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

Science involves:

• Collection of facts by observation, experiment
  • "raw" facts
  • "interpreted" facts
• Development of theories to explain facts
• Rejection, refinement, extension of theories

We have discussed examples of:

• 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 exteriors.
  • 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).
• Ages:
  • 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 planets.
• 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 was lower.
• 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 one-on-one collisions.
• 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

• 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 systems.
• 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 period planets.
• 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 eclipsing planets.

SURPRISES

• ~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 sustained effort.
• 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 survive?
• 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.