Are there planets outside our solar system? This is a question that human beings have had for thousands of years.
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Already in antiquity, the Greek mathematician Metrodorus (400-350 BC) said that a universe where Earth is "the only world" is as plausible as a "large field containing a single plant."
About 2,000 years later, in the 16th century, the Italian philosopher Giordano Bruno proposed something similar. “Countless suns and countless earths” existed elsewhere, he said, all revolving “around their sun in exactly the same way as the planets of our system.”
Scientists now know that both Metrodorus and Bruno were right. Today, astronomers are still investigating this question, using new tools.
There is now evidence that points to the existence of "exoplanets," that is, planets orbiting stars other than our Sun.
This evidence is based on the discoveries of the Kepler space telescope, launched by NASA in 2009. For four years, the telescope continuously monitored a single region of space within the constellation Cygnus.
Kepler has 42 cameras, similar to those on a smartphone you use to take photos. In this region, the telescope detected more than 150,000 stars.
About every half hour, he observed the amount of light emitted by each star. Back on Earth, a team of scientists analyzed the data. For most stars, the amount of light remained about the same. But for about 3,000 planets, the amount of light decreased repeatedly, by small amounts and for several hours. These drops in brightness occurred at regular intervals.
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This, astronomers concluded, was caused by a planet orbiting the star, periodically blocking some of the light that Kepler's cameras would otherwise detect. This phenomenon is known as a transit.
Although 3,000 planets look very similar, it is certain that many more within this region remain undetected.
However, because of the number of transits observed by Kepler and astronomers' knowledge of the geometry, we can make a good estimate of the total number of exoplanets.
For example, our Milky Way has at least 100 billion stars. That means it has at least 100 billion planets.
But remember: The universe holds up to 2 trillion galaxies. And each galaxy contains tens or even hundreds of billions of stars.
Thus, the number of planets in the universe is truly astronomical, roughly equivalent to the number of grains of sand on each beach on Earth.
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Some of these planets are gas giants, like Jupiter in our solar system. Others are hot like Venus. Others may be filled with water or ice. And some are like Earth.
Scientists have discovered that about 50% of Sun-like stars in the Milky Way host an Earth-like planet. That adds up to billions of potentially habitable worlds in our galaxy alone.
So the question is, is there life on other planets? Although scientists have yet to find evidence, many now believe that it is unlikely that Earth is the only planet where life evolved.
However, whether evidence of life will ever be found or whether we will ever communicate with an extraterrestrial civilization is still unknown.
