Today’s APOD
Begin Chapter 8 on Monday– Terrestrial Planets
Hand in homework todayQuiz on Oncourse
The Sun Today
A100 – Ch. 7Extra-Solar Planets
Today’s TopicsSolar systems around other stars
How do we find themWhat are they like?How are they similar to our solar
system? How do they differWhat kinds of stars have planets?
Worlds around other SunsEvidence exists for
planets around other nearby stars
The new planets are not observed directly, but rather by their gravitational effects on their parent star
These new planets are a surprise - they are huge planets very close to their parent stars
Note: All of these images are artists’ conceptions
Our Solar System
Gas Giants
Ice Giants
Terrestrial Planets
Plutoids
Searching for PlanetsMore than 300 “extra-solar”
planets have been discoveredHow are planets discovered?
Radial velocityTransits
Gravitational lensingWobbles in stars’ positions
o The Doppler Shift!o Absorption lines shift left or right if
stars move toward or away from uso Planetary orbits cause stars’ radial
velocities to change
Discovering Planets
from Spectra
The Electromagnetic Spectrum
What is Wavelengt
h???
Some phenomena are periodic They repeat in equal intervals of time The time between successive peaks is called
wavelength Wavelength = Color
Red light has longer wavelengths Blue light has shorter wavelengths
Time
Where does light come from?
Thermal radiators emit light at all wavelengths
Atomic emission occurs only at particular wavelengths
THERMAL EMISSION
ATOMIC EMISSION
The Doppler Shift
Police SirenDoppler Pumpkin
Johan Christian Doppler
The Doppler EffectHow does light tell us the speed of
a distant object?How does light tell us the rotation
rate of an object?
Doppler Principles (Police Traffic Radar Handbook)
The Doppler Shift of Sound Waves
What's Happening?
The Doppler Shift for LightAstronomers us
the Doppler effect to measure the “radial” velocities of astronomical objects
Radial velocities are motions toward or away from us
Measuring the Shift
Stationary
Moving Away
Away Faster
Moving Toward
Toward Faster
Doppler shift tells us ONLY about the part of an object’s motion toward or away from us
The amount of blue or red shift tells us an object’s speed toward or away from us:
The Doppler Shift in YOUR Life
Periodic velocity changes due to orbiting planet
VERY high precision is needed to measure these very small velocity changes
Velocity vs. Time
Velocity of 51 Peg
-80
-60
-40
-20
0
20
40
60
80
0 10 20 30 40
Time (days)
Vel
oci
ty (
met
ers
per
sec
on
d)
About 7 orbits in 30 daysP=4.2 days
A Planet around Eridani A planet orbits the star Eridani at a
radius of 3.2 A.U. Eridani is similar to our Sun Eridani is only 10.5 light years away The planet is similar to Jupiter The planet orbits Eridani in 7 years Eridani has at least one more planet
The starAnd has at least 3 planets
terrestrialplanets
If the Earth lies in the same plane as the orbit of a planet we see a transit
•The planet passes across the face of the star
•Some of the starlight is blocked by planet and the star appears dimmer
Planetary Transits
Seeing planets
near stars is hard
Looking for an Earthlike planet around a nearby star is like standing on the East Coast of the United States and looking for a pinhead on the West Coast — with a VERY bright grapefruit nearby.
Very large telescopes will help
Imaging Planets?
This photo shows an image of the faint star GQ Lupi taken in the infrared. The faint object to the right of the star is a possible planetary companion. It is 250 times fainter than the star itself and it located 0.73 arcsecond west. At the distance of GQ Lupi, this corresponds to a distance of roughly 100 astronomical units. The planet probably has a mass of about 2 x Jupiter.
Locationof browndwarf
Possible planet
Another possible planet
Orbiting the brown dwarf ~225 light years away
Young, temp near 1000K
Further from its “sun” than Pluto is from ours
(brown dwarf is blocked out)
Properties of KNOWN Properties of KNOWN Extra-Solar PlanetsExtra-Solar Planets
SURPRISES!SURPRISES!
All are gas giants like Jupiter and SaturnAll are gas giants like Jupiter and SaturnMost are larger than JupiterMost are larger than JupiterMany orbit very close to their parent starsMany orbit very close to their parent starsSome are in systems with multiple planetsSome are in systems with multiple planets
Many Known Planets
Are Close to Stars
The Sun
(Masses are given as Jupiter masses)
Hot Jupiters
Did these hot Jupiters form further out, and migrate inward as they eject smaller bodies from their planetary systems
BUT: Selection EffectsClose-in, massive planets are easier
to detectFar-out planets and light-weight
planets are MUCH HARDER to detectSo far, we’ve only been able to
detect massive, close-in planetsTechniques, sensitivity are improvingTerrestrial planets soon!
Planet Mass
Distribution
The number of planets discovered decreases as planet mass increases
There are few planets much more massive than Jupiter
Most planets are <2 Jupiter masses
How Far Are
Planets from their
Stars?
Planets with semi-major axes > 3AU have periods comparable to or longer than the length of most Doppler surveys, so the distribution is incomplete beyond that distance.
inco
mple
te
Period Distribution
for Hot Jupiters
A “pile-up” of planets with orbital periods near 3 days – very close to central star
Why???
What are the
planets’ orbits like?
What are the eccentricities of our sun’s planets? Planets within 0.1 AU are probably tidally circularized Beyond 0.1 AU, the distribution of eccentricities
appears essentially uniform between 0 and 0.8 Very different from our solar system!
Looking for Another Earth
So far, we don’t have the right instruments to detect Earth-like planets
Requires large ground-based telescopes or large, specially equipped space telescopes
Stay tuned!
ASSIGNMENTSthis week
Begin Chapter 8 – Terrestrial Planets
HAND IN HOMEWORK
Dates to Remember