The Search for (Habitable) Planets

C. Beichman, JPL

From Greek Philosophers ...

“There are infinite worlds both like and unlike this world of ours...We must believe that in all worlds there are living creatures and plants and other things we see in this world.”--- Epicurus (c. 300 B.C)

…to Medieval Scholars...

“I [regard]… as false and damnable the view of those who would put inhabitants on Jupiter, Venus, and Saturn, and the moon, meaning by ‘inhabitants’ animals like ours and men in particular.”

…and Medieval Martyrs...

"There are countless suns and countless earths all rotating around their suns in exactly the same way as the seven planets of our system. We see only the suns because they are the largest bodies and are luminous, but their planets remain invisible to us because they are smaller and non-luminous. The countless worlds in the universe are no worse and no less inhabited than our Earth”Giordano Bruno (1584) in De L'infinito Universo E Mondi

…To Hollywood Producers…Klaatu Borada Nikto

“Your choice is simple. Join us and live in peace or pursue your present course and face obliteration. We shall be waiting for your answer. The decision rests with you.”

NASA’s Origins Theme Has Two Defining Questions

Are We Alone?

Search for Life Outside the Solar system

• Remote detection of the signposts of biological activities on extra- solar planets

Where Did We Come From?

Tracing Our Cosmic Roots• Formation of galaxies, stars, heavy elements, planetary systems and ….. life on the Early Earth

NASA Origins Science GoalsNASA Origins Science Goals

Understand How Stars

and Planetary Systems

Form and Evolve2

Determine Whether Habitable

or Life-bearing Planets Exist Around Nearby Stars

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Understand How Galaxies Formed in the

Early Universe

Some Fundamental Scientific Facts To Remember

• The necessary ingredients of life are widespread – Observation reveals uniformity of physical and chemical laws – Origin of the elements and their dispersal is well understood

• Life on Earth can inhabit harsh environments– Micro- and environmental biology reveal life in extremes of

temperature, chemistry, humidity

• Life affects a planetary environment in a detectable way– Our own atmosphere reflects the presence of primitive through

advanced life

• Planets are a common outcome of star formation– Modern theory of star formation makes planet formation likely

Organic Chemistry Ubiquitous: Comets

IR, submm, mm spectra reveal gas phase, ices, mineralogical signatures of many species, incl:H2O, CO2, CH3OH, CO, CH4, formic acid (HCOOH) and formaldehyde (H2CO), etc.

…Star Forming

Regions…

…and distant galaxies

• Polycyclic Armomatic Hydrocarbons (PAHs)– Complex 2-D carbon

molecules (>25 carbon atoms)

– Found in many active galaxies

Pierre et al 2001

z=1.5??

Barthel 2001

PAH

• Perhaps in distant quasar at z~1.5 (wait for SIRTF)

• CO detected in a very distant quasar (z=4.1!)– Found with more complex species in

more nearby objects

Life is Hardy

• Life needs water, a source of energy, and cosmically abundant elements

•Extremophiles can live in hot (~120 C!) acid lakes, near undersea volcanic vents, in underground aquifers, and within rocks in Antarctica

Life Affects The Earth’s Atmosphere

Earth’s Gases With And Without Life

Tim Lenton, Centre for

Ecology and Hydrology

Kant-Laplace Theory of Star Formation Favors Planet Formation

Star Formation & Protoplanetary Disks• The formation of planets is an integral part of our theory of how stars

form– Hundreds of planetary masses of gaseous and solid material in the protostellar

disk

• Solar System-scale dust disks found around nearby stars

Fomalhaut

Debris DisksFrom the Ground

• Sub-millimeter (SCUBA/JCMT) observations of disks reveal evacuated cavities the size of our solar system as well as clumps that may be structures associated with planets

• Many groups searching for planets using AO

Beta Pic

Eps Eri

SIRTF Observations of Disks• NASA’s next Great Observatory will map disks, survey 100s stars

– single, binary–with, w/o planets–ages from 1 million to 5 billion yr

• SIRTF launches April 18 after 25 years!

SIRTF Is 10 days From Launch

Finding Planets Indirectly

• Gravitational Effects on Parent Star– Radial Velocity Changes– Positional Wobble

(Astrometry)

• Effect of Planet on Star’s Brightness– Transits of edge-on systems– Gravitational micro-Lensing

Radial Velocity Searches

Mayor et alMarcy et al.

Gas Giant Planets

• Over 100 planets found using radial velocity wobble

– ~10% of stars have planets – Most orbits < 2-3 AU– Half may be multiple systems

???

