Science 3210 001 : Introduction to Astronomy
Lecture 7 : Extrasolar Planets and Star
Formation
Robert Fisher
Items
 Reading/Homework set 7 has been posted to the website.
 Lunar eclipse… (mostly) clouded-out.
 March 23 is week of spring break -- no class!
 Midterm Review.
Lunar Eclipse, March 3 2007
 The lunar eclipse was almost completely clouded-out from
Chicago, but provided spectacular images from elsewhere.
Lunar Eclipse, March 3 2007
 The view from Austria was amazing :
Midterm Grading and Review
Review of Last Week
 The Outer Planets
Today -- Extrasolar Planets and Star Formation
 Planetary “Discoveries” Worthy of The Onion
 Properties
 Hot Jupiters
 Planet Migration
 Extrasolar Planetary Atmospheres
 Methods of detection and observation
 Direct Imaging
 Doppler (spectroscopic) method
 Transits (photometric) method
 Star Formation -- The Big Picture
Planets Outside Our Solar System?
 For centuries people have speculated that planets exist around
other stars.
 In 1584, Giordano Bruno wrote
 “There are countless suns and countless earths all rotating around
their sun exactly the same way as the seven planets of our system…
The countless worlds in the universe are no worse and no less
inhabited than our Earth.”
 This idea was not always as popular as it is today -- Bruno was
burned at the stake in 1600, partially because he held this view -among many other radical views for the 16th century.
Statue of Bruno in Campo de’ Fiori, Firenze
Worlds Galore?
 The basic ideas for the detection and scientific study of planets
around other stars has existed for at least fifty years, but it has
only become technologically feasible in the last few years.
 Among the key questions…
 How common are extrasolar planets?
 How common are Earthlike planets?
 Is is possible to detect the existence of life outside our solar system?
 How do these lessons inform our understanding of our own solar
system?
Early Ideas for Planet Detection
 The primary method of detection and study of extrasolar planets
used today was first suggested by the Russian-American
astronomer Otto Struve in the early 1950s.
Other Worlds Around Other Stars…
 Science fiction images of other worlds around other stars are
pervasive in our culture, yet prior to 1990, no such planets were
known.
Pulsar Planets
 The first evidence for planets outside our own solar system came
from an unusual place -- around a rapidly rotating dead star,
known as a pulsar.
 Detailed timing studies of the radio signals emitted from pulsars
can be made to extraordinary precision.
 Extraordinary precision of pulsar measurements have made
pulsars unique timeclocks which allow for high-precision physics
measurements… and also permitted the first discovery of a planet
outside the solar system.
Pulsar Structure
 The magnetic field surrounding a dead, extremely-dense, rapidlyspinning neutron star sends a beamed radio pulse at a regular
interval of time.
Crab Pulsar
 The Chandra Space Telescope produced a spectacular image of
the Crab Pulsar in X-rays, showing the high-energy gaseous
nebular powered by the emission from the pulsar.
Pulsar Signal
 100 pulses from the first pulsar to be discovered, PSR 1919+21
were used for the cover of the Joy Division album Unknown
Pleasures.
A Planet Around Pulsar PSR1829-10?
 In 1991, a British team announced the first-ever detection of the
first extrasolar planet in the prestigious journal Nature.
Oops … Not!
 Less than a year later, Lyne and colleagues retracted their
announcement.
Pulsar PSR 1257+12
 Given the controversy surrounding PSR 1829-10, it is incredibly
remarkable that within a year, the first actual discovery of an
extrasolar planet was announced -- around another pulsar!
 In 1992, while studying an isolated millisecond pulsar PSR
1257+12, Alex Wolzczan of Penn State University made a
stunning discovery of planets surrounding the pulsar.
 Eventually three planets surrounding PSR 1257+12 were
discovered and confirmed, including a possible minor body -- an
asteroid, comet, or Kuiper belt-like object.
