Transcript Formation of the Solar System

Formation of the Solar System
and Extrasolar Planets
1
How did the Solar System Form?
We weren't there. We need a good theory. Check it against other
forming solar systems. What must it explain?
- Solar system is very flat.
- Almost all moons and planets orbit and spin in the same direction.
Orbits nearly circular.
- Planets are isolated in space.
- Terrestrial - Jovian distinction (esp. mass, density, composition).
- Leftover junk and its basic properties (comets, asteroids, TNOs).
Not the details and oddities – such as Venus’ and Uranus’ retrograde
2
spin.
General theory – the Nebular Model
• Idea goes back to Descartes (1664), then Kant and
LaPlace in late 18th century.
• Interstellar cloud of dust and gas
• Slow rotation, original quasi-spherical shape
• Gravitational collapse, dissipation into a plane due
to conservation of angular momentum
• Differing temperature environments
3
A cold, relatively dense cloud of interstellar molecular gas
a few pc,
or about 10,000
times bigger than
Solar System
The associated dust blocks starlight. Composition mostly H, He.
Collisions cause rotational transitions in molecules – emission lines at
mm wavelengths. From Doppler shifts, clouds rotate at a few km/s.
Some clumps within clouds collapse under their own weight to
form stars or clusters of stars. Clumps spin at about 1 km/s.
4
So how can you get a flat, rapidly rotating Solar System?
Conservation of Angular Momentum
For an isolated
spinning object:
L=IΩ
where L is angular momentum, I is moment of inertia, and Ω is
angular rotation rate = 2πν = 2π/P.
For uniform density sphere,
In general
2
I  MR 2
5
L  MR 
2
L is conserved if object contracts. R decreases, so Ω increases.
(For orbiting object, L  MR 2 is conserved.)
5
Clump within cloud starts collapsing under its own gravity.
It’s pressure cannot support it.
It spins more rapidly as it collapses (conservation of
angular momentum). Gravity strong enough at center –
forming star is spherical. But rest flattens into a disk.
We observe these now in star
forming regions, using the
Hubble.
6
How does the nebular model explain
planets and “debris”?
• Solar Nebula composed of 71% H (by mass), 27% He,
traces of heavier elements in gas, and dust grains (only
about 2% of mass).
• After collapse, now so dense that solid material can grow
by collisions and accretion. In warm inner nebula,
growth of dust grains.
• Further from Sun, ice mantles on dust grains form
readily. Lots of gas to make ice from => much more
solid material. But most matter still gas.
7
Condensation temperature
Temp (K)
Elements
Condensate
>2000 K
All elements
gaseous
1600 K
Al, Ti, Ca
Mineral oxides
1400 K
Fe, Ni
Metallic grains
1300 K
Si
Silicate grains
300 K
C
Carbonaceous
grains
300-100 K
H, N
Ices (H2O, CO2,
NH3, CH4)
8
The “snow line"
• Rock and metals forms where T<1300 K
• Ices form where T<170 K
• Inner Solar System is too hot for ices
• In the Outer Solar System ices form beyond the
"snow line"
9
Temperature distribution in Solar Nebula at time of
formation of the planets
Snow line
10
Grains => planetesimals => protoplanets
The planets formed by the collision and sticking of solid
particles, leading to km-scale planetesimals (few 106 yrs).
Collisions of planetesimals enhanced by gravity, growth of
proto-planets. Larger ones grew faster – end result is a few
large ones.
11
Observations of disk around
young star with Hubble and
ALMA, showing ring
structure. Presumably
unseen planets sweeping
out gaps.
12
Terrestrial planets
• Only rocky planetesimals inside the frost line
• Energy of collisions heats growing protoplanets. Along
with heat from radioactivity, they become molten
• Hotter close to the Sun, and lower gravity protoplanets
=> they cannot capture H, He gas and retain thick
atmosphere.
