Phil Armitage, University of Colorado

lecture01web.pptPlanet FormationPlanet Formation Phil Armitage, University of Colorado
XIII Ciclo de Cursos Especiais
How do planets form?
Very detailed: only one system
Many systems: limited individual information
Historically: observation that the planets orbit the Sun in (approximately) the same orbital plane
Nebula Hypothesis: planets formed from a rotating disk of gas and dust orbiting the proto-Sun (Kant, Laplace in the 18th century)
…fundamentally correct concept
Quantitatively: Terrestrial planet formation: Victor Safronov - “Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets” (1969)
Giant planet formation: Hiroshi Mizuno (~1980) building on many earlier ideas
New developments
2. Discovery of the Solar System’s Kuiper Belt
3. Discovery of extrasolar planetary systems
…partially confirm earlier ideas, but also point to the unexpected importance of planetary system evolution and reveal a great diversity of planetary systems
Outline
1. Observations of planetary systems 2. Protoplanetary disks 3. Formation of planetesimals (km-scale bodies) 4. Formation of terrestrial and giant planets 5. Evolution and stability of planetary systems
Today: mostly introductory Generally, mix of basic ideas + open questions
Feel free to ask questions at any time!
Solar System Observations
Terrestrial planets
Low mass (up to 1 Earth mass = 6 x 1027 g), mostly rocky objects.
Found in the inner Solar System (Mercury 0.39 AU, Mars 1.52 AU)
Giant planets - found only in the outer Solar System
Gas giants: primarily gaseous objects but not made of the same composition as the Sun… enriched in heavy elements
Jupiter: ~10-3 Msun, ~300 MEarth
Ice giants: ~10 Mearth of rock and ice, plus large (several Earth masses) contributions from gas
2 (or maybe 3) classes of planet that need to be explained…
Tristan Guillot
Comparison of the multipoles of gravity field (J2 - J6) with internal structure models
High density EOS for H / He (c.f. Militzer & Hubbard 2008 work)
Could do much better on the observations: JUNO mission
Integrated properties
Mass: planets are ~0.2% of the mass of the Sun
Sun is ~2% “metals” (not H / He) - most of the heavier elements are also in the Sun… planet formation need not be 100% efficient
Angular momentum:
Solar rotation
-1
..segregation of mass from angular momentum during the formation of the Solar System
Minimum mass Solar Nebula
How much mass was needed to form the planets?
1. Take mass of heavy elements in each planet 2. Augment the mass with enough H / He to restore
Solar composition 3. Spread the mass into an annulus around each orbit
Jupiter’s orbit
spread Jupiter’s augmented mass (~3 x real mass) across this annulus to yield a surface density
Minimum mass Solar Nebula (Weidenschilling 1977)
!
1 AU
Comparable to the masses of disks measured around other stars
BUT… this is at best a lower limit - could have been more gas / could have been a different radial profile…
Minor bodies in the Solar System Asteroids, Kuiper Belt objects, comets…
Dynamical clues as to the early evolution of the Solar System
Most stable orbits in the Solar System are populated with minor bodies
a
!
= i
j …with i, j integers. “Plutinos”, include Pluto itself. Who ordered that!?
2. Apparent edge at ~47 AU (not just selection function)
The puzzle of Earth’s water…
Liquid water is stable on the Earth today because the temperature at atmospheric pressure is 0 C < T < 100 C
BUT - when minerals that later formed the Earth condensed, pressure in the disk was very low. Water would be vapor, would not form water-rich rocks…
Chemical evidence:
Ratio of deuterium to hydrogen:
Earth’s water: 153 parts per million Meteorites known as carbonaceous chondrites: 159 ppm Comets: 309 ppm
These meteorites appear to originate from the outer asteroid belt (beyond about 2.7 AU)
Evidence for radial transport - mass dynamically negligible but critical for life
How common is water on planets in the “habitable zone”?
Extrasolar Planets
Contrast ratio in reflected light:
!
Astronomical units: 24-25 magnitudes
Contrast at the peak of the planet’s thermal emission is less: about f ~ 10-6 at 20 µm for the Earth
Imaging planets would allow measurement of atmospheric spectra - biomarkers such as oxygen / ozone…
Tinetti et al. (2006)
Tinnetti et al. (2006) Future goal: NASA’s Terrestrial Planet Finder / ESA’s Darwin proposals
All detections of extrasolar planets to date are via indirect methods
Radial velocity searches
Observable: time dependent radial velocity of star (via Doppler shift of spectral lines), due to perturbation from orbiting planet
For planet on circular orbit:
!
!
Massive planets are easier to detect
Planets at small a easier to detect
Usually unknown: derive lower limit on mass
Jupiter: 12.5 m s-1
High resolution astronomical spectrographs: R ~ 105 (3 km s-1)
!
1 2
dNph /dv
Can detect small RV shift if (a) high S/N and (b) spectrum has plenty of structure (limited by thermal broadening)
!
1 2
Can measure very small RV shifts against shot noise if calibration is stable - need only resolve the lines…
Actual noise sources include: • stellar activity • stellar oscillations (the signal for helioseismology)
Sub-m s-1 very challenging
!
a sini
If a survey could detect K > Kmin for some sample of stars:
Detectable
Undetectable
log a
In fact no survey is anything like this simple… but basic selection function is of this form with Kmin ~ 20 m s-1 for complete samples…
Example of real data, measure:
• orbital period • MP sin i (with stellar mass) • orbital eccentricity e
…all that is known for most extrasolar planets
Multiple planet system
Results #1: eccentricity vs semi-major axis
Results #2: mass vs semi-major axis
Results #3: eccentricity vs mass
Summary of radial velocity findings
1. Planet frequency among “Solar type” stars is at least 7%
2. “Hot Jupiters” - massive planets at a < 0.1 AU
3. Typical planet is eccentric: <e> = 0.27 #2, #3 are different from Solar System expectations
4. Mass function favors low mass planets, radial distribution increases to large orbital radius
Note: only very limited information on planet population with M and a similar to that of Jupiter…
Fischer & Valenti (2005)
Abundance of (detected) planets is a strongly increasing function of the metallicity of the host star measured from the spectrum Giant planet formation process “knows” about the trace abundance of heavy elements (~1-2%)
Transits
!
!
+ Rp
a
About 10% for hot Jupiters, 0.5% for Earth in Earth’s orbit
Ground based data quality (TrES project)
Space based data quality (COROT mission results)
Giant planets: direct measurement of planetary radius - confirms that these are gas giant planets - limited information on structure
Terrestrial planets: Kepler mission should be sensitive to planets with Earth radius
Also possible to observe the secondary eclipse / phase modulation in the infra-red (Spitzer):
Harrington et al. (2006)
Measure of temperature on the day / night side of the planet
Torres et al. (2008)
A radius mystery
Measured Rp are not a one parameter family with planet mass
What is the additional physics at work in setting the radius?
• planetary structure? • dynamics (heating
How do terrestrial and gas giant planets form?
How can we understand their orbits:
• in the Solar System? • in extrasolar planetary systems?
Hope is that this will inform questions such as:

Phil Armitage, University of Colorado

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