Figure 1: Original notes of the discovery of Jupiter's moons by Galileo, 1610 .

The search for new worlds started a long time ago. It is a story of struggle, boldness and great suffering. Finally today, it has a chance to become the biggest story of all times.

It was 400 years ago when the first time, Galileo Galilei, an Italian physicist, mathematician and astronomer, pointed a telescope towards the sky to observe the second brightest object in our night sky, the planet Jupiter. With his refined telescope and scientific skills he was able to identify Jupiter as a sphere just like earth and the little bright spots close to Jupiter as moons orbiting the planet (see Figure 1).

Today we know that Jupiter has 16 major moons, including the four Galilean moons (as discovered by Galileo) Callisto, Europa, Ganymede and Io (see Figure 2), as well as 46 tiny moons between 3km and 9km in diameter.

Figure 2: The four Galilean moons of Jupiter

This was the first time in the history of mankind that an alien world was found. The word alien comes from the Latin word alius and means other. To speak of alien world means to imagine a world different from ours. A widely used term for alien world is extraterrestrial which comes from Exo-/ex- (Greek or Latin, outside of, outer, outer part) and the word terra (Latin for Earth).

Galileo Galilei was born 5 February 1564 in Pisa, Tuscan (Italian). Galileo is most famous for supposedly having dropped balls of different masses from the Leaning Tower of Pisa. Galileo, however, is one of the first who puts the experiment as the only means to figure out the laws of nature. Today Galileo is called “the father of modern science”.

Galileo showed that the universe consists of heavenly objects which are very similar to our planet earth. As profound as his discovery was, Galileo was by no means the first human to believe in the existence of alien worlds.

A little bit earlier than Galileo, another Italian, the philosopher Giordano Bruno, published theories about alien worlds in the universe.

Giordano Bruno was born in Nola (in Campania, then part of the Kingdom of Naples) in 1548, he was originally named Filippo Bruno.

Bruno believed that the Earth rotates around its axis and therefore not the heavens around the Earth. He also saw no reason to believe that the stellar region was finite, or that all stars were the same distance away from a single center of the universe. For Giordano Bruno the Earth was just one more heavenly body just like the Sun. In his views God cared as much about the infinite universe as He does for Earth. According to Bruno, God is an immanent; God is always close to His creation.

Bruno’s insight let him write this remarkable accurate (especially in modern standards) passage:” There are countless suns and countless earths all rotating around their suns in exactly the same way as the seven planets of our system. We see only the suns because they are the largest bodies and are luminous, but their planets remain invisible to us because they are smaller and non-luminous. The countless worlds in the universe are no worse, and no less inhabited than our Earth.”

Figure 3: The philosopher Giordano Bruno (1548 - 1600)

It is an irony of history that Giordano, as apt and visionary his insight was, became a martyr of science. As a result of the Inquisition, Giordano Bruno was burned at the stake in Rome in 1600.

The situation was slightly better for Galileo. The Inquisition didn’t execute him; however, he was forced to recant his theories and spent the last years of his life under house arrest on orders of the Inquisition.

A long time before Giordano and Galileo, mankind had an era of free thinking. In the Greek antiquity philosophers revolutionized thinking and led us to the dawn of modern science during the Renaissance. One Greek philosophers in particular envisioned alien worlds. So Epicurus (341-270 B.C.) writes: “There are infinite worlds both like and unlike this world of ours ... we must believe that in all worlds there are living creatures and plants and other things we see in this world. “

Inspired by such bold thinking, the Romans took Greek philosophy and incorporated it into their poetry. In this tradition the Roman poet Lucretius (c 99-55 B.C.) writes: "Granted, then, that empty space extends without limit in every direction and that seeds innumerable are rushing on countless courses through an unfathomable universe ... It is in the highest degree unlikely that this earth and sky is the only one to have been created ... So we must realize that there are other worlds in other parts of the universe, with races of different men and different animals."

Despite such a long history of imagining alien worlds the question about our own world, our own solar systems was not understood until the late Renaissance (16th to 17th century), a period of significant changes in the way the universe was viewed and the methods with which philosophers sought to explain natural phenomena. The revolutionary new way of learning about the world focused on empirical evidence, the importance of mathematics.

Important roles in the modern understanding about our solar system, i.e. the sun and the planets surrounding it, played the German-Polish astronomer Nicolaus Copernicus, the Danish astronomer Tycho Brahe(pronounced Tie-ko Bra-hey) and the German astronomer Johannes Kepler. Each of them laid the foundation of modern astronomy and our understanding of our solar system.

In the next part of the article series, I will talk about how the three astronomers