Transcript Lecture 19 – Detection of Extrasolar Planets

Stellar Classification & Planet
Detection
Meteo 466
Reading for this week
• “How to Find a Habitable Planet”, James Kasting.
Chapter 10, 11 & 12.
• For this lecture, part of Chapter 10 & all of Chapter 11
• Cassan et al., Nature (2012)
•
Udry & Santos, Ann. Rev. Astron. Astrophys. (2007))
How bright is a star ?
• The brightness of a star is specified in magnitudes.
•
Hipparchus (190 B.C – 120 B.C) based it on how bright a
star appeared to the unaided eye.
• Brightest stars are Magnitude 1 & dimmest stars are
Magnitude 6 (barely visible)
• Refined definition: Difference of 5 magnitudes (1 to 6)
corresponds to a factor of 100 times in intensity (flux).
m1 – m2 = -2.5 log(flux2 / flux1)
Color- (apparent) magnitude
Apparent Magnitude: Brightness Measure as seen from Earth
Color: The difference in apparent magnitudes in different filters
Color- (absolute) magnitude
If the distance ‘d’ to the star is known:
Absolute magnitude = app. Magnitude – 5 Log(d)
Main
sequence
White dwarfs
Giants
Hertzsprung-Russell (HR) Diagram
O and B
stars
Main sequence
Sun
G stars
M-stars
See also The
Earth System,
p. 194
O
B
A
F
G
K
M
http://www.physics.howard.edu/students/Beth/bh_stellar.html
Evolution of
Sun like star
Is white-dwarf
“The” end ??
Evolution of Sun like star
Center for Interdisciplinary Exploration and
Research in Astrophysics (CIERA),
Northwestern University
Evolution of 10 MSun star
Fission
Fusion
Evolution of 10 MSun star
Center for Interdisciplinary Exploration and
Research in Astrophysics (CIERA),
Northwestern University
Stars in the solar neighborhood
Within 12.5 light years, there are 33 stars. Most of them
Red dwarfs.
• Ultimate goal: To find Earth-like planets,
if they exist, and to search for evidence
of life
– So, how do we do that?
Exoplanet detection methods
(ordered by the number of detections )
Indirect
• Radial velocity (Doppler method)
• Transits
• Gravitational microlensing
• Pulsar planets
• Astrometric
Direct
• Optical imaging
• Infrared interferometry
Pulsar planets (Doppler technique
in time)
• Alex Wolszczan
(1991): 3 planets
around pulsar PSR
B1257+12
• Arecibo radio
telescope
http://www.astro.psu.edu/users/alex/
pulsar_planets_text.html
Extrasolar planet geometry
• One must think about geometry at which the system is
being observed
• Let
i = inclination of the planet’s orbital plane with respect to
the plane of the sky
 = angle of the planet’s orbital planet with respect to the
observer
Radial (Doppler) Velocity
http://www.eso.org/public/videos/eso1035g/
Radial (Doppler) Velocity
Jupiter: K = 12.6 m/s
Earth : K = 0.1 m/s
http://www.astro.sunysb.edu/mzingale/software/astro/
First detection around sun like star :
51 Pegasi b ~ 0.5 Jupiter (Mayor & Queloz, 1995)
Velocity curve for 51 Pegasus
(Mayor & Queloz, 1996)
• Mass of the planet is only a lower limit because the plane of the
planet’s orbit is uncertain (Msin I = 0.47 MJ in this case)
http://obswww.unige.ch/~udry/planet/51peg.html
Radial (Doppler) Velocity
elliptical orbit
Velocity curve for HD66428
• More often than not, the velocity curves are not symmetric
 orbit is eccentric (e = 0.5 in this case)
http://exoplanets.org/figures.html
RV around M-dwarfs
Mahadevan et al.(2011)
Currently known exoplanets
exoplanet.eu
J
E
V
Planet eccentricity vs. semi-major axis
(Jan 27, 2012)
Extrasolar Planet Encyclopedia
88 Known
Multi - Planet Systems
Kepler - 11
Planetary systems allow for more
detailed analysis
• 2 massive planets
orbiting HD 168443
• Planetary masses
– 8 MJ
– 18 MJ
HD 69830
b : 0.61 neptune
c : 0.70 neptune
d : 1.07 neptune
Gliese 581 system
• Spectral type: M3V
(0.31 M, 0.0135 L)
• 4 planets discovered
by radial velocity:
b
c
d
e
a (AU) Mass (M)
0.041 >15.6
0.073 >5.06
0.253 >8.3
0.028 >1.7
Ref.: S. Udry et al., A&A (2007)
(Image from Wikkipedia)
Tentative conclusions for the
Gliese 581 system*
• Gliese 581c (> 5.1 M) is probably not habitable
– Stellar flux is 30% higher than that for Venus
• Gliese 581d (>8.3 M) could conceivably be
habitable, but it is probably an ice giant
– Near the (poorly determined) outer edge of the HZ
*Selsis
et al., A&A (2007)
*von Bloh et al., A&A (2007)
Gliese 581g ?
