Astro 101 – 003 Fall 2012

Lecture 13

Life in the Universe Part I

T. Howard

•  Ideas about life in the universe

•  SETI – the Search for Extra-terrestrial intelligence

•  Extrasolar planets

•  Astrobiology

Giordano Bruno (1548 – 1600) proposed that the Sun is a star, and that life existed elsewhere in the universe. He was convicted of heresy and burned at the stake.

•  Ideas that other beings exist elsewhere in the universe date to ancient times

•  Popular theme in historical literature

•  Among those who believed that ET life exists:

-- Benjamin Franklin (1706 – 1790) -- Immanuel Kant (1724 – 1804) (philosopher) -- William Herschel (1738 – 1822) (astronomer)

Cyrano de Bergerac (1619 – 1655) wrote early science fiction about lunar inhabitants.

Percival Lowell and the “canals” of Mars (c. 1906)

“War of the Worlds” radio drama (1938) •  Styled like a contemporary radio newscast •  Based on famous novel (1898)

by H. G. Wells •  Believed to be real by some people

ETIs in Science Fiction •  Alien life an essential theme of much science fiction from 1930s – today •  ETIs portrayed as benign, hostile, and everything in between

Example sci-fi novels featuring ETI

The Day the Earth Stood Still (1951)

UFOs – meaningful ??

•  Unidentified Flying Objects – sightings common from 1948 – 1970s, still some sightings today

•  Believed by proponents to be evidence of visitation by ETI in spacecraft (“flying saucers”)

•  NOTE: interstellar travel does not violate any laws of physics … … BUT, it would be difficult, lengthy, and very, very expensive

•  NO definitive evidence that UFOs even represent a real phenomenon •  Most professional scientists disregard the entire topic •  BUT, a small fraction (~ 5 – 6%) of all historical UFO reports

cannot be adequately explained as other phenomena •  Some prominent people and a few foreign organizations support the

investigation of UFOs, a few believe in the ETI theory •  The entire topic is fraught with controversy, hoaxes, inconsistency,

foolishness and partisanship, making it difficult to study seriously

Ideas about Interstellar Travel (1)

•  Government (NASA) and private studies (British Interplanetary Society) of concepts for nuclear-powered interstellar spacecraft, 1950s – 1970s •  Many other studies also •  Unmanned missions taking years to reach a nearby star

Orion: 100 yr to Alpha Centauri (4.4 LY) at .033 c (10000 km/sec) Daedalus: 50 yr to Barnard’s star (5.9 LY) at 0.12 c

•  Technically feasible though very expensive and difficult •  Backed by large amounts of detailed technical studies and designs

Orion concept (above) Daedalus concept (below)

Ideas about Interstellar Travel (2)

•  Interest in interstellar flight continues today in aerospace engineering and space enthusiast communities •  Other design concepts include beamed energy (laser propulsion) and solar sails •  Currently there is a study project underway to design a 100-year mission to a nearby star (100 Year Starship, see website http://100yss.org), partly funded by U.S. Govt. (DARPA)

SETI and the Development of Radio Astronomy [ SETI = Search for Extra-Terrestrial Intelligence ]

•  Radio astronomy “discovered” in 1930s by K. Jansky – galactic radio emission from the plane of the Milky Way

•  Development proceeded rapidly after 1940s with availability of radar technology developed in WW II

•  Serious scientific thoughts about possibility of ETI and methods of communication started late 1950s

•  Radio seen as best method of sending long-distance (interstellar) signals •  Project OZMA (Frank Drake) c. 1960, search for ETI signals from nearby stars •  OZMA II c. 1973 – 76 •  NASA studies through mid-80s •  Now, privately funded

Karl Jansky and early radio telescope

To address question of whether other intelligent life exists in the Milky Way, Frank Drake formulated the Drake Equation.

Life Elsewhere in the Milky Way

The equation is usually written: N = R* • fp • ne • fl • fi • fc • L Where, N = The number of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable. R* =The rate of formation of stars suitable for the development of intelligent life. fp = The fraction of those stars with planetary systems. ne = The number of planets, per solar system, with an environment suitable for life. fl = The fraction of suitable planets on which life actually appears. fi = The fraction of life bearing planets on which intelligent life emerges. fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space. L = The length of time such civilizations release detectable signals into space.

The Drake Equation

number of technological, intelligent civilizations in the Milky Way

rate at which new stars are formed = x

fraction of stars having planetary systems

x average number of habitable planets within those planetary systems

x fraction of those habitable planets on which life arises

x fraction of those life-bearing planets on which intelligence evolves

fraction of those planets with intelligent life that develop technological society

average lifetime of a technological civilization

x x

Each term is less certain than the preceding one! Only in last ten years have we addressed the second term.

