Download Light: The Cosmic Messenger

Planets and ExoPlanets
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Earth, as viewed by the Voyager spacecraft
What does the solar system look
like?
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• There are eight major planets with nearly
circular orbits.
• Pluto and Eris are smaller than the major
planets and have more elliptical orbits.
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What are the major features of
the Sun and planets?
Sun and planets to scale
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Sun
• Over 99.9% of solar system’s mass
• Made mostly of H/He gas (plasma)
• Converts 4 million tons of mass into energy each second
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Mercury
• Made of metal and rock; large iron core
• Desolate, cratered; long, tall, steep cliffs
• Very hot and very cold: 425C (day)–170C (night)
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Venus
• Nearly identical in size to Earth; surface hidden by clouds
• Hellish conditions due to an extreme greenhouse effect
• Even hotter than Mercury: 470C, day and night
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Earth
Earth and Moon with
sizes shown to scale
• An oasis of life
• The only surface liquid water in the solar system
• A surprisingly large moon
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Mars
• Looks almost Earth-like, but don’t go without a spacesuit!
• Giant volcanoes, a huge canyon, polar caps, more
• Water flowed in distant past; could there have been life?
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Jupiter
• Much farther
from Sun than
inner planets
• Mostly H/He;
no solid surface
• 300 times more
massive than
Earth
• Many moons,
rings
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Jupiter’s moons
can be as
interesting as
planets
themselves,
especially
Jupiter’s four
Galilean moons.
• Io (shown here): active volcanoes all over
• Europa: possible subsurface ocean
• Ganymede: largest moon in solar system
• Callisto: a large, cratered “ice ball”
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Saturn
• Giant and gaseous like Jupiter
• Spectacular rings
• Many moons, including cloudy Titan
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Rings are
NOT solid;
they are made
of countless
small chunks
of ice and
rock, each
orbiting like a
tiny moon.
Artist’s conception
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Cassini probe arrived July 2004 (launched in 1997).
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Uranus
• Smaller than
Jupiter/Saturn;
much larger than
Earth
• Made of H/He gas
and hydrogen
compounds
(H2O, NH3, CH4)
• Extreme axis tilt
• Moons and rings
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Neptune
• Similar to Uranus
(except for axis
tilt)
• Many moons
(including Triton)
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Pluto (and Other Dwarf Planets)
• Much smaller than major planets
• Icy, comet-like composition
• Pluto’s main moon (Charon) is of similar size
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Motion of Large Bodies
• All large bodies
in the solar
system orbit in
the same
direction and in
nearly the same
plane.
• Most also rotate
in that direction.
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Two Major Planet Types
• Terrestrial planets
are rocky, relatively
small, and close to
the Sun.
• Jovian planets are
gaseous, larger, and
farther from the
Sun.
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Swarms of Smaller Bodies
• Many rocky
asteroids and
icy comets
populate the
solar system.
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Notable Exceptions
• Several
exceptions to
the normal
patterns need to
be explained.
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Why is it so difficult to detect
planets around other stars?
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Brightness Difference
• A Sun-like star is about a billion times
brighter than the light reflected from its
planets.
• This is like being in San Francisco and
trying to see a pinhead 15 meters from a
grapefruit in Washington, D.C.
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Planet Detection
• Direct: pictures or spectra of the planets
themselves
• Indirect: measurements of stellar properties
revealing the effects of orbiting planets
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How do we detect planets around
other stars?
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Gravitational Tugs
• The Sun and Jupiter
orbit around their
common center of
mass.
• The Sun therefore
wobbles around that
center of mass with
same period as
Jupiter.
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Gravitational Tugs
• The Sun’s motion
around the solar
system’s center of
mass depends on tugs
from all the planets.
• Astronomers around
other stars that
measured this motion
could determine the
masses and orbits of
all the planets.
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Astrometric Technique
• We can detect planets
by measuring the
change in a star’s
position on sky.
