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ASTR 330: The Solar System
Announcements
• Homework assignment #5: results today.
• Class average was: 38.5/50.0 (77 %)
• Overall class course average is: 75%.
• Extra credit term paper: due Dec 5th (next
Tuesday).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 26: Extra-solar planets
Figure credit: Stephan Simon, SCIENCE-WORLD.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Other planetary systems
• Amazingly, despite countless sci-fi films to the contrary, no-one knew
for sure whether any other stars in the galaxy had planets at all until the
1990s.
• However, most of the astronomical community believed that many solartype stars, would have planetary systems. (There is much less
agreement as to whether these systems are inhabited or not!)
• It would be rather incredible if, out of 100 billion stars in the Milky Way
galaxy (and 100 billion galaxies), our Sun had nine planets and no other
stars had any at all!
• However, until such systems were found and counted, we could not
ascertain whether such systems were common or rare.
• Why had no planets outside our solar system been found?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
An unexpected start
• Of course, we are most interested in planets around solar-type stars,
which are the most likely to harbor life.
• In 1991, the first planets discovered since Pluto in 1930 were
announced. But, rather than orbiting a Sun-like star, they were discovered
orbiting a pulsar, a rapidly rotating neutron star.
• Neutron stars are the remains of
giant stars, which explode at the end
of their lives in supernovas.
• Pulsars send out two beams of radio
energy as they rotate, like a
lighthouse. Timing these pulses shows
that pulsars are amongst the best time
keepers in the universe.
Figure credit: A. Wolszczan, Penn State
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Pulsar Planets
• Because pulsars are so regular, it was with surprise that Penn State
professor Alexander Wolszczan noticed in 1991 that PSR B1257+12 was
showing regular deviations in its pulsing. How could this arise?
• If another object were orbiting the pulsar, the gravitational pull would
move the pulsar in a small circle. This wobble would cause the pulses to
arrive first late and then early.
• In this case, the pulses showed two
regular deviations, which showed the
definite effect of two bodies in orbit
about the pulsar.
• This was later shown to be three
bodies: two Earth-sized and one
Moon-sized.
Figure credit: A. Wolszczan, Penn State
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Why pulsars?
• Pulsars are the last place astronomers would think to look for planets.
Why?
• During the supernova explosion, most of the mass of the star is blown
off into space. Besides the intense heating which would kill off any life on
nearby worlds, the massive reduction in gravity should allow the planets
to immediately escape the system.
• Why were the pulsar planets the first detected?
• The answer is simple: technology. Accurate timing is relatively easy to
achieve now on the Earth. If other types of stars behaved like pulsars, we
could have found planets around them as well!
• In the case of solar-type stars, the problem was much harder however,
and the eventual success had much less to do with luck than good
planning.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Can we see extrasolar planets?
• How would we go about finding planets around other stars? Can we see
them orbiting other stars?
• Ideally we would like to see the point of light in a telescope. When
Neptune was tracked down by mathematics in the 1800s, the finding was
not considered definitive until the moving dot had been located.
• However, for extra-solar planets, this may be the last method we use.
Why?
• Quite simply, the star is too bright (more than a million times brighter
than any planet) and the planets too close to the star, for direct imaging
in visible light to be successful with current technology.
• However, this is a distant goal that astronomers are already working
towards.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
How to find planets
• So, we can’t image a planet at the present time. How can we find
planets then?
• The answer is that we must use an indirect detection method, by
looking at changes in the star caused by the planet.
• Three methods are currently feasible for finding planets around MS
stars:
1. Astrometric method.
2. Radial Velocity Method.
1. Transit Method
• We’ll examine each in turn.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Astrometric Method
• The first of the indirect methods is to use the fact that the planet will pull
the star into a small circle about the center of mutual mass, called the
system barycenter. On the sky, the star will move from side to side.
Figure credit: wikipedia/Zhatt
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Astrometric Method
• Clearly, we are looking for a very small effect! The mass of the star
far outweighs the mass of the planet, and the ‘wobble’ will be very
small.
• For example, the barycenter of the following systems:
• Sun-Earth: barycenter is 450 km from the center of the Sun.
• Sun-Jupiter: barycenter is 777,000 km from the Sun’s center.
This is just outside the surface of the Sun.
• This was the first method used to try to find planets. Over several
decades of research, no positive results were found. However, it was
clear from calculations that the effect would very likely be too small to
measure.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Doppler Effect
• The Doppler Effect can also be used to find planets. How?
