LECTURE 21: THE ORIGIN OF PLANETARY SYSTEMS
- What are the relations between facts and theories in science?
- What are the basic elements of the standard picture of planet formation?
- Why did smaller, rocky planets form in the inner solar system and
massive planets with hydrogen/helium form in the outer solar system?
- How did Jupiter's moon system form?
- What methods can be used to search for planets around other stars?
- What has been found by these searches to date, and why are
the results surprising in light of our standard theory of planet formation?
AN INTERLUDE: FACTS AND THEORIES IN SCIENCE
With familiarity, some theories get effectively re-categorized as facts,
e.g., Kepler's laws.
- Collection of facts by observation, experiment
- "raw" facts
- "interpreted" facts
- Development of theories to explain facts
- Rejection, refinement, extension of theories
Facts are a means to an end: development of explanatory, predictive theories.
We have discussed examples of:
Successful scientific theories
- Well tested, highly successful theories of broad scope
- Plate tectonics
- Newton's theory of gravity
This theory remains powerful and broadly applicable
BUT it is only a limiting case of Einstein's
broader, more accurate theory.
- Successful theories of smaller scope
- Greenhouse warming on Venus
- "Orbiting ice chunk" model of Saturn's rings
- Tentatively accepted theories
- Giant impact theory for origin of Moon
- Theories that survive in heavily modified form
- Heliocentric theory of Copernicus
- Firmly rejected theories
- Geocentric theory of Ptolemy
- Tentatively rejected theories
- Fission theory for origin of Moon
- Theories under construction
- Theory of planet formation
- Explain phenomena in natural (not supernatural) terms.
- Are motivated largely by empirical evidence.
- Make predictions that can be tested with empirical data ---
scientific theories can be falsified.
- Usually suggest routes for further experimentation and testing.
FORMATION OF THE SOLAR SYSTEM: CLUES AND FACTS TO BE EXPLAINED
- Orbits and rotations: Planetary orbits are nearly circular,
nearly parallel to Sun's equator.
- Contents and arrangement:
- Inner, terrestrial planets are low mass, dense, iron and rock.
- Outer, Jovian planets are high mass, rock/ice cores, hydrogen/helium
- Largest planet (Jupiter) is near the middle (5 out of 8).
- Also rocky asteroids in inner solar system (asteroid belt),
icy comets from outer solar system (Kuiper belt, Oort cloud).
- Oldest Earth rocks ~ 4.2 Gyr.
- Oldest Moon rocks ~ 4.5 Gyr.
- Meteorites: all ~ 4.6 Gyr.
- Meteorite age spread of less than 0.02 Gyr (20 million years).
- Sun age is ~ 4.5 Gyr, from completely independent method.
- Lunar cratering implies heavy bombardment for first ~ 0.7 Gyr.
- Observations of forming stars:
- Many forming stars surrounded by disks of gas and dust.
- Disks appear to last ~ 0.01-0.03 Gyr (10-30 million years);
older stars don't have them.
FORMATION OF THE SOLAR SYSTEM: THE "STANDARD" THEORY
- Proto-sun surrounded by extended disk of gas and dust.
- Dust grains stick, build planetesimals.
- Gravity of planetesimals accelerates growth.
- Terrestrial vs. Jovian planets:
- Key idea: the "frost line"
- Inner solar system: too hot for ices (H2O, CO2,
CH4) made from most abundant heavy elements.
- Build rocky planets from grains of metals, silicon compounds.
- Outer solar system: ices survive, much more solid material.
- Build rock/ice cores several times Earth mass.
- Massive enough to attract and retain hydrogen/helium envelopes
in cooler outer solar system.
- Planets sweep up or eject most remaining debris, ending heavy
bombardment after ~ 700 Myr.
- Asteroid belt, Kuiper belt are leftover debris, unable to form large
- Oort cloud is ejected debris.
JUPITER'S MOON SYSTEM: A SMALL-SCALE EXAMPLE
- Jupiter formed from its
own collapsing gas cloud, orbiting around Sun.
- Gravitationally contracting planet heated nearby disk.
- Moons formed like terrestrial planets.
- Rocky moons (Io/Europa) close to planet,
icy moons (Ganymede/Callisto) further out, where temperature
- Largest moon (Ganymede) near middle of moon system.
- Similar story for Saturn.
