Extra Solar PlanetsJust some introductory materials. A very fast moving field.

My favorite website: http://www.exoplanets.org

Touch on masses of stars/planets

Some of the results concerning exoplanet discovery

Several techniques for searching, Kepler the new King

Also have star system/planet building webpages:

http://curriculum.calstatela.edu/courses/builders/lessons/less/les1/choose.html

Binary StarsMore than 50% of all

stars in our Milky Way are not single stars, but

belong to binaries:

Pairs or multiple systems of stars which

orbit their common center of mass.

If we can measure and understand their orbital

motion, we can estimate the stellar

masses.

The Center of Masscenter of mass =

balance point of the system.

Both masses equal => center of mass is in the middle, rA = rB.

The more unequal the masses are, the more

it shifts toward the more massive star.

Estimating Stellar MassesRecall Kepler’s 3rd Law:

Py2 = aAU

3

Valid for the solar system: star with 1 solar mass in the center.

We find almost the same law for binary stars with masses MA and MB different from 1 solar mass:

MA + MB = aAU

3 ____ Py

2

(MA and MB in units of solar masses)

Examples:a) Binary system with period of P = 32 years

and separation of a = 16 AU:

MA + MB = = 4 solar masses.163____322

b) Any binary system with a combination of period P and separation a that obeys Kepler’s

3. Law must have a total mass of 1 solar mass.

Visual Binaries

The ideal case:

Both stars can be seen directly, and

their separation and relative motion can be followed directly.

Spectroscopic Binaries

Usually, binary separation a can not be measured directly

because the stars are too close to each other.

A limit on the separation and thus the masses can

be inferred in the most common case:

Spectroscopic Binaries:

Spectroscopic Binaries (II)

The approaching star produces blueshifted lines; the receding

star produces redshifted lines in the spectrum.

Doppler shift Measurement of radial velocities

Estimate of separation a

Estimate of masses

Spectroscopic Binaries (III)

Tim

e

Typical sequence of spectra from a spectroscopic binary system

Eclipsing Binaries

Usually, inclination angle of binary systems is

unknown uncertainty in mass estimates.

Special case:

Eclipsing Binaries

Here, we know that we are looking at the

system edge-on!

Eclipsing Binaries (II)

Peculiar “double-dip” light curve

Example: VW Cephei

Extra-Solar Planets• Hard to see faint planet right next to very bright star• Two main indirect techniques available

(Like a binary star system but where 2nd “star” has extremely low mass)– Watch for Doppler “wobble” in position/spectrum of star– Watch for “transit” of planet which slightly dims light from star

• More than 700 planets discovered since 1996– See http://exoplanets.org/ or several other sites

• Initially tended to be big (Jupiter) and very close to star (easier to see), but starting to find others now.

51 Peg – the first extra-solar planet discoveredHD 209458 – Transit of planet across star

Radial Velocity or “Wobble” Method

• 51 Peg back in 1996, followed by hundreds of others, primarily from Geoff Marcy’s group out of California (Lick and Keck Observatories). Marcy went on Letterman wearing a Hawaiian shirt we both bought in Kona…tried mine on and it’s a little too small now 15 years later. Hmmm.

• Depends on techniques to get ultra high spectral resolution (meters per second) via iodine cells and other “tricks”

• Need stars closer to edge on, has mass uncertainties because of unknown viewing angle

• Works, but need long time, long surveys, mostly one target at a time.

First Extrasolar Planet

• Doppler shifts of the star 51 Pegasi indirectly revealed a planet with 4-day orbital period.

• This short period means that the planet has a small orbital distance.

• This was the first* extrasolar planet to be discovered (1995).

Insert TCP 6e Figure 13.4a unannotated

First Extrasolar Planet

• The planet around 51 Pegasi has a mass similar to Jupiter’s, despite its small orbital distance.

Insert TCP 6e Figure 13.4b

Other Extrasolar Planets

• Doppler shift data tell us about a planet’s mass and the shape of its orbit.

Doppler Technique

• Measuring a star’s Doppler shift can tell us its motion toward and away from us.

• Current techniques can measure motions as small as 1 m/s (walking speed!).

Planet Mass and Orbit Tilt

• We cannot measure an exact mass for a planet without knowing the tilt of its orbit, because Doppler shift tells us only the velocity toward or away from us.

• Doppler data give us lower limits on masses.

Transit Method

• Astronomers do photometry well and can detect small, periodic changes in light level. Small telescopes can do this.

• Need very close to edge-on systems, usually within a degree given planet sizes, separations, and geometry.

• More than a thousand candidates here or coming (Kepler mission!), dozens confirmed.

• Can detect Earth-like planets, but needs long timescales to see planets far out from their suns.

Transit Missions

• NASA’s Kepler mission was launched in 2008 to begin looking for transiting planets.

• It is designed to measure the 0.008% decline in brightness when an Earth-mass planet eclipses a Sun-like star.

Transits and Eclipses

• A transit is when a planet crosses in front of a star.• The resulting eclipse reduces the star’s apparent

brightness and tells us planet’s radius.• No orbital tilt: accurate measurement of planet mass

Direct Imaging Problem:Brightness Difference

• A Sun-like star is about a billion times brighter than the light reflected from its planets.

• This is like being in San Francisco and trying to see a pinhead 15 meters from a grapefruit in Washington, D.C.

Direct Imaging

A VLT infrared image of a hot young planet around a brown dwarf star.

Keck adaptive optic image showing planets orbiting HR 8799. http://apod.nasa.gov/apod/ap081117.html

An HST coronograph image of a planet around Fomalhaut.

http://apod.nasa.gov/apod/ap081114.html

What have we learned about extrasolar planets?

Orbits of Extrasolar Planets

• Most of the detected planets have orbits smaller than Jupiter’s.

• Planets at greater distances are harder to detect with the Doppler technique.

Orbits of Extrasolar Planets

• Orbits of some extrasolar planets are much more elongated (have a greater eccentricity) than those in our solar system.

Multiple-Planet Systems

• Some stars have more than one detected planet.

Orbits of Extrasolar Planets

• Most of the detected planets have greater mass than Jupiter.

• Planets with smaller masses are harder to detect with Doppler technique.

Hot Jupiters

Revisiting the Nebular Theory

• The nebular theory predicts that massive Jupiter-like planets should not form inside the frost line (at << 5 AU).

• The discovery of hot Jupiters has forced reexamination of nebular theory.

• Planetary migration or gravitational encounters may explain hot Jupiters.

Planetary Migration

• A young planet’s motion can create waves in a planet-forming disk.

• Models show that matter in these waves can tug on a planet, causing its orbit to migrate inward.

Planets: Common or Rare?

• One in ten stars examined so far have turned out to have planets.

• The others may still have smaller (Earth-sized) planets that current techniques cannot detect.

• Kepler seems to indicate COMMON

Take Aways

• Very likely all stars, or nearly all stars, have planets based on our current detection rates, keeping in mind our limitations.

• At least a few percent of systems with planets, and likely more, have Earth-like planets. Worst case scenario: tens of millions in the Milky Way.

• A little early to say if our Solar System is typical, but there exists quite a range out there different from our own: http://www.space.com/7916-strange-zoo-worlds.html

– Hot Jupiters– Big planets farther out, Cthonian worlds, water worlds, super Earths, rogues– Some highly eccentric orbits– “Tatooine” – planets in binary star systems (which are common)

– FIELD IS CHANGING FAST –CHECK THE WEB/APPS!