Why is it hard to find planets?

Transcription

1 Admin. 11/28/17 1. Class website 2. Optional Discussion sections: Tue. ~11.30am in Bryant 3; Thur. ~12.35pm, start in Pugh 170, then Bryant 3 3. Office hr: Tuesday pm; Wed pm, Bryant 302 (but me if coming on Wed.). 4. Homework 11 due Wed. 29th Nov. by 11.59pm. 5. Reading this week: Ch , 5-17, 18, me Astro-news, jokes, tunes, images: ast1002_tanl@lists.ufl.edu 7. Printed class notes? Name tags? 8. Final Exam: Tue., 5th December, 10.40am, in class (about 1/2 the questions on material since midterm 2).You are NOT allowed calculators: questions will only require simple arithmetic. You will be given a list of all formulae used in the class (see next slide). Exam is multiple choice format on a scantron so bring a pencil. Bring your UF ID. In class review session on Thur. 1st Dec. - bring questions for discussion. 9. Extra review session - Monday 4pm-5.30pm - Bryant 217 Key Concepts: Lecture 36: The Search for Planets and Life Search for planets: Radial Velocity Search; Transit Technique; Microlensing Technique; Direct Detection The Habitable Zone Estimates from Drake Equation SETI and Fermi s Paradox Speed = distance / time Angular size: θ = size / distance Kepler s 3rd Law: P 2 = a 3 [ Newton s version of Kepler s 3rd: P 2 a 3 /(m 1 +m 2 ) ] Newton s 2nd Law: F = m a Newton s Law of Gravity: F m 1 m 2 / r 2 Density = mass / volume Volume of a sphere = (4/3)πr 3 Surface area of sphere = 4πr 2 Momentum = mass x velocity Angular momentum mass x rotation rate x size 2 Frequency: f = 1/Period Speed of wave (light) = frequency x wavelength: c = f λ Energy of Photon: E = h f Wien s Law: λ max = 0.29cm / T(K) Parallax distance: d = 1/ p Flux: F = L / (4πd 2 ) Stellar Luminosity: L = 4πr 2 σt 4 Doppler Shift: Δ λ / λ = v / c Main Sequence Luminosity: L M 4 Stellar lifetime M / L M -3 Mass-Energy Equivalence E = m c 2 Light Gathering Power = Area x Exposure time Resolving Power (angular resolution) = 0.25 λ (microns) / diameter (m) Orbits in Galaxies: M galaxy + M sun a 3 /P 2 a v 2 Hubble s Law: v = H 0 d Drake Equation: N tc = R sf f wp N sfl f lb f il f ts L t All Formulae for Final The Search for Planets Beyond the Solar System (=Extrasolar Planets = Exoplanets) Why is it hard to find planets? 1. Planets are faint (small; low temperature): Internal luminosity L = 4πr 2 σt 4 Reflected luminosity may be more important than internal 2. Habitable planetary systems will be close to a star, which is much brighter than the planet

2 L sun compared to L earth Radial Velocity Search Look for periodic Doppler shift due to wobble of star caused by orbiting planet L = 4π r 2 σ T 4 Planet Search Techniques Planet Searches: (Many more extrasolar planets are now known than there are planets in our own solar system) 1. Radial velocity search: gravity of planet causes star to wobble. Look for the blue and redshift of spectral lines from the star Most sensitive to massive planets that are close to their stars detected >~500 planets to date by this method

