Download Planets Around Other Stars

Planets Around
d Other Stars
How to makke a planet
How to mak
ke a planet
• Large, cool cloud of gass and dust
– Gas makes the star, dus
, st is necessary for planet y
p
formation
– Dust is usually made of
Dust is usually made off metals (Fe, Ni, Al), rocks f metals (Fe Ni Al) rocks
(silicates) and ices (solid H2O, CH4, NH3)
– Mostly H and He (these
Mostly H and He (thesee two elements make up e two elements make up
about 98% of our Solarr System)
• Cloud begins to collapsse under its own self‐
gravity
How to makke a planet
How to mak
ke a planet
• Collapse causes
ll
– Increase in temperaturre (conservation of energy)
– Increase in rate of rotattion (conservation of angular momentum)
• Result is a rapidly rotatting disk of gas and dust
– Again, dust means rock
Again, dust means rockks, metals and ices
ks, metals and ices
– at sufficient distance frrom the parent star, hydrogen compounds (H
hydrogen compounds (
( 2O, CH
O, CH4 NH3) are ) are
important in formation
n of giants (Jovian planets are far from the Sun))
How to makke a planet
How to mak
ke a planet
• Accretion
– Dust grains collide to fo
g
orm larger particles ‐‐>
g p
– Collide small particles tto form still larger particles ‐‐> – Collide large Collide large “boulders
boulders” to form planetesimals to form planetesimals
(asteroids, comets, Kuiper Belt Objects)‐‐> – collide planetesimals to
ll d l
l o form planets
f
l
How to makke a planet
How to mak
ke a planet
•
•
•
•
•
Nebular H
b l Hypothesis
h i
Solar Nebula
Contraction into rotatting disk w/ hot center
Dust grains accrete to
Dust grains accrete to
o form larger and larger
o form larger and larger particles
L
Large particles sweep
i l
out more material to i l
form planetesimals
Planetesimals eventuaally collide to form p
planets
Basic prroblem:
A good scientific theoryy has to be tested, but we only have one So
y
olar System and we y
cannot go back in tim
me.
We needed to fin
We needed to fin
nd other planets!
nd other planets!
So we did and …
Exoplanets
• A planet that orbits a sstar other then the Sun
• Also can be called an Also can be called an “eextrasolar
e
planet”
planet
• There are 1890 exoplan
nets as of February 2015
– Many more than in ourr Solar System
• First planet (similar to t
p
(
those in our Solar System) discovered in ~~1995
– Detection is very challe
Detection is very challeenging!
Exoplanets
• Known Exoplanets characteristics
– Largest ~50 Jupiter mas
g
p
sses
– Smallest ~1 Earth masss
– Closest to parent star ~
Closest to parent star ~0.01 AU ~0
~ 01 AU
• Period less than 1 day
– Furthest from parent st
h f
tar ~1000 AU
• Very wide range!
y
g
Examples of Sttellar Systems
Examples of St
tellar Systems
• Gliese
Gli
8 6
876
a) Parent star:
– M3.5 V, T=3480 K, L=0.01
M3 5 V T 3480 K L 0 01
124 Lsun
124 L
b) Gas Giant
– M=2MJ,, a=0.208 AU
c) Gas Giant
– M=0.62 MJ, within orbit o
of Gliese 876 b
d) Super Earth
– Inside orbits of both Gliese 876 b and c
Examples of Sttellar Systems
Examples of St
tellar Systems
• Gliese
Gli
581
a) Parent star:
– M3 V, T
M3 V T=3480
3480 K, L
K L=0
0.013 013 Lsun
b) Gas Giant (Neptune‐siized)
– M=16 ME, a = 0.04 AU
c)) Rocky Planet
R k Pl t
– M=5 ME, a = 0.07 AU, witthin habitable zone (Temperature could be as low as ‐3 oC or as high as 500 oC, due to runaway greenhouse akin to Venus)
greenhouse akin to Venu
d) Super Earth
– M=7 ME, a = 0.22 AU, witthin habitable zone
e) Super Earth
– M=1.9 ME, a=0.03 AU
Upsilon An
p
dromedae
Two planets are several times moree massive than Jupiter
The third planet mass 75% that of Jupiter, is so close to the star that The third planet, mass 75% that of J
Jupiter
J
is so close to the star that
it completes a full orbit every 4.6
6 Earth days
Why is it so hard to find planets around other
d th r stars?
t ?