• Solar wind also dispersing nebula from the inside,
removing H & He
=> Rocky terrestrial planets with few ices
13
Jovian planets
• Addition of ices increases masses of grains - large protoplanets of rock and ice result (few to 15 MEarth)
• Larger masses & colder temps: can accrete and retain H &
He gas from the solar nebula
Form large Jovian planets with massive cores of rock and
ice and heavy H, He atmospheres
• Alternative: formed directly and rapidly by gravitational
collapse in disk. Denser material sunk to center. Should
take 100’s to 1000’s of years only!
14
Moons & Asteroids
Some gas attracted to proto-Jovians formed disks:
• “Mini solar nebula” around Jovians
• Rocky/icy moons form in these disks (later, more detail
on four Galilean moons of Jupiter as mini “solar” system)
• Later moons added by asteroid/comet capture
15
Planetary Migration
Essentially friction with remaining gas and dust may have
caused Jovian planets to migrate. Gravitational
interactions with each other and smaller objects may have
led to exchanges of energy, causing inward or outward migration.
Early Migration
Thought that in first few 100,000 years, Jupiter migrated inward
to 1.5 AU, scattering planetesimals that would have made a larger
Mars. Then migrated outward, deflecting planetesimals inward
to form Asteroid Belt. Explains why some asteroids are icy.
16
Planetary Migration
Late Migration
All Jovians thought to have formed within 20 AU, in order to
have enough accreting material. Generally migrated outward
due to gravitational interactions with planetesimals over few
108 yrs. Possible that Neptune initially closer to Sun than
Uranus, and they switched via an interaction.
Deflection of planetesimals by Jupiter created the large Oort
cloud.
Neptune’s outward migration flung out planetesimals to
create the TNOs. Also some flung inward to explain “Late
Heavy Bombardment” of Terrestrial planets and Moon.
“Nice model”
17
Icy bodies and comets
• Leftover bodies from planet building in Jovian planet
zone. Hence more icy than asteroids.
• Oort Cloud and TNOs are sources of comets. For
example, a TNO may encounter Neptune and get sent
into inner Solar System, where they start to
evaporate, grow a tail, and appear as comets.
18
What evidence do we have for
the Nebular model?
• Theory is reasonable, but needs to make
predictions to compare with observations. Saw
some evidence from other forming systems.
• But what about other formed planetary
systems?
19
Extrasolar Planets
• Test solar system formation process
• Possibility of life on other planets
•
•
•
•
•
Techniques:
Direct detection (images)
Transit of star by planet
Detection of star’s wobble by spectroscopy
Detection of star’s wobble by imaging
Microlensing
About 2000 found by Fall 2015
See exoplanet.eu
20
Detecting a star’s wobble
Idea: a planet and its star both orbit around their common
center of mass, staying on opposite sides of this point.
21
The massive star is closer to center of mass, and moves
more slowly than the planet, but it does move!
Example: ignore planets other than Jupiter. Then Sun
and Jupiter orbit their common center of mass every
11.86 years. Jupiter’s orbit has semi-major axis of 7.78 x
108 km, while Sun’s semi-major axis is 742,000 km.
(Compare to Sun’s radius of 696,000 km).
22
A wobbling star might be seen by careful observations of
its position, called “astrometry”:
No confirmed
detections this way
23
More successful method: Use the Doppler shift of star’s
spectral lines due to its radial (= back and forth) motion:
Over 600 found
this way by Fall 2015
24
Calculate orbital speed of Sun, assuming Jupiter is only
planet:
Moves in nearly circular orbit of radius 742,000 km
2r 2 (742,000 km)
V

 12.5 m/s
P
11.86 years
How much Doppler shift? Consider H-alpha absorption
line, at rest wavelength 656 nm:
V
12.5 m/s
5
  0 
656 nm  2.7 10 nm
8
c
3 10 m/s
This is tiny!