3 The Hot Jupiters About 5% of surveyed stars have Hot Jupiters How are these planetary systems different from our own solar system? They have massive planets very close to the star in the warm/ hot region we expect terrestrial planets to form. Maybe these planets formed further from the star and migrated inwards. Often the orbits are eccentric. Why are we finding these kinds of systems? The radial velocity search technique is most sensitive to massive planets that are close to their parent star: they produce a large wobbling of the star. (Recall Newton s Law of Gravity: F m 1 m 2 /r 2 ) Searches have only been in progress for ~20 years, so we have only had time to find planets with orbital periods shorter than this. Nov 2005: By end of 2005 about 160 planets found by radial velocity method. By 2014 we have found ~1000 planets in total by the radial velocity method. Artist s Impression (John Whatmough) The glacier covered moon of 47 Ursae Majoris' planet. (note we do not know if the planet has a moon) Deep below the icy surface may exist an ocean of liquid water, and possibly, life. The second "Hot Jupiter" to be discovered, the planet orbiting 55 Cancri, like its cousin 51 Pegasi B, is awash in the glare of its nearby sun. On the night side of this seething world, we see super bolts of lightning arcing through the tumultuous atmosphere. 2. Transit Search A planet passes in front of its star. We see a small dip in the brightness of the star. If we know the size of the star, we can then measure the size of the planet.

4 Depth of Transit Depends on Relative Sizes of Star and Planet Transit search technique measures the size of the planet (if we already know the size of the star) Planetary Transits First Planetary Transit Detected in 1999 Star is called HD Radius of planet is 1.3 times Jupiter s radius Mass of planet (from radial velocity technique) is 0.63 times Jupiter s mass Why is the planet so big? Probably because it is so close to its star, is strongly heated, and so is puffed up. NASA s Kepler telescope Space telescope that stared continuously at ~160,000 stars towards constellation Cygnus Kepler has found >3000 planet candidates via the transit method. Many multi-planet systems have been found, often with Earth to 10x Earth sizes in orbits within 0.3 AU. The formation of these compact planetary systems, which are very different from our Solar System, is not yet understood.

5 Hot Jupiters Super Earths/ Hot Neptunes found by transits Systems with Tightly-packed Inner Planets (STIPs) 3. The Microlensing Technique Fabricky et al. (2014) Microlensing is when a star (the Lens star) passes in front of a background star (the Source star) and acts as a gravitational lens, bending the light to our telescopes (recall Einstein s General Relativity). This causes the source star to appear to brighten and then fade over time (see fig. on right). If the lens star has a planet, this can cause an extra blip in the magnification. This method is good at finding small planets that are far from their star (in contrast to the radial velocity method). But it requires monitoring millions of source stars and generally the planets that are found are so far away from us they are difficult to study further with other techniques. Figure 1. Systems of three or more planets. Each line corresponds to one system, as labelled on the right side. Ordering is by the innermost orbital period. Planet radii are to scale relative to one another, and are colored by decreasing size within each system: red, orange, green, light blue, dark blue, gray.

6 4. Direct Detection Why is it hard to find planets? 1. Planets are faint (small; low temperature): Internal luminosity L = 4πr 2 σt 4 Reflected luminosity may be more important than internal 2. Habitable planetary systems will be close to a star, which is much brighter than the planet The Drake Equation N tc = R sf f wp N sfl f lb f il f ts L t N tc = number of technological civilizations now present in the Milky Way R sf = rate of star formation over lifetime of the Galaxy ~ 10 f wp = fraction of stars with planetary systems (>0.05) ~ N sfl = average number of planets suitable for life f lb = fraction of habitable planets where life arises f il = fraction of life-bearing planets where intelligence evolves f ts = fraction of intelligent-life planets that develop technology L t = average life time of a technological civilization HR 8799 planetary system, Nov Number of Planets Suitable for Life: The Habitable Zone Need liquid water Small Temperature Range: K Only planets with certain distances from their star can maintain liquid water alternative heat sources tidal forces volcanoes

7 Habitable Zones around different stars Fraction with Intelligent Life Intelligent life arose on Earth despite many catastrophic mass extinctions once life forms, intelligence is inevitable fil ~ 1 Anthropic Principle - Earth must have intelligent life for us to ask this question Earth is not representative! fil <<< 1 Even on Earth it took several billion years for intelligent life to develop. Fraction Where Life Arises Life on Earth arose very rapidly flb ~ 1 Anthropic Principle we would only exist in a solar system where life arose Earth need not be representative flb <<< 1 Technological Civilizations? Advanced civilizations developed in several places on Earth chances are likely that any one such civilization would have developed communication technology How long does this communication phase last? Very difficult to predict. On Earth - only ~100 years so far!