• Faint planet glimmer is lost Faint planet glimmer is lost in glare from parent star
in glare from parent star
– Planets are small, close to their parrent star, and shine by reflected starlight
• Thought experiment:
Thought experiment:
– Imagine grain of rice an inch from aa 100 Watt light bulb. Someone standing at end of a long dark hall would seee only the light bulb, not the grain of rice. • Consider the case of Jupiterr and the Sun:
– As seen from the nearest star, Alpha Centauri, Jupiter would appear a billionth as bright as the Sun.
ose to the Sun, only 4 arc sec away.
– Jupiter would also be extremely clo
• Si
Since all other stars are fart
ll th t
f ther than Alpha Centauri, h th Al h C t i
Jupiter would be even hardeer to detect from other stars
Planets are very hard to
o observe directly (by detecting their own light)
detecting thei
r own light)
• Planets are too faint compa
pared with their star
– This brown dwarf star is barely visible ‐ and its star is faint
• Planets shine by reflected liight
– Planets close to parent stars aare brightest, but hardest to see
Gravitational Motions
• The Sun and Jupiter orbit around their common center of mass.
• The Sun therefore wobbles around that center of mass with same period as Jupiter.
Gravitational Motions
• Th
The Sun’s motion S ’
ti
around the solar system’ss center of system
center of
mass depends on tugs from all the planets.
l t
• Astronomers around other stars that
other stars that measured this motion could determine the masses and orbits of all the planets
all the planets.
Astrometric Motion
• W
We can detect d
planets by measuring the change in a star’ss the change in a star
position on sky.
• These tiny motions These tiny motions
are very difficult to measure (~ 0.001 measure (
0 001
arcsecond).
Doppler Te
pp Technique
q
• M
Measuring a star’s i
’
Doppler shift can tell us its motion toward
us its motion toward and away from us.
• Current techniques can measure motions
can measure motions as small as 1‐2 m/s (walking speed!)
(walking speed!).
First Extrasolar Planet: 51 Pegasi
P
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 – well within
orbit of Mercury.
Mercury
• This was the first
extrasolar planet
discovered around a
normal stars (1995).
51 Peg b (1995)
Half the mass of Jupiter, yet
y orbiting much closer to
the Sun tha
an Mercury!
Other Extrasolar Planets
• Doppler
Doppler shift data tell us shift data tell us about a planet
about a planet’ss mass and mass and
the shape of its orbit.
Planetaryyy signatures
g
Size depe
ends on
mass of planet
Low mass, high mass
Circular, eccentric
Periodicity
y depends
on period of planet
Shape depends on eccentricity of planet
Small period, large period
Changes in RV depend on mass, period and
eccentricity of planet
Dopple
pp er shift
• Look for periodic shift in st
Look for periodic shift in sttar
tar’ss spectrum
spectrum
– Does not depend on distan
nce of star
– Need massive planet near d
l
star
he faster the orbital speed (of • the closer the planet, th
b h l
both planet and star)
d
)
– Need very good spectrum
• Many planet detections
• Incredibly hard measurem
Incredibly hard measurem
ments have now become ments
have now become
standard
Limitations
Limitations of Do
of Do
oppler technique
oppler technique
From ground‐based g
observatories, detect shifts > 1‐2 m/sec
The nearer to the star and The
nearer to the star and
the more massive the planet, the easier to
planet, the easier to detect
Radial velocity method doesn’t give all the orbital info
ll th
bit l i formation
ti
• Doppler
Doppler shift only detects
shift only detectss velocity along line of sightt
– Can’t distinguish massive g
planet (or brown dwarf!) in tilted orbit from less massive planet in edge on
massive planet in edge‐on
n orbit
– They both have the same y
line‐of‐sight velocity
• The only way to resolve this ambiguity is to hi
bi i i
observe using another method
Tran
nsits
• As planet moves across face of star, it blocks a y
g
tiny bit of starlight
• Watch for periodic dimm
ming of star
Planet detected around the star HD209458 by transit metthod
• Planet
Planet is 70% mass of Jupiter, is 70% mass of Jupiter
but orbits in just 3.5 days
• So it is very close to its parent star
• Many planets have been found this way
found this way
• Amateur astronomers have organized to watch for g
fluctuations in star brightness
– http://www.transitsearch.org/
Transits and Eclipses
Transits and • A transit is when a planet ccrosses in front of a star.
