Lecture 18 - Planets Around Other Stars

Lecture 18 - Planets Around Other Stars - Planets around...

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Unformatted text preview: Planets around other Stars ASTRONOMY 3 Lesson 18 1 Depiction of planets in orbit around the pulsar PSR 1257+12 (Credit: Robert Hurt, SSC) NATURE of the Universe Astronomy - Planets around other Stars Review of Lesson 17 The interstellar medium can get compressed into dark (molecular) clouds. Star formation starts when part of a molecular cloud becomes gravitationally unstable, and starts to collapse. The contracting protostar heats up. Once it becomes hot enough to be observable in the infrared, the pre-main sequence phase starts. Protoplanets might form in a disk of gas and dust around the protostar, Once density and temperature in the core get high enough, hydrogen fusion starts. The star is now on the main sequence. The Orion nebula is a nearby region where star formation is taking place. 2 NATURE of the Universe Astronomy - Planets around other Stars Questions from Lesson 17 Think about (questions from Lesson 17): How do we know about planets around other stars? Could (intelligent) life exist on extra-solar planets? 3 NATURE of the Universe Astronomy - Planets around other Stars Today's Topics Stars, Disks, and Planets Indirect Detections: Doppler methods, astrometry, and transits Direct Detection of Exoplanets Life on Other Worlds? Summary & Announcements Textbook: Ch. 4.4 - see also Lab 4 on "Gravity & Extrasolar Planets" 4 NATURE of the Universe Astronomy - Planets around other Stars Stars, Disks & Planets More than 2,500 years ago, Greek philosophers speculated that many worlds might exist. Democritus Epicurus In the 1,700s, Immanuel Kant and Pierre-Simon Laplace formed our modern view that the planets of the solar system formed in a disk. Kant Laplace 5 See, e.g., http://www.ucolick.org/~bryden/ for more details on current planet formation theory NATURE of the Universe Astronomy - Planets around other Stars Stars, Disks & Planets The Orion nebula houses many young stars with disks. Let's take an imaginary flight through the Orion nebula ... 6 The volume visualization movie of Orion can be found at http://www.vislab.uq.edu.au/ research/vistools/index.html NATURE of the Universe Astronomy - Planets around other Stars Stars, Disks & Planets Challenges of direct detections of planets around stars Mercury transit - the view in white light (photosphere of the Sun) and in the light of the red hydrogen line (chromosphere of the Sun) 7 Notice how tiny Mercury is compared to the Sun! NATURE of the Universe Astronomy - Planets around other Stars Stars, Disks & Planets The angle between the Sun and Jupiter as seen from Centauri is 4 arcsec. This corresponds to the angle between the two headlights of a car at a distance of 75 km (50 miles). 8 NATURE of the Universe Astronomy - Planets around other Stars Stars, Disks & Planets A further complication .... (or can you see Jupiter during daylight?) In the visual the Sun is 1 billion times brighter than Jupiter. This is comparable to the contrast between car headlights and the glow from a firefly. Firefly Car headlights The challenge in directly detecting a Jovian planet around the most nearby star is similar to trying to spot a firefly next to car headlights seen at a distance of 75 km (50 miles). The direct detection of a planet in orbit around even the most nearby stars is challenging! 9 NATURE of the Universe Astronomy - Planets around other Stars Indirect Detections In a binary star, the two stars orbit their common center of mass. Observer The spectra of both stars alternate between blueand red-shifted as they orbit their common center of mass. 10 Credits: R. Pogge, OSU NATURE of the Universe Astronomy - Planets around other Stars Indirect Detections In 1952, Otto Struve (UC Berkeley) suggested that a planet with 10 times the mass of Jupiter in a close orbit (3 million km) around a star would cause the radial velocity of the star to change by 2 km/s every day. What could we learn about such a planet detected by the Doppler method? (remember Lab 4) Mass of the Planet Orbital period (semi-major axis) Eccentricity of the orbit 11 NATURE of the Universe Astronomy - Planets around other Stars Indirect Detections More than 40 years later in 1995, the Swiss astronomers Michel Mayor and Didier Queloz announced the discovery of a planet with 0.45 times the mass of Jupiter and an orbital period of 4.2 days orbiting the star 51 Pegasi. The radial velocity of the star changed by just 57 m/s. 12 Note: the discovery of the planet orbiting 51 Pegasi was subsequently confirmed by Geoff Marcy and Paul Butler (see plot). In 1992, Aleksander Wolszczan had detected planetary mass objects orbiting a pulsar. NATURE of the Universe Astronomy - Planets around other Stars Indirect Detections In 1999, Paul Butler, Geoff Marcy and others announced that the star Ups Andromedae has 3 planets in its orbit. The complex radial velocity signal of Ups Andromedae can be explained by the presence of 3 planets orbiting it. 13 NATURE of the Universe Astronomy - Planets around other Stars Indirect Detections Today (Nov. 2006) almost 200 planets around other stars have been detected using the Doppler method. Planets orbit stars at separations between 0.03 and 8 Astronomical Units. The apparent sparseness at larger separation is due to the long orbital periods at these separations, and the fact that Doppler surveys have only been running of 15 years thus far. About 12% of the nearby solar-type stars seem to have at least one Jovian planet! 