Search for Extraterrestrial Life

The Search for Extraterrestrial Intelligence

Final Exam: reminder

• April 17, 7–9pm • Cumulative, 70 questions, 2 hours • Natural Sciences Rooms 1, 7, 145

• sorted by last names: • NS 1: Abdel – Kirleis • NS 145: Kirupakaran – Starr • NS 7: Steer – Zubairi

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Today’s Topics

• Review of last lecture • Extrasolar planets (Ch. 11) !

• Classifying stars (Ch. 11.4) • SETI (Ch. 12)

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Rate of exoplanet discoveries: 1995–2014

1779 exoplanets known

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Planet Detection Methods

• Indirect: Measurements of stellar properties revealing the effects of orbiting planets !

• Direct: Pictures or spectra of the planets themselves

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The Challenge to the Direct Imaging of Exoplanets: Brightness Difference

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Gravitational Tugs

• Sun and Jupiter orbit around their common centre of mass !

• Sun therefore wobbles around that centre of mass with same period as Jupiter

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Gravitational Tugs

• Sun’s motion around solar system’s centre of mass depends on tugs from all the planets !

• Astronomers around other stars that measured this motion could determine masses and orbits of all the planets

© 2008 Pearson Education Inc, publishing as Pearson Addison-Wesley

Doppler Technique

• Measuring a star’s Doppler shift can tell us its motion toward and away from us !

• Current techniques can measure motions as small as 1 m/s (walking speed!)

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How does light tell us the speed of a distant object?

The Doppler Effect

sound (speed in air ≈ 300 m/s ≈ 700 mph)

light (speed in air ≈ 300,000 km/s)

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Measuring the Wavelength Shift

• We generally measure the Doppler Effect from shifts in the wavelengths of spectral lines

• The amount of blue or red shift tells us an object’s speed toward or away from us.

Stationary

Moving Away

Away Faster

Moving Toward

Toward Faster

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First Extrasolar Planet• Doppler shifts of star

51 Pegasi indirectly reveal a planet with 4-day orbital period !

• Short period means small orbital distance !

• First extrasolar planet to be discovered (1995)

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Transits and Eclipses

• A transit is when a planet crosses in front of a star • The resulting eclipse reduces the star’s apparent brightness and tells

us planet’s radius • Essentially no orbital tilt: accurate measurement of planet mass

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All Kepler candidatesSun + Earth

© 2008 Pearson Education Inc, publishing as Pearson Addison-Wesley Kalas et al. (2008, Hubble telescope)

Fomalhaut b

Exoplanets Imaged !

Marois et al. (2008, Keck telescope)HR 8799 bcd

HR 8799 Movieb

c

d

HR 8799 (masked)

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Planets: Common or Rare?

• One in six stars examined so far have turned out to have planets !

• The others may still have smaller (Earth-sized) planets that current techniques cannot detect !

• Likely half of stars have planets.

© 2008 Pearson Education Inc, publishing as Pearson Addison-Wesley

Modifying the Nebular Theory

• Observations of extrasolar planets have shown that nebular theory was incomplete !

• Effects like planet migration and gravitational encounters might be more important than previously thought – depends on how quickly the solar wind clears the

protoplanetary disk

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Planetary Migration

• A young planet’s motion can create waves in a planet-forming disk !

• Models show that matter in these waves can tug on a planet, causing its orbit to migrate inward

© 2008 Pearson Education Inc, publishing as Pearson Addison-Wesley

Revisiting the Nebular Theory

• Nebular theory predicts that massive Jupiter-like planets should not form inside the frost line (at << 5 AU)

• Discovery of “hot Jupiters” has forced reexamination of nebular theory

• “Planetary migration” or gravitational encounters may explain “hot Jupiters”

Today’s Topics

• Review of last lecture • Extrasolar planets (Ch. 11) !

• Classifying stars (Ch. 11.4) • SETI (Ch. 12)

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Stellar Classification

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Stellar Classification

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Habitable zone dependence on stellar luminosity

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Habitable zone dependence on galactic location?

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How could we detect life

on an exoplanet?

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© 2008 Pearson Education Inc, publishing as Pearson Addison-Wesley

What have we learned?• Are planetary systems common around other stars?

– One in six stars examined have been discovered to host planets

– As many as half of stars in galaxy may host planets !

• How can we detect life on exoplanets? – Through direct spectroscopy of their atmospheres – Looking for biosignatures: oxygen, ozone, methane

Today’s Topics

• Review of last lecture • Extrasolar planets (Ch. 11) !

• Classifying stars (Ch. 11.4) • SETI (Ch. 12)

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How Common Are Intelligent Civilizations?

The Drake Equation

N = NHP * flife * fciv * fnow!37

Frank Drake, SETI Institute

The Drake Equation• NHP – number of habitable planets in our Galaxy

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• flife – fraction that are inhabited !

• fciv – fraction that have technological civilizations !

• fnow – fraction of technological civilizations living now

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The Drake Equation• NHP – number of habitable planets in our Galaxy

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• can potentially be answered by Astronomy • considerations:

• planet frequency in stellar habitable zones (10%?) • galactic habitable zone (1%–10% of Galaxy?)

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• NHP ~ 100 billion * 10% * 1 % = 100 million

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The Drake Equation• NHP – number of habitable planets in our Galaxy

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• flife – fraction that are inhabited • is life a probable event? If so, flife ~ 100% • but so far, observationally, flife ~ 0%

• fciv – fraction that have technological civilizations !

• fnow – fraction of technological civilizations living now

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The Drake Equation

NHP * flife = number of inhabited planets • ~ 100,000? (if flife ~ 0.1%)

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N = NHP * flife * fciv * fnow

The Drake Equation• NHP – number of habitable planets in our Galaxy !

• flife – fraction that are inhabited !

• fciv – fraction that have technological civilizations • technology took 4.5 Byr to develop on Earth • but most stars in Milky Way are older than Sun • is intelligence an expected outcome of evolution? • does intelligence spawn technology? !

• fnow – fraction of technological civilizations living now

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The Encephalization Quotient: an Approximate Measure of Intelligence

• EQ = 1 means brain mass proportionate to body weight !

• humans: EQ = 7 • dolphins: EQ = 6

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Evolution of the Encephalization Quotient: Intelligence Is Evolutionarily Favoured

• Still: does intelligence spawn technology?

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The Drake Equation

NHP * flife * fciv = number of technologically advanced civilizations

• ~ 100? (if fciv ~ 0.1%)

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N = NHP * flife * fciv * fnow

The Drake Equation• NHP – number of habitable planets in our Galaxy

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• flife – fraction that are inhabited !

• fciv – fraction that have technological civilizations !

• fnow – fraction of technological civilizations living now • how long will we survive our technology? • 60 years form invention of radio transmissions to

nuclear weapons.

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The Drake Equation

If communication technology has existed in our Galaxy for 10 billion years, but a technologically advanced civilization survives only 100 years, then fnow = 100 years / 10 billion years = 0.00001%

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N = NHP * flife * fciv * fnow

The Drake Equation

N ~ 0.00001 to 100

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N = NHP * flife * fciv * fnow

What we Have Learned?

• None of the parameters of the Drake Equation are well known. !

• However, they illustrate the directions that we need to research to find other intelligent civilizations.

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N = NHP * flife * fciv * fnow