There are likely over 100 billion planets in our galaxy, according to a review of studies spanning over the past six years. This amounts to about 1.6 planets per star according to a research article published in Nature this week. With this new data, physicists are one step closer to solving a formula used to calculate the amount of extraterrestrial life: the Drake equation.
Named for its creator, physicist Frank Drake, the equation attempts to predict the number of alien civilizations in our galaxy. Drake proposed the equation in 1961 as a way to show how difficult it would be to find alien life. The equation, however, is fairly straightforward—it consists of multiplying a number of variables such as the rate of star formation and the fraction of stars that have planets.
While the equation is easy to calculate, it's extremely difficult to find definitive values for the eight variables in the equation. The variables progressively narrow down the conditions for detectable life. For instance, one has to estimate the number of planets per star; then the number of those planets that support life; then the number of this smaller group of life-supporting planets that develop intelligent life. Values for these variables are highly speculative, but the new research has at least given scientists a better starting point by knocking out the first part of the equation.
The research team reviewed a sample of papers that detected planets with gravitational microlensing. When a massive body, such as a planet, passes in front of a star, the planet's gravity bends the light sent toward observers on Earth. Scientists can detect this gravitational effect on the light and estimate the size of the distant planet.
After reviewing the past research, the astronomers found that most of the planets in the sample were small, like Earth. This agrees with data from other planet-detecting methods, such as those that measure the dip in light when a planet passes in front of a distant star.
This new review of data should take out at least some of the guesswork that goes into calculating the Drake equation, but plenty of work remains. Even with a solid prediction for one of the variables, values for the other parts of the equation, such as the amount of life-supporting planets that go on to produce intelligent life, are still highly speculative. Depending on a scientist's level of pessimism or optimism, Drake equation answers can range from around one alien civilization to hundreds of thousands.
With the recent surge of detected exoplanets over the past decade, however, it seems like only a matter of time before astronomers find a life-supporting planet—and maybe intelligent life—outside of our solar system.