Detectability of Detectability of Habitable Planets Habitable Planets

with the with the Space Space

Interferometry Interferometry MissionMission

Evan Bierman, Chris McCarthy, Evan Bierman, Chris McCarthy, Debra Fischer, Geoff MarcyDebra Fischer, Geoff Marcy

San Francisco State UniversitySan Francisco State University

OverviewOverview

What is SIM and why is it important?What is SIM and why is it important? How does SIM find Planets?How does SIM find Planets? What is a Habitable Planet?What is a Habitable Planet? Can SIM find a habitable planet the Can SIM find a habitable planet the

size of earth around another star?size of earth around another star?

The Space Interferometry Mission (SIM)

SIM has a 5 year mission in a heliocentric earth trailing orbit

Unparalleled Unparalleled PrecisionPrecision

SIM has an astrometric precision of SIM has an astrometric precision of

11asas or 1/3600000000 or 1/3600000000thth of a of a degreedegree

~ 1000 times better than HIPPARCOS ~ 1000 times better than HIPPARCOS and current ground based technologies and current ground based technologies

~100 times better than Hubble~100 times better than Hubble Will be able to determine actual Will be able to determine actual

massesmasses of extrasolar planets of extrasolar planets

How does SIM find How does SIM find planets?planets?

AstrometryAstrometry An observing technique to measures the precise An observing technique to measures the precise

PositionPosition of star. of star. The Star’s “Wobble” is caused by a gravitational The Star’s “Wobble” is caused by a gravitational

perturbation as a planet orbits the starperturbation as a planet orbits the star

Most of the Astrometric Most of the Astrometric Signature is Signature is NOTNOT due to due to

EarthEarth

0.03Pluto

158.57Neptune

89.28Uranus

281.25Saturn

512.62Jupiter

0.05Mars

0.31Earth

0.18Venus

0.01Mercury

Signature at 10pc (as)Planet

What What does the does the

Sun’s Sun’s AstrometrAstrometric Wobble ic Wobble look like?look like?

The wobble of the Sun's projected position due to influence of all the planets in the

Solar System as it would appear from 10pc, courtesy of NASA ORIGNS

What is a Habitable What is a Habitable Planet?Planet?

•Liquid water must exist on the Liquid water must exist on the planet planet

•Temperatures ~ 0ºC-100ºCTemperatures ~ 0ºC-100ºC

Given the Luminosity of a star, it is possible to compute Given the Luminosity of a star, it is possible to compute an approximate Habitable Zone (HZ)an approximate Habitable Zone (HZ)

First Order Calculation of a First Order Calculation of a Habitable ZoneHabitable Zone

Star’s LuminosityStar’s Luminosity Planet’s Bond AlbedoPlanet’s Bond Albedo Temperature range of the habitable zone; Temperature range of the habitable zone; 0ºC-0ºC-

100ºC100ºC Neither the size nor mass of the planet is a factor Neither the size nor mass of the planet is a factor

Star'sLuminosity, Lstar

Planet'sBondAlbedo, AbEarth .3 Temperature rangeof theHabitable Zone, 0°C 100°C

Neither thesizenor themass of theplanet isa factor

ahz 1 AbLstar

16 Tplanet4

Astrometric Signature of a Astrometric Signature of a Habitable Planet Increases with Star Habitable Planet Increases with Star

MassMass

a HZ middle MStar; 2

Lstar Mstar; 4

HP MStar

dstar; 1

star a planet Mplanet

dstar Mstar

The Two SIM Key Science projects (PIs The Two SIM Key Science projects (PIs Shao, Marcy) selected 120 optimal target Shao, Marcy) selected 120 optimal target stars for low mass planet detectionstars for low mass planet detection

The habitable zones were calculated for The habitable zones were calculated for all of these starsall of these stars

Assume that there is a planet orbiting in Assume that there is a planet orbiting in the middle of the habitable zone: what is the middle of the habitable zone: what is the minimum detectable habitable the minimum detectable habitable planet massplanet mass

Given the Given the 1as precision, precision, what planets could be what planets could be

detected???detected???

