Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 43: Other Solar Systems

Key Ideas:

Selection Effects
Properties of Extrasolar Planets: Giant planets very close to their parent stars; Close and eccentric orbits; Evidence for migration or interactions?
Properties of the parent stars: Planets rarer around small stars; Planets rarer around metal-poor stars

Strange New Worlds

Almost none of the systems found so far resemble our Solar System
The biggest surprise is Jupiter-sized planets so close to their parent stars: Deep inside the "Frost Line" where Jupiter-sized planets should not be able to form; Many have orbits smaller than Mercury's!
What is going on?

Selection Effects

Vast majority of exoplanets have been found using the Doppler Wobble method
Orbital speeds will be largest for: Massive planets close to their parent stars; In stars with lots of spectral lines (iron-rich)
Implies that unusual systems will be easier to find than ones like our solar system
This is an example of a Selection Effect

How long to detect Jupiter?

Distribution of Orbital Periods

Basic Selection Ranges

Doppler Wobble experiments started in the early 1990s, so only about 15 years of data:: Would only find planets with P<15 years, really want to see at least 2 periods; Recall: Jupiter has P=11.9 years, Saturn has P=29.5 years
Mass Limit: larger mass corresponds to faster orbital motion: Current limits are ~3 meters per second; Limited to Jupiter/Saturn masses at a few AU; Down to Uranus/Neptune masses at <0.03 AU

Distribution of Exoplanet Masses

Hot Jupiters

Jupiter-size planets close to their parent stars: Periods less than 10 days; Well inside the orbit of Mercury; Transits give densities like Jupiter and Saturn, so they are gas giants
Selection effect: Easiest type of planet to find, but
Why are they there at all?: How does a Jupiter-size gas planet get so close to its parent star?

Orbit Distribution of Exoplanets

Many systems have Jupiter-size planets closer to their star than in our Solar System
Two Possibilities:: Formation: there is a way of forming gas giants close to stars; Migration: Planets move during/after the formation process
Enhanced by selection effects: Solar Systems like ours are still hard to find

Migration and Other Ideas

Migration:: Jovian planets form far from the star; Spiral inwards due to drag from the primordial stellar nebula; Migration stops at the inner edge of the disk; Could be more likely in very heavy stellar nebulae
Formation:: Close Jupiters just formed differently than in our Solar System; Might be very different from Jupiter in detail?

Distribution of Orbital Eccentricities

Very elliptical orbits are interesting

Solar System: Jovian and Terrestrial orbits are nearly circular; Objects with very elliptical orbits (comets, scattered KBOs) had those orbits perturbed by gravitational interactions
Among Extrasolar Planets:: Very elliptical orbits are common; A few show strong resonances; Implies strong gravitational perturbations; Lends support to the migration idea

Planet Frequency

Planets are found in about 8% of the stars in the solar neighborhood so far: Selection effects: systems more like our solar system are harder to find, could still be common
Basic patterns are emerging:: Don't often find planets around low-mass stars (stars smaller than the Sun); Don't find planets around metal-poor stars

Low-mass Stars

Planets would be easiest to find around low-mass stars: Smaller stars would have a bigger orbital reflex motion
We don't find many planets around stars much less massive than the Sun: Even though low-mass stars are very common!
May not be that surprising:: Small stars have small proto-stellar disks; Not enough raw materials to make big planets

Metallicity and Extrasolar Planets

Astronomers call all heavy elements "metals" (heavier than H and He): Metallicity measures their abundance; High-metallicity = lots of rock, ice for planets
The Sun is metal-rich for stars of its age: Stars with planets have unusually high metallicity even relative to the Sun; Could mean that unusual planets are caused by unusually high metallicity?

Is Solar System Different?

Systems with high metallicity have more ice and rock to form giant planets: Did they initially form more gas giants?; Did gravitational interactions among them re-arrange the orbits to what we see now?
Did the Sun have a smaller than usual proto-stellar disk?: Don't form as many gas giants?; Don't have strong migration due to the disk?

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.