Terrestrial Planet (and Life) Finder AST 309 part 2: Extraterrestrial Life
The Drake Equation: N = N * f pl n hab f L f C f T L/T Stars? Planets? Habitable Origin Complex Intelligence, Lifetime planets? 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 f i and f c (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 f P (0.1), n P (1), and f L = (0.1). Then N life ~ 10 11 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 O 2 M 10 B 20 M 3 A 500 M 1.5 F 4 B 1 G 10 B 0.7 K 17 B 0.2 M 56 B M = million, B = billion
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 Pi 3 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):
α Cen A [G2 V] V=0.01 V=0.01 T eff = 5790 K M = 1.10 M α Cen B [K1 V] V=1.33 V=1.33 T eff = 5260 K M = 0.93 M Binary: P = 79.91 yrs a = 23 AU e = 0.52 i = 79 Introducing the Neighbors: d = 1.34 parsecs = 4.37 lyr = 227,600 A.U. ~ 40 10 12 km Angular separation: 2 22 arcsec (2009: 7.5 )
The challenge of detecting Earth-mass planets α Cen A α Cen B 1 M E 10 M E 1 M E 10 M E 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. Significance of signal depending on N (total number of measurements): classic signal detection problem: S << σ, N ~ 10 4 10 5 Systematic errors under control! Mostly white noise (= random, in time uncorrelated errors) Greg Laughlin s proposal to observe α Cen with N~10 5 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 Displacment (µas) 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 µas V-mag 20: 200 µas Launch: 2012 Space Interferometry mission (SIM) 60 solar-type stars precision of 4 µas
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 µas and noise floor of 0.3 µas 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, sensitivity, as the Doppler technique struggles to detect lower-mass planets. 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.
uses Nulling Interferometry in the infrared: Earth TPF-I: Venus Mars Simulated Solar System detection With TPF-I searching for terrestrial planets around as many as 500 nearby stars!
External Occulter 50000 km θ 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 is the simultaneous detection of large amounts of oxygen as well as a well as a reduced gas, such as methane or nitrous oxide, which can be can be produced by living organisms. Oxygen, methane, and nitrous oxide nitrous oxide are produced in large amounts by plants, animals, and and bacteria on Earth today, and they are orders of magnitude out of 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 the nonbiological generation of planetary characteristics that that mimic biosignatures. For example, while atmospheric atmospheric methane may be a possible biomarker on a planet planet like Earth, especially when seen in the presence of of oxygen, on a body like Titan it is simply a component of the of the atmosphere that is non-biologically-generated. Theoretical and experimental research and analysis are necessary to secure a detailed understanding of the biosignatures 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