Are there planets outside our solar system? This is a question that human beings have had for thousands of years.
From ancient times, the Greek mathematician Mitrodoros (400-350 BC), he said that a universe where the Earth is "the only world" is as believable as a "large field containing a single plant".
About 2.000 years later, in the 16th century, the Italian philosopher Giordano Bruno suggested something similar. "Countless suns and countless earths" existed elsewhere, he said, all revolving "around their sun in exactly the same way as the planets in our system."
Scientists now know that both Mitrodoros and Bruno were right. Today, astronomers are still exploring this question, using new ones tools.
There is now evidence that there are "exoplanets", that is, planets orbiting stars other than our Sun.
These facts are based on his discoveries Kepler Space Telescope, launched by NASA in 2009. For four years, the telescope continuously monitored only one area of space within the constellation Cygnus.
The Kepler has 42 cameras, similar to those of 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 them data. For most stars, the amount of light remained almost the same. But for about 3.000 planets, the amount of light was reduced repeatedly, in small amounts, and for several hours. These drops in brightness occurred at regular intervals.
This, astronomers concluded, was caused by a planet orbiting the star, which periodically blocked part of the light that Kepler's cameras would otherwise detect. This phenomenon is known as transit.
Although 3.000 planets are very similar, it is certain that many others within this region remain undetected.
However, due to the number of transits observed by Kepler and the astronomers' knowledge of geometry, we can make a good estimate of the total number of exoplanets.
For example, our galaxy 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.
See also: Exoplanet with black snow
Some of these planets are gas giants, like Jupiter in our solar system. Others are warm like Aphrodite. Others may be filled with water or ice. And some are like the Earth.
Scientists have discovered that about 50% of the Sun-like stars in the Galaxy host an Earth-like planet. This adds up to billions of potentially habitable worlds to our galaxy alone.
So the question is, is there life on other planets? Although scientists have not yet found evidence, many now believe that the Earth is unlikely to be the only planet where life evolved.
However, whether evidence will ever be found that there is life or whether we will ever communicate with an alien civilization is still unknown.