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Distilling the Planets - Observational Data
Mercury, Venus, Earth and Mars all have an
iron core, rocky silicate crust and a thin
atmosphere, almost no hydrogen or helium.
Jupiter and Saturn are much larger, made
mostly of hydrogen and helium, have a thick
gaseous atmosphere and small rock/iron core.
Uranus and Neptune are about 15 times the
mass of Earth, made of rock and volatile ice.
Space Junk
between Mars and Jupiter: Asteroids are
lumps of rock or nickel/iron. Some are
chondrites with millimetre-sized spherical
silicate granules called chondrules which
appear to have been flash melted and frozen.
beyond Pluto: Comets are lumps of dirty ices,
water, carbon dioxide, methane etc.
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Distilling the Planets - a Model
1. Heat from the protostar probably destroys
gas molecules, but in the disc they can be
recreated more easily due to high density.
2. New dust grains condense out of the gas.
3. Tens of AU from the Sun the temperature
is below 150 K (-123oC), so volatiles can
condense to ices, H2O, carbon dioxide,
alcohols etc. (1 AU = Earth - Sun distance)
4. Within 5 AU the temperature reaches
1400 K and only silicates, iron and
rocky materials can condense.
5. Violent electrical storms may have
occurred within 5 AU to create chondrules.
6. Particles grow by attraction and collisions this theory is not yet secure.
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Distilling the Planets - a Model
7. In a few hundred thousand years the lumps
are up to a few tens of kilometres across.
8. These bodies continue to collide, eventually
forming the planets we see today. Some of
the collisions must have been extremely
violent - the Moon is thought to have been
created in a collision between Earth and a
Mars-sized object.
9. Jupiter and Saturn grew in the same way,
but also collected the ices that had
condensed and the clouds of hydrogen and
helium gas which solar radiation had
removed from the inner solar system.
10. Neptune and Uranus grew more slowly as
the density of icy planetesimals was lower,
and by the time they were heavy enough to
attract gas, the solar wind and radiation
had flushed it clear of the Solar System.
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Planet Hunting - Dust Discs
1984:
InfraRed Astronomical Satellite, IRAS saw
extended infrared emission surrounding the
star Beta Pictoris.
IR emission most likely due to dust particles
warmed up by light from the star.
Ground-based telescopes confirmed that
Beta Pictoris had an edge-on disc reaching
400 AU from the star.
1997:
The Beta Pictoris disc is found to be warped this may be due to the gravitational pull of one
or more planets…
… but could be the effect
of a close encounter with another star or a
dense cloud of young comets.
Figs. Z22.14 & K7-15
2004:
Three separate dust rings are mapped - could
be held apart by planet(s).
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Planet Hunting - Reflex Motion
Planets too faint and too small to be seen
directly make the star they orbit wobble.
Look at star’s position - astrometry
Look at star’s spectrum - Doppler shift
Big planets have strongest effect on star, so
are likely to be found first.
Astrometry looks for star moving from side
to side - easiest to detect for nearest stars
with large planets far from the star.
Doppler shifts are biggest for large planets
close to the star. Biggest shift can be
observed if the orbit is viewed edge-on, such
that we measure its true “Radial Velocity”.
Jupiter causes the Sun to move around a
circle of radius about 1/100 th of Mercury’s
orbit at a speed of 45 km per hour.
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Planet Hunting - 51 Pegasi
10/1995: Mayor and Queloz at the HauteProvence Observatory found a Doppler shift
with a Sun-like star, 51 Pegasi.
• Measured velocity of up to 216 km per hour
• Orbital period of candidate planet 4.2 days
• Mass about half that of Jupiter
• Just 0.05 AU from star (1/20th of Earth-Sun)
• Surface temperature probably about 1300 K
• Confirmed by Marcy and Butler
Nothing like Mercury / the solar system.
How did it get there? Massive planet formed
further out and dragged in by gas and dust?
If so, any terrestrial planets would have been
kicked out into interstellar space - not good
for life as we know it!
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Planet Hunting - Radial Velocity
~ 300 planets discovered with this technique
1997: 16 Cygni B - a “classical Jupiter” planet
found around a Sun-like star - but highly
eccentric orbit, whereas solar system planetary
orbits are very close to circular.
2001: 47 Ursae Majoris - planetary system. Has
a 2.6 Jupiter mass planet in a near circular
orbit at 2.1 AU from the star. If this were in our
solar system it would lie between Jupiter and
Mars and might prevent an Earth-type planet
forming in the “habitable zone”.
04/2009: Gleise 581e - smallest exoplanet
around a normal star. Minimum 1.9 Earth
masses. Just 0.03 AU from parent red dwarf
star in a 3.15 day orbit and therefore outside
the “habitable zone”.
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Planet Hunting - Transits
~ 70 planets discovered with this technique
A planet passing directly in front of a star
along our line of sight blocks its light and
reduces the star’s apparent brightness.
Relative change in = Area of planet
brightness
Area of star
For an Earth-like planet brightness drops
by 0.01% for a few hours in a year.
Can measure orbital period and physical
size of planet.
Likelihood of transit depends on viewing
geometry - 0.5% if Earth-like. Easiest
planets to detect are very large and close to
the star - “Hot Jupiters”.
The Kepler mission launched on 6/3/09 will
stare at a patch of sky containing 100,000
target stars for 3.5 years. See
http://www.kepler.arc.nasa.gov
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Planet Hunting - atmospheres
As transit begins starlight filters through
the planet’s atmosphere and we can look for
spectral absorption lines.
2004: oxygen and carbon detected around a
0.7 Jupiter mass planet orbiting its parent
star at 0.04 AU - thought to be the core of
an evaporating gas giant.
Occultation: when a planet moves behind
it’s star we see only star light. If we subtract
this spectrum from the light measured when
planet and star are side by side the tiny
difference is emission from the planet.
2005: first detection of a planet’s thermal
emission. Used Spitzer Space Telescope’s
infrared camera so the star was only 400 
brighter than the planet (would swamp it by
 10,000 in visible light).
Want to search spectra for “biomarkers” methane, ozone, water…
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Planet Hunting - Imaging
Direct Imaging is difficult because
(i) stars are typically a million to ten billion
times brighter than the planet. Need to use a
coronagraph to block out the star so light
reflected by the planet can be seen.
(ii) on an astronomical scale planets are
very close to their parent stars so a high
resolution is needed to separate them.
2008: First visible light image of an
extrasolar planet, Fomalhaut b, recorded by
Hubble Space telescope. Infrared images of
a 3 planet system taken with 8m groundbased telescopes. In both cases the motion of
the planets shows they are orbiting objects.
Detection of extrasolar planets is a goal of
the European Extremely Large Telescope - a
proposed 42m dish costing €960 M.
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Planet Hunting - Status
As of 20th October 2009 more than 400
extrasolar planets have been identified.
See http://planetquest.jpl.nasa.gov
09/2002: radio telescope detects water
molecules in the Upsilon Andromedae
planetary system.
10/2002: a planet is detected in a binary star
system - most stars are in binaries, so possible
sites of planet formation greatly increased.
05/2007: Gleise 436b - this planet’s density is
found to be consistent with a 50% rock + 50%
water composition (but its orbit is eccentric).
02/2008: Spitzer measures warm dust around
other stars indicative of colliding rocky bodies
in orbits of 1 to 5 AU - suggests at least 20% of
Sun-like stars in our galaxy (~ 5 billion stars)
could have rocky planets.
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