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PLATO: Planets - foreign worlds - 9 Exoplanets
Exoplanets
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Artist’s impression of Kepler 11, a 6-planet system (NASA)
PLATO: Planets - foreign worlds - 9 Exoplanets
Planet Demographics• What kinds of planets have been
found?
! 547 planet systems total
! Mostly massive planets
! Mostly close to star
! High eccentricities
• Jupiter-size planets at close distance to star:
! Called “Hot Jupiters”
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average orbital distance (AU)0.10 1.00 10.0 100
Plan
et M
ass
(Jupi
ter
mas
ses)
0.001
0.010
0.100
1.000
10.00
100.0
Jupi
ter’s
orb
it
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• Orbital types:
! Many have high eccentricity!
! Much higher than Solar System
⇒ Many exoplanets:
! On elliptical orbits
Planet Demographics
PLATO: Planets - foreign worlds - 9 Exoplanets
Bias:• The way we design observations influences the results
• We must be careful to account for this
• Example:
! We are much more likely to find objects that produce a large radial velocity signal, i.e., planets that...
" ...have a large mass (more gravity)
" ...are close to the star (more gravity)
! Basically: Radial velocity searches are likely to find hot Jupiters.
! That does not mean that all exoplanets are hot Jupiters.
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PLATO: Planets - foreign worlds - 9 Exoplanets
Direct Imaging• Extremely challenging:
! Planets shine by reflected starlight
! Planet much dimmer than star
• Best solution:
! Use a “coronograph” to make a virtual eclipse
! So far limited to big planets (Jupiters)
! Far away from star
! Many year away from being possible for Earth-like planets
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PLATO: Planets - foreign worlds - 9 Exoplanets
Transits! Example HD 209458: 1% decrease in brightness
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HD 209458
1%
PLATO: Planets - foreign worlds - 9 Exoplanets
Transits• No eclipse unless the telescope (Earth) is almost in the planet’s
ecliptic plane
• Transits only tell us the planet size - need radial velocity for mass
• Planets close to the star are more likely to eclipse
! Bias to find close, big planets
• Possible to find Earth-like planets if you monitor enough stars...
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PLATO: Planets - foreign worlds - 9 Exoplanets
Hot Jupiter Bias• Hot Jupiters are easy to find by transit as well
! Big planet = big eclipse signal
! Close to the star = like to eclipse the star
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PLATO: Planets - foreign worlds - 9 Exoplanets
Rotation vs. orbit
• Stars rotate
• Solar nebula theory:
! Stellar rotation should be in the same sense as planet orbits
• Transits can measure rotation (Rossiter-McLaughlin effect):
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tow
ards
us
away
from
us
star
PLATO: Planets - foreign worlds - 9 Exoplanets
Rotation vs. orbit
• Transits can measure rotation (Rossiter-McLaughlin effect):
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tow
ards
us
away
from
us
star
no blockingapproaching (blue)side blocked
no blocking middle (unshifted)side blocked
receding (red)side blocked
PLATO: Planets - foreign worlds - 9 Exoplanets
Retrograde Orbits!• Kepler planet transit survey finds...
! ...many hot Jupiters have orbits that are misaligned with the star’s rotation!
! 6 out of 27 exoplanets studied have retrograde orbits - they orbit in the opposite direction from the star’s rotation
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PLATO: Planets - foreign worlds - 9 Exoplanets
Planet Formation Upgrade• Solar nebula theory needs an upgrade to explain...
1. ...hot Jupiters (big planets close in) - inside the ice line!
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PLATO: Planets - foreign worlds - 9 Exoplanets
Planet Formation Upgrade• Solar nebula theory needs an upgrade to explain...
1. ...hot Jupiters (big planets close in) - inside the ice line!
2. ...retrograde, eccentric orbits
• Most likely candidate:
! Orbital migration because of...
! ...gravitational interactions with other big planets (slingshot)
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PLATO: Planets - foreign worlds - 9 Exoplanets
Planet Formation Upgrade• Migrating Jupiters are bad news
! On its way in, Jupiter would eject planets in its way
• In planetary systems with hot Jupiters:
! don’t expect habitable Earth-like planets :(
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PLATO: Planets - foreign worlds - 9 Exoplanets
• But this doesn’t mean that our Solar System is special
! We are biased to find hot Jupiters (so they don’t have to be common)
! And they have to be different from us
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Planet Formation Upgrade
PLATO: Planets - foreign worlds - 9 Exoplanets
Kepler Spacecraft
• Launched 03/2009
• 3.5 yr lifetime
• 95 cm mirror
• 42 x 2megapixel CCDs
• Staring at the same piece of sky constantly
• Constantly monitoring brightness of over 100,000 stars
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PLATO: Planets - foreign worlds - 9 Exoplanets
Kepler Spacecraft
• Launched 03/2009
• 3.5 yr lifetime
• 95 cm mirror
• 42 x 2megapixel CCDs
• Staring at the same piece of sky constantly
• Constantly monitoring brightness of over 100,000 stars
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PLATO: Planets - foreign worlds - 9 Exoplanets
Kepler: 1236 planet candidates
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All multi-planet candidates detected by Kepler as of 02/02/2011
PLATO: Planets - foreign worlds - 9 Exoplanets
Kepler finds “Super Earths”• Can detect 0.01% eclipses
• 1235 planet candidates found
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PLATO: Planets - foreign worlds - 9 Exoplanets
Kepler finds “Super Earths”
• Earth-size planets are common:
! 9% of stars have a planet with ~2.4 Earth radii
! likely many more smaller planets per star!
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Kepler survey not yet finished in this area - wait for exciting discoveries!