• In a familiar example, an ambulance racing down the street past us has
a distinctly higher pitch as it approaches, and a lower pitch as it recedes.
Why is this?
• The sound waves given off by the siren are affected by the forward
motion of the vehicle.
• As it approaches, the waves get bunched up more closely together, and
arrive closer together. We perceive that as a higher sound pitch.
• The opposite occurs as the vehicle moves away. The wave peaks get
stretched further apart, and we hear a lower pitch.
• Thus the familiar pitch drop, from high to low, as an ambulance passes.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Red Shift and Blue Shift
• The Doppler Effect also occurs with light.
• The light emitted from a star moving towards us is shifted to a higher
frequency than we would see if the object was at rest. We see this as a
shift to the blue end of the spectrum for visible light.
• Similarly, if a star or
galaxy is moving away
from us, the light is
shifted to lower
frequencies, which we
call a red-shift.
• Incidentally, red-shifts
of distant galaxies were
our first evidence that the
universe is expanding.
Figure credit: John Fix, McGraw-Hill
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Spectroscopic Method
(or radial velocity method)
• We can use the Doppler Effect to find planets! How?
• We have just discussed how a planet orbiting a star will cause it to
move in a small circle about the system’s center of mass.
• As the star moves
first towards us, then
away from us, its light
is first blue-shifted,
then red-shifted in a
cyclical fashion. Of
course, the period of
this cycle may be days,
months or years!
Figure credit: physicsweb
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Doppler Searches
• Starting in the early 1990s, several groups of scientists began to look for
such a shift.
• The size of the shift they were searching for was very small, but even
so, this is an easier task than measuring the movement of the star on the
sky.
• The key was in measuring very small shifts of dark stellar absorption
lines, repeatably and accurately over months and years.
• A yardstick was needed: a pattern of fixed features, which the stellar
lines would show movement against.
• For this, a gas cell was also inserted into the optical path, to add fixed
lines which would not move over time. The positions of the stellar lines
would be measured relative to the gas lines.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Photometric Method
(or transit, or eclipse method)
• The final of the three methods we will discuss is the photometric (transit
or eclipse) method.
• This method is simple: it looks for the dimming of the starlight as the
planet passes in front of the star.
• The main disadvantage of this
method is that the eclipse takes
just a few hours as seen from
the Earth, and it may not occur
again for years!
• So, out of all the stars in the
sky, how do we know where and
when to look?
Figure credit: Don Kalinski, UCAR STARE Project
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
First Success
• In 1995, two teams were pursuing the radial velocity (Doppler) method to
search for planets around main-sequence (solar-type) stars.
• The first success was
announced by Michel Mayor and
Didier Queloz of Geneva
Observatory, Switzerland.
• They had found a roughly
Jupiter-sized planet in orbit
around the star 51 Pegasi.
• The figure (right) shows the
sine-wave pattern of frequency
shift as the star is pulled in a
circle by the planet.
Figure credit: Mayor and Queloz, Geneva Observatory.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Confirmation and Criticism
• The announcement by Mayor and Queloz took the astronomical
community by surprise. After so many years of negative findings, no-one
was paying attention to this corner of astronomy, which suddenly became
front-page news.
• The 51 Pegasi finding was quickly confirmed by Geoff Marcy (UCB) and
Paul Butler (Carnegie Institute) who had been observing 51 Pegasi and a
bunch of other nearby stars, but had not yet examined their data.
• The team of Marcy and Butler went on to examine the rest of their
observations, and quickly found the signature of planetary companions
around two other stars.
• However, some scientists dissented, saying that the sinusoidal signal
could occur naturally in a star, if it was pulsing in and out like a giant
heart. Over time, this theory was shown to have flaws, and the planetary
detections were accepted.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
A Multitude of Worlds
• After the 51 Peg announcement, the number of planets
found exploded from 1996.
• In just a few years, the number of planets known outside
our solar system first equaled our total of nine, then rapidly
overtook and is still climbing.
• As of November 2006, 197 planets around main sequence
stars have been found using the Doppler technique, 121 by
the Marcy and Butler team alone!
• Let’s now look at what information we can glean about
these planets.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Information From Doppler Studies
• By studying the Doppler motion of the parent star, quite an impressive
amount of information about the planet can be obtained. Firstly, of course
we find the orbital period.
