Uniqueness of Earth?
•
•
•
Sun has sufficient Main Sequence lifetime for life to develop and evolve.
•
The size of Earth large enough—formed with significant but not too large
atmosphere. Varying luminosity of Sun compensated by greenhouse effect.
•
•
Has large moon that produces significant tides and stabilized Earth’s spin axis.
•
•
But Jupiter also prevented a planet from forming, resulting in asteroids.
•
Existence of plate tectonics also
contributes to climate stability, being
an important part of the CO2 cycle.
Plate tectonics are possible for
Earth-sized planets containing water.
Sun is a single star.
Earth has the right distance from the Sun, i.e., is in “Habitable Zone”. Limits
determined by runaway greenhouse and refrigerator effects.
Jupiter and other giant planets removed asteroids and comets from Earth-crossing
trajectories, reduced cometary impact rate during last few billion years.
Solar composition has C/O<1 (0.55),
so free oxygen is available.
Lattimer, AST 248, Lecture 22 – p.1/43
Future Habitability of Earth
•
Sun brighten as it ages. This is caused by increasing amount of He ash in Sun’s
core which reduces abundance of H fuel. Also, number of pressure-producing
particles decreases (4H→He). Sun’s core temperature and density must increase
to compensate, resulting in an increase in luminosity.
•
Since Sun was born, its luminosity has increased about 30%.
Lattimer, AST 248, Lecture 22 – p.2/43
Greenhouse Effect
Important greenhouse gases: H2 O (50%),
clouds (25%), CO2 (20%), O3 (2%), CH4 (1%),
N2 O (1.5%), fluorocarbons (0.5%)
Clathrate greenhouse: release of CH4 from
Siberian tundra could trigger large change
Lattimer, AST 248, Lecture 22 – p.3/43
Global Warming
•
Studies of ice core samples have shown there to be a good correleation between
temperature and CO2 abundance in the atmosphere, and that CO2 levels are
higher now than anytime in last half million years.
•
The major contributor to the greenhouse effect in Earth’s atmosphere is H2 O, not
CO2 , but H2 O has a short residency in the atmosphere and its concentration is
determined by temperature.
Lattimer, AST 248, Lecture 22 – p.4/43
Global Warming
•
These studies also indicate that the rise in CO2 occurs an average of 800-2000
years after the rise in temperature begins.
•
This indicates that a rise in CO2 does not initially trigger global warming. However,
800 years is very short to typical CO2 cycles (around 100,000 years) so it does not
prove that CO2 increases don’t also lead to further global warming.
•
Climate modeling generally shows that enhanced CO2 levels result in higher
global temperatures.
•
Models also show that global warming results in a rising of the tropopause and a
shrinking of the thermosphere as observed.
•
•
It is observed that the O2 /N2 ratio is slowly decreasing, as predicted.
A further point is that if the CO2 change was a result of rising temperatures,
concentrations of CO2 in ocean should be decreasing. However, they are
observed to be increasing and oceans are also becoming more acidic.
•
The extra CO2 in the atmosphere almost certainly is human-induced: the
abundance of 14 C in atmospheric CO2 is comparitively low, showing it originates
from buried or stored carbon.
•
Expected effects of higher average temperatures include increased sea levels,
more violent storms in the Gulf of Mexico, shifts in agriculture.
•
Release of large amounts of CH4 from melting Siberian tundra could result in even
more climate forcing than CO2 .
Lattimer, AST 248, Lecture 22 – p.5/43
Search Techniques for Other Solar Systems
•
Satellite and space telescope infrared observations (IRAS, HST), of dust emission
from protosolar disks.
•
•
Radio telescope searches for Jupiter-like bursts of radio waves.