Howard et al. 2011
PLATO: Planets - foreign worlds - 9 Exoplanets
Life in the Universe
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Not this... (this is proof of intelligent life on Earth, sort of...)
PLATO: Planets - foreign worlds - 9 Exoplanets
Life: One Possible Definition! Living organisms...
" ...respond & adapt to changes in their environment, e.g., movement, chemistry
" ...have physical presence (unlike artificial intelligence)
" ...develop towards greater complexity (guided by natural selection)
" ...reproduce
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PLATO: Planets - foreign worlds - 9 Exoplanets
Life: Bare Necessities• Guided by our understanding of life on Earth:
! Life is based on complex molecules, which requires Carbon, Oxygen, Nitrogen, Hydrogen
! Molecules must be stable, which requires temperatures below boiling and low flux of ionizing radiation and particles
! Chemical reactions require solvent. Best: liquid water
! Planets must exist long enough for life to form (~billion years?)
• A location that satisfies these criteria is called habitable
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PLATO: Planets - foreign worlds - 9 Exoplanets
Life is Resilient• Hydrothermal vents
(“Black smokers”)
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PLATO: Planets - foreign worlds - 9 Exoplanets
Life is Resilient• Cyanobacteria survive extreme temperatures and chemistry
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PLATO: Planets - foreign worlds - 9 Exoplanets
Habitability• The region in the Solar system where surface water can be liquid
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sun
bigger stars
smaller stars
Habitable zone
PLATO: Planets - foreign worlds - 9 Exoplanets
Life• Goldilocks problem:
! Conditions change over the age of the Solar System, so initially friendly planets would become un-inhabitable
⇒ We should not exist
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PLATO: Planets - foreign worlds - 9 Exoplanets
Life• Solution:
! The greenhouse effect and the carbon cycle can generate a habitable planet over a range of distances from a star
! Atmospheres act as thermal buffers
! Life is not completely improbable
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PLATO: Planets - foreign worlds - 9 Exoplanets
Habitable Earths?• Some of the Super Earths are
Neptune-like...
• ...but some are Earth-like (rocky)
• Kepler so far:
! Planets too close to star
! Too hot for life
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Howard et al. 2011
PLATO: Planets - foreign worlds - 9 Exoplanets
Habitable Earths?• This is expected:
! Need a second eclipse
! This takes time (bias!)
• Trend:
! More planets at larger distances
! This is promising!
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closer to star
Mercury: 90 day orbit
Howard et al. 2011
PLATO: Planets - foreign worlds - 9 Exoplanets
Life on Moons• Satellites might be the better breeding grounds for life
! Additional heating from tides
! Lots of water ice in outer planetary systems
⇒ Liquid water?
⇒ This widens the habitable zone
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PLATO: Planets - foreign worlds - 9 Exoplanets
Finding Life• Signatures of life:
! Look for spectral features of products of life
! Oxygen
! Methane
• This is hard:
! The star light is overwhelming
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PLATO: Planets - foreign worlds - 9 Exoplanets
Finding Life• Transits help - again
! When exoplanet transits, its atmosphere absorbs more strongly in lines
! Difference in star spectrum before and after transit: Atmospheric absorption
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HD 189733b
PLATO: Planets - foreign worlds - 9 Exoplanets
Finding Life• Doing this with Earth-like planets will
require much more sensitive space telescopes
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PLATO: Planets - foreign worlds - 9 Exoplanets
Life• If we found life, what could it be like?
! How do we distinguish it from life on Earth (contamination)
! Would it be intelligent?
! Would it communicate?
! Would we want to communicate? (i.e., would it be friendly?)
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PLATO: Planets - foreign worlds - 9 Exoplanets
How likely is life?
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• Could life have developed multiple times on Earth?
! All life on Earth is DNA based
! All DNA is “right-handed”
• If DNA had developed multiple times
! We would expect both left- and right-handed DNA
! But maybe only one family of organisms survived and others did not
PLATO: Planets - foreign worlds - 9 Exoplanets
The Fermi Paradox
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• If life on a habitable planet is likely, we should expect many inhabited planets in the Galaxy
• If many other intelligent civilization exist, why are there no signatures? Maybe...
a) ...civilizations don’t last long once they become technologically advanced - we seem to set a good example for that
b) ...they realize it’s better not to engage in communication
c) ...it’s just not likely
PLATO: Planets - foreign worlds - 9 Exoplanets
Hawking:• Contact with extraterrestrial life might be like Columbus
discovery of new world
• Maybe we should avoid contact
! “I would say the biggest argument FOR intelligent life elsewhere in the universe is that they have not tried to contact us”
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PLATO: Planets - foreign worlds - 9 Exoplanets
SETI (“search for extraterrestrial intelligence)
• Communication best done in radio band
! relatively little contamination from astronomical sources
! no absorption
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Allen telescope array
PLATO: Planets - foreign worlds - 9 Exoplanets
SETI (“search for extraterrestrial intelligence)
• Communication best done in radio band
! relatively little contamination from astronomical sources
! no absorption
• So far:
! No luck
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PLATO: Planets - foreign worlds - 9 Exoplanets
SETI (“search for extraterrestrial intelligence)
• Maybe we should try something else: MASER/LASER
" Requires very special conditions, rare in nature
" Extremely concentrated, travels far
" Can easily encode a signal (MORSE-code-like)
• Pencil beam:
! Requires target pre-selection
" We would have to pick target planets,
" Other civilizations would have to point at us
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PLATO: Planets - foreign worlds - 9 Exoplanets
A long wait...• From the time we send a signal
(which we already did), it would many take decades to reach even nearby planets.
• So: unless others are looking for us, it is going to be a while before we have any chance to hear back.
• In the meantime, maybe we should focus on figuring out how not to destroy civilization all by ourselves.
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