14 NATURE of the Universe Astronomy - Planets around other Stars Indirect Detections The masses of the detected extrasolar planets range from about 10 times the mass of Earth to 15 times the mass of Jupiter. Radial velocity surveys (Doppler surveys) would not be sensitive enough to detect terrestrial planets with masses comparable to Earth. 15 NATURE of the Universe Astronomy - Planets around other Stars Sun Indirect Detections 1.0 MJup The properties of most planetary systems are quite unlike the properties of the solar system. In particular, Jovian planets tend to orbit other stars at much smaller separations. 16 NATURE of the Universe Astronomy - Planets around other Stars Are there other ways to detect planets indirectly? Indirect Detections Similar to the periodic change in radial velocity as a star orbits the common center of mass with its planet, one can detect the astrometric motion ("wobble") of a star on the sky. For an astronomer in a planetary system 30 light years away, the "wobble" of the Sun as it orbits the common center of mass with all the planets would amount to less than 0.001 arcsec. 17 NATURE of the Universe Astronomy - Planets around other Stars Indirect Detections Astrometric measurements with a precission better than 0.001 arcsec are very difficult from the ground, and easier done from space. GAIA is expected to detect between 10,000 and 50,000 Jovian planets around stars within 150 light years of the Sun. The launch is planned for 2011 18 NATURE of the Universe Astronomy - Planets around other Stars Indirect Detections For an observer looking from the die on a planetary system, the planets transit in front of the star, similar to the Mercury and Venus transits in front of the Sun we can observe from Earth. Sunspot Mercury Jupiter has about 1/10 the diameter of the Sun. For an alien astronomer observing a transit of Jupiter in front of the Sun, the Sun would dim by 1%. Earth has about 1/100 the diameter of the Sun, and hence would cause a dimming by 0.01%! 19 NATURE of the Universe Astronomy - Planets around other Stars Indirect Detections Transits of Jovian exoplanets in front of their star have been observed. As Earth's atmospheric absorption varies (because of clouds and dust in Earth's atmosphere), such measurements are best done from space. As the planet passes in front of the star, it blocks some of the light, causing a slight dimming. Observations of a transit by the Hubble Space Telescope. 20 NATURE of the Universe Astronomy - Planets around other Stars Indirect Detections 1.4m mirror of the Kepler satellite NASA is currently preparing a satellite mission,which aims at detecting a few Earth size planets and 1000s of Jupiter size planets around other stars. The launch is planned for the end of 2008. 21 NATURE of the Universe Astronomy - Planets around other Stars Direct Detection How does the mass of a planet compare to the mass of its star? Lower mass stars tend to have lower mass planets 22 NATURE of the Universe Astronomy - Planets around other Stars Direct Detection Planets more massive than Jupiter are all about the same size as Jupiter (i.e. Jovian planets increase in average density with higher mass). Planetary mass objects Brown dwarfs Stars Jupiter Jupiter The direct imaging detection of a Jovian planet is equally difficult for planets with 0.3 times the mass of Jupiter (like, e.g., Saturn) or 10 times the mass of Jupiter. 23 NATURE of the Universe Astronomy - Planets around other Stars Direct detection It is easier to search for planetary mass companions around brown dwarfs. The first direct imaging detection of a planetary mass object in orbit around a brown dwarf was achieved in 2004 by a team including UCLA astronomer Ben Zuckerman. The brown dwarf and its companion are 10 million years old. At this age the companion with a mass of 8 MJup has still a temperature of 1,600 K. This and the large separation of 55 Astronomical Units makes the direct detection possible. Still, one of ESO's 8m telescopes equipped with adaptive optics was required to image it. 24 See http://www.eso.org/outreach/press-rel/pr-2004/pr-23-04.html and http:// www.eso.org/outreach/press-rel/pr-2005/pr-12-05.html NATURE of the Universe Astronomy - Planets around other Stars Which is the most nearby star with a planet? Direct detection Radial velocity variations Eridani The planet around Eps Eridani is currently close to periastron. In 4 years (in apastron) a direct detection might be possible. See http://hubblesite.org/newscenter/archive/releases/2006/32/ Orbit of its planet 25 NATURE of the Universe Astronomy - Planets around other Stars Direction Detection A Hubble Space Telescope follow-up is currently getting developed. It will be able to detect directly Jovian planets with ages up to 1 billion years. Launch of the James Webb Space Telescope is foreseen for 2013 at the earliest. 