the small star opportunity to find and characterize habitable planets
DESCRIPTION
The Small Star Opportunity to Find and Characterize Habitable Planets. Jacob Bean. Harvard-Smithsonian Center for Astrophysics. Collaborators:. Texas Fritz Benedict Chris Sneden Barbara McArthur Amber Armstrong (ugrad, now STScI) Germany Andreas Seifahrt (now UC Davis) Ansgar Reiners - PowerPoint PPT PresentationTRANSCRIPT
The Small Star Opportunity to Find and Characterize Habitable
Planets
Jacob BeanHarvard-Smithsonian Center for Astrophysics
Texas
Fritz Benedict
Chris Sneden
Barbara McArthur
Amber Armstrong (ugrad, now STScI)
Germany
Andreas Seifahrt (now UC Davis)
Ansgar Reiners
Stefan Dreizler
Derek Homeier
Günter Wiedemann
Sweden
Henrick Hartman
Hampus Nilsson
Japan
Tomonori Usuda
Bunei Sato
Ichi Tanaka
Harvard
David Charbonneau
Jean-Michel Désert
Zachory Berta (grad)
MIT
Sara Seager
UC Santa Cruz
Eliza Miller-Ricci Kempton
Jonathan Fortney
Princeton
Nikku Madhusudhan
Georgia State
Todd Henry
Funding from: NASA, German DFG, ESO, & the EU
Collaborators:
data compiled by Jean Schneider
Planets detected with RV and transit
Key Results: gas giants in focus
•statistical properties
•first-order atmospheric characterization of hot planets
•feedback to how we view the outer Solar System
Key Results: gas giants in focus
•statistical properties
•first-order atmospheric characterization of hot planets
•feedback to how we view the outer Solar System
Key Questions for the Future: towards other Earths•statistical properties
•basic physical properties
•atmospheric properties
•habitability
•inner Solar System in context
strongly coupled
Key Results: gas giants in focus
•statistical properties
•first-order atmospheric characterization of hot planets
•feedback to how we view the outer Solar System
Key Questions for the Future: towards other Earths•statistical properties
•basic physical properties
•atmospheric properties
•habitability
•inner Solar System in context
strongly coupled
Low-mass stars offer a shortcut using RV and transit methods
Summary
• Initiated a comprehensive search for planets around nearby, very low-
mass stars (M* < 0.2 Msun)
• NIR radial velocities with CRIRES at the VLT and IRCS at Subaru using a
new gas cell
• Paved the way for a new instrument that will be capable of finding
characterizable habitable worlds
Detecting planets: near-infrared radial velocities
Characterizing planets: transit spectroscopy
• First atmospheric study of a “super-earth” exoplanet – only possible
because the planet orbits a very low-mass star
• Measurements obtained using a new ground-based technique
• First results guide new theoretical and observational work
The shortcut to habitable planets
#1 Low-mass advantage for dynamical methods
RV signal ∝ M*-2/3
Example – 1 Mearth at 1 AU
K = 0.09 m/s for M* = 1.0 Msun
K = 0.42 m/s for M* = 0.1 Msun
Current state-of-the art is 1 m/sTransit depth ∝ R*-2
R* = 0.2 Rsun for M* = 0.15 Msun
The shortcut to habitable planets
#1 Low-mass advantage for dynamical methods
Transit Spectroscopy
Reflection & Emission
Transmission
both ∝ R*-2
The shortcut to habitable planets
#2 Low-mass brings in habitable zone
better for RV
signal ∝ a-1/2
better for transits
probability ∝ a-1
frequency ∝ a-3/2
(Kasting et al. 1993)
P = 3 d P = 35 d(Selsis et al. 2007)
The shortcut to habitable planets
#3 Low-mass stars most numerous
75% M dwarfs
50% M* < 0.2 Msun
The shortcut to habitable planets
(Deming et al 2008)
light, small, low luminosity, ubiquitous
Best chance to find a transiting habitable planet around a nearby star, and study its atmosphere
Low-mass stars…
Part I. Planet detection with the radial velocity method
Part II. Planet characterization with transit spectroscopy
Part I. Planet detection with the radial velocity method
Part II. Planet characterization with transit spectroscopy
The problem
faintness
Planet Detection: Technical Approach
The problem
faintness
normal RV measurements
Planet Detection: Technical Approach
@ 10 pc
Sun V=4.8
M0 V=9.0
M8 V=18.7
The problem
faintness
normal RV measurements
more flux in the red/NIR
Planet Detection: Technical Approach
The solution – the NIR
But there is another problem…
calibration!