• Gliese 581g (~ 3 M),
“Zarmina’s world”,
apparently exists in the
HZ (Vogt et al 2010)
• The Swiss group with
HARPS instrument
found it doesn’t exist !
Planet Mass Distribution
Occurrence rate α M-0.48
(for periods < 50 days)
Howard et al. (2011), Science
Packed Planetary systems
Planetary systems form in such a way that the system
could not support additional planets between the orbits of the
existing ones (gaps with stable orbits contain an unseen planet)
Barnes et al.(2005)
Kopparapu et al.
(2009)
HD 74156 (Barnes et al.2005)
HD 47186 (Kopparapu et al. 2009)
Gravitational microlensing
dL
ds
• Planets can also be detected by gravitational microlensing
• This method takes advantage of the fact that, according
to general relativity, light rays are bent by a gravitational
field
-- or, equivalently, space-time is distorted and light travels
along straight paths in the distorted reference frame)
http://www.astro.cornell.edu/academics/courses/astro201/microlensing.htm
A microlensing event
• When the lensing star passes in front of the source star, the light
from the source star is amplified by a factor of as much as 10-20
• The typical duration of a microlensing event is minutes to hours
http://www.nasa.gov/topics/universe/features/planet20110518-video.html
An event with planets
• If the lensing star has
planets, then the light
curve can be distorted
(i.e., you get spikes)
• The planets must be near
the Einstein ring radius to
be detected
– Typically, the ring radius is
outside of the habitable
zone, so this technique is
not that useful for finding
habitable planets
http://exoplanet.eu/catalog-microlensing.php
Planet Mass Distribution
(Microlensing)
Sensitivity:
0.5 to 10 AU
5 Earth to 10 Jup
• The majority of all
detected planets have
masses below that of
Saturn, though the survey
sensitivity is much lower
for those planets
• Low-mass planets are
thus found to be much more
common than giant planets.
Cassan et al.(2012)
Planet Mass Distribution
(Microlensing)
• 17% of stars host Jupiter
mass planets
• 52% of stars host Neptune
mass
• 62% of stars host SuperEarths
• On average, every star in
the Milkyway has 1.6
planetstwithin 0.5 to 10 AU !!
• Planets around stars in
our Galaxy thus seem to
be the rule rather than the
exception.
Historical astrometry: Barnard’s star
• Second closest star to Earth (6
light yrs), in Ophiucus
• Red dwarf (M3.8)
• Largest stellar proper motion
(10.3”/yr)
– Moving towards us. Will be
closest star (3.8 l.y.) in about
12,000 yrs
• Discovered by Edward
Emerson Barnard (1957-1923)
• Studied hard by Peter van de
Kamp from 1938 until his death
in 1995. Thought to have a
planet, but this hypothesis was
later proved to be incorrect
1985
1990
1995
2000
2005
The sexagesimal system of angular
measurement
unit
value
symbol
abbreviations
degree
1/360 circle
°
deg
arcminute
1/60 degree
′ (prime)
arcmin, amin,
MOA
arcsecond
1/60 arcminute
″ (double
prime)
arcsec
milliarcsecond
1/1000
arcsecond
mas
conversion
17.45 mrad
,
290.89 µrad
4.8481 µrad
4.848 nrad
• Equivalently, there are 1,296,000 arcsec in a circle
http://en.wikipedia.org/wiki/Arcsecond
Determination of parallax
• A star’s parallax, p, is the
angle by which it appears
to move as the Earth
moves around the Sun
• A star that moves by 1
arcsecond when Earth
moves by 1 AU relative to
the Sun is defined to be
at at distance of 1 parsec
• 1 pc = 1 AU/sin p
= 3.0857×1013 km
= 3.262 light years
http://en.wikipedia.org/wiki/Parsec
Astrometric method
• Calculated motion of the Sun
from 1960 to 2025, as viewed
from a distance of 10 pc, or
about 32 light years above the
plane of the Solar System, i.e.,
at i = 0o
• Scale is in arcseconds
• You get the actual mass of the
planet because the plane of
the planet’s orbit can be
determined
• Can do astrometry from the
ground, but the best place to
do it is in space 
http://planetquest.jpl.nasa.gov/Navigator/
material/sim_material.cfm
Astrometric missions
• Hipparcos 1989 – 1993
(ESA)
• Precise proper motion &
parallax for 118,000 stars
(1 milli-arc sec)
Sun-Earth
0.3 microarc sec
• Gaia 2013 (ESA)
• Parallax for 1 billion stars
(20 micro-arc sec)
• 3-D map of our Galaxy
SIM – Space Interferometry Mission
• This mission will do extremely accurate astrometry from space
http://planetquest.jpl.nasa.gov/SIM/simImageGallery.cfm