THE SPIRAL WAVE SCENARIO
- A possible problem: Calculations of standard theory suggest ~ 100 Myr
required to build Jovian planets
- Inferred ages of proto-planetary disks arouond other stars only
~ 10 Myr.
- Competing idea: Spiral waves in disk, caused by gravity,
might have allowed faster
formation of proto-planets, in few thousand years.
- Substitutes rapid, wave-assisted growth for slow assembly by
- This idea can now be studied with simulations on fast computers.
- True scenario may be hybrid: spiral wave growth accelerates early
phases, traditional accretion growth follows.
SEARCHES FOR EXTRA-SOLAR PLANETS
Radial velocity method:
- Earth reflects about 1 billionth of Sun's light, Jupiter 4 billionths.
- With current technology, can't detect such faint objects close to
overwhelming light of parent star.
- Conclusion: direct detection doesn't work, though might be possible
in 1-2 decades.
- Alternative: detect planet's effect on parent star.
- Planet's gravity causes parent star to move.
- Star's orbit smaller than planet's by mass ratio, e.g. 1/1000
for Sun and Jupiter.
- Small motions (few meters per second) detectable with high
precision spectroscopy, using Doppler shifts. (Light wavelengths
shifted by ~ 0.001 percent.)
- Look for periodic variation as signature of orbiting planet.
- Detect stellar "wobble" with careful position measurements.
- Transiting planets produce periodic eclipses of small fraction
of star's light.
- Gravitational microlensing:
- Gravity bends light (Einstein).
- Foreground star can act as "gravitational telescope," amplifying
light of background star if it passes directly in front.
- Planet around foreground star would perturb this signal.
- Requires finding microlensing events, following them closely over
several weeks to look for small perturbations.
- OSU astronomers are leaders in microlensing method, becoming leaders
in transit method.
EXTRA-SOLAR PLANETS: RESULTS TO DATE
- ~150 extra-solar planets now discovered. Several multiple planet
- Nearly all by radial velocity (Doppler shift) method.
- Most have masses of several x Saturn to several x Jupiter.
- Some masses as low as Neptune.
- Most have short periods, few days to 1-2 years.
- Easier to detect massive planets. Why?
- Easier to detect short periods. Why?
- Sensitive observations since early 1990s, now finding more long
- Transit method has discovered three planets.
- Microlensing method has discovered two planets.
- Microlensing method has produced interesting "null result": absence
of more detections shows less than half of typical stars have
Jupiter mass planet at several AU.
- Transit observations measure sizes, atmospheric signatures of
- ~5% of solar type stars have Jupiter-mass planets within 1-2 AU.
- Orbits are often quite eccentric.
- Very different from our solar system!
- Standard theory predicts: Jovian planets only form beyond
"frost line" at several AU. Circular orbits.
- Best guess for now: Massive planets form at several AU, then
"migrate" in because of gravitational interactions with disk.
- Migration process may also increase orbital eccentricities.
- Mild version of this process may have occurred in our solar system,
moving Jupiter in and Uranus and Neptune out.
- This would explain why Pluto and many other Kuiper belt objects
have period exactly 3/2 Neptune period; "swept up" by Neptune's
gravity as it moved out.
LIFE IN THE UNIVERSE
- Key requirements for Earth-like life:
- Roughly circular orbit.
- Liquid water.
- Promising environments: Terrestrial planets, massive moons, ~1 AU
from parent star.
- Don't yet have technology to find such planets, don't know whether
they are common or rare.
- Hope to develop such technology over next decade or so.
- More ambitious goal: develop technology to measure spectra of
Earth-like, extra-solar planets. Probably takes 2 decades of
- Could look for spectral signatures of oxygen atmospheres,
other signs of biological effects on surface and atmosphere.
Is there other intelligent life in the universe?
- Development of intelligent life could be extremely improbable.
- But space is big, time is long.
- Billions of stars in galaxy, trillions of galaxies in universe.
- Most astronomers think odds favor other intelligent life,
because so many chances.
How likely are we to find it?
- Space is big, time is long.
- If development of intelligent life is rare, nearest example could
be very far away.
- A key unknown: How long do technologically advanced civilizations
- Suppose 100,000 years (20 times longer than human civilization to date).
- This is 1/100,000 of the age of the Milky Way Galaxy (10 billion years).
- Even if there were tens of thousands of technologically advanced
civilizations in the history of the Milky Way, they might not have
overlapped in time.
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Updated: 2005 May 31[dhw]