to continue our search of the Universe using probability. The Drake equation is:

\begin{align*}N=R^* \cdot f_p \cdot n_e \cdot f_1 \cdot f_i \cdot f_c \cdot L\end{align*}

where \begin{align*}N\end{align*} is the number of civilizations in our galaxy with which communication might be possible. According to Drake's calculations, there were/are/will be between 1,000 and 100,000,000 civilizations able to communicate in the Milky Way Galaxy. So, the probability of other planets having life is between \begin{align*}\frac{1,000}{Number \ of \ planets \ in \ the \ Milky \ Way}\end{align*} and \begin{align*}\frac{100,000,000}{Number \ of \ planets \ in \ the \ Milky \ Way}\end{align*}. These probabilities are high enough to encourage scientists to search for extraterrestrial life through programs like SETI (Search for Extra-Terrestrial Intelligence).

Credit: NASA
Source: http://commons.wikimedia.org/wiki/File:Andromeda_galaxy_2.jpg

One of the biggest problems with the Drake equation is that it only accounts for life as we know it—life made up of carbon, hydrogen, and oxygen that survives on planets with liquid water and a thick atmosphere. Because our sample size is so small (we only know of one planet that has life and that's us!), scientists aren't able to make better predictions about the likelihood of other planets supporting life. Moreover, scientists have only been able to visit the other planets in our solar system to find out why they can't support life, so we have an extremely small sample set of planets compared to the rest of the Universe (scientists estimate there are between 100-200 billion planets in the Milky Way Galaxy alone!). But this doesn't stop them from trying. Scientists continue to use their findings from exploring other planets to refine the Drake equation, expand their sample size, and continue to search for life beyond Earth.

Read for yourself about the history and calculation of the Drake equation:
http://en.wikipedia.org/wiki/Drake_equation

#### Explore More

Use the link below to learn more about what makes Earth just right for life and why it is so difficult to find planets that might be a good home for life as we know it.

Describe some of the problems with our current planet sample and why this might make it harder to predict the probability of life on other planets.

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