Download PHYS178 2008 week 11 part-1

Extrasolar planets
Lecture 2: Planetary formation theory and
detection techniques
A/Prof. Quentin A Parker
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Formation of a Star and proto-planetary disk
(a) Dense cores form within a molecular cloud.
(b) A protostar with a surrounding disk of material forms at the centre, accumulating
additional material from the molecular cloud through gravitational attraction.
(c) A stellar wind breaks out, confined by the disk to flow along the stellar poles.
(d) Eventually this wind sweeps away the cloud and halts the accumulation of
additional material, and a newly formed star, surrounded by a disk, becomes visible.
The diameter of a typical accreting envelope is about 5000 astronomical units.
The typical diameter of the disk is about 100 AU.
Disks around protostars
These Hubble Space Telescope infrared images show disks around
young stars in the constellation of Taurus, in a region about 450 LY
away.
In some cases we can see the central star (or stars—some are
binaries). In other cases, the dark horizontal bands indicate
regions where the dust disk is so thick that even infrared radiation
from the star embedded within it cannot make its way through.
The bright glowing regions are starlight reflected from the upper
and lower surfaces of the disk, which are less dense that the
central regions.
Fig 20-12, p.450
•Tracks are plotted on the H–R diagram to show how stars of
different masses change during the early parts of their lives.
•The numbers next to each dark point on a track are the rough
number of years it takes an embryo star to reach that stage.
•You can see that the more mass a star has, the shorter the time it
takes to go through each stage.
•Stars that lie above the dashed line would typically still be
surrounded by infalling material and would be hidden by it.
Disks around protostars
These HST images show 4 disks around young stars in the Orion Nebula.
The dark, dusty disks are seen silhouetted against the bright backdrop of the
glowing gas in the nebula.
The size of each image is about 30 times the diameter of our planetary system;
this means the disks we see here range in size from two to eight times the orbit of Pluto.
The red glow at the center of each disk is a young star, no more than a million years
old.
(credit: M. McCaughrean, C. R. O’Dell, and NASA)
• The currently favoured scenario for planet formation is that of core
accretion
• Initially planetary cores form from condensed material in the
protoplanetary disc around a star
• In an inner hotter zone only grains of dust and small particulates
aggregate together
• Planet formation is further supported by the presence of icy snowballs in
a cooler zone outside the so-called
“ice boundary”
• Planets forming there are likely to grow to gas giants by accreting
hydrogen and helium
• They can then migrate inwards
• A decent fraction end up in close orbits
• These constitute the hot inner planets.
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Dust Ring Around a Young Star
This near infrared HST image shows a narrow ring of dust around the very young
star HR 4796A, ~220 lyr away in the constellation of Centaurus. The ring is very
narrow, spanning the same distance as that which separates Mars from Uranus
Though the ring is much further from its star, lying at what would be about twice
the distance of Pluto from our Sun. The image was taken with a coronagraph, a
device that covers the bright star which allows faint structures to be seen.
(B. Smith, U. of Hawaii; G. Schneider, U. of Arizona; and NASA)
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(at least not at the moment)
Planets do not produce (much of )their own light
They are very far away
The stars they orbit are too bright
We have to rely on indirect
methods
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The Doppler shift
Astrometry
Planet transits
Gravitational microlensing
Direct imaging
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Planets do not orbit stars,
they orbit each other
around the common
centre of mass
This causes the
star to “wobble”
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In 1842, the Austrian physicist Christian Johann Doppler
noted that the wavelength of light, sound, or any other kind of
propagating energy measured by a moving observer will be
shifted by a factor of: v/c where v is the velocity at which the
observer is approaching or receding from the source and c is
the speed at which the wave propagates
This effect occurs for any kind of radiation not just
electromagnetic radiation
We may all be familiar with the effect with sound – a
mechanical wave transmitted through air
We note the doppler effect with sound by a change in pitch of
the sound
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Consider truck approaching with constant velocity clanging bell
once a sec.
When bell clangs first the sound reaches our ears and 1 sec later
the truck has moved forward and the sound from the bell has a
shorter distance to travel
Note the circles of expanding sound from the position of each bell
causing a compression of sound ahead of the truck and an
expansion behind due to the truck’s movement
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(a)
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v
(b)
light waves emanate from origin
If source is moving forward relative to observer (a) then the
wavefront is compressed – frequency is increased and
wavelength decreased (blue shift)
If source receding from observer (b) then wavefront is
stretched out, i.e. the frequency is decreased and wavelength
is increased (red shift)
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Measure this wobble
using spectroscopy
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Convert Doppler shift into velocity
Derive Period,
Eccentricity &
minimum Mass
Velocity measured in metres per second
Required precision is ~3 m/s or
1 part in 100,000,000!
