Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 40: Finding Extrasolar Planets

Key Ideas:

All stars form surrounded by disks of material
Extrasolar planets are hard to find
Successful Search Techniques: Astrometric Wobble; Doppler Wobble (most successful so far); Planetary Transits; Gravitational Microlensing

Are We Alone?

The question of the existence of other planets beyond the Solar System is an old one in Astronomy: Are there solar systems around other stars?; Are such solar systems like ours or different?; Are any of the planets like the Earth?; Has life arisen on other planets?; Has intelligent life arisen on other planets?

Scientific Questions

How can we address these questions scientifically?
Break these questions into more manageable pieces:: How do solar systems form?; How many stars have planets?; What kind of planets do they have?; What kind of planets could support life?; What kind of planets could support intelligent life?

Formation of Solar Systems

Disks of material are commonly found around very young stars in our Galaxy
Our Solar System formed from a similar disk of material 4.5 billion years ago

Planets are Hard to See

The Sun is a billion times brighter than Jupiter and ten billion times brighter than the Earth
Seeing the planets against the bright glare of the Sun would be extremely difficult, especially from 100 light years away!

Searches for Extrasolar Planets

There are two basic search strategies:
Direct detection:: Images of planets orbiting other stars; See planets transit their parent star, causing a characteristic drop in brightness
Gravitational detection:: Orbital motions of the star because of the planet's gravity; Gravitational microlensing by the planet

Wobbling Stars

Newton's form of Kepler's Laws:: Planets orbit stars with the center-of-mass at one focus; The star orbits at a much smaller distance with a slower speed because of its greater mass
The star appears to "wobble" around the center-of-mass of the star-planet system

Astrometric Wobble

Star wobbles back and forth on the sky relative to more distant background stars
Problem:: The wobble is very small; Best seen looking down on the orbital plane
From 18 light years away, the Sun's astrometric wobble is <0.001 arcseconds!

Doppler Wobble

Look for orbital motions using the Doppler Effect
Measuring the orbital speed and period gives an estimate of the unseen planet's mass.
The greater mass of the star makes its orbital speed very small.
Example: Sun and Jupiter: Jupiter: 13 km/s; Sun: 13 m/s
Need to be able to measure the Doppler shifts with extremely high precision

51 Pegasi

1995: Michel Mayor and Didier Queloz (Geneva) observed a wobble in the star 51 Pegasi: Sun-like star; ~40 ly away in Pegasus; Wobble is 56 m/s; Period: 4.23 days!
This implies a 0.5 Jupiter mass planet only 0.05 AU from its parent star!

Planetary Transits

Planet's orbital plane along the line of sight:: The planet periodically crosses (transits) the face of its parent star; Star dims slightly during transit; Biased towards close-in Jupiter-sized planets
About 5 transit candidates found so far from searches
Transiting Planet HD 209458b: Mass of ~0.7 Jupiters; ~0.045 AU from its star; Orbital period of ~3.5 days

Gravitational Microlensing

Two stars line up:
Light from the background star is amplified by the gravity of the foreground "lensing" star: Brief brightening of the background star as they pass
Microlensing by planets:: If there is a planet around the lensing star, it will amplify the light as well.; Two planets have been found this way, one this summer by a collaboration led by Ohio State.

Roster of New Planets

As of January 2007, Doppler techniques have found 172 candidate planets around more than 147 nearby stars:: Most are single Jupiter-mass planet detections; About a dozen are multi-planet systems; One, 55 Cancri, has 4 planets!; All are Jupiter-sized or larger (up to 13 times Jupiter's mass), with a few of Neptune or super-Earth mass (6-8 times more massive)
All orbit withing 5-6 AU of their parent star

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