• Secondly, if we know the approximate mass of the star (which we can
tell from its spectrum) , we can use Newton’s Law of Gravity to find the
radial distance (semi-major axis) of the orbit.
• Thirdly, we can tell the eccentricity of the orbit from the shape of the
oscillations.
• A sinusoidal oscillation shows a circular orbit, whereas a saw tooth
shape on the other hand shows an elliptical orbit. The amount of ellipticity
can be determined mathematically by fitting the shape.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Eccentric Planetary Orbit
• This figure shows the distinctive signature of a planet in a very eccentric
orbit about the parent star, in this case, the 16 Cygni system, with an
eccentricity of 0.68.
Figure credit: Cochran et al, Ap J 453.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Upsilon Andromeda: Multiple System
• The star Upsilon Andromeda was the first main-sequence star to show a
multiple planetary system.
• The right figure below shows the oscillations due to two of the three
planets detected. The left figure shows the orbits of the three planets.
Figure credit: Marcy and Butler/Berkeley
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Planet Mass
• The fourth fact we can discern from the radial velocity graph is the
minimum mass for the planet, from the amplitude of the star’s reflex
motion.
•Why the minimum mass? Consider two extreme cases.
• If the planetary system was face-on as seen from Earth, the motion of
the star would be side-to-side on the sky, not forwards and backwards. In
this case, we would see no Doppler shift of the starlight at all.
• However, if the system was edge on, we would see the star moving
forwards and backwards in the sky.
• The intermediate cases occur where the system is tilted partly towards
us. In this case, we measure only the part of the stellar motion that is in
the radial direction (forwards-backwards) not transverse (side-to-side).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Minimum Mass
• By measuring the radial motion only, we can underestimate
the mass of the star.
• In general, we can find only part of the effect that the planet
has on the star, not all of the effect.
• So, when we measure the radial effect of the planet to
determine its mass, we can tell only the minimum mass that
the planet could have.
• Note that, if we were able to detect the transverse motion
as well, using astrometry, we could then find the exact mass!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Planet or Brown Dwarf?
• When is a planet really a planet, and how big does it have to be to
become a star?
• A true star is at least 70 Jupiter Masses (MJ). This is the minimum mass
required to compress the core sufficiently to ignite true hydrogen fusion,
the chain of adding protons together to make helium, which powers stars.
• However, between 13 and 70 MJ, an object can fuse deuterium (heavy
hydrogen) to make helium, which occurs at lower temperatures and
pressures.
• Such objects will never be able to fuse normal hydrogen like the Sun,
and will glow very dimly: a ‘brown dwarf’.
• Below 13 MJ, there is no fusion at all, although of course Jupiter-sized
planets can generate some energy by gravitational contraction and
differentiation (‘helium rain’).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Mass Distribution
• If stars and planets were formed in the same way, then we would expect
to see a continuous distribution of objects at all masses.
• However, most of our theories hold that stars form in the central
condensation of the nebula, whereas planets form in the disk.
• The figure (right) shows the
mass distribution of the first fifty
or so exoplanets to be
discovered.
• There is a distinct gap at
around 13 MJ, which seems to
indicate that different processes
give rise to planets, or stars and
brown dwarves.
Figure credit: Geoff Marcy/Physics Today
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
51 Pegasi
• Once the 51 Pegasi system was unravelled, the most startling thing
about the planet turned out to be not the size, but the orbital period.
• At just 4.2 days (compare to Mercury’s 88 days), this planet makes an
orbit just 0.05 AU in radius about its parent star!
• Initially, there was some skepticism that a planet could possibly survive
in a near-circular orbit so close to a star.
• However, calculations show that, although this planet would be very hot
(1200 K) its minimum mass of 0.46 MJ (Jupiter Masses) was enough to
hold its gases from totally evaporating.
• Many other extrasolar planets turned out to be ‘hot giants’ as well: large
planets relatively close to the parent star. Why could this be?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Selection Effects
•
We must be careful when search methods not to delude ourselves
that we are seeing all there is to see!
•
The Doppler search method has two biases:
1. Large planets: the bigger the planet, the further the stellar
absorption lines will move, and the more obvious will be the effect.
We could not detect Earth-sized planets with this method: the line
movements are too small.
2. Close-in planets: the smaller the orbit about the parent, the less
time we will have to wait to see the effect. For example, Saturn
takes around 30 years to orbit the Sun. We would have to wait at
least that long to see one full oscillation of the stellar lines.
