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There are ~200 billion stars in our galaxy… …one of them is our Sun. Are there other planets in the universe? Is there another Earth out there? 2 Giordano Bruno (1548-1600) Believed that the Universe was infinite and that other worlds exists. He was burned at the stake for his beliefs. Extra-Solar Planets Exoplanet - A large body orbiting a star other than the Sun. Topics in this lecture: •Review of planet formation •The Habitable Zone •Basic properties of discovered exoplanets •Hot Jupiters •Super-Earths How to make a planet • Large, cool cloud of gas and dust – Gas makes the star, dust is necessary for planet formation – Dust is usually made of metals (Fe, Ni, Al), rocks (silicates) and ices (solid H2O, CH4, NH3) – Mostly H and He (these two elements make up about 98% of our Solar System) • Cloud begins to collapse under its own selfgravity How to make a planet Nebular Hypothesis a) Solar Nebula b) Contraction into rotating disk w/ hot center c) Dust grains accrete to form larger and larger particles d) Large particles sweep out more material to form planetesimals e) Planetesimals eventually collide to form planets The Habitable Zone The region around a star within which the requisite conditions for the existence of life can be met. Typically these conditions include: – Suitable temperature for liquid water – Existence of water itself – Appropriate cosmo-chemical composition – Are there other conditions or different ones? Are these necessary and sufficient conditions for life? The Habitable Zone Depends on: • Distance to parent star • Type of parent star • ATMOSPHERE – Albedo – Greenhouse effect – Dynamics (=atmospheric motions) Overview of Exoplanets Planet (IAU definitions of Planet, Dwarf Planet and Small Solar System Bodies) (1) A "planet” is a celestial body that: (a) is in orbit around the Sun, (b) Reached hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit. (1) A "dwarf planet" is a celestial body that: (a) is in orbit around the Sun, (b) Reached hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite. (1) All other objects except satellites orbiting the Sun shall be referred to collectively as "Small Solar System Bodies". Limited by search size... Imagine, if you shrunk our solar system to a little larger than a quarter: Our whole Solar System Our Milky Way Galaxy would be this big would be the size of the United States. From: http://planetquest.jpl.nasa.gov/Navigator/material/sim_material.cfm And the neighborhood where we’ve found new planets would only be the size of Manhattan. You can even see some of the stars that have planets in the night sky… From: …if you know where to look From: Overview of Exoplanets Discovery • First discovered extrasolar planet: 1988 • Number to date: 405 • Largest: 25 MJ • Smallest: 0.006 MJ (7x10-5 MJ possible comet) • Closest to parent star: 0.0172 AU • Furthest from parent star: 670 AU • Detection methods: numerous Exoplanet Detection Methods: 5 proven methods - Astrometry & Radial Velocity Method : 235 systems - Transit Method: 46 systems - Microlensing Method: 6 systems - Pulsar Timing Method: 3 systems 1) Astrometric Method Astrometric Method: Astrometry consists of measuring a star's position in the sky and observing the ways in which its position changes over time. If the star has a planet, then the gravitational influence of the planet will cause the star to move in a tiny circle. Astrometric Method Astrometric Method: Pro’s and Con’s Pro’s: 1 telescope can search many stars at a time Con’s: Does not work for far away stars (Can’t see the motion) Difficult to detect Terrestrial planets (not sensitive enough) Process is slow (needs to observe multiple orbits) Radial Velocity Method Doppler Effect - The change in frequency of a wave as perceived by an observer moving relative to the wave source. If a wave source is moving TOWARD an observer, the waves tend to “pile up”, INCREASING the frequency, or pitch. If a wave source is moving AWAY FROM an observer, the waves tend to “spread out”, DECREASING the frequency, or pitch. Radial Velocity Method Blue Shift - Object is approaching Red Shift - Object is receding Radial Velocity Method Radial Velocity Method (Doppler Method) This method measures slight changes in a star's velocity as the star and the planet move about their common center of mass. Astronomers can detect this motion by analyzing the spectrum of starlight due to the Doppler shift in the stars spectrum. The larger the planet and the closer it is to the host star, the faster the star moves about the center of mass, causing a larger color shift in the spectrum of starlight. That's why many of the first planets discovered are Jupiter-class (300 times as massive as Earth), with orbits very close to their parent stars. Radial Velocity Method Radial Velocity Method: Pro’s and Con’s Pro’s: Can be used on far away stars Con’s: Can only observe 1 star at a time (with high tech spectrographs) Difficult to detect Terrestrial planets (not sensitive enough) Process is slow (needs to observe multiple orbits) 3) Transit Method From the vantage point