No NIR RV precision like in the visible
Best previous precision around 200 m/s
Planet Detection: Technical Approach
Calibration methods
Emission Lamps
• few lines in the NIR (ThAr)
• existing instruments have small wavelength coverage
• doesn’t track image motion
• requires a highly stabilized instrument
Planet Detection: Technical Approach
Calibration methods
Emission Lamps
• few lines in the NIR (ThAr)
• existing instruments have small wavelength coverage
• doesn’t track image motion
• requires a highly stabilized instrument
Gas Cells
• iodine only works in the visible
• no existing NIR gas cell
• tracks all important effects for non-stabilized instruments with varying illumination
Planet Detection: Technical Approach
Calibration methods
Emission Lamps
• few lines in the NIR (ThAr)
• existing instruments have small wavelength coverage
• doesn’t track image motion
• requires a highly stabilized instrument
Gas Cells
• iodine only works in the visible
• no existing NIR gas cell
• tracks all important effects for non-stabilized instruments with varying illumination
?
Planet Detection: Technical Approach
A NIR gas cell
Important considerations for the gas cell method:
• cell should provide lines in a region where stars also have lines
• avoid telluric lines
• temperature stabilization necessary?
• gas mixture not toxic, explosive, or corrosive
Planet Detection: Technical Approach
18 cm
5 cm
wedged windows to eliminate fringing
filled with 50 mb ammonia (NH3)A NIR gas cell
Planet Detection: Technical Approach
First implementation in CRIRES at the VLT
ESO
Planet Detection: Technical Approach
• cryogenic, vacuum
• λ = 1 – 5 μm, Δλ = 50 nm
• R ≤ 100,000
• AO fed
• long-slit
• no gas cell temperature stabilization possibleESO
gas cell goes here
Planet Detection: Technical Approach
Gas cell lines overlap for in situ calibration
stellar linesgas cell lines
Planet Detection: Technical Approach
Adaptation of the “iodine cell” method
instrumental profile and sampling
Planet Detection: Technical Approach
Velocity precision tests
(Bean et al. 2010b)
Planet Detection: Results
A giant planet around VB10?
(Pravdo & Shaklan 2009)
Star Properties•spectral type: M8V
•M* ~ 0.075 Msun
•distance = 5.9 pc
•V = 17.6
•K = 8.8
Planet Properties•period = 272 days (0.744 yr)
•mass = 6 ± 3 Mjup
•inclination ~ edge-on
•e, ω, and Tp not constrained
•expected K ~ 1 km/s
Planet Detection: Results
A giant planet around VB10?
(Bean et al. 2010a)
Planet Detection: Results
A giant planet around VB10?
(Bean et al. 2010a)
Planet Detection: Results
A giant planet around VB10? – probably not
(Bean et al. 2010a)
Planet Detection: Results
Compare to other results
Visible: Anglada-Escudé et al. 2010
Magellan + MIKE
rms = 250 m s-1
NIR: Zapatero Osorio et al. 2009
Keck + NIRSPEC
rms = 560 m s-1 | 200 m s-1
CRIRES + ammonia cell rms = 10 m s-
1
Planet Detection: Results
Planet Detection: Outlook
• Initial 2 yr VLT survey complete
• Identified gas giant planet candidates that need to be followed up
• Started a northern hemisphere survey with Subaru + IRCS (Seifahrt PI, with Japanese collaborators)
• Next step is to build a specialized instrument to get to 1 m s-1
Planet Detection: Outlook
Will enable the large-scale detection of planets down to a few times the mass of the Earth in the habitable zones of nearby M dwarfs
PI: A. Quirrenbach, Heidelberg
Operational in 2014
low-mass planet statistics and characterization
Spectral coverage: 0.5 – 1.7μm
Precision: 1 m s-1 for late M dwarfs
Telescope: Calar Alto 3.5m
Part I. Planet detection with the radial velocity method
Part II. Planet characterization with transit spectroscopy
Part I. Planet detection with the radial velocity method
Part II. Planet characterization with transit spectroscopy
Planet Characterization
Reflection & Emission
Transmission
both ∝ R*-2
Recall small size advantage for transits…
Planet Characterization
(Charbonneau et al. 2009)
GJ 1214b
Detection of a “super-earth” around a low-mass star
planet properties:
M = 6.5 Mearth
R = 2.7 Rearth
Teq < 550 K
star properties:
M = 0.16 Msun
R = 0.20 Rsun
super-earth ≡ 1 < mass < 10 Mearth
Planet Characterization
(Charbonneau et al. 2009)
Detection of a “super-earth” around a low-mass star
Comparison to models should reveal composition…
planet properties:
M = 6.5 Mearth
R = 2.7 Rearth
Teq < 550 K
star properties:
M = 0.16 Msun
R = 0.20 RsunKepler-10b
Planet Characterization
(Charbonneau et al. 2009)
Detection of a “super-earth” around a low-mass star
H/He
H2O75% H2O / 22% Si / 3% Fe
