Download From Big bang to lives on planets

1pc = 2x105 AU
M.Hogerheijde1998, after Shu et al. 1987
Formation of Kuiper belt and Oort cloud
Brett Gladmann Science 2005
What have we learned so far?
 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.
Big bang
Nuclear fusion in stars
Supernova nucleosynthesis
Exotrasolar planets
6. Extrasolar Planets
Planetary formation
Current Solar System
• Do other stars have planetary systems?
• What are they like?
• Could they have life on the planets?
• An Extrasolar planet, or exoplanet, is a planet outside the
Solar System.
• First exoplanet was confirmed indirectly at G-type star 51
Pegasi.
• As of May 11, 763 exoplanets were confirmed through the
astronomical observations.
• In this chapter, we study the detection method, the
structure and the evolution of exoplanets.
Detection Methods
• Exoplanets are an extremely fainter than those of central
stars. Glares from the stars obscure the faint light sources.
• Thus only a very few extrasolar planets have been
observed directly.
• The following are the indirect method that have been
applied for the detection of known exoplanets.
1.
2.
3.
4.
5.
6.
Doppler technique
Astrometric technique
Transit technique
Gravitational microlensing
Circumstellar disk
Direct detection
1. Doppler method*
• A planet exerts a small gravitational force on its parent star,
causing the star to wobble (비틀거림, 흔들림).
• The motion amplitude depends on the orbital distance and
the mass of the planet.
• The motion of star is detectable with the Doppler Effect.
•
The light coming from a star moving toward the Earth will
be Doppler shifted to bluer (shorter) wavelengths, whereas
a star moving back from the Earth will emit light shifted to
redder (longer) wavelengths.
Wavelength:
Sound:
Light:
Short
High
Blue
Long
Low
Red
/0 = v/c
• By making precise measurements of the frequency
(wavelength) of absorption lines in the star's spectrum, it is
possible to see this alternate blue- and red-shift effect.
• The Doppler spectroscopy allows one to estimate the
distance between planet and central star, and place a lower
limit on the mass of the planet.
• The first exoplanet was found at 51 Pegasi in 1995 by
using the Doppler technique. The Doppler spectroscopy
technique has been the most successful so far in finding
extrasolar planets.
Constellation: Pegasus
5.5 mag star
 orbital period = 4.2 days
 semi-major axis = 0.05 AU
 evaluated mass = 0.5 Jupiter Masses
• The wobbling effect is very small.
• In our Solar System, Jupiter exerts the strongest force on
the Sun with a radial velocity of 12 m/s. On the other hand,
Earth have the effect only 10 cm/s, over a period of a year.
• For the reason, more massive planets and the closer
distance from the central stars are selectively found .
• The Dopple method is most effective for edge-on view, but
not for face-on view.
Planetary Systems, Ollivier et al., Springer
12.5 m/s = 45 km/h
2. Astrometry method
• 'Astrometry' is a measurement of stellar position.
• With the help of astrometry, astronomers study the precise,
periodic wobble that a planet induces in the position in the sky
of its parent star.
• Unlike the Doppler method, astrometry method works best
when the orbit of the planet around the star is perpendicular to
the viewer. Also, a planet that orbits far away from its star will
be more easily detected as it will cause a greater shift in the
position of the star.
• However, planets at greater distances from the star require
longer time. In total it could take many years, perhaps decades.
Motion of the Sun as a function of time (in years) on the plane of the
sky as it would appear from a location 10 parsecs away and
perpendicular to the plane of the ecliptic. This movement is mainly
dominated by the giant planets (Jupiter, Saturn, Uranus, and Neptune).
Twinkle, twinkle, little star,
How I wonder what you are!
Up above the world so high,
Like a diamond in the sky.
Twinkle, twinkle, little star,
How I wonder what you are!
반짝 반짝 작은 별,
난 네가 무엇인지 궁금해!
저 세상 위에 너무 높이,
하늘의 다이아몬드처럼.
반짝 반짝 작은 별,
난 네가 무엇인지 궁금해!
3. Transit Method*
• This method uses the fact that when a smaller object passes in
front of a host star, the star appears to fade in luminosity. Even
if the reduction is very small (typically between 0.01% and
1%) astronomers can detect it through the photometry
(measurement of the brightness of stars).
• The event could be regarded as an eclipse or an "occultation".
Transit by Venus
Solar Eclipse
부분일식 (2012 May 21)
http://www.kidd.co.kr/news/149163
• The photometric transit method has an disadvantage in that
the star which is being studied needs to be edge-on.