25
51 Pegasi
• Mayor & Queloz
at Geneva
Observatory saw
observed
wobble in star 51
Peg in 1995
• Sun-like star
~40 ly distance
• Wobble was 53 m/s, period 4.15 days
• Implied a planet with 0.5 Jupiter mass orbiting at
0.05 AU!
• First planet found around sun-like star
26
Selection effects
• Doppler wobble biased towards massive planets close
to their star (leads to larger velocities and shorter
periods). But now detecting Earth-mass planets!
• Inclination of binary orbit unknown (unless transits
observed). More likely to be close to edge-on for
detection. If not, wobble is larger than measured and
so is planet.
27
28
Second technique: detect eclipse of planet. Over 1200 found.
29
Kepler mission. Launched March 2009. Several years of data – still
being analyzed. Detected eclipses. Yields only radii, not masses.
30
Third technique: direct imaging of planet.
59 found by July 2015.
HR 8799
31
Fourth technique: Gravitational microlensing
• If two stars line up, one near and one far, the light
from the background star will bend around the
foreground star (due to gravity)
• A planet around the foreground star will cause an
intense amplification if passing close to the line of
sight
• 37 thus detected as of July 2015
32
33
Most planets found are more massive than Jupiter – many are closer to
star than 1 AU (“hot Jupiters”)! Did they form there or migrate there?
10 Jupiter
Masses
1 Jupiter
Mass
34
Earth
Jupiter
Most planets have eccentric orbits. Circular might be better for life!
35
0.01
(Earth)
0.1
1
Habitable planets
Habitable Exoplanets Catalog
36
Habitable planets
From statistics of detections, we estimate about 20% of
Sun-like stars have habitable planet near Earth-size (1-2
Earth radii)!
Most are probably around the most common kind of
star, red dwarf (M class) stars.
Since red dwarfs are dim, planets have small orbits to
be in Habitable Zone. Leads to strong tidal forces.
Planets probably “tidally locked” to their stars like our
Moon. Could make life less likely.
37
Next NASA Mission – TESS
(Transiting Exoplanet Survey Satellite)
•
•
•
•
Like Kepler, but all-sky
3 year mission
Launch ready Aug 2017
>500,000 of the nearest and brightest stars, making followup ground-based and JWST observations easier than Kepler
• Should find >3000 exoplanets, include 500 Earth-size or
“Super-Earths”
38
39
40
For a little fun, see:
http://tauceti.sfsu.edu/~chris/SIM/
http://vo.obspm.fr/exoplanetes/encyclo/catalog.php
41
Problem 5.31
• The bright star Sirius in the constellation of
Canis Major has a radius of 1.67 Rsolar, and
a luminosity of 25 Lsolar.
• Use this information to calculate the energy
flux at the surface of Sirius.
• Use your answer in part a) to calculate the
surface temperature of Sirius.
42
Atmosphere composition
• 78% N2, 21% O2, 1% everything else
• Initial atmosphere had more H, CH4, NH3
• Composition depends on
– Original formation (what gases were trapped)
– Chemical processes (including life)
– Escape speed
43
Molecular weights
Hydrogen
Helium4
Methane
Ammonia
Water
Neon
Nitrogen
Oxygen
Argon
CO2
2
16
17
18
20
28
32
40
44
Air: 29 (Ar, CO2, Ne, He and other
rare gases).
Early atmosphere: CO2, steam, NH3
and CH4 (volcanoes). No oxygen.
Steam condensed to water => seas
Photosynthesis: CO2 and H2O to
O2. NH3 and CH4 reacted with O2.
44
45
Problem 7.24
a) Find the mass of a hypothetical spherical
asteroid 2 km in diameter and composed of rock
with average density 2500 kg m-3.
b) Find the speed required to escape from the
surface of this asteroid.
c) A typical jogging speed is 3 m/s. What would
happen to an astronaut who decided to go for a
jog on this asteroid?
46
47