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The Traveling Exhibit Science Background Part C: Planet Quest prepared by Dr. Cherilynn Morrow for the Space Science Institute Boulder, CO C. Planet Quest Are there planets orbiting distant stars where life could exist? Could we see them so far away? For the first time in human history, we have detected planets orbiting distant stars. During the past decade, we have discovered over 150 (and counting) of these extra-solar planets. We do not generally “see” these planets, but infer their presence using clever techniques to observe how they affect their parent stars. Almost all of these planets are like the gas giant Jupiter rather than Earth, but new missions are planned to detect Earth-sized worlds. Some planets were known to the ancients who watched them move against the night sky. Courtesy NASA’s Navigator Program 2 Mercury, Venus, Mars, Jupiter, and Saturn were the “Wandering Stars.” “Planet” comes from the Greek word for “wanderer.” Courtesy NASA’s Navigator Program Over the centuries, telescopes got better and better… Galileo and his Refractive Telescope, 1609 Herschel’s Reflecting Telescope, 1789 The Hooker Telescope Mount Wilson, ca 1920 Courtesy NASA’s Navigator Program And other planets were “discovered.” Uranus The year 1781 The first planet “discovered.” William and Caroline Herschel Neptune The year 1846 First observed by Galle and d'Arrest (based on calculations by Adams and Le Verrier). Pluto The year 1930 Discovered by Clyde Tombaugh Courtesy NASA’s Navigator Program 5 But what about more distant worlds? Thousands of years ago, Greek philosophers speculated. “There are infinite worlds both like and unlike this world of ours...We must believe that in all worlds there are living creatures and planets and other things we see in this world.” Epicurius c. 300 B.C Courtesy NASA’s Navigator Program And so did medieval scholars. The year 1584 "There are countless suns and countless earths all rotating around their suns in exactly the same way as the seven planets of our system . . . The countless worlds in the universe are no worse and no less inhabited than our Earth” Giordano Bruno in De L'infinito Universo E Mondi Courtesy NASA’s Navigator Program 4 In 1995, a breakthrough: the first planet around another star. Didier Queloz and Michel Mayor A Swiss team discovers a planet – 51 Pegasi – 48 light years from Earth. Courtesy NASA’s Navigator Program7 Artist's concept of an extrasolar planet (Greg Bacon, STScI) Methods to Detect Planets Spitzer, for the first time, captured the light from two known planets orbiting stars other than our Sun. But so far, most of the extra-solar planets are being detected using INDIRECT methods. Artist Concept: NASA’s Spitzer Infrared Telescope Methods to Detect Planets There are several complementary methods for detecting planets orbiting distant stars. Wobble Doppler – detecting the star wobbling in the line of sight due to the planet’s gravitational pull Astrometry – detecting tiny wobble of stars against other stars in the background. Planet Transit – detecting a tiny drop in brightness of the star as a planet passes in front Coronograph – blotting out the light of the star so planets can be “seen” Transit Coronograph Astrometry Inside the Planet Quest Area Finding Planets Orbiting Distant Stars An Atlas of new worlds discovered Keeping count of the new planets being discovered Planet Transit Coronograph Star Wobble 16 Alien Earths: Spin the wobbling star-planet toys When the child chooses two wooden balls and spins them, one ball does not orbit around the center of the other ball. Rather, each ball orbits around a common center of mass or balance point. Which ball is more massive, blue or red? If the balls have different masses, the balance point moves closer to the more massive ball. The more massive ball wobbles as the less massive ball goes around it. Massive Planets Cause Stars to Wobble • Stars and their planets also move about the common center of mass. • Since the mass of a star is so much greater than the mass of a planet, the “center of mass” (i.e. “balance point”) is located close to (but not at the center) of the parent star. • This means that stars with planets in orbit around them are not stationary. Rather, they move slightly about this balance point producing a gravitational wobble! • The gravitational wobble of the Sun is dominated by the gravity of the most massive planet Jupiter. The animation illustrates a planet's orbital affect on the position of its parent star. The effect is greatly exaggerated. Star moving toward observer (positive velocity) far side of orbit near side of orbit Source of animation Star moving away http://astronautica.com/detect.htm copyright 1997 Garber Astronautics NASA’s SIM PlanetQuest Mission Are there terrestrial planets orbiting nearby stars? NASA’s SIM PlanetQuest Mission will survey nearby stars for Earth-size planets by measuring the wobble of stars against other stars in the background. Artist Conception: NASA’s SIM PlanetQuest This method of detection is called astrometry. Scientists use the Doppler shift to measure the tug of planets on stars. Here is how it works: So far, nearly all extrasolar planets have been discovered with this technique If an unseen planet tugs the star back and forth… Courtesy NASA’s Navigator Program …the light from the star shifts slightly to the red as the star moves away from you. …and slightly to the blue as it moves toward you. Astronomers can detect these shifts by very carefully observing the spectra (or colors) of the stars. Detecting Planets: Transit Method Kepler Alien Earths: Turn-the-Crank Transit Device Check out the dips in starlight when a planet passes by! NASA’s Kepler mission will use the transit method. “Star” Crank “Planets” Graphic data When a planet passes by (or transits) a star, we can detect a slight decrease in the amount of light from the star. NASA’s Kepler Mission Are there planets orbiting distant stars where life could exist? Could we see them so far away? NASA’s Kepler Mission is specifically designed to survey our area of the Milky Way Galaxy to detect and characterize hundreds of Earth-size and larger planets in or near the habitable zone. The “habitable zone” encompasses the distances from a star where liquid water can exist on a planet's surface. Artist Conception: NASA’s Kepler Spacecraft Coronograph: We will block out the bright light from the star. Telescopes that block the light from the central star can take images of planets that might be in orbit around them. Keck Interferometer The Keck Interferometer Terrestrial Planet Finder will combines the light of two 10search from space for planets meter telescopes to take as small as Earth and for images of hot Jupiter-size signs about whether they can planets that shine bright in support life. infrared light. The Terrestrial Planet Finder Courtesy NASA’s Navigator Program The Constellation of Orion Using the Doppler method, two planets have been discovered around a Sun-like star in Orion that can be seen with the naked eye! Betelgeuse – red giant Bellatrix – blue giant HD 38529: Sun-like star 2 planets detected Orion Nebula – star forming region Rigel – massive blue giant Artist’s Concept of the two planets in orbit around the star called HD 38529 in the Orion Constellation (Lynette Cook) NOTE: the giant worlds we are detecting in orbit around other stars may well have moons (like Jupiter’s Europa) that might be habitable, but we would not be able to detect their presence because they are so small and distant. Other Star Systems are Different COMPARE The orbits of planets in our solar system (dashed blue lines) 6 years to orbit the star 2 weeks to orbit TO The orbits of the two Jupiter-sized planets detected in the star system HD 38529 (solid blue & green lines) WHAT DO YOU NOTICE? The orbit of planet “b” is closer to its star than Mercury is to the Sun! The orbit of planet “c” is more elliptical than Jupiter’s orbit. C. Planet Quest SUMMARY For the first time in human history, we have detected planets orbiting distant stars. During the past decade, we have discovered over 150 (and counting) of these extra-solar planets. We do not generally “see” these planets, but infer their presence using clever techniques to observe how they affect their parent stars (e.g Doppler & Transit methods). Almost all of these planets are like the gas giant Jupiter and can have an important impact on whether there are habitable planets in the system. New missions are planned to detect Earth-sized worlds.