Transcript Slides from the third lecture

The Search for Extrasolar Planets
Since it appears the conditions for planet formation are common, we’d like to know
how many solar systems there are, and what they look like.
Indirect Methods:
1) Doppler shift of the star’s orbit
this is the main one so far
2) Astrometric wobble of the star’s orbit
Semi-direct Methods:
1) Transits (light blocked by the planet)
might also see phases
2) Microlensing (planet’s gravity)
Direct Methods:
1) Planet imaged directly (perhaps with coronograph)
reflected or emitted (IR or radio) light
2) Planet imaged by interferometer
Precision Radial Velocity Searches
Shift is
1 part in
100
million
Discovery of Extrasolar planets
We get the orbital
period, semimajor
axis, and a lower
limit on the mass of
the planet. This can
only do giant
planets relatively
close in (but could
see Jupiter).
A Big Surprise : Close-in Jupiters
It is easiest to find a massive planet that is close to the star (it repeats
quickly and has a large velocity amplitude). The first discovery, 51
Peg, had a 4 day orbit (0.05 AU!) and the mass of Jupiter. Many are
now known, but that doesn’t mean they are most common, just
easiest to find (and present in some numbers).
Properties of the systems found: 1
Properties of the systems found: 2
Astrometry
This works best for large orbits (which
take a long time) and stars that are nearby.
Interferometry would allow very small
motions to be measured.
“Microlensing” : Gravitational lenses
In principle, this method could
even see Earth-mass planets.
You have to have a huge and
long-time monitoring program
with high time resolution and
good photometric precision.
The downside is that you will only
detect the planet once, and can’t
learn anything more about it. One
detection has been claimed
(but how to confirm it?).
The Problem with Direct Imaging
1) The host star is FAR
brighter (106) than any
planet (except very
young Jupiters in the
infrared).
Reflected light
Thermal emission
2) The planet is VERY
close in angle (microarcsecs) to the star, so
any stray light from the
star can overwhelm the
light from the planet.
Interferometric Missions
Perhaps a decade from now we will be able
to directly image older extrasolar giant
planets.
Darwin
Terrestrial Planet Finder
Nulling Interferometry
You can try to keep the star
at a destructive null fringe,
while the planet will be
slightly off the fringe and
so still visible. Might be
able to reduce the star’s
brightness by a million
times?
Planetary Transits
A transit is like an eclipse, only
smaller… This has been seen for a
few cases (confirming the radial
velocity detections).
PHOTOMETRY CAN DETECT EARTH-SIZED PLANETS
•
The relative change in brightness is equal to the relative areas
(Aplanet/Astar)
Jupiter:
1% area of the Sun (1/100)
Earth or Venus
0.01% area of the Sun (1/10,000)
•
To measure 0.01% must get above the Earth’s atmosphere
•
This is also needed for getting a high duty cycle
•
Method is robust but you must be patient:
Require at least 3 transits, preferably 4 with same brightness
change,
duration and temporal separation
(the first two establish a possible period, the third confirms it)
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HST measurement of HD209458
Information from Transits
The distance and size of the planet come out
directly. If you have radial velocity as well
you get the mass, and thus the density. It is
unlikely you could ever image the planet or
get its spectrum, but you can get the thermal
spectrum and something about the
atmosphere during eclipses.
Summary of Kepler Mission Goals
• Find the frequency of terrestrial planets in the Galaxy
• Characterize the properties of inner planetary systems.
• Determine the properties of stars
(single & multiple) hosting planets.
• Discover terrestrial planets in habitable zones
•
(or show that they are rare).
• Detect true Earth analogs
A NULL result would also be very significant
(frequency of stars with terrestrial planets is less than 5%)
Kepler is uniquely qualified to detect Earth-sized
extrasolar planets “before this decade is out”!
Kepler MISSION CONCEPT
• The Kepler Mission is optimized for finding habitable planets
( 0.5 to 10 M ) in the HZ ( near 1 AU ) of solar-like stars
• Continuously and simultaneously monitor 100,000 dwarf stars
using a 1-meter Schmidt telescope: FOV >100 deg2 with 42 CCDs
• Photometric precision of < 20 ppm in 6.5 hours on
Vmag = 12 solar-like star  4s detection for one Earth-sized transit
Focal plane electronics
15 minute integrations
Sunshade
42 CCDs
read every
3 seconds
1.4 m diameter
primary mirror
0.95 m diameter
Schmidt corrector
Focus
mechanisms
105 sq deg FOV
Focal plane assembly:
CCDs, field flattening lenses
fine guidance sensors
Radiator and heat pipe
for cooling focal plane
Graphite cyanate
structure
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Kepler CCDs on the Sky
•
Full
Moon
Transit Detectability
The strict periodicity of planetary transits provides an extremely powerful filter
against misleading stellar signals. You need 3 transits to be sure you’ve seen it.
The Easy False-Positives Problems
There are several common sources of false positives. They
produce the right signal for the wrong reasons but some
are easy to deal with:
1. Grazing eclipses of one star by another
2. M dwarfs transiting giants and supergiants
3. White dwarfs transiting solar-type stars
A full eclipse is flat-bottomed, a grazing eclipse is more bowl or “V” shaped.
Giants and supergiants can be known from their spectra and photometric behavior.
Gravitational focussing makes a white dwarf transit into a bump instead of a dip!
The Hard False-Positives Problem
The other types generate the right signal for the
wrong reasons and are harder to remove:
1. Full eclipses in a faint background binary whose light is
combined with a foreground bright star
2. Triple star systems with a bright primary and a faint eclipsing
secondary pair
+
=
Potential for Planetary Detections
10000
1000
# of Planet
Detections
100
10
1
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Orbital Semi-major Axis (AU)
Expected # of planets found, assuming one planet of a given size &
semi-major axis per star and random orientation of orbital planes.
THE HABITABLE ZONE BY STELLAR TYPES
The Habitable Zone (HZ) in green is the distance from a star where liquid
water is expected to exist on the planets surface (Kasting, Whitmire, and
Reynolds 1993).
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Search Methods :
what they can find
Detections by 2005