Marcy et al.

• Planets on longer periods starting to be identified

– 55 Cancri is solar system analog

• Radial velocity technique not sensitive to terrestrial planets

Number of PlanetsIncreasing as Mass-1

The Royal Society sent Captain Cook along with Joseph Banks to Tahiti to observe a transit of Venus on June 3, 1769 to set the scale of the Solar System

Planetary Transits: Then and Now

Fundraising: Cook, Joesph Banks, and Lord Sandwich.

Transit Determines Planet’s Properties

• Transits of HD 209458 determine properties of another Solar System– Confirmation of planet interpretation – Inclination= 85.9– Mass= 0.69 ± 0.07 Mjup

– Radius =1.35 ± 0.06 Rjup

– Density= 0.35 g/cc <Saturn

• Active ground based efforts using 10 cm to 10 m telescopes

• COROT, Kepler and Eddington will find fewhundreds of Earths, thousands of Jupiters

• Spectroscopy probes atmosphere– Cloud heights, heavy-element abundances, temperature

and vertical temperature stratification, and wind velocities

Astrometric Search for Planets

• Astrometry measures positional wobble due to planets

• Interferometry enables measurements at the micro-arcsecond level

• Result of new observing systems will be a census of planets down to a few Mearth

over the next 10-20 years

Interferometery Is One Key to Planet Detection

• Enables precision astrometry, high resolution imaging, starlight nulling

• Make astrometric census of planets • Detect “Hot Jupiter’s” • Detect exo-zodiacal dust clouds• Image protostellar disks

• Break link between diameter, baseline

Space Interferometer Mission (SIM) Will Make Definitive Planet Census

A Deep Search for Earths• Are there Earth-like (rocky)

planets orbiting the nearest stars?

• Focus on ~250 stars like the Sun (F, G, K) within 10 pc

• Sensitivity limit of ~3 Me at 10 pc requires 1 µas accuracy

A Broad Survey for Planets • Is our solar system unusual?• What is the range of planetary system architectures?• Sample 2000 stars within ~25 pc at 4 µas accuracy

Evolution of Planets• How do systems evolve?• Is the evolution conducive to the

formation of Earth-like planets in stable orbits?

• Do multiple Jupiters form and only a few (or none) survive?

What We Don’t Know• Are planetary systems like our own common?• What is the distribution of planetary masses?

– Only astrometry measures planet masses unambiguously

• Are there low-mass planets in ‘habitable zone’ ?

But What is a Habitable Planet?

• Not too big– Avoid accreting

material to become gas giant

• Not too small– Lose atmosphere

• Not too hot or too cold– No liquid water

• Not too close to star– Avoid tidal lock

Finding Terrestrial Planets• Detecting light from planets

beyond solar system is hard:– Planet signal is weak but

detectable (few photons/sec/m2)

– Star emits million to billion more than planet

– Planet within 1 AU of star– Dust in target solar system

300 brighter than planet

• Finding a firefly next to a searchlight on a foggy night

>109>106

Four Hard Things About TPF • Sensitivity (relatively easy)

– Detection in hours spectroscopy in days. – Integration time (distance/diameter)4

– Need 12 m2 of collecting area (>4 m) for star at ~10 pc

• Angular resolution (hard)– 100 mas is enough to see ~25 stars, but requires >4 m coronagraph or >20 m

interferometer– Baseline/aperture distance

• Starlight suppression (hard to very hard)– 10-4 to 10-6 in the mid-IR– 10-8 to 10-10 in the visible/near-IR

• Solar neighborhood is sparsely populated– Fraction of stars with Earths (in habitable zone) unknown– Unknown how far we need to look to ensure success– Surveying substantial number of stars means looking to ~15 pc

Signatures of Life• Oxygen or its proxy ozone is most reliable biomarker

– Ozone easier to detect at low Oxygen concentrations but is a poor indicator of quantity of Oxygen

• Liquid water on a planet’s surface is considered essential to life.

• Carbon dioxide indicates an atmosphere and oxidation state typical of terrestrial planet.

• Abundant Methane can have a biological source– Non-biological sources might be confusing

• Find an atmosphere out of equilibrium• Expect the unexpected

Mars Odyssey Looks Back at Earth

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Christensen and Pearl 1997

Earthshine Reveals Visible Spectrum

Woolf, Traub and Jucks 2001

Goals for Terrestrial Planet Finder• Primary Goal: Direct detection of emitted or reflected radiation

from Earth-like planets located in the habitable zones of nearby solar type stars. – Determine orbital and physical properties – Characterize atmospheres and search for bio-markers– Search a statistically meaningful sample of stars (30-150)

• The Broader Scientific Context: Comparative Planetology– Understand properties of all planetary system constituents, e.g. gas giant

planets, terrestrial planets and debris disks.