• Eclipse
l
is when a planet goe
h
l
es behind a star
b h d
• From both, can learn aboutt atmosphere of planet (b
(beginning and end of trans
d d f
sit, eclipse)
l
)
Transitin
ng planets
gp
Orbital inclination must be
Orbital
inclination must bee closely aligned to our line
e closely aligned to our line‐
of‐sight
This makes many addition
This
makes many additional interesting planetary al interesting planetary
properties possible to meaasure
size, mass, temperature and sp
pectra
Can test the atmospheric mode
p
els that have been developed for p
planets in our solar system
Some of these planets are subjeect to more exciting conditions than the ones in the Solar System:
Small distance from star
Extreme eccentricities
NASA’s Kepler Space Mission to detect transitingg planets
March 2009: NASA mission Keepler was launched
Repeated images ~100,000 stars in the constellation Cygnus for transiting planet siignals
Has found thousands of possible planets
some have been confirrmed
some have been confir
not called a planet until follow‐up is executed NASA’s Kep
pler Mission
Determining the frequency of Earth-size
E
and larger planets
in the habitable zon
ne of Sun-like stars
Kepler 10‐b
• Earth‐sized planet
• Very close to its parent star, so very
parent star, so very hot
Transit and
d Doppler
pp
Measurementss Yield Density
+
10b Size
Dennsity
Mass
Volume
= 8.8 g/cm3
Composition o
off Kepler
Kepler-10b
10b
Gravitational Microlensing g
• Background: Microlensing Background: Microlensing around a star (or black hole)
around a star (or black hole)
Needs
N
d almost
l
t
perfect
alignment
between source
and lens.
Planet detection: ffine structure on microlensingg light curve
g
• A few planets have been p
discovered this way
Potentially very useful:
• Potentially very useful: can detect planets at large distances from us
large distances from us
– Even farther away than transit method can
– Much farther than radial velocity or astrometry can
• Samples a different p p
population of stars than other techniques
Direct Imaging
Direct I
• Use planet
Use planet’ss own light
own light
• Take image of it
– Can
Can reconstruct full orbit reconstruct full orbit
by watching it go around
• Can also obtain spectra
p
– Learn about physical conditions, atmosphere, maybe even presence of b
f
life
• Jupiter
Jupiter is a billion times is a billion times
fainter than the Sun, in visible light!
visible light!
First Images of
o Exoplanets:
Exoplanets:
HR 8799 So
Solar
l System
S t
Marois et al. 2008, Science Magazine
First images of exopla
g
p nets: Fomalhaut
Star
location
Neptune-sized
orbit
http://dps.aas.org/education/dpsdisc/
Hubble Space Telescope Hubble
Space Telescope
visible image of the star Fomalhaut (whose light was blocked) with a
was blocked), with a dust belt similar to the Kuiper belt. Inset: Images taken ~2 years apart show a planet moving around the star.
Old list of other planetary systems
Characteristics of extra‐solar planets
p
• Much higher eccentriciity in most of their orbits than in our Solarr System
y
• Much higher fraction o
of planets very close to their parent stars
their parent stars
• Many planets are “super‐Jupiters” (up to 10 times more massive than Jupiter)
• Microlensing suggests Microlensing suggests systems like our own systems like our own
may be around ~1/3 off all stars
Eccentric O
Orbits
Page
Many extrasolar planets are very close to pareent stars
very close to pare
ent stars
• This is a selection effect
(caused by detection
method))
• But it does show that such
planets
l
t exist!
i t!
• Our Solar System is very
different (green points) Mercury is farther away from
m
Sun than many of the extra-solar planets are from their
stars
Patterns in ecceentricity
Patterns in ecce
• Most new planets are in elliptical orbits
• Short period planets: Short period planets:
– Very close to parent stars very low
stars, very low eccentricity
– Same process that S
h
moved planets close to star circularized their i l i d h i
orbits
Parent stars of exxtrasolar planets
Parent stars of ex
• Hi
High in elements heavier hi l
t h i
than hydrogen and h li
helium (usually > Sun)
(
ll > S )
– reasonable: planets
form from dust
form from dust
• Probability of finding a planet increases as heavy l ti
h
element content of parent star increases
t t i
1.6
Pplanet ~ (NFe/ NH)
What are theese planets?
What are the
ese planets?
