Planetary Systems:
Ours and Others
Inventory of the Solar System
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Sun: 99.9% of total mass. = 330,000 M_earth
Planets: Jupiter = 318 M_earth
Minor Planets (asteroids, ice dwarfs)
Satellites
Comets
Small stuff (< 100-m): meteoroids, gas, dust
Inventory of the Solar System
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Sun: 99.9% of total mass. = 330,000 M_earth
Planets: Jupiter = 318 M_earth
Minor Planets (asteroids, ice dwarfs)
Satellites
Comets
Small stuff (< 100-m): meteoroids, gas, dust
Except Sun, none are self-luminous (at visible wavelengths)
Except Sun, Moon, & some comets, none are resolvable
without telescopes
SS Organization & Orbits
Systematics of Planetary Orbits
• All lie close to ecliptic plane
• All nearly circular
• Direction of revolution same for all (counter-clockwise
from above Earth N pole)
• Systematically increasing spacing (“Bode’s Law”)
Systematics of Planetary Orbits
• All lie close to ecliptic plane
• All nearly circular
• Direction of revolution same for all (counter-clockwise
from above Earth N pole)
• Systematically increasing spacing (“Bode’s Law”)
• Because none of these features is required by
Newton’s laws,
These are clues to planet formation
Another Clue:
Segregation of physical properties of planets.
There are TWO primary kinds of planets:
– Terrestrial (Earth-like, near Sun)
– Jovian (Jupiter-like, distant from Sun)
Major Solar System Bodies (Sizes to Scale)
Terrestrial
Jovian
Earth and Jovian Planets to Scale
Uranus
Neptune
Earth
Jupiter
Saturn
Terrestrial Planets to Scale
Venus
Mercury
Mars
Earth
Thin atmosphere
Solid surface
Mars
Jupiter and Satellite Io
Jupiter’s “Red Spot”
compared to the Earth
Cloud layers in thick atmosphere
No solid surface
Jupiter’s “Red Spot”
compared to the Earth
"Red Spot"
A permanent anti-cyclonic
weather system
Cloud layers in thick atmosphere
No solid surface
Jupiter’s “Red Spot”
compared to the Earth
Saturn
Only planet with conspicuous rings, but all Jovians have them
Terrestrial Planet Cross-Sections
Jovian Planet Cross-Sections
Comparison of Properties
“Planets” vs. “Satellites”
“Planets” vs. “Satellites”
S
S
S
S
P
S
S
S
P
S
"Interplanetary" material
"Interplanetary" material
"Interplanetary" material
("Ice Dwarfs")
Comet Hale-Bopp (1997)
Origin of Planetary Systems
“Tidal” Theory: Near-collision between stars
expels stellar material
“Nebular Theory”: Planets are a
by-product of normal star formation
Implications of Formation Theories
• “Tidal” Theory: planets are RARE
• “Nebular” Theory: planets are COMMON
Implications of Formation Theories
• “Tidal” Theory: planets are RARE
• “Nebular” Theory: planets are COMMON
Implications of Formation Theories
• “Tidal” Theory: planets are RARE
• “Nebular” Theory: planets are COMMON
 Our galaxy contains ~100 billion planets
Contemplate any picture of the night sky
and realize that there are at least as many planets
in that view as there are stars
Star Birth
• Stars are continually forming from gas and "dust"
that fills the interstellar space in our Galaxy (at low
density)
• About 2% of the mass of our Galaxy now resides
in this "interstellar medium"
• Star formation is driven by the self-gravity of
interstellar gas clouds, which ultimately causes
collapse of the clouds.
Puzzlah #33
Stars condense from the very dilute "interstellar" gas, which has a
number density of about 1 atom per cubic centimeter. The Sun, a
typical star, has an average mass density of about 1 gram per
cubic centimeter. How large a volume of the interstellar medium
would have to be compressed to yield a single cubic centimeter of
solar material? Give your answer as the side of the cube
containing the material.