The “WOW!” Signal

•  Detected Aug. 15, 1977, at “Big Ear” radiotelescope, Ohio State U. •  Narrowband signal at 1420 MHz (21-cm, neutral hydrogen line) •  Characteristic of small source, 30 x stronger than bkgd. (in “top 20”) •  Duration about 70 sec., modulation unknown

•  Not known to be associated with a manmade source, moved like a true sky source •  Detection never repeated since

Allen Telescope Array – radio SETI

Optical SETI [Harvard U.]

SETI League (Amateur Radio Astronomers)

SETI in the Movies

Contact (1997)

based on Carl Sagan novel

The Arrival (1996)

Exoplanets •  Definition – planets around stars other than the Sun •  Sometimes referred to as “exosolar” or “extrasolar” planets •  Speculated about for many decades in astronomical community •  First actual, confirmed detections occurred beginning in 1992 •  Method: radial velocity technique using miniscule Doppler shifts •  (explained later … see following pages) •  Requires exquisitely stable, precise measurement of spectra

•  Initiated serious thoughts about detecting planets around other stars

Planets around HR7899b Starlight blocked by VV coronagraph NASA/Palomar Observatory

Requirements for Exoplanets that could Support Life

•  Astronomers generally believe that life most likely to exist in a “habitable zone” where liquid water can form at least sometimes

•  This restricts the distance at which a life-supporting planet could orbit the parent star •  Too close too hot, no liquid water •  Too far too cold, no liquid water •  c.f. Venus and Mars compared to Earth

•  Other requirements •  Probably only F, G, K stars are really good candidates … why?? •  O – A stars generally are large and have short stellar lifetimes •  M stars probably too cool, have very small habitable zones •  Stellar system probably must be in disk of Galaxy … why?? •  Halo stars too old, little gas/dust – planets unlikely •  Bulge too much radiation (life molecules are fragile)

Difficulties of Exoplanet Detection

•  Imagine a star like the Sun, with an Earthlike planet at 1 AU from it •  Imagine this star is about 5 LY away (slightly more than the nearest star) •  Seen from the Solar System, the planet would be how far away from the image of the star?

1 AU = 1.496 x 1011 m 1 LY = 9.46 x 1015 m

So, planet would appear about [ 1.5 x 1011 / (5 * 9.5 x 1015) ] radians from the parent star [note, 1 radian is about 57 degrees], or 3.15 x 10-6 radians [microradians] which is ~ 0.6 arcseconds At visual wavelengths, separating the planet “spot” from the star “spot” would require only about a 1m telescope in principle … … but other “realism” factors, e.g., noise, scattered light, turbulent atmosphere, imperfect optics make this unrealistic …. PLUS (next page)

Detecting Terrestrial Planets Directly is HARD Brightness differences make this a very tough problem

Parent stars are 106 to 109 brighter than planets

0.1 1 10 100 1 .1031 .10 141 .10 131 .10 121 .10 111 .10 101 .10 91 .10 81 .10 71 .10 61 .10 51 .10 41 .10 3

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SunSpec

ER10pcQ

Exozody

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star

planet (reflection)

Zodiacal dust (reflection + emission)

planet+dust total

(log

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wavelength [microns]

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Example Stellar Debris/Planetary Disk

Beta Pictoris system near-infrared light Candidate planet “Beta Pic b” is estimated to be ~ 8 Jupiter masses at a distance of ~ 8 AU from star It is approx. 1/1000 the brightness of the star Composite image using 3.6 m and 8.2 m telescopes [European Southern Observatory]

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Terrestrial Planet Finder (TPF) Mission

Image courtesy NASA/JPL

•  TPF Science goals –  Survey stars within ~ 30 pc for planets within “habitable zone”

•  Looking for “Earth analogues” –  Characterize any detected planets

•  Orbit & Spectrum

•  Past, current, future TPF studies –  Two main designs possible –  Preliminary concept studies underway for several years –  Followed eventually by “Life Finder” and “Planet Imager”

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Roddier-Guyon Synthesis Imager Earth-Sun system at 10pc

2.7h exposure time, 5 to 15 µm

Exoplanets Seen Through an Advanced Space Telescope (simulation)

http://planetquest.jpl.nasa.gov/science/finding_planets.cfm Astrometric method

Exoplanet-finding methods

Extrasolar Planets -- Methodology

More than 800 discovered. Main technique: detect Doppler Shift due to wobble of star caused by unseen planet. Biased – easier to detect heavier (Jupiter-class) planets. Second technique: detect eclipse (transit) of planet – Kepler mission. Third technique: detect wobble in star's position in sky due to unseen planet (astrometric method). Fourth technique: direct imaging of planet. Difficult due to brightness ratios. Fifth technique: microlensing

Future missions hope to detect Terrestrial planets