For thousands of years, humans have been asking this question.
Here’s what Metrodorus, an ancient Greek mathematician (400-350 B.C.), said. Metrodorus (400-350 B.C.) put it this way: “A universe in which Earth is the only world” is as plausible as “large fields containing one stalk.”
A little over 2,000 years later in the 16th century Giordano Bruno, an Italian philosopher, suggested something similar.
He said that “Countless suns and innumerable earths” existed somewhere else, each rotating in the exact same way as our planets.
Scientists now know that Bruno and Metrodorus were both correct. Astronomers such as me continue to explore this question today, using new tools.
Evidence is available to support the existence of exoplanets, that is, planets orbiting stars other then our Sun. This evidence is based upon the Kepler space telescope’s discoveries, which was launched by NASA in 2009.
The telescope was aimed at one region of space in the constellation Cygnus for four years. It takes up less that 1% of your skyview from Earth.
How the telescope worked
Kepler carried 42 cameras aboard, which is similar to the type of smartphone camera you use to take photos. The telescope was able to detect more than 150,000 stars in that region.
It observed the amount light radiating from each star approximately every half hour. The data were analyzed by a group of Kepler scientists back on Earth.
The amount of light that most stars receive remained pretty constant.
For approximately 3,000 stars, however, the amount light decreased over time. It was not a sudden decrease, but it did so for a few hours. Like a clock, these drops in brightness occurred at regular intervals.
Astronomers concluded that the drops were caused by a planet orbiting its Star, which periodically blocks some of Kepler’s light.
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100 billion stars, 100 billion planets
For instance, our Milky Way galaxy has at least 100 billion stars; that means it has at least 100 billion planets too.
But remember: The universe holds up to 2 trillion galaxies. That’s 2,000,000,000,000! And each galaxy contains tens or even hundreds of billions of stars.
So the number of planets in the universe is truly astronomical, roughly equivalent to the number of grains of dry sand on every beach on Earth.
Some of those planets are gas giants, like Jupiter in our solar system. Others are boiling hot, like Venus. Others may be water worlds or ice planets. And some are Earth-like.
In fact, the Kepler team calculated the abundance of Earth-like planets in the “habitable zone,” a sector of space around each star where a world might have moderate temperatures and liquid water.