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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 4: Other Solar Systems

Key Ideas

There are direct and indirect methods of searching for planets.
Indirect methods include:
--Doppler wobble (most successful method so far)
--Gravitational lensing
--Astrometric wobble
First planet orbiting a main-sequence star discovered in 1995
Now know of about 220 planets orbiting other stars
Most of these planets look nothing like our solar system
Formation and evolution of planets not well understood
Just becoming sensitive to Earth-mass planets

Where are the planets?

How common are planets around other stars?
How does the frequency of planets vary with the kind of host star?
How many Earthlike planets are there?
How many planets in the habitable zones of stars?
Are moons of planets common?

Things to Keep in Mind

Planets are (comparatively) very small!

--Jupiter/Sun ~ 0.1%
--Earth/Sun ~ 0.0003%
--Jupiter/Sun ~ 10%
--Earth/Sun ~ 1%
--Jupiter ~ 3 billionths of the Sun!
--Earth ~ one billionth of the Sun

Finding Exoplanets

Detecting Exoplanets

There are two basic detection methods:
Direct Detection:
--Detect the light from the planet directly.
Indirect Detection:
--Detect the planet through an effect on its parent star, or some other effect not associated with light from the planet
--Orbital motions of the star because of the planet's gravity.
--Gravitational lensing by the planet.
--The decrease in the parent star flux when the opaque planet passes in front of it.

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.

Doppler Effect

Change in wavelength due to motion of object

Doppler Wobble

Look for orbital motions using the Doppler Effect
As the star moves toward us, the spectrum is shifted blueward.
As the star moves away from us, the spectrum is shifted redward.

Measuring the orbital speed and period gives an estimate of the unseen planet's mass.

Doppler Wobble Measurements

The greater mass of the star makes its orbital speed very small.
Example: Sun & Jupiter
--Jupiter: 13 kilometers / second
--Sun: 13 meters / second
Earth: 10 centimeters per second
Need to be able to measure the Doppler shifts with extremely high precision.

Doppler measurements can find planets around FGKM stars within a few hundred parsecs.

Astrometric Wobble

Star wobbles back & forth on the sky relative to more distant background stars.
--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!

Astrometric measurements can find planets around AFGKM stars within a few hundred parsecs.


Planet's orbital plane along the line of sight:
The planet periodically crosses (transits) the face of its parent star.
Star dims slightly (fraction of a %) during transit.
Biased towards close, big planets.

Transiting Planet HD 209458b
-- Mass of ~0.7 Jupiters
-- ~0.045 AU from its star
-- Orbital Period of ~3.5 days

Transits can find planets around FGKM stars within a few kiloparsecs.


Light from a background star is magnified by the gravity of the foreground "lensing" star.
Brief brightening of the background star as they pass
If there is a planet around the lensing star, it will amplify the light as well.
Four planets have been found this way by the MicroFUN team led by OSU!

Microlensing can find planets around GKM stars within a few tens of kiloparsecs.

Strange New Worlds

51 Pegasi

1995: Michel Mayor & Didier Queloz (Geneva) observed a wobble in the star 51 Pegasi.
51 Pegasi Attributes
--Sun-like star
--~40 ly away in Pegasus
--Wobble is 56 m/sec
--Period: 4.23 days!!

Roster of New Planets

As of May 2007, all of these techniques have found more than 220 planets around other stars:
--Most are single-planet detections
--A number of multi-planet system have turned up
--Most are Jupiter-sized or larger (up to 13 times Jupiter's mass), down to ~ Neptune mass (15ME) and a few "super-Earth" (6-8ME)
Almost all orbit within ~5 AU of their parent star!

Planet Properties

Planet Masses
--From 1.6 times the mass of the Moon to 13 times the mass of Jupiter
Semimajor Axes
--From 0.02 AU to 275 AU
--From circular orbits (eccentricity zero) to very eccentric orbits (eccentricity of 0.93)
Host Star Masses
--from 25 Jupiter masses to 5 Solar masses
--from 3.2 parsecs to 6.5 kiloparsecs

Selection Effects

Doppler Wobbles and Transits are only detectable for large planets like Jupiter or Saturn, maybe down to Neptune masses.
Doppler techniques are currently sensitive to Jupiters out to ~5 AU from their stars.
Transit technique is currently strongly biased towards finding the closest-in Jupiter-like planets.

Current methods are not yet sensitive to Earths

Strange New Worlds

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

The Future

Continuing search for other systems
--Find systems more like our own
--How common are planetary systems?

Future Goals:
--Find Earth-sized planets.
--Find Earth-sized planets in habitable zones


Launch late 2008
Should be sensitive to habitable, Earth-sized planets
FGKM host stars

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

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