• This method could work on great distances.
• An advantage of this method has is that during the
occultation, the composition of the planet's atmosphere could
be detected.
• The study of such an occultation would produce a light curve,
which would show how much a star had faded due to the
passage of the planet. If the curve is precise enough, it could
even reveal the presence of moons around the planet and
astronomers would know immediately if the planet is in the
habitable zone.
Diagram illustrating the principles of a planetary transit in front of a star,
and the associated photometric light-curve
• The transit will be visible only if the line of sight intercepts
the cylinder, constructed on the orbit, of radius ap and
height 2r .
• For a circular orbit, the following relationship may be
derived:
2paP ´ 2r* r*
P=
=
2
4paP
aP
• Example:
Let us derive the probability of the transit of the Earth seen
from the other planetary systems.
2r* = 1.4x106 km
aP = 1 AU= 1.5 x 108 km
PE =(1.4x106)/2/(1.5x108)~ 10-2 = 0.5%
observable only in 13 hours per year!
4. Gravitational Microlensing Method
• According to general relativity (일반 상대론), mass "warps"
space–time to create gravitational fields and therefore bend light
as a result.
• Microlensing is a phenomenon that occurs when an object with
enough mass passes between us and a background star. If a planet
and a star would happen to pass in front of a background star, the
background star's luminosity would appear to increase (because
light is bent by the planetary-system's gravity).
• This is a very promising and new method, though the chance is
low that a planet-star system would pass between us and a
background star. For this reason, it is more efficient to study a
background with many stars (e.g. the galactic center).
Detection via microlensing
OGLE-2003-BLG-235
[day]
Circumstellar Disk
• Disks particles surround many stars (debris disks or circumstellar
disk). The dust can be detected because the dust particles have a
large total surface area.
• Dust disks have now been found around more than 15% of nearby
sun-like stars.
• The dust is believed to be generated by collisions among asteroids
or sublimating of ice from comets. Radiation pressure from the
star drags the dust particles into the stars (Poynting-Robertson
effect). Therefore, the detection of dust indicates continual
replenishment by new collisions, and provides strong indirect
evidence of the presence of small bodies around the parent star.
Poynting-Robertson Effect
Stationary
Moving train
Moving train loses the momentum by the
collision with droplet
콩가루
콩
Kalas et al. 2005
Advantages of the Doppler Method
–Most successful method
–About 85% of known exoplanets are detected by the technique
–The Doppler method is sensitive to massive planets around
relatively nearby stars
Advantages of Transits
–Transits offer the only way we currently have to make a
direct measurement of the radii of exoplanets
–Gives an estimate of the density (with Doppler Method)
–Densities are important clues to the composition of the
exoplanet (gas giant, ice giant, rocky planet, etc.)
–The only way we have to probe the atmospheres of exoplanets
–The latest application of the Transit Method from space holds out
the possibility of detecting Earth-mass planets.
e.g. the European COROT satellite and the US KEPLER mission.
Diversity of Exoplanets
• As of December 2, 2011, all of these techniques have found 707
planets.
• Most are Jupiter-sized or larger (up to 13 times Jupiter's mass),
with some recent detections getting into the Neptune range, and
a couple of tantalizing "Super Earths" down to the several Earth
mass range.
• It is now known that a substantial fraction of stars have
planetary systems, including at least around 10% of sun-like
stars. It follows that billions of exoplanets must exist in our own
galaxy alone.
2
1
log (mass)
0
-1
-2
-3
-4
-5
-2
-1
0
1
log (distance)
2
3
• None of the planetary systems found so far resembles our Solar
System.
• The biggest surprises are findings of many Jupiter-sized planets
very close to their parent stars
• Some, called "Hot Jupiters", are on orbits smaller than that of
Mercury, and have periods less than 10 days!
• What is going on? This is a subject of much current research.
• The discovery of extrasolar planets has great interest in the
possibility of extraterrestrial life. As of September 2010, Gliese
581 g, fourth 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.
M-type
Gliese 581g:
orbital period=37 day
>3.1 Earth mass
• It is estimated that the average global equilibrium
temperatureof Gliese 581 g ranges from 209 to 228 K (-64
to -45°C) for Bond albedos from 0.5 to 0.3.
• Adding an Earth-like greenhouse effect yields an average
surface temperature in the range of 236 to 261 K (-37 to
−12 °C).
• A factor that could potentially give Gliese 581 g a
greenhouse effect greater than Earth's is the possibility the
more massive planet also has a more massive atmosphere.
Hot Jupiters