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Phys 214. Planets and Life Dr. Cristina Buzea Department of Physics Room 259 E-mail: [email protected] (Please use PHYS214 in e-mail subject) Lecture 30. Habitability in star systems. Extrasolar planets. March 28th, 2008 Contents Textbook pages 329-332, 338-353, 360-392 The evolution of surface habitability in a star system Stars and their classification Which stars do make good stars Detection of extrasolar planets The nature & evolution of habitability A star’s habitable zone = the range of distances from the star where liquid water can be stable on the surface of a suitable planet. Being in a star habitable zone is NOT enough to make a world habitable. The size of the planet is very important! E.g. The Moon is in the habitable zone of the Sun but is not habitable (too small to retain an atmosphere necessary for liquid water to be stable). E.g. Europa is located outside the Sun’s habitable zone and yet may be habitable (because is tidally heated, allowing liquid water to exist beneath its icy surface) The nature & evolution of habitability Habitable zones evolve with time. Over time the Sun’s habitable zone has widened and moved farther from the Sun. When the Sun was younger its habitable zone was narrower and closer to the Sun. In the future, the Sun’s habitable zone will be wider and farther from the Sun. The end of habitability of Earth: conservative estimates ~ 1 billion years from now. optimistic estimates ~ 3-4 billion years from now. The nature & evolution of habitability The Sun brightens gradually with time because as H is converted into He in the core, the number of H nuclei decreases, decreasing the fusion rate. To maintain the balance with the gravity of the outer layers, the core compensates by shrinking and heating up. The outer layer surrounding the core (that still contains unburned H) becomes very hot that ignites nuclear fusion. The total rate of fusion is so high that the star increases in size and emits more light. The Sun will expand into a red giant when will run out of nuclear fuel. The Earth surface will heat to 700oC, oceans will evaporate, runaway greenhouse effect followed by the total loss of its atmosphere. Life not able to survive even beneath the surface. When the Sun ejects its outer layer into space to become a planetary nebula, most likely the Earth will be destroyed. Surface habitability factors Critical factors that determine the habitability of a planet: 1) Size E.g. Mars currently lacks surface habitability mostly because of its small size Large size enough to allows plate tectonics to exist. Venus (similar size to Earth) but lacks plate tectonics because of the runaway greenhouse effect - too close to the Sun. 2) distance from parent star (the planet must be not too close or too far from its star. Must lie within the habitable zone) 3) presence of an atmosphere Without enough atmospheric pressure, liquid water cannot be stable & protection against harmful solar radiation. To have a long lasted atmosphere, the planet must have enough gasses trapped in interior to be outgassed, a magnetic field, and at least a moderate rotation. Global warming The recent gradual rise in the Earth’s average surface temperature is commonly referred to as global warming. The detailed causes of the recent warming remain an active field of research, but the scientific consensus is that the increase in atmospheric greenhouse gases due to human activity caused most of the warming observed since the start of the industrial era. Earth - Global warming Measurements of CO2 concentrations over the past 400,000 years show a direct correlation with global surface temperatures. The atmospheric CO2 concentration is higher than it has been at any time during the last 400,000 years. While there is no doubt that global temperatures are increasing, it is now becoming clear that human activity is indeed causing global warming. Earth - Global warming The green stripe shows the variation between different models that take into account both: -natural factors (changes in sun’s brightness, effects of volcanic eruptions) & - emission of greenhouse gases by humans. Global warming These changes are expressed in terms of radiative forcing = a measure of the influence that a factor has in altering the energy balance of the climate system. Positive forcing tends to warm the surface while negative forcing tends to cool it. (check http://www.physics.queensu. ca/~phys214/Links.htm Climate change 2007 Changes in the atmospheric abundance of greenhouse gases and aerosols, in solar radiation and in land surface properties. Aerosols Natural and anthropogenic aerosols. The amount of man-made aerosols is considerable and can induce weather cycles over populated regions, where they can affect cloud formation. NASA research: anthropogenic aerosols induce clear weekly cycles over New York City. The aerosols were thickest midweek and lightest on weekends, affecting clouds formation (J. of Geophysical Res. 110, 2005) STRATOSPHERE TROPOSPHERE St. Helen, 1080 40 km plume Solar irradiance Some researchers estimated that the Sun may have contributed about 25–50% of the increase in the average global surface temperature during 1900–2000. [Geophysical Research Letters 33 (5)] Movie: Solar flares Some researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing and underestimate the cooling effects of volcanic dust. However, even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases!!! Stars Finding a planet on which life might arise is selecting a sun that can provide sufficient light and heat to support habitable worlds. Stars spend 90% of their lives fusing hydrogen into helium in their cores and slowly brightening. A star that exhausts its hydrogen core begins to grow larger and brighter, becoming a giant or supergiant star. Habitable planets are around stars that are in the long-lasting H-fusing stage of their life. When stars finished the fusion fuel they die. Relatively low-mass stars like our sun eject their outer layers into space as planetary nebulae, leaving behind a type of dead star called white dwarf. Higher mass stars die in titanic explosions – called supernovae, in which their cores collapse in either neutron stars or black holes. Stars One of the fundamental principles of stellar evolution is that the more massive a star is the faster it evolves. A star less massive than our Sun will have a longer lifetime. A star more massive than our Sun will have a shorter lifetime. How do we classify stars? Because all stars are born with basically the same composition (98% H & He), the physics that determines the star characteristics is straightforward. During the H burning phase, the star’s surface temperature (or spectral type) and total luminosity is determined almost entirely by one thing – star mass! There are seven major spectral type of stars: O, B, A, F, G, K, M. The spectral sequence OBAFGKM runs from hot to cool in terms of surface temperature of stars. Mnemonic Oh, Be A Fine Girl, Kiss Me Which stars do make good suns? O-type stars (most luminous) have lifetimes too short for planet formation. O-type stars in our galaxy are very rare (less than 1%) A- and F-type stars (more luminous than the Sun) have lifetimes long enough for planets to form and for simple life to appear, but not long enough for advanced life to develop. B-type stars (much more luminous than our Sun) have lifetimes long enough for planets to form but not for life to appear. Stars like our Sun have lifetimes long enough for advance life to evolve. K- and M-type stars (less luminous than the Sun) have lifetimes long enough for advanced life to evolve. However, they might not have many habitable planets around them because their habitable zones are very narrow. Which stars do make good suns? K- and M-type stars have lifetimes long enough for advanced life to evolve. However, they might not have many habitable planets around them because their habitable zones are very narrow. Objections to their habitability: Closer planets might be locked in a synchronous rotation, with one side facing the star. Smaller stars have frequent and energetic flares that might negatively affect life on closer planets. Which stars do make good suns? • Depending on the stellar type (mass and luminosity), habitable planets will be at different distances from the parent star Which stars do make good suns? Many brown dwarfs in constellation Orion. Infrared image of a Jupiter-size planet orbiting a brown dwarf. Brown dwarfs are substellar objects with insufficient mass to sustain nuclear fusion in their cores. They have higher surface temperatures than planets and masses between 10to 80 times that of Jupiter. Brown dwarfs have no habitable zones because they are so dim. However, recent infrared observations suggest that planets are forming around them, and one brown-dwarf has a Jupiter-size planet orbiting it. Habitability of planets orbiting a multiple star system Multiple star systems in our galaxy are fairly common, making up around 30% of the total stars. Stable planetary orbits: - A large orbit around both stars in a close binary system - An orbit close to one of the stars in a wide binary system Detection of extrasolar planets Two ways to search for extrasolar planets: 1) Directly – pictures or spectra of planets 2) Indirectly – by measuring stellar properties (position, brightness, spectra) • astrometric technique • Doppler technique • transit technique • Gravitational lensing The few extrasolar planets that have been detected directly to date are very large and at great distances from their parent star. Detection of extrasolar planets - Astrometry Astrometry uses regular changes in the positions of the parent stars with respect to more distant stars as they move across the sky to detect extrasolar planets around other star systems. Most extrasolar planets have been detected by observing the gravitational tug they exert on the stars they orbit. All the objects in a star system, including the star, orbit the system’s center of mass. Orbital path of the Sun around the center of mass of the Solar System as it would appear from a distance of 30 light-years away. 1960-2025 The center of mass of the solar system is close to center but not exactly at the center of the Sun. Detection of extrasolar planets - Doppler technique Detection of Doppler shifts in the spectra of the parent stars has been the MOST successful in detecting extrasolar planets around other star systems. Stars exhibit Doppler shift only if they are moving toward or away from us along the line of sight. The wavelengths