Gravitational effects on motion of star due to planets
•
•
•
Doppler shifts
Ground and space-based continuous monitoring of millions of stars (Kepler)
•
•
•
“wobbling” (astrometry)
Planetary transits
Gravitational lensing (microlensing)
Imaging
•
•
•
Speckle imaging especially in infrared with adaptive optics
Nulling with interferometry and coronographs
Large space-based infrared interferometers (Ex-NPS) for direct imaging
Lattimer, AST 248, Lecture 22 – p.6/43
Disks
Lattimer, AST 248, Lecture 22 – p.7/43
Gravitational Effects
•
•
“wobbling”
Doppler shifts
Lattimer, AST 248, Lecture 22 – p.8/43
Limits of Doppler Technique
Lattimer, AST 248, Lecture 22 – p.9/43
Transits
Brief, periodic
dimming of star
Lattimer, AST 248, Lecture 22 – p.10/43
HD 209458
H2 O also identified
Lattimer, AST 248, Lecture 22 – p.11/43
Atmosphere
Lattimer, AST 248, Lecture 22 – p.12/43
SWEEPS
Lattimer, AST 248, Lecture 22 – p.13/43
Lattimer, AST 248, Lecture 22 – p.14/43
Gravitational
Lensing
Brief, one-time
brightening of star
Lattimer, AST 248, Lecture 22 – p.15/43
Gravitational
Lensing
Lattimer, AST 248, Lecture 22 – p.16/43
Direct Imaging
• Existing methods have inherent limitations
• Doppler method requires large orbital velocity; small period or orbital radius
(a < 5 AU)
•
•
Transit method requires smaller orbit still (a < 1 AU)
Extracting atmospheric information is indirect and quite difficult
Lattimer, AST 248, Lecture 22 – p.17/43
Direct Imaging
• Existing methods have inherent limitations
• Doppler method requires large orbital velocity; small period or orbital radius
(a < 5 AU)
•
•
Transit method requires smaller orbit still (a < 1 AU)
Extracting atmospheric information is indirect and quite difficult
• Direct Detection
• Permits direct atmosphere studies with
spectroscopy
• Very difficult: planets are intrinsically faint and very
close (in angle) to bright star
• Detectability greater if in larger orbits (a > 5 AU)
• Brightness of planet increases with mass (> 5 MJup )
• Brightness of Jovian planet greater if very young
(< 10 Myr)
Lattimer, AST 248, Lecture 22 – p.18/43
Lattimer, AST 248, Lecture 22 – p.19/43
Lattimer, AST 248, Lecture 22 – p.20/43
Technical Challenge
Diffraction from telescope aperture
Currently not a limitation
Could be reduced with coronagraphy
Scattering from atmosphere
Can be compensated by adaptive optics
Scattering from optics
Small defects on optical surfaces
Speckles produced are very problematic
Lattimer, AST 248, Lecture 22 – p.21/43
What’s Been Found
Lattimer, AST 248, Lecture 22 – p.22/43
What’s Been Found
Transits
Radial Velocity
Lattimer, AST 248, Lecture 22 – p.23/43
What’s
Been
Found
•
“Hot Jupiters” are large planets in relatively close orbits; most of them have highly
eccentric orbits so they come close to parent star during part of their orbit.
•
Most planets found to date are hot Jupiters, probably because selection effects
favor finding massive planets in close orbits.
•
Hot Jupiters form by migration of gas giants towards inner part of system;
migration caused by momentum transfer between dust or rocky debris and planet
(slingshot or gravity-assist effect).
•
•
Migration or eccentric orbits disrupt inner planets, ejecting them from system.
However, in some cases, simulations indicate terrestrial planets can reform in
larger, stable orbits, possibly within habitable zones.
Lattimer, AST 248, Lecture 22 – p.24/43
Lattimer, AST 248, Lecture 22 – p.25/43
Lattimer, AST 248, Lecture 22 – p.26/43
Lattimer, AST 248, Lecture 22 – p.27/43
Lattimer, AST 248, Lecture 22 – p.28/43
Lattimer, AST 248, Lecture 22 – p.29/43
Gliese 581
Lattimer, AST 248, Lecture 22 – p.30/43
Kepler’s Planets
Lattimer, AST 248, Lecture 22 – p.31/43
Lattimer, AST 248, Lecture 22 – p.32/43
Lattimer, AST 248, Lecture 22 – p.33/43
Lattimer, AST 248, Lecture 22 – p.34/43
Recent Successes of Direct Imaging
•
•
•
•
•
•
•
•
•
GQ Lupi b
AB Pic A b
CT Cha b
DH Tau b
CHXR 73 (Cha I) b
1RXJS J1609-2104 b: A giant planet candidate around a young solar analogue
HR 8799 bcd: A system of three giant planets
Fomalhaut b: Opticatl detection of a giant planet inside a dusty disc
β Pic b: A giant planet at only 8 AU
Lattimer, AST 248, Lecture 22 – p.35/43
Lattimer, AST 248, Lecture 22 – p.36/43
Multi-Planet Detection: HR 8799
Lattimer, AST 248, Lecture 22 – p.37/43
Fomalhaut
Lattimer, AST 248, Lecture 22 – p.38/43
β Pictoris
Lattimer, AST 248, Lecture 22 – p.39/43
Spectra
Lattimer, AST 248, Lecture 22 – p.40/43
Terrestrial Planet Finder
Lattimer, AST 248, Lecture 22 – p.41/43
The Future
Lattimer, AST 248, Lecture 22 – p.42/43
Lattimer, AST 248, Lecture 22 – p.43/43