• So, it is not surprising that we tend to find large, close-in planets: hot
giants!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Distance
Distributions
• This figure shows the
dramatic concentration of
exoplanets at small semimajor axis values.
• We can also see that
several systems
discovered so far are
multiple systems.
Figure credit: exoplanets.org
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
What is a typical solar system?
• The detections we have made so far, even allowing for the biasing of
the methods, show that our system may be far from typical.
• Our paradigm has been of terrestrial planets inside 5 AU from the star:
small rocky worlds. Outside 5 AU are the gas giant planets.
• But in many exoplanet systems we see large giants close in, which we
assume to be gas planets because off their size.
• This is not something our formation scenarios predict. Remember that
theories were based on a core of icy planetesimals for the outer planets,
followed by accretion of hydrogen and helium.
• These processes should have occurred at temperature of 30-100 K.
• How then can we account for the hot giants?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Density
• If the hot giant planets turned out to be rocky, not gaseous, perhaps we
don’t have a problem after all. We have no problem with rock and metal
condensed inside of 5 AU.
• If only we could find the actual size of the planet, then we could
calculate the density and find out which is right.
• But how to find the planet’s size?
• At this stage, a second of our indirect techniques came to the rescue:
the occultation or photometry method.
• This method would show the existence of a planet, due to dimming of
starlight as the planet passed in front. But, now that we know which
systems have planets (from Doppler technique) we can see if any of them
eclipse!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
First Transit
• In 1999, two groups almost simultaneously discovered the first transit, in
the system of star HD 209458. The odds that a given system will be in the
correct orientation for an eclipse to occur (ever) are about 1 in 10.
• This figure
shows the
distinct dip in the
light curve as the
planet passed
slowly in front of
the stellar disk.
• The radius of
the planet was
1.3 times that of
Jupiter.
Figure credit: STARE project/UCAR
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
HD 209458
• Since the initial detection using
a small scope, the HST was
trained on the system, showing
the beautifully smooth lightcurve
(right). The lower image shows
an artists impression of the
transit occurring.
• Using the derived radius, and
the exact mass (which was
known from the system
orientation), the density was
found to be 0.4 g/cm3, even less
than that of Saturn. Although in a
four-day orbit, the planet is
indeed a gas giant world.
Figure credit: (i) HST/APL (ii) Lynette Cook
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
How could hot giants form?
•
There are two plausible theories at the present time.
• The first theory is that the planets form very early, even before the
star is burning at full strength.
• If the planets can accrete much of their mass by then, then they
stand a good chance of being able to hang onto to the mass once
the star really gets going.
• In this scenario, the planets can form inside the ‘snowline’: the 5 AU
distance inside of which volatiles are not able to condense: after the
star is shining properly.
•
I.e. the planet condenses before the ‘snowline’ boundary is in place.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Planet Migration
• In the second scenario, the planet forms outside the snowline, as
‘normal’, but then migrates inwards. There is even some evidence that
Jupiter too underwent this migration process, due to the enrichment in
argon, which must have condensed at 30-40 K.
• How could migration occur? One theory involves the presence of the
nebular disk, which could linger after the planet had formed.
• So long as disk material exists both inside and outside the planet’s orbit,
the gravitational effect on the planet is neutral.
• But if the inner disk material is removed, by accreting onto the star for
example, then the gravitational forces acting on the planet become
unbalanced.
• The net effect is that the planet loses energy and falls into a smaller
orbit.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Migration Schematic
Figure credit: physicsweb
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
An End to Migration
• The migration can end in one of two ways.
• In one scenario, the migration is stopped by tidal forces. When the
planet comes close enough to the star to experience strong tides, it
becomes synchronously rotating.
• Once its rotation period becomes equal to its orbital period (at about 0.1
AU or less), it can no longer drift further inwards.
• There are some systems which seem to defy this description however,
having giant planets ‘stopped’ at 0.3 AU for example.
• A second possibility is that the planet simply continues to migrate
inwards all the way until it is consumed by the star. Ths could possibly
explain the high metalicity of many of the stars which have planets.
• In summary, our theories are still embryonic and require much further
work.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Secondary eclipses
•
As a planet orbits a star, two eclipses occur:
1. When the planet goes in front of the star (primary eclipse).
2. When the planet goes behind the star (secondary eclipse).
•
Astronomers were very keen to observe a secondary eclipse when the
planet is invisible - much harder to see than the primary eclipse when
the starlight is blocked.