• Our Solar System has small rocky planets close to star, large gas giants further awaay
– no experience of large massivve planets close to sun in our S l S t
Solar System
• Theory of giant planet form
mation says they have to form outside “frost
form outside frost line
line”
Page
Hot Jupiters
p
How are “Hot
Hot Ju
upiters” formed?
upiters
Theory for our Solar System:
Stellar wind from young Sun
blew volatiles outwards
Ice condensation at 5 AU where
water-ice solidified
Fast accretion of large icy planet
(~10 MEarth) which then
collected H/He atmosphere
Gas giants Jupiter, Saturn
just outside “frost line”
Small rocky planets inside
Slowly accreting icy planets
in outer system (Uranus,
Nept ne)
Neptune)
Extrasolar giant planets:
Do they form in situ?
looks impossible: too hot for
i
ices,
ttoo little
littl material
t i l ffor rock
k
Do they form outside frost line and
g
inwards?
migrate
planet forms in gas/dust disc
around star
drag from remaining gas/dust
causes it to spiral inwards
Does scattering from other giant
planets causes migration
why does it stop?
Interesting questions!
Interesting
• O
Original theories of solar sy
i i l h i
f l ystem formation developed f
i d l
d
when our own Solar System
m was the only one
– Mostly circular orbits
M tl i l
bit
– Giant planets in outer solar ssystem, terrestrial planets inside
• Many new Solar Systems a
Many new Solar Systems are (in general) not like ours
re (in general) not like ours
• Needs new ideas
• How to arrive at a new parradigm?
– Use computer simulations to
o develop ideas, test hypotheses, make predictions
make predictions
– Test predictions against obseerved young solar systems, disks
Page
Theories for how giant planets got so close to t
l
heir stars
1. Interactions between individual new planets and gaseous disk. “Migration””
2. After gas disk cleared awaay, several giant planets in outer parts of solar system
m were left
– Three‐body gravitational intteractions between them
– One giant planet got slung o
outwards, a second was slung inwards and got “captured”” by the star in a close orbit
– But why isn’t the close orbitt very elliptical?
• Why didn’t Jupiter migrate inwards close to Sun?
What have we learned so far?
What have we learned so far?
• Most stars have planets h
l
of some sort
f
y y
ar to ours
• Many systems not simila
• A large fraction do have systems similar to ours though
– Roughly 1/3
• M
More work is needed to ki
d d t understand new systems d t d
t
and to better refine how
w many stars have systems similar to ours
i il t
– Also need more work to find Earth analogs
What is a Habitaable Planet?
What is a Habita
able Planet?
A good planet is:
• N
Not too big
g
• Not
N too small
• Not
N too hot or too cold
What does “habittable” mean
•
Right temperature
•
Liquid water
•
Free oxygen to breath
•
Light for warmth and
perception
•
Radiation shield
•
Some form of
asteroid/comet protection
Things That Affect TTemperature
Want temperature so you can ha
Want
temperature so you can haave liquid ave liquid
water on the surface of the p
planet
280°
260°
240°
220°
Water boils
->
200°
Temperature of star 180°
160°
160
Distance from the star
140°
120°
p
p
or Shape of planet’s orbit: circular o
elliptical
p
g
e Planet’s atmosphere: greenhouse
gases
100°
80°
60°
40°
Water freezes ->
20°
20
0°
-20°
-40°
The Habitable Zone for Stars of Various Masses
The Habitable Zone (HZ) in grreen is the distance from a star
where liquid water is expected
d to exist on the planets
surface.
Summ
mary
• The
The ~ 2000 planets we have
2000 planets we havee detected to date are only e detected to date are only
a sub‐set of potential planeets out there
• These new solar systems ha
These new solar systems haave raised big questions ave raised big questions
about how planets formed in general
• Future search methods hav
F t
h
th d h ve high probability of finding hi h
b bilit f fi di
more (and more varied) plaanets
• It’s hard to find Earth‐like p
planets
– But prospect of finding Earth‐‐like planets is exciting!
Page
Suppose you found a st
f
d tar with the same mass h h
as the Sun moving back and forth with a period of 16 months. What could you conclude?
a.
b.
c
c.
d.
The star has a planet orrbiting at less than 1 AU
The star has a planet orrbiting at greater than 1 AU
The star has a planet orrbiting at exactly 1 AU
The star has a planet, but we do not have enough
information to know its o
orbital distance
What do you think of the observation that other planets
may exist in the habitable zo
ones of other stars?