The Five Best CandidatesThe Five Best Candidates

Star

Alpha Centauri

A

Alpha Centauri

B

Altair Vega Proxima Centauri

Spectral Type G2V K1V A7V A0Va M5.5Ve

Distance (pc) 1.3 1.3 5.1 7.8 1.3

Mass (Msun) 0.9 0.8 1.7 2.7 0.2

Habitable Zone, Middle (AU) 0.9 0.6 3.4 7.1 0.1

Tidal Locking Radius, 4.5Gyr(AU) 0.5 0.4 - - 0.3

Lifetime (Gyr) 12 22 1.6 0.6 300

Minimum detectable planet mass; 4 detection (Mearth) 1.8 2.3 3.4 3.9 4

SIM's habitable planets with a 4 sigma detection threshold

0

5

10

15

20

25

30

1 4 8 12 16 20 24 28 32

Minimum Detectable Mass (earth masses)

Num

ber

of T

arge

ts

SIM should be able to find several large SIM should be able to find several large habitable “earths” and dozens of habitable “earths” and dozens of habitable “Neptunes” although the habitable “Neptunes” although the likelihood of finding a habitable “Earth” likelihood of finding a habitable “Earth” is lowis low

SIM will also discover and confirm the SIM will also discover and confirm the existence of hundreds of planets not existence of hundreds of planets not restricted to the habitable zone!!restricted to the habitable zone!!

Conclusion:

ReferenceReferencess

• California & Carnegie Planet Search, California & Carnegie Planet Search, exoplanets.orgexoplanets.org

• Hart, M. H., "Habitable Zones about Main Hart, M. H., "Habitable Zones about Main Sequence Stars", Icarus, vol. 37, 351-357.Sequence Stars", Icarus, vol. 37, 351-357.

• Kastings, J.F., Whitmire, D.P., Reynolds, R.T. (1993) Kastings, J.F., Whitmire, D.P., Reynolds, R.T. (1993) "Habitable Zones around Main Sequence Stars", "Habitable Zones around Main Sequence Stars", Icarus 101, 108-128 Icarus 101, 108-128

• SIM: Key Science Project: Discovery of Planetary SIM: Key Science Project: Discovery of Planetary Systems with SIM, Systems with SIM, www.physics.sfsu.edu/SIMwww.physics.sfsu.edu/SIM

• http://www.unm.edu/~astro1/ET109/http://www.unm.edu/~astro1/ET109/zones/zones.htmlzones/zones.html

Second Order Second Order Corrections*Corrections*

Star’s Luminosity and Temperature over the Star’s Luminosity and Temperature over the entire lifetime of the main sequence stageentire lifetime of the main sequence stage

Greenhouse Effect (GE): Warms the planet by Greenhouse Effect (GE): Warms the planet by trapping IR radiationtrapping IR radiation

Wavelength dependent and time varying Wavelength dependent and time varying AlbedoAlbedo

Atmospheric Composition and Pressure: The Atmospheric Composition and Pressure: The effects on the GE, atmospheric lapse rates, and effects on the GE, atmospheric lapse rates, and atmospheric convection ratesatmospheric convection rates

Whether the planet started frozen and had to Whether the planet started frozen and had to be thawed or whether the planet was always be thawed or whether the planet was always “warm”“warm”

Ocean Cover and Ocean Cover and Cloud formationCloud formation*Kasting et. al 1993

Other Considerations on the Other Considerations on the HZHZ

Tidal Locking of close planetsTidal Locking of close planets Spin Axis tiltSpin Axis tilt Companion StarsCompanion Stars Carbonate-Silicate cycle as a feedback Carbonate-Silicate cycle as a feedback

mechanism on the Greenhouse Effect (CS cycle)mechanism on the Greenhouse Effect (CS cycle) Organic feedback mechanismsOrganic feedback mechanisms Plate Tectonics (PT): A key component of the CS Plate Tectonics (PT): A key component of the CS

cyclecycle Planet’s Mass, Radius, and composition Planet’s Mass, Radius, and composition

determine the length of time and magnitude of determine the length of time and magnitude of PTPT

Jupiter Sized Objects: Can stunt the formation of Jupiter Sized Objects: Can stunt the formation of planets. Example: Marsplanets. Example: Mars

Planetary Formation Zone* vs. Habitable ZonePlanetary Formation Zone* vs. Habitable Zone* Weatherill 1996