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