of radiation from a star that is moving toward us are shorter. The wavelengths of radiation from a star that is moving away from us are longer. Detection of extrasolar planets - Doppler technique The radial velocity curve of a star with an extrasolar planet is a plot of radial velocity against time. If a star has an extrasolar planet - the amplitude of its radial velocity curve is related to the planet’s mass. - the wavelength of its radial velocity curve is related to the planet’s orbital period. - the symmetry of its radial velocity curve If a star has a high mass planet is related to the planet’s orbital shape. at a small distance form it, its (The uneven nature of the change in velocity radial velocity curve would indicates that the planet is in a highly elliptical orbit.) have a large amplitude, short wavelength. Massive planets closed to their parent star -easiest to detect using the Doppler shift. A low mass planet far from its parent star would be the most difficult to detect using the Doppler shift method. Detection of extrasolar planets - Doppler technique If we view a planetary orbit face-on, we will not detect any Doppler shift at all. We can detect Doppler shift only if the planet and star have a part of their orbital velocities directed toward or away from us. When we measure the mass of a planet using the Doppler shift method, we know that it is mass could well be larger. The Doppler shift method of detecting extrasolar planets only give us the minimum mass of a planet because we don’t necessarily know the angle the planet’s orbit makes with our line of sight. Most of the extrasolar planets detected to date are found very close to the parent stars. Detection of extrasolar planets - Transit technique The transit method for detecting extrasolar planets is based on detection of brightness changes in a star as a planet passes in front of it. These changes depend on the planet size. When Mercury or Venus passes in front of the disk of the Sun, we call this a transit. For the transit of an extrasolar planet to be observed, the orbital plane of the planet has to be aligned along our line of sight. This method detects planets orbiting closed to their star. Detection of extrasolar planets - Transit technique •Hubble Space Telescope field of view in the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS). •Half of these stars are bright enough for Hubble to monitor for any small, brief and periodic dips in brightness caused by the passage of an exoplanet passing in front of the star. •The green circles identify 9 stars that are orbited by planets with periods of a few days. Planets so close to their stars with such short orbital periods are called "hot Jupiters." Detection of extrasolar planets - Gravitational lensing Gravitational lensing is the process by which a massive object magnifies and distorts the light from an object behind it. Mass distorts space -> light rays bend slightly when passing near objects with large mass. Gravitational lensing detection has discovered the lowest-mass planet to date – only 5 times the mass of earth planets in large orbits. Properties of extrasolar planet discovered so far Hot-jovian describes the most common type of extrasolar planet discovered to date. The orbits of most extrasolar planets detected to date are highly elliptical. Most of the extrasolar planetary systems discovered to date are very different than our own solar system having Jovian-sized planets close to their parent stars. Properties of extrasolar planet discovered so far The existence of giant planets in sub-Mercurian orbits and in excentric orbits has come as a surprise and is forcing theorists to revise their understanding of how young planetary systems evolve. According to our current theory of planet formation, Jupiter-like planet cannot form close to its parent star because it would be too hot for gases to condense. However, they can form farther out and then migrate inward. The inward migration of a Jovian-like planet in an extrasolar planetary system will alter the probability of life appearing on inner terrestrial planets - it would greatly decrease the chance because the orbits of the inner, terrestrial-like planets would be disrupted How to detect life on extrasolar planets Spectra from different Earth-like planets The simplest way we might detect life on an extrasolar planet is to analyze the spectrum of reflected light from the planet. Galactic constraints Green – habitable zone Both the amount of heavy elements AND the amount of radiation from the center determine the position of a galaxy’s Galactic Habitable Zone. It is difficult for planets with life to form around Sun-like stars in the inner parts of the disk of our galaxy because there would be too much harmful radiation It is difficult for Earth-like planets to form around Sun-like stars in the outer parts of the disk of our galaxy because there would be insufficient amounts of heavy elements. Impact rates and Jupiter If our solar system hadn’t formed with a Jupiter-sized planet, the rate of impacts on the Earth may have been much higher, possibly preventing the appearance of life. Next lecture • • • • The search for extraterrestrial intelligence The Drake equation Interstellar travel Conclusions • The final review lecture to be posted online later today!