26 NATURE of the Universe Astronomy - Planets around other Stars Direct Detection The ultimate goal is direct detection of Earth-like planets and a study of their properties H 2O H 2O Measurements of the atmospheric composition of terrestrial planets around other stars could reveal life signatures (Methane, Ozone). 27 NATURE of the Universe Astronomy - Planets around other Stars Summary Summary: Planets around other Stars The first indirect detections of planets around other stars are based on the Doppler effect. The variation in radial velocity of the star as it orbits its common center of mass with the planet is measured. Other indirect methods include astrometry and the search for transits. The direct detection of planets around stars is much more difficult. Planetary mass objects in orbit around brown dwarfs have been directly detected using ground-based telescopes. The detection and study of Earth-like terrestrial planets and the search for signs of life requires space missions, which are currently in various planning stages. 28 NATURE of the Universe Astronomy - Planets around other Stars Homework Reading assignment: Chapter 12 No online Homework this week! Homework 29 NATURE of the Universe Astronomy - The Solar System Homework Review Review of Homework 5: dose of statistics Percentage of correct answers 100 75 50 25 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Question Questions #10, #12, and #15: less than 85% correct answers 30 Answer key to homework 5 is available at http://web.physics.ucla.edu/class/06F/ 3_BRANDNER/homework/index.html - select "Homework 5" and then "key". NATURE of the Universe Astronomy - The Solar System Homework Review Review of Homework 5 10. Which statement is true? Compared to K- and M-type main sequence stars, O- and B-type stars are hotter and more massive The spectral sequence in order of decreasing temperature is OBAFGKML O B A F G K M L mass in units of the mass of the Sun 31 NATURE of the Universe Astronomy - The Solar System Homework Review Review of Homework 5 12. Suppose two stars have the same luminosity and spectral type. Star A is at a distance of 5 light years and star B is at a distance of 15 light years. How bright will star B appear compared to star A? Star B will be 1/9 as bright as star A The apparent brightness of a star scales inversely with the square of its distance. Star B is 3 times farther away as star A. Thus Star B is 1/32 = 1/9 as bright as star A. 15. A main sequence star has 3 times the mass of the Sun, and 120 times its luminosity. The life span of this star will be 250 million years lifetime = mass (M ) / luminosity (L ) * 10 billion years 3 / 120 * 10 billion years = 1/40 * 10 billion years = 1/4 billion years = 250 million years 32 NATURE of the Universe Astronomy - Terrestrial Planets Mid-term Exam 2 Mid-term 2: Wed, Nov 15 at 12:00 pm (noon), in class 45 multiple choice questions (select 1 out of 4 possible answers!) Answer all the questions! 45 minutes (no one may leave the classroom during the first 30 min of the exam - bring homework or a book to read) Bring a type 2 pencil. Scantrons will be provided. No headphones (iPods, ...) are allowed during the exam. Turn cell phones off. In case you should not feel well on the day of the exam, please see or contact me before the start of the exam. See the syllabus for what happens and/or is required in case you should miss one of the mid-term exams! 33 NATURE of the Universe Astronomy - Earth: Atmosphere Mid-term Exam 2 Mid-term 2: Wed, Nov 15 at 12:00 pm (noon), in class Topics cover Lessons 12 to 18 (including some repeats, like, e.g, Stefan's law or the Doppler effect) Use copies of the lecture slides and the notes you took in class as a basis for your studies. Use the textbook to read up difficult points. Don't try to memorize all the numbers! You should, however, know a few "key numbers" or ratios like, e.g., the length of the sunspot cycle, or the mass of the Sun compared to Jupiter. You should also be familiar with the H-R diagram, and the location of the main sequence etc. 34 NATURE of the Universe Astronomy - Earth: Atmosphere Homework Mid-term 2: Wed, Nov 15 at 12:00 pm (noon), in class Some questions will involve simple math, quite similar to homework questions. No calculator is required! about 15% are repeated homework questions See discussion board on class web page for additional details: http://web.physics.ucla.edu/class/06F/3_BRANDNER/wwwboard/index.shtml 35 ...
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This note was uploaded on 10/13/2008 for the course ASTR 3 taught by Professor Hauser during the Fall '07 term at UCLA.

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Lecture 18 - Planets Around Other Stars - Planets around...

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