# Presentation: 645952

## 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...

- ...To Hollywood Producers...

***“******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

## NASA Origins Science Goals

**NASA Origins Science Goals**

## 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

## ...Star Forming Regions...

- 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

- ...and distant galaxies

- Polycyclic Armomatic Hydrocarbons (PAHs)
- Complex 2-D carbon molecules (>25 carbon atoms)
- Found in many active galaxies

- 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 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 

- 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

## Debris DisksFrom the Ground  

- 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

## 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 al
- Marcy 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

-   Planets on longer periods starting to be identified
-  55 Cancri is solar system analog
-  Radial velocity technique  not sensitive to terrestrial  planets

## Number of Planets

**Number of Planets**
**Increasing 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

## 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 fewhundreds 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

## 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

***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

- +

**Cost ($$), Launch Date**

## How many stars to avoid mission failure (Np = 0)
How many stars to ensure enough planets (Np >5)

- How many stars to avoid mission failure (Np = 0)
- How many stars to ensure enough planets (Np >5)

-    # Stars Dist(Aperture, Baseline)CostSchedule

-  

- 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 sizereflectivity
- 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 ************m***

## 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

-  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

## 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 __	__Time_
- Planet? 	R=3/SNR=5	 2.0 hour
- Atmosphere?   R=20/SNR=10 2.3 day
-    CO2, H2O
- Habitable? 	R=20/SNR=25 15.1 day
-   O3, CH4

## Nulling Interferometry

****

**B**

******/B**

## 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

-  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

## 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*