(A) 10 centimeters
(B) 1000 centimeters
(C) 100,000 centimeters (one kilometer)
(D) 100 million centimeters (1000 kilometers)
[Hint: use Avogadro's number]
Puzzlah #33
Stars condense from the very dilute "interstellar" gas, which has a
number density of about 1 atom per cubic centimeter. The Sun, a
typical star, has an average mass density of about 1 gram per
cubic centimeter. How large a volume of the interstellar medium
would have to be compressed to yield a single cubic centimeter of
solar material? Give your answer as the side of the cube
containing the material.
(A) 10 centimeters
(B) 1000 centimeters
(C) 100,000 centimeters (one kilometer)
(D) 100 million centimeters (1000 kilometers)
[Hint: use Avogadro's number]
Raw Material for Star Birth
• Interstellar gas: 74% H, 24% He; 2% other elements
• "Dust": tiny solid grains, smoke-like. Act to absorb light
and refrigerate gas clouds
– Dust concentrations are visible as dark clouds
(1/250,000 inch)
Interstellar “dust grains”: tiny, smoke-like particles which
absorb light and act to shield/cool gas.
Typical gas-to-dust ratio is 100:1 by mass.
"Whirlpool Galaxy", M51
"Whirlpool Galaxy", M51
(Dark) dust lanes
(Red) glowing gas
Dust clouds in our Galaxy are seen as “Dark Nebulae” when projected
against the bright star background of the Milky Way
“Pillars of Creation”:
elongated cold, dusty
regions surround by
hot gas
Dense knots possibly
contain protostars
Newborn stars
dissipating natal cloud
Orion star-birth
zone
The Orion Nebula: Gas Cloud Surrounding
Massive, Young Stars
Puzzlah # 34
What causes the reddish-pink tone in the color images
of this and other star-forming regions?
(A) Hotter materials are red-colored
(B) Prismatic effects in Earth's atmosphere
(C) Hydrogen gas
(D) Optical illusion caused by distance of objects shown
(E) Reflection from dust grains
Puzzlah # 34
What causes the reddish-pink tone in the color images
of this and other star-forming regions?
(A) Hotter materials are red-colored
(B) Prismatic effects in Earth's atmosphere
(C) Hydrogen gas
(D) Optical illusion caused by distance of objects shown
(E) Reflection from dust grains
Star formation begins with the collapse of a
cold, dense molecular cloud
Causes flattening of cloud
A “solar nebula” or “protoplanetary disk” is
a natural by-product of star formation
Note: scale several 1000x smaller than preceding slide
ProtoSun and Solar Nebula
Core collapses to form Sun
Residual disk (only ~ 1/1000 of mass
of Sun) accretes to form planets.
Inside the Solar Nebula
Continual accretion of solid particles
to form proto-planets
Proto-Sun heats inner nebula;
nebula is cooler at larger distances
Composition of solids changes with
distance from protosun
Warmer inner disk
Cooler outer disk
Origin of segregated planetary types
Planetary accretion is
a violent process
“Grinding Down”: vast amounts of collision debris
are still present in solar system
Bombardment melts terrestrial protoplanets
The nebular model explains almost all of the
systematics of the solar system:
Common orbital planes and direction of motion
.
Near-circular orbits
Segregation of physical properties
Can we find protoplanetary disks
around other stars? Yes.
Orion Nebula
HST discovery of
“Proplyds” in Orion
Nebula
HST discovery of
“Proplyds” in Orion
Nebula
Protoplanetary disk
Protoplanetary disks
(Eroding)
Polar jets are a
common signature
of protoplanetary
systems
Puzzlah #35
A new planet the size of the Earth’s Moon is suddenly discovered
orbiting between Jupiter and Saturn. What materials are likely to be
most common in the planet? [Hint: base your answer on the part of
the protoplanetary disk where the planet formed.]
