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First, we need to make some assumptions. The Solar System is the only place we’ve studied deeply, so we could have many more planets than other star systems, or a fraction of the number of planets they have. We’ll never know until newer and better telescopes that can find all the planets in nearby star systems.
Some extrasolar systems will have more planets and some will have less, but let’s use the 8 planets in our Solar System as an average. Then, let’s multiply this number with the number of stars in the Milky Way: 580 million. This gives us a total of 4.6 billion planets in the galaxy. Remember, that’s a really rough estimate.
At the time of this writing, astronomers have only discovered 342 planets orbiting 289 stars. But that number is probably much higher at the time that you’re reading this.
Astronomers have a few techniques for detecting planets. The radial velocity method measures the velocity a planet exerts on its parent star as it goes around its orbit. Imagine the planet is yanking its parent star back and forth as it goes around. The transit method searches for stars that have planets passing directly across their surface, slightly dimming their light.
In the next few years, new space telescopes will be launched that will be able to detect smaller planets orbiting stars, and maybe even determine if there’s life on them. Once these new missions get going, we’ll get better estimates for the size and number of planets that orbit a typical star, and we’ll be able to come up with a much better estimate for the number of planets in the galaxy.
We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.