Earth-like
Planet is 0.5 Rearth too large to be 100% solid --> substantial gas envelope
Radiu
s of
pla
net
(Reart
h)
planet properties:
M = 6.5 Mearth
R = 2.7 Rearth
Teq < 550 K
star properties:
M = 0.16 Msun
R = 0.20 Rsun
Kepler-10b
Planet Characterization
Three models for GJ1214b
(Rogers & Seager 2010)
Planet Characterization
Three models for GJ1214b
Mini-Neptune
solar composition
H2O
FeMgSiO3
Fe
Primordial envelope approximately few percent by mass
(Rogers & Seager 2010)
Planet Characterization
Three models for GJ1214b
Mini-Neptune Water World
solar composition
H2O
FeMgSiO3
Fe
H2O
FeMgSiO3
Fe
Primordial envelope approximately few percent by mass
Water vapor atmosphere from sublimated ices, H lost or never accreted
(Rogers & Seager 2010)
Planet Characterization
Three models for GJ1214b
Mini-Neptune Water World true Super-Earth
solar composition
H2O
FeMgSiO3
Fe
H2O
FeMgSiO3
FeFe
FeMgSiO3
H
Primordial envelope approximately few percent by mass
Water vapor atmosphere from sublimated ices, H lost or never accreted
Secondary atmosphere, formation interior to the snow line
(Rogers & Seager 2010)
Planet Characterization
Wavelength (micron)
Tra
nsi
t D
epth
(%
)
Transmission spectroscopy predictions for GJ1214b
(Miller-Ricci & Fortney 2010)
H-rich
“metal”-rich
Planet Characterization
Transmission spectroscopy indirectly probes the atmospheric mean molecular weight
scale height
strength of features
(Miller-Ricci, Seager, & Sasselov 2009)
Planet Characterization
Transmission spectroscopy indirectly probes the atmospheric mean molecular weight
scale height
strength of features
(Miller-Ricci, Seager, & Sasselov 2009)
Planet Characterization
Transmission spectroscopy indirectly probes the atmospheric mean molecular weight
(Miller-Ricci, Seager, & Sasselov 2009)
scale height
strength of features
Nature
low mmw
high mmw
Planet Characterization
Wavelength (micron)
Tra
nsi
t D
epth
(%
)
Transmission spectroscopy predictions for GJ1214b
(Miller-Ricci & Fortney 2010)
H-rich
“metal”-rich
Planet Characterization
A transmission spectrum for GJ1214b – from the ground!
(Bean, Miller-Ricci Kempton, & Homeier 2010, Nature)
Planet Characterization
A transmission spectrum for GJ1214b – from the ground!
(Bean, Miller-Ricci Kempton, & Homeier 2010, Nature)
Planet Characterization
A transmission spectrum for GJ1214b – from the ground!
(Bean, Miller-Ricci Kempton, & Homeier 2010, Nature)
•H-rich ruled out at 5σ
•>70% water by mass needed to be consistent
Planet Characterization
Three models for GJ1214b
Mini-Neptune Water World true Super-Earth
solar composition
H2O
FeMgSiO3
Fe
H2O
FeMgSiO3
FeFe
FeMgSiO3
H
Primordial envelope approximately few percent by mass
Water vapor atmosphere from sublimated ices, H lost or never accreted
Secondary atmosphere, formation interior to the snow line
(Rogers & Seager 2010)
Case closed?
Planet Characterization
Three models for GJ1214b
Mini-Neptune Water World true Super-Earth
solar composition
H2O
FeMgSiO3
Fe
H2O
FeMgSiO3
FeFe
FeMgSiO3
H
Primordial envelope approximately few percent by mass
Water vapor atmosphere from sublimated ices, H lost or never accreted
Secondary atmosphere, formation interior to the snow line
(Rogers & Seager 2010)
Case closed?
Planet Characterization
Three models for GJ1214b
Mini-Neptune Water World true Super-Earth
solar composition
H2O
FeMgSiO3
Fe
H2O
FeMgSiO3
FeFe
FeMgSiO3
H
Primordial envelope approximately few percent by mass
Water vapor atmosphere from sublimated ices, H lost or never accreted
Secondary atmosphere, formation interior to the snow line
(Rogers & Seager 2010)
Case closed?
Planet Characterization
Case closed? -- not exactly…low mmw
Nature
high mmw
low mmw with clouds
Clouds at <200 mbar in a H-rich atmosphere also consistent with the data
Planet Characterization
(Désert, Bean, et al. 2011)
Spitzer observations
•Spitzer data fully consistent with VLT data
•>80% water by mass now required in cloud-free atmospheres
•Clouds still possible
CH4
Planet Characterization
•Clouds or haze remain a possibility although no
species/model has been proposed – this is an outstanding
theoretical problem
•Further observations are planned/ongoing to fill in
between the VLT optical and Spitzer IR measurements:
VLT, Magellan, HST, & etc.
•The next frontier will be comparative studies – this is an
important general aspect of exoplanet science
Final Thoughts on GJ1214b…
Outlook
Characterization of a habitable exoplanet by 2020
transit
radial velocity
individual masses and radii
transmission spectrum
census