Transcript Extra-Solar Planets

The Planets of Other
Stars
The Astronomy Diagnostic Test (ADT):
The Sequel
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As before, the survey won’t be graded, but participation
will count towards your homework grade. Just answer
the questions the best you can. (Taking the test may
even help you study for the final!)
The test is 30 minutes long, and the link will disappear
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Detecting Other Planetary Systems
For 50 years, astronomers have been looking for planets around
other stars. Since planets are at least 100,000,000 times fainter
than stars (because all they do is reflect a bit of the star’s light),
direct detection is not (yet) possible. But planets can be found
in 5 different ways:
• Astrometrically (via a
positional “wobble”)
• Spectroscopically (via
blueshifts and redshifts
of absorption lines)
• Photometrically (via
transits)
• Timing phenomenon
• Gravitational lensing
Astrometric (Wobble) Detections
Because our Sun (and other stars) are moving through space,
the positions of stars on the sky change ever so slightly each
year. This is called proper motion.
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Astrometric (Wobble) Detections
If a star’s proper motion wobbles with time, it could be due to
an unseen companion. Only Jupiter-mass planets have enough
mass to be detected in this way.
Astrometric (Wobble) Detections
The greater the mass of the unseen companion, or the closer the
separation, the greater the wobble. Detecting binary stars in this
way is tedious, but do-able. Detecting planets astrometrically is
extremely difficult.
If one were to observe the Sun
from 10 pc away, its wobble
(due principally to Jupiter and
Saturn) would be less than
0.002 over 30 years. (Recall
that the atmosphere blurs
things out by 1, and, with our
best measurements, we can
measure parallaxes to 0.003)
0.001 arcsec
Spectroscopic Detections
Planets can be detected via measurements of the Doppler effect.
The planet won’t be detected, but the reflex motion of the star
might. (Thus, the star is like a single-line spectroscopic binary,
with an extremely low-mass companion.)
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The Sun’s reflex motion
due to Jupiter is about 13
meters/sec. For reference,
the absorbing gas in the
Sun’s atmosphere moves
about 13 km/sec, due to
thermal effects alone.
Only Jupiter-mass planets
can be detected in this way.
Transit Detections
If a planet moves in front of
its star, the light from the star
will decreases very slightly,
(less than 1%), depending on
the size of the planet. Only
Jupiter-sized planets are big
enough to be detected.
Mercury transit
Venus transit
Jupiter transit (artist conception)
Timing Detections
If a star system contains a very accurate clock, you can tell
when the star is closer to you (or further away) by timing
when the clock’s signal arrives. (In practice, the only objects
that this can be applied to is millisecond pulsars.)
The faster the pulsar,
the more accurate the
timing. In theory,
objects as small as
Mercury could be
detected around a
millisecond pulsar by
the gravitational force
it exerts on its parent
star.
Gravitational Lens Detections
If a star/planet moves
exactly in front of a
background star, the
brightness of the
background star can
be greatly magnified
by the gravitational
lens effect.
Gravitational Lens Detections
In principle, the gravitational lens technique can detect planets
of any mass. However, once the event is over, the planet is
lost forever (since we are only seeing the background source).
It is impossible to learn anything more about the system.
History of Extra-Solar Planets
• 1960’s – 1990’s: Numerous claims (and retractions) of
planet detections via astrometry and spectroscopy
• 1991: First extra-solar planetary system (accidentally) found
by timing a millisecond pulsar (PSR B1257+12)
PSR B1257+12
Orbiting the 6.2 millisec pulsar are (at
least) 4 small planets, with masses of
0.02, 4.3, 3.9, and 0.0004 M. These
objects were mostly likely formed
after the supernova, and after the
pulsar evaporated its companion star.
The orbits of planets B and C are in a
3:2 resonance.