Artist’s Rendition of the PSR 1257+12 System
PSR 1257+12 Planetary System
 Three planets around PSR 1257+12 have been confirmed :
 PSR B1257+12A
 Orbit at .19 AU, Period 25 Days, Mass about Twice Earth’s Moon
 PSR B1257+12B
 Orbit at .36 AU, Period 66 Days, Mass about that of Earth
 PSR B1257+12C
 Orbit at .46 AU, Period 98 Days, Mass about 4 times that of Earth
 In addition, a tiny body (asteroid? comet? Kuiper-belt-like object?)
may exist at about 2.6 AU, with a mass less than Pluto
Origin of PSR 1257+12 System?
 The unusual nature of the pulsar planetary system, which must
have formed after the explosion of the central star in a
supernova, has fueled intense speculation.
 One possibility is that these planets are left over from a solar
system that existed before the star exploded.
 More likely, however, is that these planets formed after the
supernova, from a nebula of material blown off from a companion
star, which then reformed into a new solar system.
 Because pulsars systems are relatively rare, this system is an
exceptional case which may be dissimilar from solar systems
around sunlike stars.
Direct Detection
 The most obvious method of detecting a planet around another
star is to directly image the planet, just as we image stars.
 Unlike stars, planets are both much smaller and observable only
in reflected light, and so are far fainter. The glare of their parent
star makes it incredibly difficult to see them.
Question
 If you had to photograph a planet next to a star, and the star were
literally billions of times dimmer than the star, how would you go
about doing it?
Coronagraph
 An astronomical instrument known as a coronagraph is designed
to artificially block out the light from the sun in much the same
way that the moon does during a total solar eclipse.
 Using this instrument it is possible to see much fainter details
around the sun. For instance, the detection of a comet near the
sun --
Direct Detections
 To date, however, direct detection methods have been almost
entirely unsuccessful.
 The only direct detection to date is the planet orbit brown dwarf
2M1207 and its companion planet 2M1207b.
2M1207
2M1207b
51 Peg -- The First Extrasolar Planet Around a
Sunlike Star
 The next major detection technique is the Doppler, or spectroscopic
technique.
 The first planetary system discovered with the Doppler technique was 51
Pegasus or 51 Peg.
 This was the first planet detected around a normal star like our sun.
 The system was first announced in the October 6, 1995 isue of Nature by
Swiss astronomers Michael Mayor and Didier Queloz of the University of
Geneva using the ELODIE spectrograph at an observatory in Provence,
France.
Echelle Spectrograph
 The modern spectrographic instrument used in planet searches is
the echelle spectrograph, which is made from a special kind of
diffraction grating called an echelle grating.
 These spectrographs are capable of extraordinarily high
resolution, equivalent to tens of thousands of pixels across the
spectrum -- essential for measuring the small motions of the
central star in a planetary search.
ELODIE echelle
spectrograph
ELODIE Sample Spectrum
 The spectrum is broken up into a number of bands and imaged
with a CCD detector, which is the same kind of detector in a
digital camera.
The Discovery of 51 Peg
 The discovery of 51 Peg was confirmed immediately by Geoff
Marcy and Paul Butler (both then at UCSF) on October 12, 1995.
 Marcy and Butler and their team went on to discover the vast
majority of the extrasolar planets known to date (70 of first 100!).
Doppler Shift
 We are all familiar with the shift in pitch of a moving source of
sound either towards or away from us.
 The shift is due to a “bunching up” of the emitted soundwaves
when moving towards us, and a “stretching out” of soundwaves
when moving away from us
Optical Spectrum Shifts
 Visible light also undergoes a Doppler shift when the source is
moving.
 When the source is moving away from us, we see the spectrum
shift “to the red” -- a redshift. Similarly, when moving towards us,
we see the spectrum shift “to the blue” -- a blueshift.
 The shift is only detectable when the spectrum has definite lines.
Doppler Planet Detection
 In a stellar system with a planet, both the star and the planet
revolve around the center-of-mass.
 Even if the planet is not directly visible, the star can be observed
to rotate around the center-of-mass of the combined system.
Doppler Detections
 The motion of the central star can be detected by a periodic
repeating Doppler shift in its spectrum -- through red, blue and
back again.
 Measurements reveal the period of the planet as well as the mass
of the planet.
