exoplanets - uw madison astronomy departmentheinzs/homepage/plato_2011... · plato: planets -...

PLATO: Planets - foreign worlds - 9 Exoplanets

Exoplanets

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Artist’s impression of Kepler 11, a 6-planet system (NASA)


Radial velocity

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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

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PLATO: Planets - foreign worlds - 9 Exoplanets 4

• Orbital types:

! Many have high eccentricity!

! Much higher than Solar System

⇒ Many exoplanets:

! On elliptical orbits

Planet Demographics


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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Direct Imaging

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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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Transits

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Transits! Example HD 209458: 1% decrease in brightness

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HD 209458

1%


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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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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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


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


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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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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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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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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• 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


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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Kepler’s Orbit

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Kepler: 1236 planet candidates

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All multi-planet candidates detected by Kepler as of 02/02/2011


Kepler finds “Super Earths”• Can detect 0.01% eclipses

• 1235 planet candidates found

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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


Life in the Universe

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Not this... (this is proof of intelligent life on Earth, sort of...)


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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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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Life is Resilient• Hydrothermal vents

(“Black smokers”)

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Life is Resilient• Cyanobacteria survive extreme temperatures and chemistry

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Habitability• The region in the Solar system where surface water can be liquid

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sun

bigger stars

smaller stars

Habitable zone


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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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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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


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


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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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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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


Finding Life• Doing this with Earth-like planets will

require much more sensitive space telescopes

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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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The Tree of Life

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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


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


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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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



• Communication best done in radio band

! relatively little contamination from astronomical sources

! no absorption

• So far:

! No luck

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• 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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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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Closing comments• Thank you for your interest in Astronomy!

• Astronomy depends on public interest more than almost any other science

• Stay in touch - I will likely organize future PLATO lecture series

! Next up: Cosmology

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exoplanets - uw madison astronomy departmentheinzs/homepage/plato_2011... · plato: planets -...

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