8 SETI The Drake Equation Ntc = Rsf fwp Nsfl flb fil fts Lt Rsf = rate of star formation over life time of galaxy = 10 per year fwp = fraction of stars with planetary systems = Nsfl = Average number of planets suitable for life = ~0.2? flb = fraction of habitable planets where life arises = ??? fil = fraction of life bearing planets where intelligence evolves = ?? fts = fraction of intelligent life planets that develop technology = 1-0.1? Lt = average life time of a technological civilization = years? Ntc = number of technological civilizations present in the Milky Way Ntc = civilizations Jodie Foster in Contact Attempt to detect radio signals from extraterrestrial civilizations Began in 1960s Lost federal funding in early 1990s Period of private funding - e.g. Paul Allen of Microsoft, recently discontinued... And restarted in 2015! Breakthrough Listen! How many star systems have heard us? How Close is our Nearest Neighbor? For our extremely optimistic value of 1010 communicative civilizations average distance to nearest neighbor 6 light years For less optimistic values of ~ 1000 civilizations (this is still very optimistic) average distance to nearest neighbor ~ 1000 ly We have been sending radio signals into space for about 70 years. These have now traveled 70 light years in every direction. On average there is 1 star in every 4x4x4 ly3 =64 ly3 The volume of the radio sphere is 4/3 πr3 = 1.4x106ly3 So there are about 22,000 stars in this volume, that could have detected our radio/tv. 4 light years 70 light years schematic diagram not to scale

9 Fermi s Paradox If there are a multitude of advanced extraterrestrial civilizations in our Galaxy, the Milky Way, then, where are they? Why haven't we seen any traces of intelligent extraterrestrial life, such as probes, spacecraft or transmissions? e.g. an intelligent civilization should be able to spread out and colonize the entire Galaxy within tens of millions of years. HR 8799 planetary system, Nov Perhaps they are already here - studying us discretely (Zoo hypothesis) - Perhaps we are the only ones - formation and survival of intelligent life is difficult - Perhaps communication & interstellar travel are much more difficult than we imagine Search for Earth-like Planets Problem planets faint stars bright Solution Very precise optics Telescopes in Space Optical and infrared wavelengths TPF: Terrestrial Planet Finder brightness Searching for Earth-like Planets Our solar system as it would be seen by TPF from a distance of 10 parsecs Earth By the year 2030 or so, we hope!

10 Search for Life - Oxygen Oxygen in planetary atmosphere is a signature of biological life processes Oxygen can be detected from the spectrum at mid-infrared wavelengths via OZONE Astro News Breakthrough Starshot UFOs Question Einstein Special Relativity nothing can travel faster than speed of light severely limits interstellar travel and visitation Probably would involve large scale colonization expeditions that would take thousands of years Nearest Neighbors? 6 (very very optimistic) to several thousand (still optimistic) light years away. But we do not really know. Most UFO sightings are explainable as astronomical objects such as Venus You encounter a UFO and make contact with ET. You are only allowed to ask one question. What would that question be??

11 The Scientific Method - review Propose theories to explain nature. The theory should make some prediction which has a chance to tested. Test theories against experiments or observations Reject theories that fail to explain the data Keep those theories that work, but admit there is the possibility they are still wrong - there is always some uncertainty/doubt. Generally prefer the simplest theories that can explain the data. Further Reading and Info Astronomy News: www. space.com, www. spaceflightnow.com US Astrophysics Policy for the next decade: Astronomy at UF ( Public lectures, telescope observing and Starry Night events AST2037 Life in the Universe AST3043 History of Astronomy Consider a B.A. in Astronomy -> Law, Business, Medical school, etc, or Science Education (UFTeach) Consider a B.S. in Astrophysics -> as above, and/or career in research