So Many Planets!
Did you ever learn how many planets there were in the universe when you were in school? Chances are that if you are in your twenties or older, you learned that nine planets existed. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto were the only planets anyone had ever seen. Of course poor Pluto has since been demoted to a "dwarf planet" (a cold, lonely, little snowball).
Starting in the 1990s, astronomers developed ways to find planets orbiting other stars. The first and perhaps simplest method is to watch how bright a star appears in telescopes. Whenever a planet passes between the star and us in its orbit, the star's brightness will be slightly (around 1%) lower.
Another method is to watch the speed that the star travels towards or away from us in space. As the planet orbits, its gravity will tug the star back and forth slightly. We can watch how fast the star moves and see if it periodically speeds up and slows down just a little. This is discovered by looking at the color of the starlight; the light is bluer when the star comes faster towards us, and redder when it moves more away.
In 1995 the first planet orbiting around a regular star like our own sun, Sol, was found. By the turn of the century, roughly forty of these planets, called "exoplanets" or "extrasolar planets" had been found. The very first picture of an exoplanet was taken in 2010, an astounding feat of astronomical skill and technology.
NASA Image of a planet around a distant star, Fomalhaut b. The star is blacked out in the center of the image.
Currently we have directly found somewhat more than 700 alien worlds for sure, and more than one thousand more that are probably out there. Astronomers realize that they can probably never figure out how many planets exist by finding them all one by one. However, if we know how many stars there are, then if we also know how many planets an average star has, we can make a rough guess of how many planets exist in our galaxy.
Fortunately, we do know roughly how many stars there are in our galaxy. This is one of those unimaginably huge numbers, around 100 billion. Something like the number of blades of grass on 75 American football fields.
Scientists have just completed their estimate of how many planets the average star has, finding roughly 1.6 stars per planet. So, (100 billion stars) X (1.6 planets per star) = About 160 billion planets! Now if the chance of life arising on a planet is greater than one in 160 billion...