(A) Metals
(B) Ices
(C) Rocky materials (like the Earth's Moon)
Puzzlah #35
A new planet the size of the Earth’s Moon is suddenly discovered
orbiting between Jupiter and Saturn. What materials are likely to be
most common in the planet? [Hint: base your answer on the part of
the protoplanetary disk where the planet formed.]
(A) Metals
(B) Ices
(C) Rocky materials (like the Earth's Moon)
Detecting Other
Planetary Systems
First discovery of planets around other
normal stars (“exoplanets”):
October 1995
Exoplanet Detection Methods?
Exoplanet Detection Methods
Direct imaging?
Difficult/impossible with present technology:
planets are too faint and too near their
parent stars.
Exoplanet Detection Methods
Radial velocity (“Doppler”) method:
Detect motion of star induced by planet's gravity
Limitations: difficult; biased toward large planets
in small orbits
Transit Method:
Detect eclipses of star by planet
Limitations: Earth must lie in orbital plane of planet;
must go to space for detection of small planets
Exoplanet Detection Methods
Radial velocity (“Doppler”) method:
Detect motion of star induced by planet's gravity
Limitations: difficult; biased toward large planets
in small orbits
Advantage: get estimate of planet mass
Transit Method:
Detect eclipses of star by planet
Limitations: Earth must lie in orbital plane of planet;
must go to space for detection of small planets
Advantage: get estimate of planet radius
Radial Velocity Method:
Mutual gravitational attraction causes motion
of a star and its planets about their common
“center of mass”
Motion of the “center of mass” of
our solar system
“The Doppler Effect”
• Change in WAVELENGTH with MOTION of source
toward or away from observer
• Allows spectroscopic detection of “radial velocity”
• Doppler Effect Java demo
• Doppler Effect in Sound Waves (video)
• Doppler Effect in Sound Waves (video 2)
Doppler shift due to motion of
star about the center of mass
Blue when
approaching
Red when receding
Velocity of star
induced by planet
First detection of exoplanet around normal star
Early Detections: “Hot Jupiters”
(Jupiter mass planets in small orbits)
Early Detections: “Hot Jupiters”
(Jupiter mass planets in small orbits)
Surprise! Jupiter-mass planets
nearer parent star than Mercury!
Completely unexpected!
Is this contrary to nebular model?
Early Detections: “Hot Jupiters”
(Jupiter mass planets in small orbits)
Surprise! Jupiter-mass planets
nearer parent star than Mercury!
Completely unexpected!
Is this contrary to nebular model?
No. Now believe hot Jupiters form
at large distances but migrate
inward through gravitational interactions between planets.
Exoplanet "Transit" Eclipse
Light curve of a “transit” eclipse
From space, can detect transits of Earth-sized planets
Kepler Mission Launch
(March 2009)
Transit-detection mission
Kepler Mission
On Orbit
Kepler continuously monitors
brightnesses of 150,000 stars.
Transit Light Curve (Kepler 10-b)
Darkening is only 3 parts in 10,000
Earth-size planet: 1/10,000
Jupiter-size planet: 1/100
Current Statistics (3/2014)
1692 planets
1024 planetary systems
441 multiple planet systems
Confirmed Planets
Hot Jupiters
Super Earths
AlphaCenBb
Earth
Kepler transiting sources: many near
Earth size or smaller.
Earth
Earth-size Kepler planets compared
“Habitable Zone”: region where planet surface
temperature permits liquid water
“Habitable Zone”: region where planet surface
temperature permits liquid water
Habitable Zone: some Super Earths
Consensus: Discovery of
Earth mass/size planets in
Habitable Zone is inevitable
: Two super-Earths in habitable zone
“Habitable Zone”: region where planet surface
temperature permits liquid water
Sun
"M dwarf" primary star
Kepler 186 system: 5 planets in orbit around an M1
dwarf;
K186-f is an Earth-sized planet in the habitable zone
Announced 17 April 2014
K186-f, artist's concept
More Exoplanet Art
Melting Exoplanet
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