Curve for 51 Peg
 High-precision measurements of the spectrum of 51 Peg revealed
that the star was oscillating back and forth, with a period of 4
days.
51b Peg -- The Planetary Companion to 51 Peg
 The properties of the planetary companion to 51 Peg turned out
to be highly unusual.
 It orbits 51 Peg with a period of 4.2 days, placing it at just .05 AU
away -- far closer than even Mercury is in our solar system (.40
AU).
 Much stranger still, its mass is much larger than any of the inner
planets in our solar system, with nearly half the mass of Jupiter -qualifying it as a giant planet.
 Its extreme proximity to 51 Peg implies a surface temperature of
nearly 1200 (!!) degrees Celsius.
Hot Jupiters
 Since the discovery of 51 Peg, however, many more such “hot
Jupiters” have been detected -- over 200 planets have been
detected to date.
 Some extrasolar planets the size of Saturn and even Neptune
have been detected, though none are terrestrial planets like the
Earth.
 While the surveys are biased (they are currently unable to detect
Earthlike planets, even if they do exist), this implies that our own
solar system may be atypical in some ways.
 The million dollar question was -- how could such a strange hot
Jupiter system could form?
Giant Planet Shipping Express -- Disk Gaps and
Disk Migration
 Fundamental work done on protostellar accretion disks by Doug
Lin of University of California Santa Cruz and colleagues
demonstrated that as it is forming, a protoplanet exchanges mass
and angular momentum with the protostellar accretion disk.
 Giant planets are able to open up a gap in the disk, at which point
they are expected to migrate inwards.
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Artist’s Conception of 51 Peg
 The proximity of “hot Jupiter”-like planets like 51b Peg to their
parent star suggests that they are slowly leaking their
atmospheres through evaporation.
51 Peg in Popular Culture
 The definite sign that a discovery has hit the big time…
Transit Detections
 Both Mercury and Venus occasionally pass directly between the
line-of-sight joining the Sun and the Earth, resulting in a
spectacular “transit” of the planetary disk across the solar
surface.
2004 Transit of Venus
Transit Detections
 In our own solar system, transits were very important historically
in establishing the actual physical distance from the Sun to the
Earth.
Transit of Mercury in 2003
The Hypothetical Planet Vulcan
 Even more interestingly, a number of amateur and professional
astronomers have observed unexplained transits, beginning with
the physician and amateur astronomer Edmond Lescarbault in
1859, who wrote Urbain Leverrier of his discovery of a “intraMercurial” planet.
 Leverrier named the hypothetical planet Vulcan, and predicted an
orbit and future transits for it, though the “planet” itself was never
seen.
 Interest in the hypothetical planet Vulcan peaked by around 1900,
when Mercury was shown to deviate substantially from its
predicted orbit.
The Hypothetical Planet Vulcan
 In 1915, Albert Einstein published his General Theory of
Relativity, which explained the deviations in Mercury’s orbit.
 Since then, the general consensus is that the transits of bodies
other than Mercury and Venus are likely a population of asteroids
interior to the orbit of the Earth.
 Work continues to this day in detecting a possible family of
vulcanoid asteroids, which probably do exist, but are incredibly
difficult to observe if they do exist.
Vulcan
 Interest in the planet Vulcan continued in science fiction well after
1915…
Extrasolar Transit
 If a planet transits over the disk of the star, by carefully measuring
the amount of light emitted from a star over time, one can see a
dip in the light emitted, even if the planet itself is too faint to see
directly.
HD209458 Transit
 The first detection of an extrasolar planet transit was made in
1999 in the HD209458 system by David Charboneau, then a
graduate student at Harvard.
 While an amazing achievement, it was made by observing a
system already known to have an extrasolar planet from a
Doppler survey.
See an Extrasolar Planet in Your Backyard!
 The shift of about 1% in the brightness of the central star is
observable even in larger amateur-sized telescopes.
 The first amateur detection was made less than a year after
Charboneau’s announcement at an observatory in Finland.