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1952
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Conception  Reality ~ 40 years
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Use very stable spectrographs, either
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Very temperature/pressure stable; or
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With very precise reference
Several long-term programmes
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Measure the absolute position
of an object over time
Look for a regular wobble as
the star drifts through space
Very hard to do - need to
account for all other effects
Need accuracy to 1 part in
10,000,000!
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Similar to a solar eclipse
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A star dims when a planet passes in front
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Brightness change depends on the planet’s size
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Small planet  small change
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Large planet  large change
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Only a small fraction of planets transit
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Almost any telescope can be used to find them
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An amateur astronomer discovered a planet around
HD149026 with a 14” Celestron
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Space telescopes have a huge advantage
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From Einstein’s Theory
of General Relativity
The gravity of
massive objects can
act as a “lens”
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Stars can do this on a
smaller scale
A background star will
brighten when a star
passes in front of it
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More sensitive than other techniques to small-mass earth-like
planets
Most sensitive to planets that have orbits of just several AU’s
(such as for Mars or Jupiter/Saturn)
The most common stars will be the most likely candidates for
lensing
Capable of detecting multiple planets in a single light curve
Can be used to study the statistical abundance of extra solar
planets in our own Galaxy with properties akin to those in the
Solar System.
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Millions of stars must be monitored to find the few that are microlensing
at any given time
Planetary deviations in a light curve are short-lived and could be easily be
missed
Quite high probability that any planet will not be detected in a lensed
system, even if present
Deviations in microlensing light curves due to planets will not repeat (as
they are due to a chance alignment that will not recur
Planetary parameters (such as mass, orbit size, etc) depend on the
properties of the host star, which are typically unknown
The microlensing technique requires intensive use of telescope time, and
is unsuitable for continued detailed study of individual extra solar
planets
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Table 20-2, p.457
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Astronomers would, however, prefer to obtain a direct image of an
exoplanet, allowing them to better characterize the object's physical
nature. This is an exceedingly difficult task, as the planet is generally
hidden in the "glare" of its host star.
To partly overcome this problem, astronomers study very young objects.
Indeed, sub-stellar objects are much hotter and brighter when young and
therefore can be more easily detected than older objects of similar mass.
Based on this approach, it might well be that last year's detection of a
feeble speck of light next to the young brown dwarf 2M1207 by an
international team of astronomers using the ESO Very Large Telescope
(ESO PR 23/04) is the long-sought bona-fide image of an exoplanet. A
recent report based on data from the Hubble Space Telescope seems to
confirm this result. The even more recent observations made with the
Spitzer Space Telescope of the warm infrared glows of two previously
detected "hot Jupiter" planets is another interesting result in this context.
This wealth of new results, obtained in the time span of a few months,
illustrates perfectly the dynamic of this field of research.
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On several occasions during the past years, astronomical images revealed faint objects, seen near
much brighter stars. Some of these have been thought to be those of orbiting exoplanets, but after
further study, none of them could stand up to the real test. Some turned out to be faint stellar
companions, others were entirely unrelated background stars. This one may well be different.
In April of this year, the team of European and American astronomers detected a faint and very red
point of light very near (at 0.8 arcsec angular distance) a brown-dwarf object, designated
2MASSWJ1207334-393254. Also known as "2M1207", this is a "failed star", i.e. a body too small for
major nuclear fusion processes to have ignited in its interior and now producing energy by
contraction. It is a member of the TW Hydrae stellar association located at a distance of about 230
light-years. The discovery was made with the adaptive-optics supported NACO facility [3] at the 8.2m VLT Yepun telescope at the ESO Paranal Observatory (Chile).
The feeble object is more than 100 times fainter than 2M1207 and its near-infrared spectrum was
obtained with great efforts in June 2004 by NACO, at the technical limit of the powerful facility. This
spectrum shows the signatures of water molecules and confirms that the object must be comparatively
small and light.
None of the available observations contradict that it may be an exoplanet in orbit around 2M1207.
Taking into account the infrared colours and the spectral data, evolutionary model calculations point
to a 5 jupiter-mass planet in orbit around 2M1207. Still, they do not yet allow a clear-cut decision
about the real nature of this intriguing object. Thus, the astronomers refer to it as a "Giant Planet
Candidate Companion (GPCC)" [4].
