AST 309
part 2:
Extraterrestrial Life
Terrestrial Planet (and Life) Finder
The Drake Equation:
N = N*
fpl
nhab
Stars? Planets? Habitable
planets?
fL
fC
fT
L/T
Origin Complex Intelligence, Lifetime
of life? life?
technology? of civilization
Now estimate number of planets with life in our Galaxy
(not number with intelligent, communicating life)
If we leave out fi and fc (i.e. assume they are unity—all life forms develop our kind
of intelligence and technology and try to communicate), we are calculating the
number of life-bearing planets in our Galaxy at any given time (like now). We
know there has been life on our planet for 3 billion years, so take L = 3 billion.
Let’s be optimistic about fP (0.1), nP (1), and fL= (0.1). Then
Nlife ~ 1011 x 0.1 x 1 x 0.1 x (3 billion/10 billion) = 300 million
300 million planets with life in our Galaxy! That’s roughly1 out of 1000 stars. This
means that the nearest life-bearing planet might only be 10-100 light years away,
close enough that in the future we may be able to detect such planets and obtain
their spectra (that is the primary goal of astrobiology space missions for the next
decade).
This result is a major reason for exerting most of our effort toward detecting
signatures of biochemistry in the spectra of planets orbiting nearby stars.
Overview:
Terrestrial Planet Finder (TPF)
Mission goals:
Target:
Nearby stars (why?)
Sun-like stars (why)?
Detect:
1. Habitable Planets
2. Life using Biosignatures
Table 5.8
Length of Time a Star Remains on
the Main Sequence
Typical Mass Spectral Estimated
(O)
Type Lifetime
30
10
3
1.5
1
0.7
0.2
O
B
A
F
G
K
M
M = million, B = billion
2M
20 M
500 M
4B
10 B
17 B
56 B
1 AU = 1 arcsec separation at 1 parsec
There are no stars within 1 parsec (3.26 light years)
Earth : ~10–10 separation = 0.1 arcseconds for a star at 10 parsecs
Overview:
Kepler looks at stars
>1000 light years away
(too see as many as possible)
These are too far away for TPF!
The Solar Neighborhood:
The Solar Neighborhood:
Rank
Target star
Constellation
Distance
(lightyears)
Spectral type
1
Alpha Centauri A
Centaurus
4.3
G2V
2
Alpha Centauri B
Centaurus
4.3
K1V
3
Tau Ceti
Cetus
12
G8V
4
Eta Cassiopeiae
Cassiopeia
19
G3V
5
Beta Hydri
Hydrus
24
G2IV
6
Delta Pavonis
Pavo
20
G8V
7
Pi3 Orionis
Orion
26
F6V
8
Gamma Leporis
Lepus
29
F7V
9
Epsilon Eridani
Eridanus
10
K2V
10
40 Eridani
Eridanus
16
K1V
TPF Targets:
There are ~100 stars within 22 light years
Step 1: find the Earth-like planets:
- Radial velocity?
- Astrometry?
- Direct Imaging?
Probing the HZ of our closest star, Proxima Centauri (M5V):
Introducing the Neighbors:
d = 1.34 parsecs = 4.37 lyr = 227,600 A.U. ~ 40 × 1012 km
α Cen A [G2 V] V=0.01
Teff = 5790 K
M = 1.10 M
α Cen B [K1 V] V=1.33
Teff = 5260 K
M = 0.93 M
Binary:
P = 79.91 yrs
a = 23 AU
e = 0.52
i = 79°
Angular separation: 2 – 22 arcsec (2009: 7.5”)
The challenge of detecting Earth-mass planets
α Cen B
α Cen A
a (AU)
0.05
0.1
0.4
0.6
1.0
2.0
3.0
1 ME
K (m/s)
0.39
0.28
0.14
0.11
0.09
0.06
0.05
10 ME
K (m/s)
3.92
2.77
1.38
1.13
0.88
0.62
0.51
P (d)
3.88
10.99
87.9
161.5
347.5
982.8
1805.
1 ME
K (m/s)
0.43
0.30
0.15
0.12
0.10
0.07
0.05
10 ME
K (m/s)
4.26
3.01
1.51
1.23
0.95
0.67
0.55
P (d)
4.23
11.95
95.6
175.6
377.9
1069.
1964.
Significance of signal depending on N
(total number of measurements): •classic signal detection problem:
S << σ, N ~ 104 – 105
•Systematic errors under control! Mostly “white” noise
(= random, in time uncorrelated errors)
•Greg Laughlin’s proposal to observe α Cen with N~105
•Remaining systematic noise sources are stellar origin
(pulsation, star spots, magnetic cycle)
Radial velocity detection of Earth-like planets
Alpha Centauri campaign at the McLellan 1 m telescope at
Mt. John Observatory (NZ) with Stuart Barnes & John Hearnshaw
Radial velocity detection of Earth-like planets
Strategies to detect Earth-like planets around nearby stars
Doppler detection of Earth analogues
is possible with a precision of 2-3 m/s
and ~50,000 measurements over 4 -5 yrs!