of the Earth, the planet of HD 209458 moves across the face of its star once every few days. It is so close that it is being vaporized by the star. Transit Method Transit Method: Planet passes in front of parent star and dim’s the light Instuments detect this periodic dip in brightness. Period and depth of the transits, orbit and size of the planets can be calculated. Transit Method Transit Method Transit Method Transit Method: Pro’s and Con’s Pro’s: Can be used on far away stars Can detect Terrestrial Planets (Method is sensitive enough) Can determine atmospheric composition Con’s: Can only detect ~1% of all solar systems (angles need to exactly align) Process is slow (needs to observe multiple orbits) Also called Photometric Method Gravitational Microlensing Method If a planet transits EXACTLY through the center of the parent star, gravity from the planet will bend the starlight Like a lens. The lensed light is magnified, and the star appears to briefly brighten. Telescopes can watch for the brief brightening of a star to detect a planet. Gravitational Microlensing Method Pro’s: Sensitive enough to detect terrestrial planets Con’s: The chances of a planet microlensing are very rare. Extremely sensitive instruments need to be used. Hot Jupiters • Large planets, orbiting close to parent star • Too close to be in the Habitable Zone • Still some interesting atmospheric chemistry and physics – Silicate rain? – Tidal locking causing interesting atmospheric waves? – All atmospheric chemistry/physics is still speculative Super Earths • Terrestrial (“rocky”) planet • More massive than Earth, less massive than Jupiter (ranges used in the literature range from 1 ME – 10 ME) • Not necessarily “habitable”, as the name might suggest • Host stars are metal-poor • Some super-Earths have been found in the habitable zone around main sequence stars Good Examples of Stellar Systems • Gliese 876 a) Parent star: – M3.5 V, T=3480 K, L=0.0124 Lsun b) Gas Giant – M=2MJ, a=0.208 AU c) Gas Giant – M=0.62 MJ, within orbit of Gliese 876 b d) Super Earth – Inside orbits of both Gliese 876 b and c The Gas Giants in this system are within the Habitable Zone, the Super Earth is too close (hot) for liquid water. Good Examples of Stellar Systems • Gliese 581 a) Parent star: – M3 V, T=3480 K, L=0.013 Lsun b) Gas Giant (Neptune-sized) – M=16 ME, a = 0.04 AU c) Rocky Planet – M=5 ME, a = 0.07 AU, within habitable zone (Temperature could be as low as -3 oC or as high as 500 oC, due to runaway greenhouse akin to Venus) d) Super Earth – M=7 ME, a = 0.22 AU, within habitable zone e) Super Earth – M=1.9 ME, a=0.03 AU The Drake Equation Where should we search for extraterrestrial life? How should we search? What is required to have life? Complex life? Life we could communicate with? The “Drake Equation” simply organizes these supposed requirements into separate factors, a sort of list of possibilities for our consideration. We want to estimate the likelihood that there are stars with planets with life that developed into complex “intelligent” technological forms that might be sending or receiving signals. What we really want is the total number of them, because that tells us how far we might have to search. The Drake equation assigns a symbol for each one of these key factors, representing its probability of occurrence, and multiplies all of them together. It is not something that is actually solved, or that you will have to work with except to see a few basic things. The Drake Equation: At the end you should see this “equation” as a map of our class topics: N =R fpl nhab Stars ? Planets? Habitable planets? fL fC Origin Complex of life? life? fT L/T Intelligence, Lifetime technology? of civilization Kardeshev Scale • Type I Civilization: Utilize all the Energy of Planet – Widespread Use of Fusion Power – Alternative Energies • Type II Civilization: – Utilize all the Energy of Parent Star – Create and harness energy from black holes • Type III Civilization: – Harness energy from multiple star systems and galaxies Kardeshev Scale • A tiered category for civilizations based on the amount of power available to them • Type 0 • Type I • Type II • Type III • Type IV Type 0 • Primary Energy Attained From Burning Fossil Fuels • Sensitive to environmental, societal, and biological pressures • Extreme danger of extinction Example: Planet Earth at Present Type I • Utilizes resources of entire planetary system • Use of fusion power for energy needs • Still Sensitive to extinciton Type II • Extract energy from multiple stellar systems • Capable of Stellar engineering • Very Low Extinction Risk due to spread Type III • Able to utilize the resources of an entire galaxy • Travel through wormholes possible • Capable of galactic scale influence and engineering Type IV Utilize all galaxies in the entire universe as an energy source Control matter, energy, time, and space Effectively Immortal Planet Earth at Present • .7 on Kardashev Scale The Drake equation is just a symbolic way of asking what the probabilities are that a sequence of events like those below (and more) might occur in other planetary systems.