Transcript Extrasolar Planets

Extra Solar Planets
Just 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 Stars
More 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 Mass
center 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 Masses
Recall Kepler’s 3rd Law:
Py2 = aAU3
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:
3
a
____
AU
MA + MB =
Py2
(MA and MB in units of solar masses)
Examples:
a) Binary system with period of P = 32 years
and separation of a = 16 AU:
3
16
____
MA + MB =
= 4 solar masses.
2
32
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)
Typical sequence of spectra from a
spectroscopic binary system
Time
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
51 Peg – the first extra-solar planet discovered
HD 209458 – Transit of planet across 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.
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
Insert TCP 6e Figure 13.4a unannotated
• 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).
First Extrasolar Planet
Insert TCP 6e Figure 13.4b
• The planet around 51 Pegasi has a mass similar
to Jupiter’s, despite its small orbital distance.
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
Keck adaptive
optic image
showing planets
orbiting HR 8799.
http://apod.nasa.go
v/apod/ap081117.ht
ml
A VLT infrared image
of a hot young planet
around a brown dwarf
star.
An HST coronograph image
of a planet around Fomalhaut.
http://apod.nasa.gov/apod/ap0
81114.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 planetforming 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 (Earthsized) 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/7916strange-zoo-worlds.html
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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!