1. Historical View of Extraterrestrial Planets

2. The question of the existence of inhabited ( 사람이 살고 있는 ) worlds outside the Solar System goes back to ancient time. Even at the time of the Greeks, who, following the tradition of Aristotle ( 아리스토텔레스, 384 BC – 322 BC), placed the Earth at the center of the Universe ( 지구중심설, Geocentric theory), voices were raised, suggesting a heliocentric system. History has preserved the name of one celebrated pioneer, Aristarchos of Samos ( 사모스의 아리스타르코스, 310 BC – 230 BC). However, his works remained forgotten for more than a millennium, until the Copernican revolution at the end of the 15th century.

3. Firmamento : heven or sky

4. The view of the universe dramatically changed during the 16th century. In 1543, De revolutionibus (on the revolutions of heavenly spheres, 코페르니 쿠스 의 " 천체 운행 " ) by a Renaissance astronomer Nicholas Copernicus, was published when the author died. In the book, Copernicus offered an alternative model of the universe to geocentric system, that is, "heliocentric system". The heliocentric view did, however, come to convince the world of scholars and philosophers, thanks to the work of astronomers such as Tycho Brahe (1541-1601), Johannes Kepler (1571 - 1630) and Galileo Galilei (1564 - 1642).

5. Jovian satellites ( 목성의 위성 ), taken every 30 minutes

6. Galileo discovered the existence of satellites around the Jupiter. There was a symbolic astronomical event- A bright comet appeared in 1577. Tycho Brahe (1546 - 1601) could successfully obtain a parallax, which placed the comet at least four times farther away than the Moon. Tycho suggested that comets moved around the Sun on circular orbits, like Venus and Mercury. The debate on comet's trajectories could have continued for a long time. Fortunately, at the very same time as the first attempts to fit parabolas or hyperbolas to cometary paths were carried out, Isaac Newton (1642 - 1727) had almost completed his theory that predicted elliptic orbits for the planets moving around the Sun, which would occupy one of the focus. His theory also applied to the case of comets. Newton became convinced that planets and comets should obey the same laws ( 만유인력의 법칙 ), and developed a method to fit a parabola to the comet's motion given three observations more or less evenly spaced in time. This method was later included in his master work Principia (1687).

7. The Great comet of 1577, seen over Prague

8. Edmond Halley (1656 - 1742) was the first to fully explain the new theory of gravitation to the case of comets. He computed parabolas for a sample of 24 well observed comets and noted that those comets observed in 1456, 1531 and 1607, shared parabolas of similar characteristics as those of the comet of 1682 observed by himself. This observation led Halley to conclude that these were different passages of the same comet and predicted that it would return again in 1758. The comet was recovered by the German farmer and amateur astronomer on Christmas evening of that year. 1P/Halley was named after his great achievement.

9. Copernican system

10. The first attempt to account for the formation of the Earth and the Universe came with Rene Descartes ( 르네 데카르트, 1596-1650). In Principia philosophiae, which appeared in 1644, Descartes described his idea that the stars played exactly the same role as the Sun. He thus considered that there must be a number of planetary system like our solar system. Descartes's concept therefore opened the door to the possibility of an infinite number of possible worlds.

11. At the beginning of the 20th century, astronomers wondered about the presence of possible companions around nearby stars. Because it is accepted that the Sun is a perfectly ordinary star in the Galaxy, nothing ruled out the existence of numerous planetary systems, similar to our own. The problem, however, lay in detecting these systems. We have seen that direct detection was impossible due to the faintness of the planets. Another method had to be used, namely measurement of the periodic motion of the central star caused by the companion, relative to the centre of mass of the overall system (explain the detail latter). In the middle of the 19th century, the German astronomer Friedrich Bessel (1784-1846) had become famous by detecting, using this indirect method, the presence of a low-mass companion orbiting Sirius. His discovery opened the way to the search for low-mass stellar companions by using the astrometric technique.

12. In 1977, the Swiss astronomer Michel Mayor started a systematic search for stellar companions with the 1.93-m telescope at the Observatoire de Haute Provence. At the same time, other teams were carrying out similar programs. Finally, in 1995, Michel Mayor and Didier Queloz announced the discovery of the first exoplanet orbiting a solar-type star, 51 Peg. With a mass that is at least half that of Jupiter, it orbits its star in just 4 days! We will study the detection method in this class later.