• But why were they so interested in ‘not seeing’ the star?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Dr Conor Nixon Fall 2006
Vanishing planets
• Resourceful astronomers had figured out a trick for detecting the faint
glow of a planet (often compared to a firefly buzzing round a streetlamp).
• By observing the system first at elongation, when the planet is side by
side with the star, and then during secondary eclipse, scientists could
subtract the two signals and find the light due to the planet alone!
• However, this trick is
much easier to perform
in infrared light, where
the star only outshines
the planet by 400 times,
rather than in visible
light where the star is
10,000 times brighter!
Figure credit: NASA/JPL-Caltech
ASTR 330: The Solar System
Double Jackpot
• In late 2004 two teams used the Spitzer Infrared Space Telescope to
search for the secondary eclipses of two known transiting exoplanets.
• The teams announced the findings in early 2005: a double success.
Secondary eclipses were observed for both TrES-1 and HD 209458b.
• This infrared detection allowed the
temperature of the planets to be
directly measured: 1060 K for TrES-1
and 1130 K for HD 209458b - both
much hotter than Venus!
• But, we must remember that these
are both giant planets very close to
their parent stars.
Figure credit: NASA/JPL/Caltech
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Dr Conor Nixon Fall 2006
HD 209458b The mystery of the puffed-up planet
• By observing the secondary eclipse as well as the primary eclipse,
the astronomers were also able to confirm that both planets are in
perfectly circular orbits.
• This was expected: tidal forces from the parent star at such close
distances can quickly circularize the planet’s orbit, removing any orbital
eccentricity.
• However, this research has deepened another mystery: why HD
209458b is 35% larger than Jupiter, whereas TrES-1 is only 4% larger,
yet both have near identical masses and heating from their suns.
• Why is HD 209458b so puffed up? No-one knows at present!
ASTR 330: The Solar System
The Future
• The field of extrasolar planets is still in its infancy, having just arisen less
than 10 years ago. Many new telescopic searches and space missions
have been proposed, and many will doubtless be approved.
• The Kepler mission is the brainchild of
Bill Borucki at NASA AMES. This spacebased telescope would simply stare at a
patch of sky for 4 years, containing a
rich field of solar-type stars, in the
expectation of seeing many transiting
systems.
• Kepler has the advantage of being
sensitive enough to detect Earth-sized
planets for the first time. About 100 are
expected to be found, along with many
more giant planets, starting in 2008.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Space-based interferometry
• Although Kepler will enable us to detect the presence of planets as
small as Mercury, we will not be able to tell much about them.
• To really find out whether they have atmospheres and the possibility of
life, we need to be able to sense the planetary infrared radiation directly,
and perform spectroscopy.
• This will require much better spatial resolution than any single-dish
telescope can achieve: the telescope would need to be 100s of meters
wide.
• The solution is to use interferometry: the technique of combining the
light from an array of separate telescopes to achieve the spatial resoluion
of a single large scope.
• To perform interferometry at optical wavelengths, we are required to
deploy our array of telescopes in space!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Terrestrial Planet Finder
• A mission called ‘SIM’ - Space Interferometry Mission will lead the way,
in around 20011 if the mission goes ahead. SIM would be able to resolve
giant planets from starlight.
• SIM would lead the way
for TPF - the Terrestrial
Planet Finder mission, in
2020 or thereabouts.
• TPF would be able to
give us direct information
about Earth-like planets:
if the daunting technical
challenges can be
overcome.
Figure credit: JPL/NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
1. What unexpected discovery was made in 1991? What was found?
2. Why were pulsars a very dubious place to hunt for planets?
3. Can we see planets around other stars using existing technology?
4. What indirect methods can be used to find planets?
5. How does the Doppler Effect help us find planets?
6. What information about extra-solar planets can we determine from
radial velocity measurements?
7. What is a brown dwarf? Do planets and brown dwarves form in the
same ways?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
8. Why does the Doppler method give us a minimum mass for the
planet?
9. What biases do we expect to occur in our planet detections when we
use the Doppler method to find them?
10.How can we determine whether close-in massive planets are gas or
rock?
11. How could hot gas giants planets form?
12. In what ways could the migration process end for the planet?
13. What is the objective and scientific method of the Kepler mission?
14. Why do we need space-based interferometry?
Dr Conor Nixon Fall 2006