• Astrophysics: An observatory with the power to detect an Earth orbiting a nearby star will be able to collect important new data on many targets of general astrophysical interest.

TPF Candidate Architectures• Visible Coronagraph

– System concept is relatively simple, 4-10 m mirror on a single spacecraft

– Components are complex• Build adequately large mirror of

appropriate quality (/300)• Hold (/3000) stability during

observation with deformable mirror

• IR Interferometer− Components are simple: 3-4 m mirrors of average quality−System is complex: 30 m boom or separated spacecraft

The Challenge of Angular Resolution

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• Coronagraphs at >3/D• Interferometers at > 1 /B

10 mm, 28 mCoronagraph

Cost ($$), L

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• How many stars to avoid mission failure (Np = 0)• How many stars to ensure enough planets (Np >5) # Stars Dist(Aperture, Baseline)CostSchedule

How Many Planets Are Enough ?

Visible Light Planet Detection• A simple coronagraph on NGST could detect

Jupiters around the closest stars as well as newly formed Jupiters around young stars

• Advanced coronagraph/apodized aperture telescope– 2~4 m telescope (Jupiters and nearest Earths) – 8~10 m telescope (full TPF goals)

• Presence and Properties of Planets– Planet(s) location and sizereflectivity– Atmospheric or surface composition– Rotation surface variability– Radial and azimuthal structure of disks

Simulated NGST coronagraphic image of a planet around Lalande 21185 (M2Vat 2.5pc) at 4.6 mm

Control of Star Light

• Control diffracted light with various apodizing pupil and/or image plane (coronagraph) masks– Square masks– Graded aperture– Multiple Gaussian masks– Band limited masks

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• Control scattered light with deformable mirror

−10,000 actuators for final l/3000 wavefront (<1 Å)

Coronagraph Status• Current contrast limited to 10-5 due to DM

imperfections and lab seeing−New DM due from Xinetics in March

• Kodak selected to provide large (1.8m), high precision (<5 nm) Mirror

− Very similar to SNAP mirror!• Innovative ideas to improve angular

resolution by combining interferometer and coronagraph ideas

5 Airy rings

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IR Interferometer

• Interferometer with cooled two to four 3~4 m mirrors – 30 m boom – 75-1000 m baseline using formation flying

• Operate at 1 AU for 5 years to survey 150 stars

Goal Earth at 10 pc TimePlanet? R=3/SNR=5 2.0 hourAtmosphere? R=20/SNR=10 2.3 day CO2, H2O Habitable? R=20/SNR=25 15.1 day O3, CH4

Nulling Interferometry

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Interferometer Detects and

Characterizes Planetary Systems

• TPF produces image of planetary system

– Orbital location– Temperature and radius

• TPF produces spectrum to search for biomarkers• 1-2 m telescopes to find Jupiters, nearest Earths•3-4 m telescopes for full TPF goals

IR Nulling • JPL Modified Mach-Zender (Serabyn et al)– 1.4 10-6 null laser null @ 10.6 um – Aim for 10-6 null target broadband

• Add spatial filter• Active pathlength stabilization

Nulling with two detectors- only low end detector shown, ignore initial spikes

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• UofA group (Hinz et al) demonstrated nulling with BLINC instrument on MMT

Pre-TPF Study Will Span Wavelengths, Techniques, Years, Ground and Space,

Theory and Observation

Hale Bopp

Planet Finding Is A Decades-Long

Undertaking• Like cosmology, the search

for planets and life will motivate broad research areas and utilize many telescopes for decades to come

• NASA’s program for planet finding will be broad and rich, with results emerging on many time scales, from the immediate to the long-term

• There are exciting, mid-term ways to detect giant planets and the nearest Earths

Collaboration on TPF/Darwin• Strong ESA/NASA interest in

joint planet-finding mission– Collaborative architecture studies– Discussions on technology

planning and development• Joint project leading to launch

~2015– Scientific and/or technological

precursors as required and feasible

The NASA Vision– To improve life here– To extend life to there– To find life beyond

The Science Vision“Search for Life outside of earth and, if it is found, determine its nature and its distribution in the galaxy…[This] is so challenging and of such importance that it could occupy astronomers for the foreseeable future” --- NAS/NRC Report