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

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

Planet Detection Methods

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

• Direct: Pictures or spectra of the planets themselves


The Challenge to the Direct Imaging of Exoplanets: Brightness Difference


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


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


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!)


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)


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


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


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)


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.


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


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


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



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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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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



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



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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 !


• 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? !


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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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• 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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The Drake Equation

N ~ 0.00001 to 100

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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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Search for Extraterrestrial Life