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ASTA01 @ UTSC – Lecture 17
Chapter 12.
Extrasolar Systems
Extrasolar planet discovery:
- Pulsar planets
- Wobble method (radial velocity)
- Transit (occultation, eclipse) method
- Examples and statistics
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HD 1415969
Observations by Hubble Space Telescope
(NICMOS near-IR camera).
Age ~ 5 Myr,
a transitional disk
Gap-opening PLANET ?
So far out?
Only if migrated outward
R_gap~350Ad
R ~ 0.1 R_gap
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HD 14169A disk gap confirmed by new observations
(HST/ACS)
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Alpha Pisces
Austrini
(α PsA)
Fomalhaut
A disk of a
bright
southern star
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Planets Orbiting Other Stars
• Are there planets orbiting other stars?
• Are there planets like Earth?
• The evidence so far makes that seem likely
YES
• We already have found Earth-mass planets
• But we don’t yet know how closely they
resemble our planet
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Extrasolar Planets
• A planet orbiting another star is called an
extrasolar planet or an exoplanet
• Such a planet would be is usually quite faint
and difficult to detect close to the glare of its
star.
• However, there are ways to find these planets.
• To see how, all you have to do is imagine walking
a dog.
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Extrasolar Planets
• Think of someone walking a poorly trained
dog on a leash.
• The dog runs around pulling on the leash.
• Even if it were an invisible dog, you could plot its
path by watching how its owner was jerked back
and forth around the
Center of Mass
Extrasolar Planets
• In the same way, astronomers can detect
a planet orbiting another star – by
watching how the star moves as the planet
tugs on it.
Extrasolar Planets
How the star moves is revealed by either:
• It’s sinusoidal motion on a sky (astrometric
detection), or
• How its light changes frequency due to te
Doppler effect (radial velocity detection), or
• If it sends pulses as a pulsar, then by the
time delay of pulse arrival times
• When the star approaches us, we see lower
frequency of pulses, or of electromagnetic waves
Extrasolar Planets: discovery in 1992
• [In 1988, Canadian astronomers Bruce Campbell, G.A.H.
Walker, and S. Yang discovered an extrasolar planet orbiting a
binary star system, but their discovery was not confirmed until
2002.]
• The first 3 confirmed extrasolar planets were
discovered around a pulsar by the Polish
astronomer Alex Wolszczan (b. 1946)
[read: Volsh-chan] in 1992
• He studied & worked in Toruń, the
city of Copernicus, but discovered
planets with the Arecibo radio
telescope in Puerto Rico, with his
coworker D. Frail.
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Extrasolar Planets: discovery
• Pulsar’s name is PSR 1257+12
• It formed in a supernova explosion and has 3 ms period of
rotation
• 4 Planets have masses:
0.02, 4.3, 3.9, 0.0004 ME
distances 0.19, 0.36, 0.46, 2.6 AU
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Chart of three PSR 1257+12 planets & inner solar
system planets
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Extrasolar Planets: 51 Peg
• The first planet orbiting a sunlike star was
discovered in 1995 around the star 51 Pegasi.
• As the planet circles the star, the star wobbles slightly.
• The very small motions of
the star are detectable as
Doppler shifts in the star’s
spectrum.
• This is the same
technique used to
study spectroscopic
binary stars.
Michel Mayor (b.1942), Switzerland
Extrasolar Planets: 51 Peg
• From the motion of the star and estimates of
the star’s mass, astronomers can deduce that
the 51 Peg b planet has half the mass of
Jupiter and orbits only 0.05 AU from the star (
<< sun-Mercury distance)
• Half the mass of Jupiter amounts to 160 Earth
masses. A large planet, larger than Saturn.
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Extrasolar Planets
• Astronomers were not surprised by the
announcement that a planet orbits 51 Peg
• For years, they had assumed that many
stars had planets
• Nevertheless, some of them (Canadian David
Grey) greeted the discovery with skepticism
• That skepticism led to careful tests of the data and
further observations that confirmed the discovery
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Extrasolar Planets
• Over 500 planets have been discovered in
this way – including at least three planets
orbiting the star Upsilon Andromedae, and
five orbiting 55 Cancri – true planetary
system.
• More than 40 such
multiple-planet
systems have been
found.
Extrasolar Planets
• Another way to search for planets is to
look for changes in the brightness of a star
when the orbiting planet crosses in front of
or behind it.
• The decrease in light is
very small, ~Rpl 2
• Astronomers have used
this technique to detect
a few 1000s of planets as
they crossed in front of their
stars.
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Extrasolar Planets
• The Spitzer Infrared Space Telescope has
detected two planets when they passed
behind their stars.
• These planets are hot and emit significant
infrared radiation.
• As they orbit their parent stars, astronomers detect
variation in the amount of infrared from the system.
Measurements reveal that they have Jupiter-like
diameters as well as masses. So, astronomers
conclude they have Jovian densities and
compositions.
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Extrasolar Planets
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Extrasolar Planets
2012
• Planets
known
so far
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Extrasolar Systems: the first images, HR 8799
• Actually getting an image of a planet
orbiting another star is about as easy as
photographing a bug crawling on the bulb
of a searchlight miles away.
• Planets are small and dim and get lost in the
glare of the stars they orbit.
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Beta Pictoris giant planet
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Extrasolar Planets
• In 2007, astronomers discovered what
could be low-mass Earth-like planets
orbiting a red dwarf star named Gliese 581
located a mere 20.3 light-years away.
• In 2011, a team of
scientists in France
confirmed that at least
one of the planets could
have an atmosphere and
oceans, and support
Earth-like life.
Extrasolar Planets: Gliese 581
Extrasolar Planets: Kepler satellite observatory
• The main aim of the Kepler mission is to
find Earth like planets in habitable zones
around other stars.
• For the first time in the history of our search
for the worlds that resemble our own, we have
the technical capabilities to see small rocky
planets.
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Extrasolar Planets
• The sizes of the newly discovered planets
range from 1.5 times the size of Earth to
large Jupiter-sized worlds.
• Spectral analyses of trails of smaller planets
show traces of silicates (building blocks of
rocks), ice, and water.
• The Spitzer infrared telescope, which prior to the
Kepler mission discovered numerous large, hot,
Jupiter-like planets around their stars, is being
used to confirm the Kepler telescope findings.
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Extrasolar Planets: Kepler 11 system
• In the few first months of the mission more than
1200 planet candidates were detected, many of
them multiple planetary systems.
• One such system is Kepler 11, six tightly packed
planets located 2000 light-years from Earth;
planets range from 2.5 to 4.5 times Earth’s size.
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Kepler 11 system
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2011
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Extrasolar Planets
• Some unusual arrangements of planets
have been discovered by the Kepler
mission.
• Notably, astronomers were shocked to see
two planets sharing an orbit – the planets
were arranged in the exact angular distance
that theoretically allows for such an
arrangement.
• Seeing such variety of possibilities allows us to
explore and test many hypotheses about our own
solar system’s origin and formation.
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Extrasolar Planets
• The discovery of extrasolar planets gives
astronomers added confidence in the solar
nebula theory.
• The theory predicts that
planets are common.
• Astronomers are finding them
orbiting many stars.
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