History of Extra-Solar Planets
• 1960’s – 1990’s: Numerous claims (and retractions of planet
detections via astrometry and spectroscopy
• 1991: First extra-solar planetary system (accidentally) found
by timing a millisecond pulsar (PSR B1257+12)
• 1995: First planet found around a “normal” star (51 Pegasi)
using spectroscopy
51 Pegasi
51 Pegasi is a G5 main sequence star 15 pc from the Sun, whose
Doppler motion changes by  53 meters/sec over a period of 4.2
days. The data imply the presence of a planet with
 a roughly circular orbit
 a semi-major axis of 0.052 A.U. (For comparison, Mercury
is 0.38 A.U. from the Sun.)
 a mass of 0.46 MJup! It’s like a “hot” Jupiter!!
History of Extra-Solar Planets
• 1960’s – 1990’s: Numerous claims (and retractions of planet
detections via astrometry and spectroscopy
• 1991: First extra-solar planetary system (accidentally) found
by timing a millisecond pulsar (PSR B1257+12)
• 1995: First planet found around a “normal” star (51 Pegasi)
using spectroscopy
• 1995 – present: Over 250 planets found -- mostly “hot”
Jupiters
Hot Jupiter Systems
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Some of the planets’ orbits are significantly elliptical!
History of Extra-Solar Planets
• 1960’s – 1990’s: Numerous claims (and retractions of planet
detections via astrometry and spectroscopy
• 1991: First extra-solar planetary system (accidentally) found
by timing a millisecond pulsar (PSR B1257+12)
• 1995: First planet found around a “normal” star (51 Pegasi)
using spectroscopy
• 1995 – present: Over 250 “hot Jupiters” detected around
230 stars
• 1999: First extra-solar planet seen transiting its star (HD
209428)
HD 209428
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Every 3.523 days, the G0
main sequence star HD
209428 dims by about
1.7%. This indicates that
the planet is 60% larger
than Jupiter.
The star’s Doppler
measurements imply a
mass of 0.63 MJup.
The density of the planet (0.27) is much less than water. The
planet must be a gas giant that is “puffed up” by the heat from
the star.
History of Extra-Solar Planets
• 1960’s – 1990’s: Numerous claims (and retractions of planet
detections via astrometry and spectroscopy
• 1991: First extra-solar planetary system (accidentally) found
by timing a millisecond pulsar (PSR B1257+12)
• 1995: First planet found around a “normal” star (51 Pegasi)
using spectroscopy
• 1995 – present: Over 130 “hot Jupiters” detected around
~120 stars
• 1999: First extra-solar planet seen transiting its star (HD
209428)
• 2003: First probable planet found (temporarily) via a
gravitational lens
OGLE 2003-BLG-235/MOA 2003-BLG-53
The modeling of the
gravitational lens event
implies the existence of
a planet in orbit about a
0.36 M lensing star.
The planet has a mass of
about 1.5 MJup and a
distance of about 3 A.U.
from its star.
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Results from Extra-Solar Planet Studies
Reflex motion techniques
work best when the
planet is large and close
to its star. Thus the data
we have are biased.
However, it is clear that
many stars have Jupitermass planets in their
inner solar system.
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Results from Extra-Solar Planet Studies
The data also show that
the more metals in a star,
the more likely the star is
to planets around it. This
suggests that planets
cannot form out of
hydrogen and helium
alone -- even gas giants
need a solid core around
which to form.
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Metallicity (compared to Sun)
As of December 8, 2008, 267 planets are known outside our
solar system around 228 stars (not counting four gravitational
lens events).
Results from Extra-Solar Planet Studies
Many stars have hot
Jupiters, and not all are
in roughly circular
orbits. But according
to the solar nebula
hypothesis, Jupiter-type
planets cannot form
close to a star, due to
the star’s radiation
pressure and stellar
wind. How can this
be?
Explaining the Hot Jupiters
Best Model: the hot Jupiters
must have formed in the outer
regions of their star systems,
and then spiraled in due to
friction in the protostellar
disk. (But if they spiral in too
much, they collide with the
star.)
If this is the case, then smaller
terrestrial planets in the inner
part of the disk were destroyed
when the Jupiter-mass planet
passed by.