Download Historical View

2012-2학기
Historical View of
Extraterrestrial Planets
외계 행성과 생명 Exoplanets and Extraterrestrial Life (Instructor: M. Ishiguro)
Our Solar System
OurOur
Solar
System
Galaxy
• The question of the existence of inhabited worlds outside the Solar System
goes back to ancient time.
• Following the tradition of Aristotle (384 BC – 322 BC) who placed the
Earth at the center of the universe, ancient people supported the geocentric
theory (지구중심설).
• However, there were voices which suggested another idea. A Greek,
Aristarchos of Samos (310 BC – 230 BC) is a celebrated pioneer of the
heliocentric theory.
• He figured out a way to measure the relative sizes and distances of the
Moon and Sun.
• He noticed that when the Moon was eclipsed by the Earth (월식) we can
see the Earth's shadow creep across the face of the Moon. Earth's shadow is
circular, and if we assume that the Sun's rays are roughly parallel to one
another.
Firmamento : heaven or sky
• Aristarchus measured a ratio of Earth's size to the Moon's size to be 2.9
(note that the real ratio is 3.7).
• Using the angular size of the Moon (=0.5 degree) we can estimate its
distance as shown below:
tan = DM/d (when <<1,  in radian)
where DM and d denote the diameter of the moon and distance between
the Earth and the moon.
• Because tan (0.5o)=0.0087, we have a distance to the Moon of about 40
Earth Diameters (actually 30).
40 Earth diameter or 115 Moon diameter
• In addition, Aristarchus considered that when the Moon was seen to be
exactly in either 1st or 3rd Quarter Phase (exactly half lit up as seen from
Earth), we can draw a right triangle between Earth, Moon, and Sun.
• The legs of the right triangle are the distance from the Earth to the Moon
(a) and the distance between the Moon and the Sun (b). The hypotenuse
is then the distance between the Earth and Sun (c). We can measure the
angle between a and c (A), it's the angular separation between the Sun
and Moon in the sky. We derive the distance c
a/c=cos A
b
a
c
• Aristarchus measured A= 87°. This corresponds to a distance to the
Sun of c = a/cos A = 40DE/cos 87o= 760DE. (cos 87o=0.052)
• Now, the angular size of the Sun is also about 0.5° so by the same
method as for the Moon Aristarchus estimated that the Sun was 6.6 times
bigger than the Earth.
• This measurement is a difficult one to make, and the real answer is A=
89.9°. This corresponds to a distance to the Sun of c = 11,700DE and a
size of the Sun, DS = 109DE.
• Either way, Aristarchus was able to establish that the Sun was bigger
than Earth in the 3rd Century B.C.
Aristarchus of Samos, the ancient Copernicus
• Earth radius
6400 km
• Moon-Earth distance
3.8105 km
• Sun-Earth distance 1.5108 km
1 AU (astronomical unit)
• However, the work by Aristarchos remained forgotten for more than a
millennium, until the Copernican revolution at the end of the 15th century.
The view of the universe thus changed dramatically during the 16th
century.
• In 1543, De revolutionibus (on the revolutions of heavenly spheres, 코페
르니쿠스 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 come to convince the world of scholars and
philosophers, thanks to works of astronomers such as Tycho Brahe (1541
– 1601), Johannes Kepler (1571 – 1630) and Galileo Galilei (1564 – 1642).
Jovian satellites (목성의 위성), taken every 30 minutes
• Galileo Galilei discovered the existence of satellites around the Jupiter
(miniature Solar System).
• Around that term, there was a symbolic astronomical event. A bright comet
appeared in 1577. Tycho Brahe (1546 – 1601) could successfully obtain a
parallax. He found that the comet existed at least four times farther away than
the Moon. In addition, he suggested that the comet moved around the Sun on
circular orbits.
• 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).
The Great comet of 1577, seen over Prague
• 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.
Copernican system
• 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 systems
like our solar system.
• Descartes's concept therefore opened the door to the possibility of an
infinite number of possible worlds.
Rene Descartes , philosopher and mathematician
• 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 found
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 center
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.
• 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.