AST 309part 2:

Extraterrestrial Life

Terrestrial Planet (and Life) Finder

The Drake Equation:

N = N* fpl nhab fL fC fT L/T

Stars? Planets? Habitable Origin Complex Intelligence, Lifetime planets? of life? life? technology? of civilization

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.

Now estimate number of planets with life in our Galaxy(not number with intelligent, communicating life)

Overview:

Terrestrial Planet Finder (TPF)Mission goals:

Target:Nearby stars (why?)Sun-like stars (why)?

Detect:1. Habitable Planets2. Life using Biosignatures

Typical Mass Spectral Estimated(O) Type Lifetime

30 O 2 M10 B 20 M3 A 500 M

1.5 F 4 B1 G 10 B

0.7 K 17 B0.2 M 56 B

M = million, B = billion

Table 5.8Length of Time a Star Remains on

the Main Sequence

Earth : ~10–10 separation = 0.1 arcseconds for a star at 10 parsecs

1 AU = 1 arcsec separation at 1 parsec

There are no stars within 1 parsec (3.26 light years)

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 ConstellationDistance

(light-years)

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:

α Cen A [G2 V] V=0.01Teff = 5790 K

M = 1.10 M

α Cen B [K1 V] V=1.33Teff = 5260 K

M = 0.93 M

Binary:P = 79.91 yrsa = 23 AUe = 0.52i = 79°

d = 1.34 parsecs = 4.37 lyr = 227,600 A.U. ~ 40 × 1012 km

Angular separation: 2 – 22 arcsec (2009: 7.5”)

1 ME 10 ME 1 ME 10 ME a (AU) K (m/s) K (m/s) P (d) K (m/s) K (m/s) P (d) 0.05 0.39 3.92 3.88 0.43 4.26 4.23 0.1 0.28 2.77 10.99 0.30 3.01 11.95 0.4 0.14 1.38 87.9 0.15 1.51 95.6 0.6 0.11 1.13 161.5 0.12 1.23 175.6 1.0 0.09 0.88 347.5 0.10 0.95 377.9 2.0 0.06 0.62 982.8 0.07 0.67 1069. 3.0 0.05 0.51 1805. 0.05 0.55 1964.

The challenge of detecting Earth-mass planets α Cen A α Cen B

•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)

Significance of signal depending on N(total number of measurements):

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

Radial velocity detection of Earth-like planets

Strategies to detect Earth-like planets around nearby stars

Doppler detection of Earth analoguesis possible with a precision of 2-3 m/sand ~50,000 measurements over 4 -5 yrs!IF error budget is dominated by whitenoise!

Pilot Study:semi-dedicated telescope/spectrographfor 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 Displacment (as)

Jupiter at 1 AU 100

Jupiter at 5 AU 500

Jupiter at 0.05 AU 5

Neptune at 1 AU 6

Earth at 1 AU 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 as

V-mag 20: 200 as

Launch: 2012

Space Interferometry mission (SIM)

60 solar-type stars

precision of 4 as

GAIA Detection limits

Red: G-stars

Blue: M Dwarfs

Casertano et al. 2008

Its 5 year mission is to boldly go where no planet hunter has gone before:

• Demonstrated precision of 1 as and noise floor of 0.3 as 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.“

Detecting Earth-like Planets

with SIM:

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 TermsContrast Stability 2.00E-11 Thermal + JitterInstrument Stray Light 1.50E-11 Incoherent lightInner Working Angle 4 λ/Dlong 57 mas at λ=550 nm, D long = 8 mOuter Working Angle 48 λ/Dshort 1.5 arcsec at λ=550 nm, D short = 3.5 mBandpass 500-800 nm Separate observ. in 3 100 nm bands.

TPF-I:uses Nulling Interferometry

in the infrared:

Mars

Earth

Venus

Simulated Solar System detectionWith TPF-I

searching for terrestrial planets around as many as 500 nearby stars!

External Occulter

50000 km

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-biologically-generated.

• 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