IF error budget is dominated by white
noise!
Pilot Study:
semi-dedicated telescope/spectrograph
for intensive multi-year campaign on
Alpha Cen.
Future:
Network of dedicated 2-4 m class telescopes with precision velocity spectrographs
=> monitor all 10-20 nearby solar-type stars over ~5 years
=> after a decade we would have an “Input Catalog” of candidates for space missions
like the Terrestrial Planet Finder (TPF), allowing detailed follow-up observations
Astrometric Detections of Exoplanets
The Challenge:
for a star at a distance of 10 parsecs (=32.6 light years):
Source
Jupiter at 1 AU
Jupiter at 5 AU
Jupiter at 0.05 AU
Neptune at 1 AU
Earth at 1 AU
Displacment
(mas)
100
500
5
6
0.33
Space Astrometry:
Hipparcos
3.5 year mission ended in 1993
~100.000 Stars to an accuracy of 7 mas
Gaia
1.000.000.000 stars
V-mag 15: 24 mas
V-mag 20: 200 mas
Launch: 2012
Space Interferometry mission (SIM)
60 solar-type stars
precision of 4 mas
GAIA Detection limits
Casertano et al. 2008
Red: G-stars
Blue: M Dwarfs
Detecting Earth-like Planets
with SIM:
Its 5 year mission is to boldly go where no planet hunter has gone
before:
• Demonstrated precision of 1 mas and noise floor of 0.3 mas
amplitude.
• Multiple measurements of nearest 60 F-, G-, and K- stars.
• Directly test rocky planet formation
„This paucity of low mass planets is almost certainly an artfact of sensitivity, as
the Doppler technique struggles to detect lower-mass planets. Thus, we have
reached a roadblock in planetary science and astrobiology.“
Jupiter only
1 milliarc-seconds for a Star
at 10 parsecs
The previous simulation was only with one planet, but a
system will look like this…
Direct Imaging:
• Need to go to space too!
• Contrasts of 10-9 or better at very small
angles!
• 3 different concepts:
– Advanced coronagraph (TPF-C)
– Nulling Interferometer (TPF-I)
– External Occulter
TPF-I
TPF-C:
Limiting delta magnitude ~ 25!
allowing it to search for terrestrial planets
in ~150 nearby star systems.
Primary mirror: 8.0 x 3.5 m
TPF-C:
High Level TPF-Coronagraph Contrast Error Budget Requirements.
Requirement
Comment
Static Contrast
6.00E-11
Coherent Terms
Contrast Stability
2.00E-11
Thermal + Jitter
Instrument Stray Light
1.50E-11
Incoherent light
Inner Working Angle
4 λ/Dlong
57 mas at λ=550 nm, D long = 8 m
Outer Working Angle
48 λ/Dshort
1.5 arcsec at λ=550 nm, D short = 3.5 m
Bandpass
500-800 nm
Separate observ. in 3 100 nm bands.
TPF-I:
uses Nulling Interferometry
in the infrared:
Earth
Venus
Mars
Simulated Solar System detection
With TPF-I
searching for terrestrial planets around
as many as 500 nearby stars!
External Occulter
50000 km
q
At a distance of
50.000 km the
starshade subtends
the same angle as
the star
Biosignatures:
• The most convincing spectroscopic evidence for life as we know it is the
simultaneous detection of large amounts of oxygen as well as a reduced
gas, such as methane or nitrous oxide, which can be produced by living
organisms. Oxygen, methane, and nitrous oxide are produced in large
amounts by plants, animals, and bacteria on Earth today, and they are
orders of magnitude out of thermodynamic equilibrium with each other.
Biosignatures:
Biosignatures:
The visible and infrared spectrum, in conjunction with theoretical and empirical
models, can tell us about the amount of atmosphere, the gases present in the
atmosphere, the presence of clouds, the degree and variability of cloud cover or
airborne dust, and the presence of a greenhouse effect.
Biosignatures:
Simulation of low-res, low-S/N spectrum acquired in 40 days with TPF-I
Biosignatures:
• We must be able to identify potential "false-positives," the
nonbiological generation of planetary characteristics that mimic
biosignatures. For example, while atmospheric methane may be
a possible biomarker on a planet like Earth, especially when
seen in the presence of oxygen, on a body like Titan it is simply
a component of the atmosphere that is non-biologicallygenerated.
• Theoretical and experimental research and analysis are
necessary to secure a detailed understanding of the biosignatures
that might be found.
The Red Edge
Plants have Chlorophyll which absorbs
in green wavelengths. Planets are thus
more reflective in the infrared.
Biosignatures:
Biosignatures:
Lightcurve of Earth (w/o clouds):
with clouds:
Summary:
• Finding Earth-like planets is extremely difficult
• Need to target the nearest stars
• Can use three methods (RV, Astrometry, Imaging)
• Ultimate goal: collect enough photons to perform spectroscopy
and search for biosignatures