Uniqueness of Earth?• Sun has sufficient Main Sequence lifetime for life to develop and evolve.

• Sun is a single star.

• Earth has the right distance from the Sun, i.e., is in “Habitable Zone”. Limitsdetermined by runaway greenhouse and refrigerator effects.

• The size of Earth large enough—formed with significant but not too largeatmosphere. Varying luminosity of Sun compensated by greenhouse effect.

• Has large moon that produces significant tides and stabilized Earth’s spin axis.

• Jupiter and other giant planets removed asteroids and comets from Earth-crossingtrajectories, reduced cometary impact rate during last few billion years.

• But Jupiter also prevented a planet from forming, resulting in asteroids.

• Solar composition has C/O<1 (0.55),so free oxygen is available.

• Existence of plate tectonics alsocontributes to climate stability, beingan important part of the CO2 cycle.Plate tectonics are possible forEarth-sized planets containing water.

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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-producingparticles decreases (4H→He). Sun’s core temperature and density must increaseto compensate, resulting in an increase in luminosity.

• Since Sun was born, its luminosity has increased about 30%.

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Greenhouse EffectImportant greenhouse gases: H2O (50%),clouds (25%), CO2 (20%), O3 (2%), CH4 (1%),N2O (1.5%), fluorocarbons (0.5%)

Clathrate greenhouse: release of CH4 fromSiberian tundra could trigger large change

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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 arehigher now than anytime in last half million years.

• The major contributor to the greenhouse effect in Earth’s atmosphere is H2O, notCO2, but H2O has a short residency in the atmosphere and its concentration isdetermined by temperature.

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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 notprove that CO2 increases don’t also lead to further global warming.

• Climate modeling generally shows that enhanced CO2 levels result in higherglobal temperatures.

• Models also show that global warming results in a rising of the tropopause and ashrinking 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 areobserved to be increasing and oceans are also becoming more acidic.

• The extra CO2 in the atmosphere almost certainly is human-induced: theabundance of 14C in atmospheric CO2 is comparitively low, showing it originatesfrom 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 evenmore climate forcing than CO2.

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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• “wobbling” (astrometry)• Doppler shifts

• Ground and space-based continuous monitoring of millions of stars (Kepler)• 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

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Disks

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Gravitational Effects• “wobbling”

• Doppler shifts

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Limits of Doppler Technique

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Transits

Brief, periodicdimming of star

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HD 209458H2O also identified

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Atmosphere

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SWEEPS

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GravitationalLensing

Brief, one-timebrightening of star

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GravitationalLensing

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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

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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 withspectroscopy

• Very difficult: planets are intrinsically faint and veryclose (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)

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Technical Challenge

Diffraction from telescope apertureCurrently not a limitationCould be reduced with coronagraphy

Scattering from atmosphereCan be compensated by adaptive optics

Scattering from opticsSmall defects on optical surfacesSpeckles produced are very problematic

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What’s Been Found

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What’s Been FoundTransits

Radial Velocity

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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 effectsfavor 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 inlarger, stable orbits, possibly within habitable zones.

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Gliese 581

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Kepler’s Planets

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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

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Multi-Planet Detection: HR 8799

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Fomalhaut

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β Pictoris

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Spectra

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Terrestrial Planet Finder

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The Future

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