Other migration processes Previous lecture: Migration through planet-disk interactions Type I: low-mass planet (does not clear a gap in disk) Type II: high-mass panet (has cleared gap) Type III: runaway migration New transiting hot Jupiters from last week First success from CoRoT Kozai mechanism Tidal migration Mass = 1.3 Mjup Radius = 1.65 Rjup, Period = 1.5 days New transiting hot Jupiters from last week Hot Jupiter with highly eccentric orbit! HATP-2b P=5.6 days, e=0.5 Kozai mechanism Interaction with a remote binary star 1. Eccentricity oscillations in planet orbit 2. At high e and short periastron tidal dissipation can circularize the orbit, drawing the planet inwards For the orbit of a planet, relative to the binary: Is conserved 1
Tidal migration Interaction of Earth/moon 1. Earth s spin slows down (2 ms/century) 2. Moon s distance increases (3.8 cm/yr) Tidal migration Interaction of Hot Jupiter with star: Star spins slower than HJ orbits 1. Star s spin increases 2. Hot Jupiter s distance decreases Time scales seem to be too long to play an important role Long term stability of planetary orbits Stability of orbits is investigated using numerical simulations. This is not straightforward: A long-standing question is whether the solar system is stable in the long run? Some general statements: The Hill stability criterion: Planets should approach each Other no closer than the Hill radius (=Roche radius) The Hill radius Some general statements about stability 1. Stability of orbits depends primarily on the separation of the semi-major axes 2. Orbital resonances can either increase stability or lead to instabilities 3. Large eccentricities tend to destabilize systems, because bodies can approach each other more closely 4. Large inclination differences increase stability Juric & Tremaine 2007: numerical simulations of exoplanet Mean separation orbits Number of planets 2
Lecture 8: Future of exoplanet research Biomarkers for extrasolar planets Search for Extraterrestrial Intelligence Biomarkers for extrasolar planets When we have found the first Earth-like extrasolar planet in the habitable zone (have we already?), how could we investigate whether it contains life? 1. SETI (see last bit of this lecture) 2. If life on planet changes the atmospheric constituencies in a detectable way (like it does on Earth) If life does not dominate the planet, we will not be able to study it. Habitability of Earth is governed by many factors Shoemaker-Levy 9 impact on Jupiter 1. The 10-15% of stars that have a gas giant, almost all have eccentric orbits. Lower mass planets also eccentric, making huge temperature variations. This makes highly developed life unlikely 2. The presence of Jupiter is likely to play an important role in the habitability of Earth, catching off a large fraction of the comets and meteores. 3. Our moon stabilizes the Earth orbit (think of the varying climates of Mars) Atmospheric history of the Earth Atmospheric history of the Earth 3
Infrared spectrum as biomarker Infrared spectra On present Earth: 1. it is difficult to envisage oxygen as a major constituency in a planet s atmosphere without biological processes. 2. Oxygen and Methane are in extreme chemical disequilibrium 3. CH 4 is quickly oxidized by O 2 if it was not constantly produced by animals. O 2 has a weak infrared signature, but O 3 has strong absorption. Optical reflectance spectrum as biomarker On Earth, green vegetation produces a sharp rise in the spectrum at 700-800 nm, caused by chlorophyll called the red edge Alien biospheres may not have similar photosynthesis processes With biomarkers we seem to be doomed to search for life like that on Earth Galileo flyby 1992: Carl Sagan et al., 1993, Nature Future space missions such as DARWIN and Terrestrial Planet Finder are aimed at studying biomarkers (see Perryman s lecture) 4
Search for Extraterrestrial intelligence SETI is an institute in the USA currently run solely on private funding (NASA sponsored for a while) Do not consider SETI as completely insane. If successful, it will be the biggest discovery in human history (and we will be the first to say that we knew all along...) However, chances of success are so slim, that most scientist do not want to be involved. SETI has been in action for more than 40 years. No results. But they do follow the scientific method, and should therefore be taken seriously Signals from ExtraTerrestrials (ET) Passive Signals: emission we just happen to pick up think about tv/radio signals we leak into space Active signals; distant civilization tries to get our attention! Passive signals cannot be detected for a significant sample of stars yet (but in near future?) How useful is SETI? Will it be more fruitful in the future? Phoenix project Future telescopes for SETI use SETI-funded Allen Telescope Array SKA. Spent 11,000 hours on 800 nearby stars. How many civilizations do we expect? Fermi paradox Completely unknown ( 1) Drake equation: If life is not rare, it can colonize the galaxy within a few million years. We don t see any aliens, so life is rare. The completely unknown is broken into several other unknowns Assume civilizations have a limited expiring date. 5
Copernican versus anthropic principle Copernican principle: the Earth occupies no special place in the universe. One could conclude from this that life should be abundant. Antropic principle: The Earth does occupy a special place in the universe, because we as intelligent civilization populate it. If Earth was not a special place, we would not be here. The Copernican principle is excellent, as long as it deals with issues that do not influence the development and evolution of intelligent civilizations. Hence, throwing in a bit of philosophy does not help... 6