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Search for Life in the Universe
Chapter 10
Search for Habitable Worlds
(Part 2)
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Outline
• Nature of Extrasolar Planets
– Observational Summary
– Solar-System Formation
– Implications for Habitability
– Signatures of Habitability & Life
• Earth-like Planets: Rare or Common?
– Frequency of Earth-size Planets
– Impacts
– Stable Climate
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Observational Summary
• Masses
– Minimum:median:maximum = 0.12:1.7:16.9 Jupiter masses
– Solar system: minimum:median:maximum = 7x106:3x103:1
• Close orbits
– Semi-major axis: minimum:median:maximum = 0.02:1.0:5.9 AU
– Solar system: Mercury:Earth:Jupiter = 0.4:1.0:5.2 AU
• Elliptical orbits
– Eccentricities: minimum:median:maximum = 0.00:0.28:0.93
– Solar system: 0.01:0.05:0.25
• Systems with multiple planets
– 18 out of 146 (1 in 8)
• Terrestrial planets
– Possible, but transit shows Jovian size for that planet
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Solar System Formation
• Standard theory: Jovian planets form at large
distances ~ 5 AU and more
• So why are they closer?
• Theory of planet formation wrong?
– We have not found a flaw, even when looking hard
after the discovery of the extrasolar planets
• Planet migration
– The obvious way out, but how?
– Drag by a residual disk  favors circular orbits
– Multiple encounters with planetessimals (evidence
that a little of that occurred in the solar system
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Implications for Habitability
• Jovian planets
– Planets themselves: unlikely, as in the solar system
– Moons of Jovian planets: a possibility, particularly if
the planet stays in the habitable zone
• Terrestrial planets:
– Migration of Jovian planets disrupts terrestrial planets
in the habitable zone during the migration
– Final elliptic orbits  long-term disruption
• Statistic
– Migration in most of the systems found
– Partly explained as a selection effect
– Need a complete sample of nearby systems
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Signatures of Habitability & Life
• What will we look for once we find Earth-like planets?
• Distance from star
– Is it in the habitable zone?
• Imaging
– Clouds
– Diurnal changes (oceans v. continents)
– Seasonal changes (snow and/or ice)
• Spectroscopy
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Surface temperature
Surface composition
Atmospheric composition (from IR spectra)
O2
CH4
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Frequency of Earth-size Planets
• Need heavy elements
– We think that terrestrial planets are formed from rocky
planetessimals
• Low heavy element abundance
– Low heavy element abundance: some regions, e.g., globular
clusters
– Solar heavy element abundance: most of the disk stars and the
interstellar medium
• Formation process
– Looks pretty straightforward, but we don’t know the details
• Bottom line
– Earth-size planets are very likely, unless we are unaware of
something special in the formation process of our solar system
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Impacts
• Earth:
– Bombardment lasted ~ 0.5 byr and then dropped off, allowing life
to form
– Could impacts last much longer elsewhere?
• Asteroids
– Mostly around Lagrange points of Sun and Jupiter (equilateral
triangles formed by Sun, Jupiter, and Lagrange point)
• Comets
– Mostly at Oort cloud, ~10,000 AU, but originated around Jupiter
• Jupiter
– Responsible for aligning asteroids along circular orbits between
Mars and Jupiter
– Responsible for ejecting comets to the Oort Cloud
– Do other stars have such a “Jupiter”, and what about migration?
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Stable Climate
• Stable climate for several byr
– Essential for life
– Has to adapt to the rising luminosity of the star over several byr
• Plate tectonics
– Essential for the CO2 cycle which regulates the climate
– Nothing unique about plate tectonics on Earth: depends on liquid
mantle and convection due to radioactive heating
• Moon
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Stabilizes the Earth’s tilt at 2025, moderating the seasons
How rare is a moon due to impact?: cf., Charon, Pluto’s moon
Other ways to stabilize seasons: e.g., winds
Can life migrate?
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