• However, these tiny
motions are very
difficult to measure
(~ 0.001 arcsecond).
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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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First Extrasolar Planet
Insert TCP 6e Figure 13.4a unannotated
• Doppler shifts of the star
51 Pegasi indirectly
revealed a planet with 4day orbital period.
• This short period means
that the planet has a
small orbital distance.
• This was the first*
extrasolar planet to be
discovered (1995).
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First Extrasolar Planet
Insert TCP 6e Figure 13.4b
• The planet around 51 Pegasi has a mass similar to
Jupiter’s, despite its small orbital distance.
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Other Extrasolar Planets
• Doppler shift data tell us about a planet’s mass and
the shape of its orbit.
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Planet Mass and Orbit Tilt
• We cannot measure an exact mass for a planet without
knowing the tilt of its orbit, because Doppler shift tells
us only the velocity toward or away from us.
• Doppler data give us lower limits on masses.
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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.
• No orbital tilt: accurate measurement of planet mass
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Spectrum During Transit
• Change in spectrum during a transit tells us about the
composition of planet’s atmosphere.
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Surface Temperature Map
• Measuring the change in infrared brightness during an
eclipse enables us to map a planet’s surface temperature.
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Direct Detection
• Special techniques like adaptive optics are helping to
enable direct planet detection.
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Direct Detection
• Techniques that help block the bright light from stars are
also helping us to find planets around them.
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Direct Detection
• Techniques that help block the bright light from stars are
also helping us to find planets around them.
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Other Planet-Hunting Strategies
• Gravitational Lensing: Mass bends light in
a special way when a star with planets
passes in front of another star.
• Features in Dust Disks: Gaps, waves, or
ripples in disks of dusty gas around stars
can indicate presence of planets.
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What have we learned about
extrasolar planets?
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Orbits of Extrasolar Planets
• Most of the detected
planets have orbits
smaller than
Jupiter’s.
• Planets at greater
distances are harder
to detect with the
Doppler technique.
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Orbits of Extrasolar Planets
• Orbits of some
extrasolar planets are
much more elongated
(have a greater
eccentricity) than
those in our solar
system.
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Multiple-Planet Systems
• Some stars
have more
than one
detected
planet.
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Orbits of Extrasolar Planets
• Most of the detected
planets have greater
mass than Jupiter.
• Planets with smaller
masses are harder to
detect with Doppler
technique.
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Hot Jupiters
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Revisiting the Nebular Theory
• The nebular theory predicts that massive
Jupiter-like planets should not form inside
the frost line (at << 5 AU).
• The discovery of hot Jupiters has forced
reexamination of nebular theory.
• Planetary migration or gravitational
encounters may explain hot Jupiters.
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Planetary Migration
• A young planet’s
motion can create
waves in a planetforming disk.
• Models show that
matter in these waves
can tug on a planet,
causing its orbit to
migrate inward.
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Orbital Resonances
• Resonances between
planets can also cause
their orbits to become
more elliptical.
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Planets: Common or Rare?
• One in ten stars examined so far have
turned out to have planets.
• The others may still have smaller (Earthsized) planets that current techniques cannot
detect.
• Kepler seems to indicate COMMON
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Transit Missions
• NASA’s Kepler
mission was launched
in 2008 to begin
looking for transiting
planets.
• It is designed to
measure the 0.008%
decline in brightness
when an Earth-mass
planet eclipses a Sunlike star.
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Astrometric Missions
• GAIA: a European mission planned for 2011
that will use interferometry to measure
precise motions of a billion stars
• SIM: A NASA mission that will use
interferometry to measure star motions even
more precisely (to 10-6 arcseconds)
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Direct Detection
• Determining whether
Earth-mass planets
are really Earth-like
requires direct
detection.
Mission concept for NASA’s
Terrestrial Planet Finder (TPF)
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• Missions capable of
blocking enough
starlight to measure
the spectrum of an
Earth-like planet are
being planned.