The Small Star Opportunity to Find and Characterize Habitable Planets

The Small Star Opportunity to Find and Characterize Habitable

Planets

Jacob BeanHarvard-Smithsonian Center for Astrophysics

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

Tomonori Usuda

Bunei Sato

Ichi Tanaka

Harvard

David Charbonneau

Jean-Michel Désert

Zachory Berta (grad)

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 Questions for the Future: towards other Earths•statistical properties

•basic physical properties

•atmospheric properties

•habitability

•inner Solar System in context

strongly coupled

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

#1 Low-mass advantage for dynamical methods

Transit Spectroscopy

Reflection & Emission

Transmission

both ∝ R*-2

#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)

#3 Low-mass stars most numerous

75% M dwarfs

50% M* < 0.2 Msun

(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

The problem

faintness

Planet Detection: Technical Approach

The problem

faintness

normal RV measurements

@ 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

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

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

Calibration methods

Emission Lamps

Gas Cells

• iodine only works in the visible

• no existing NIR gas cell

• tracks all important effects for non-stabilized instruments with varying illumination

Calibration methods

Emission Lamps

Gas Cells

• iodine only works in the visible

• no existing NIR gas cell

• tracks all important effects for non-stabilized instruments with varying illumination

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

wedged windows to eliminate fringing

filled with 50 mb ammonia (NH3)A NIR gas cell

First implementation in CRIRES at the VLT

• 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

Gas cell lines overlap for in situ calibration

stellar linesgas cell lines

Adaptation of the “iodine cell” method

instrumental profile and sampling

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

(Bean et al. 2010a)

A giant planet around VB10? – probably not

(Bean et al. 2010a)

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-

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

Planet Characterization

Reflection & Emission

Transmission

both ∝ R*-2

Recall small size advantage for transits…

(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

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

H2O75% H2O / 22% Si / 3% Fe

Earth-like

Planet is 0.5 Rearth too large to be 100% solid --> substantial gas envelope

(Reart

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

Three models for GJ1214b

(Rogers & Seager 2010)

Mini-Neptune

solar composition

FeMgSiO3

Primordial envelope approximately few percent by mass

Mini-Neptune Water World

solar composition

FeMgSiO3

Water vapor atmosphere from sublimated ices, H lost or never accreted

Mini-Neptune Water World true Super-Earth

solar composition

FeMgSiO3

Secondary atmosphere, formation interior to the snow line

Wavelength (micron)

Transmission spectroscopy predictions for GJ1214b

(Miller-Ricci & Fortney 2010)

H-rich

“metal”-rich

Transmission spectroscopy indirectly probes the atmospheric mean molecular weight

scale height

strength of features

(Miller-Ricci, Seager, & Sasselov 2009)

scale height

Nature

low mmw

high mmw

Wavelength (micron)

Transmission spectroscopy predictions for GJ1214b

(Miller-Ricci & Fortney 2010)

H-rich

“metal”-rich

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

solar composition

FeMgSiO3

Case closed?

solar composition

FeMgSiO3

Case closed?

solar composition

FeMgSiO3

Case closed?

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

(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

•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

The Small Star Opportunity to Find and Characterize Habitable Planets

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