Download Big Bang

Final Lecture on Exoplanets and Life
on June 01, 2011
• Our universe begun with Big Bang about 13.4 billions years ago
• After the Big Bang, the Universe was extremely hot and ionized.
• t~100 seconds: Nuclear fusion sets under hight temeperature and Deuterium is
formed.
• About 25% of the mass in hydrogen is converted into helium. There is also a
little bit of Li.
• Heavier elements were formed by the nuclear fusion of stars.
• First star (Population III) must be metal poor.
• Nuclear fusion is an atomic reaction that fuels stars. In fusion, many nuclei
combine together to make a larger one.
• The net mass of the fused atomic nuclei is smaller than the sum of the
constituents. This lost mass is released as electromagnetic energy, according to
the mass-energy equivalence relationship: E=mc2 .
• Star Formation: Stars are formed within extended regions of higher density in
the interstellar medium. These regions are called molecular clouds mainly
composed of hydrogen plus helium
• Main Sequence: Stars spend about 90% of their lifetime at this stage, fusing
hydrogen to produce helium near the core (main sequence).
• Fate of Stars: Massive stars process up to iron, and finally explode in
Supernova events. Low Mass stars stop before iron, and gently blow themselves
to death forming planetary nebulas
• The "Type II" supernovae are the result of a massive star consuming all of its
nuclear fuel and then exploding.
• Elements heavier than Mg produced during explosion. Lighter elements
produced during preceding stellar evolution and gently blow themselves to
death forming planetary nebulas
• The Sun is a Population I, or heavy element-rich, star.
– Population I: metal rich
– Population II : metal poor
– Population III: metal free, which is believed to form in the early universe
• The formation of the Sun may have been triggered by shockwaves from nearby
supernovae. This is suggested by a high abundance of heavy elements in the
Solar System, such as uranium, relative to the abundances of these elements in
Population II stars.
• Formation of Solar System: The sun and the planets of our solar system formed
at the same time, and from the same material reservoir
What we learn from the formation of the Solar System?
• The sun and our planets formed concurrently 4.567 billion years ago.
• Extrasolar planetary systems can be similar to or different from the solar system.
• Dust plays a decisive role in the formation of the planets.
• The lifetime of protoplanetary disks, the birthplaces of planets, is a few million
years.
Current Solar System
• Terrestrial (or inner rocky) planets: Mercury, Venus, Earth, Mars
• Asteroid belt: Rocky objects. They are mainly classified into C-type and S-type.
Some asteroids around the outer edge may have ice deep inside the bodies.
• Gas Giant Planets: Jupiter, Saturn, Uranus, Neptune
• Trans-Neptunian Objects
Search for the Evidence of Life in the Current Solar System
•
Could life exist in our Solar System? So far we haven't found it.
•
There are/were some possible life on Mar, Europe, Titan, Enceladus other than Earth
•
Amino acids were found in carbonaceous meteorites (C-type asteroid origin) and comets.
•
Materials in Earth should experience high temperature, suggesting a volatile-depleted
young Earth. It is likely that the organics on the Earth are a mix of exogenic and
endogenically synthesized in the early atmosphere (refer to Miller-Urey experiment).
Search for the Evidence of Life in the Current Solar System (cont.)
•
It is curious to notice that Both left-handed (L) and right-handed (D) optical isomers were
created through Miller-Urey experiment, while L-enantiomeric excesses have been found
in Murchison meteorite.
• An Extrasolar planet, or exoplanet, is a planet outside the Solar System.
• First exoplanet was confirmed indirectly at G-type star 51 Pegasi in 1995
• So far, about 500 planets were confirmed through the astronomical observations.
• Exoplanets are an extremely fainter than those of central stars.
• For the reason, only a very few extrasolar planets have been observed directly.
• For the detection of exoplanets, there are
– Doppler technique
– Astrometric Technique
– Transit Technique
– Gravitational microlensing
– Circumstellar disk
– Direct observation
• None of the planetary systems found so far resembles our Solar System.
• There are many Jupiter-sized planets very close to their parent stars (Hot
Jupiters). What is going on? This is a subject of much current research.
• It seems to be a matter of time before the detection of Earth-like planets.
•
A planet of the red dwarf star Gliese 581, appeared to be the best known
example of a possibly terrestrial exoplanet orbiting within the habitable zone that
surrounds its star. The number of Earth-like planets increase all the time.
• What should we aim for in the coming decades?
Difficulty of in-situ exploration
• Given the distance that separates the Earth from even the
closest stars (ProximaCentauri about 4.3light-years away), it
seem inconceivable that one could contemplate searching
for life, insitu, at any time in the near future.
• It would take about 45 years to reach Proxima Centauri
using a spacecraft with the velocity of 1/10 light speed.
• We will therefore consider only those methods that involve
doing so from at distance through observations of nearby
stars.
Spectral Signature of Life
• Particularly, spectroscopic observation is a powerful tool to
investigate the chemical composition of exoplanets.
• Not only on Earth but also in the interstellar space, carbon exists
abundantly in several oxidized (e.g., CO2) and reduced (e.g.,
CH4), forms. This allows the formation of a considerable
number of different combinations (molecules), which in turn
simultaneously allow very diverse forms.
• The massive presence of oxygen in an atmosphere may be
considered as proof of the existence of biological activity.
• It is suggested that the triple detection of CO2 , H2O and O2 ,
or even better, O3 , would be preferable.
• Oxygen (O2), in fact, has no spectral signature in the
thermal infrared. Ozone (O3 ) is a dissociation product of
O2 caused by ultraviolet photons from the star (via the
Chapman cycle). Ozone is a good marker of the quantity of
oxygen in an atmosphere.
• Based on these assumption, we prefer to make spectroscopic
observations of explanets around 10 micron.
Infrared astronomical interferometer (TPF-I):
Multiple small telescopes on a fixed structure or on
separated spacecraft floating in precision formation
would simulate a much larger, very powerful
telescope. The interferometer would use a
technique called nulling to reduce the starlight by a
factor of one million, thus enabling the detection of
the very dim infrared emission from the planets.
Visible Light Coronagraph (TPF-C):
A large optical telescope, with a mirror three to four
times bigger and at least 100 times more precise
than the Hubble Space Telescope, would collect
starlight and the very dim reflected light from the
planets. The telescope would have special optics to
reduce the starlight by a factor of one billion, thus
enabling astronomers to detect the faint planets.
Darwin, proposed by ESA, was proposed as a
constellation of four or five free-flying spacecraft
designed to search for Earth-like planets around other
stars and analyze their atmospheres for chemical
signatures of life.
The constellation was proposed to carry out highresolution imaging using aperture synthesis in order
to provide pictures of celestial objects in
unprecedented detail.
In addition to remote sensing observations of
exoplanets, interplanetary explorations of Europe and
Titan are still under discussion by NASA, ESA,
JAXA, Russia, and so on. Make sure that these space
missions are always face on funding competition.
 Venus
 Earth
 Mars
• Finally, I would like to close by course saying…