Drake Equation: Estimating the Odds of Finding E.T.
Astronomers use the huge Arecibo Observatory, a radio telescope in Puerto Rico, to study the close flyby of Earth by asteroids.
Credit: Arecibo Observatory/NSF

The Drake Equation is used to estimate the number of communicating civilizations in the cosmos, or more simply put, the odds of finding intelligent life in the universe.

First proposed by radio astronomer Frank Drake in 1961, the equation calculates the number of communicating civilizations by multiplying several variables. It's usually written, according to the Search for Extraterrestrial Intelligence (SETI), as:

N = R* • fp • ne • fl • fi • fc • L

  • N = The number of civilizations in the Milky Way galaxy whose electromagnetic emissions are detectable.
  • R* =The rate of formation of stars suitable for the development of intelligent life.
  • fp = The fraction of those stars with planetary systems.
  • ne = The number of planets, per solar system, with an environment suitable for life.
  • fl = The fraction of suitable planets on which life actually appears.
  • fi = The fraction of life bearing planets on which intelligent life emerges.
  • fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
  • L = The length of time such civilizations release detectable signals into space.

The challenge (at least for now) is that astronomers don't have firm numbers on any of those variables, so any calculation of the Drake Equation remains a rough estimate for now. There have been, however, discoveries in some of these fields that give astronomers a better chance of finding the answer.

Astronomers certainly could imagine the existence of other planets outside the solar system in 1961, but it took until 1995 until the first confirmed exoplanet was found. Called 51 Pegasi b, the discovery ushered in a new era where astronomers were able to track down first dozens, and then hundreds, of other planets across the universe.

Traditionally, planets have been found through two methods: watching them transit across a star (which causes a dimming that can be measured from Earth) or examining the gravitational wobbles the planets induce as they orbit around their parent star. More recently, a technique called "verification by multiplicity" allows astronomers to quickly identify multiple-planet systems.

Estimating the total number of planets in the universe is difficult, but one statistical study suggests that in the Milky Way, each star has an average of 1.6 planets – yielding 160 billion alien planets in our home galaxy. (The study used a technique called gravitational lensing that observes changes in light curves when a relatively nearby star passes in front of more distant objects.) [Related: 13 Ways to Hunt Intelligent Aliens]

As of March 2014, more than 1,700 exoplanets have been confirmed. The vast bulk of them were due to an observatory called the Kepler Space Telescope, which scrutinized a single spot in the Cygnus constellation between 2009 and 2013. Plumbing the data, astronomers continue to make discoveries from the information.

To date, most of the planets that have been discovered were Jupiter-sized or larger, but this may be because these gas giants are easier to spot in telescopes. A slew of Kepler discoveries announced in February 2014 mainly contained super-Earths, or planets that are slightly larger than Earth and are considered by many astronomers to be habitable under the right conditions.

Among the planets discovered by all telescopes, however, only a tiny fraction of them are likely to have an environment suitable for life. Astronomers can't measure this metric for sure yet, but a few factors likely come into play, such as how close a planet is to its parent star and what its atmosphere contains.

As of March 2014, the Habitable Exoplanets Catalog identified 20 confirmed planets that could be suitable for life, and 69 planets (yet to be confirmed) that could also be suitable.  The project is a part of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo.

"These are artistic representations of all the planets around other stars (exoplanets) with any potential to support surface life as we know it," the catalog states below an illustration of the planets. "All of them are larger than Earth and we are less certain about their composition and habitability, but they represent the best objects of interest for future observations." [Related: 5 Bold Claims of Alien Life]

Finding life outside of Earth — even microbial life — would be an important step toward better understanding the Drake Equation. Astronomers in fact have not given up on finding life within our own solar system. There are several areas that could host habitable environments now, or did in the past, such as the planet Mars or Jupiter's moon Europa.

A next step would be determining how to send a message to extraterrestrials and whether they could receive or understand it. On a small scale, astronomers have beamed messages to the stars and in a few cases, put discs on board spacecraft (such as Voyager) for anyone in the neighborhood to read and potentially find Earth for further communications.

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