Observations will now be made to ascertain whether the motion in the sky of GPCC is compatible
with that of a planet orbiting 2M1207. This should become evident within 1-2 years at the most.
ESO PR Photo 26a/04 is a composite
image of the brown dwarf object 2M1207
(centre) and the fainter object seen near it,
at an angular distance of 778 milliarcsec.
Designated "Giant Planet Candidate
Companion" by the discoverers, it may
represent the first image of an exoplanet.
Further observations, in particular of its
motion in the sky relative to 2M1207 are
needed to ascertain its true nature. The
photo is based on three near-infrared
exposures (in the H, K and L' wavebands)
with the NACO adaptive-optics facility at
the 8.2-m VLT Yepun telescope at the ESO
Paranal Observatory.
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ESO PR Photo 26b/04 shows nearinfrared H-band spectra of the
brown dwarf object 2M1207 and the
fainter "GPCC" object seen near it,
obtained with the NACO facility at
the 8.2-m VLT Yepun telescope. In
the upper part, the spectrum of
2M1207 (fully drawn blue curve) is
compared with that of another
substellar object (T513; dashed line);
in the lower, the (somewhat noisy)
spectrum of GPCC (fully drawn red
curve) is compared with two
substellar objects of different types
(2M0301 and SDSS0539). The
spectrum of GPCC is clearly very
similar to these, confirming the
substellar nature of this body. The
broad dips at the left and the right
are clear signatures of water in the
(atmospheres of the) objects.
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"If the candidate companion of 2M1207 is really a planet, this would be the first time that a gravitationally bound exoplanet has been imaged around
a star or a brown dwarf" says Benjamin Zuckerman of UCLA, a member of the team and also of NASA's Astrobiology Institute.
Using high-angular-resolution spectroscopy with the NACO facility, the team has confirmed the substellar status of this object - now referred to as
the "Giant Planet Candidate Companion (GPCC)" - by identifying broad water-band absorptions in its atmosphere, cf. PR Photo 26b/04.
The spectrum of a young and hot planet - as the GPCC may well be - will have strong similarities with an older and more massive object such as a
brown dwarf. However, when it cools down after a few tens of millions of years, such an object will show the spectral signatures of a giant gaseous
planet like those in our own solar system.
Although the spectrum of GPCC is quite "noisy" because of its faintness, the team was able to assign to it a spectral characterization that excludes a
possible contamination by extra-galactic objects or late-type cool stars with abnormal infrared excess, located beyond the brown dwarf.
After a very careful study of all options, the team found that, although this is statistically very improbable, the possibility that this object could be an
older and more massive, foreground or background, cool brown dwarf cannot be completely excluded. The related detailed analysis is available in
the resulting research paper that has been accepted for publication in the European journal Astronomy & Astrophysics (see below).
Implications
The brown dwarf 2M1207 has approximately 25 times the mass of Jupiter and is thus about 42 times lighter than the Sun. As a member of the TW
Hydrae Association, it is about eight million years old.
Because our solar system is 4,600 million years old, there is no way to directly measure how the Earth and other planets formed during the first tens
of millions of years following the formation of the Sun. But, if astronomers can study the vicinity of young stars which are now only tens of millions
of years old, then by witnessing a variety of planetary systems that are now forming, they will be able to understand much more accurately our own
distant origins.
Anne-Marie Lagrange, a member of the team from the Grenoble Observatory (France), looks towards the future: "Our discovery represents a first
step towards opening a whole new field in astrophysics: the imaging and spectroscopic study of planetary systems. Such studies will enable
astronomers to characterize the physical structure and chemical composition of giant and, eventually, terrestrial-like planets."
ESO PR Photo 10a/05 shows
the VLT NACO image, taken in
the Ks-band, of GQ Lupi. The
feeble point of light to the right
of the star is the newly found
cold companion. It is 250 times
fainter than the star itself and it
located 0.73 arcsecond west. At
the distance of GQ Lupi, this
corresponds to a distance of
roughly 100 astronomical units.
North is up and East is to the
left.
ESO PR Photo 10c/05 shows
the NACO spectrum of the
companion of GQ Lupi (thick
line, bottom) in the nearinfrared (around the Ks-band at
2.2 microns). For comparison,
the spectrum of a young M8
brown dwarf (top, in red) and
of a L2 brown dwarf (second
line, in brown) are shown. Also
presented is the spectrum
calculated using theoretical
models for an object having a
temperature of 2,000 degrees.
This theoretical spectrum
compares well with the
observed one.