Upsilon Andromeda -- The First Extrasolar
System of Planets Around a Sunlike Star
 In 1999, three extrasolar planets were detected around Upsilon
Andromeda A -- itself orbiting its companion star Upsilon
Andromeda B, a much less massive red dwarf at 750 AU
separation.
 This was the first detection of an extrasolar system of planets
around a sunlike star, as well as the first system around a binary
star system.
 Upsilon Andromeda b -- Mass .7 Jupiter, 4.6 Day Period, .06 AU
 Upsilon Andromeda c -- Mass 2 Jupiter, 240 Day Period, .83 AU
 Upsilon Andromeda d -- Mass 3.9 Jupiter, 1290 Day Period, 2.5 AU
Upsilon Andromeda
 A comparison of the Upsilon Andromeda system with our Solar
System.
Extrasolar Planetary Atmosphere Detection
 First detection of an atmosphere around an extrasolar planet was
made by David Charboneau, using Hubble Space Telescope
observations of the transit system HD209458 we discussed
earlier.
 By carefully inspecting the spectrum of the star, Charboneau and
colleagues were able to detect sodium absorption lines during
transits of HD209458b -- a clear indication that they were
detecting the atmosphere surrounding an extrasolar planet for the
first time.
 Later observations of HD209458b also revealed absorption lines
of carbon and oxygen, which are thought to have originated from
the strong wind being driven from deep interior to HD209458b by
its star.
Extrasolar Planet Atmospheres
 These observations suggest that extrasolar planets like
HD209458b are losing their atmospheres over time.
 Some extrasolar planets may have such a high evaporation rate
they eventually lose their giant atmospheres altogether -- leaving
behind a massive rocky core.
Spitzer Space Telescope
 NASA had a grand vision of four “Great Observatories” to be
launched into space, each covering a different portion of the
spectrum.
 In December 2003, Spitzer Space Telescope, covering the
infrared portion of the spectrum became fully operational.
Lyman Spitzer, Jr. (1914 - 1997)
 The Spitzer space telescope was the last of the four Great
Observatories.
 It was appropriately named after Lyman Spitzer, Jr. -- the man
who first proposed the concept of space telescopes in the 1940s.
 Spitzer also contributed fundamental advances to our knowledge
of the interstellar medium and plasma physics.
Spectrum of an Extrasolar Planet
 Just last year, the first-ever direct measurement of the spectrum
of an extrasolar planet was announced!
 The Spitzer Space Telescope was used to measure the
spectrum of the now-familiar transiting planet HD204958b.
Looking to the Future
 What stops the inward migration of giant planets, if anything?
 If nothing stops the inward migration, then is the formation of a
solar system analogous to a game of musical chairs?
 Will we be able to infer the existence of life on other worlds?
 What will direct images of extrasolar planets reveal?
Today’s Weather Forecast for HD204958b -Hot and Dry
Formation of Stars and Planetary Systems
 Astronomers have long known about cold, dark regions on the
sky which obscure starlight.
 These regions consist of very cold, dense regions of dusty
molecular gas which will in most cases form stars.
M51 “Whirlpool Galaxy” in Optical
Collision of M51 with NGC 5195
A. Toomre, 1978
Time
Atomic and Molecular Gas
Atomic or Ionized
Single Atom (eg, H, C, O) or Ion (H II, O VI)
Trace warm (~ 1000 - 104 K) gas
in visible range of spectrum
Molecular
Two or more Atoms (eg, CO, NH3)
Trace cold (~ 10 - 100 K) gas
in radio - infrared range of spectrum
Whirlpool Galaxy M51 in Optical and Submm
HST + OVRO (Scoville et al, 2004)
Overlay of Optical and CO Emission in M51
Traffic Jam
Traffic Jam (or “Spiral Density Wave”) Theory of
Formation of Giant Molecular Clouds
Molecular -->
Gravitational Potential
<-- Atomic
Lin, C.C. & Shu, F. (1962)
Distance across spiral arm
Next Week -- Star Formation
 How does a tenuous cloud of cold gas collapse under its own
weight and form a star which ignites at millions of degrees?
 How do binary and multiple star systems form?
 How did our solar system originate?