Detectability of Habitable Planets with the Space Interferometry Mission Evan Bierman, Chris McCarthy, Debra Fischer, Geoff Marcy San Francisco State University

Overview 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 size of earth around another star? Can SIM find a habitable planet the size of earth around another star?

The Space Interferometry Mission (SIM)

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

Unparalleled Precision SIM has an astrometric precision of 1  as or 1/ th of a degree SIM has an astrometric precision of 1  as or 1/ th of a degree ~ 1000 times better than HIPPARCOS and current ground based technologies ~ 1000 times better than HIPPARCOS and current ground based technologies ~100 times better than Hubble ~100 times better than Hubble Will be able to determine actual masses of extrasolar planets Will be able to determine actual masses of extrasolar planets

How does SIM find planets? Astrometry An observing technique to measures the precise Position of star. An observing technique to measures the precise Position of star. The Star’s “Wobble” is caused by a gravitational perturbation as a planet orbits the star The Star’s “Wobble” is caused by a gravitational perturbation as a planet orbits the star

Most of the Astrometric Signature is NOT due to Earth 0.03Pluto Neptune 89.28Uranus Saturn Jupiter 0.05Mars 0.31Earth 0.18Venus 0.01Mercury Signature at 10pc (  as)Planet

What does the Sun’s Astrometric Wobble 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 Planet? Liquid water must exist on the planet Liquid water must exist on the planet Temperatures ~ 0ºC-100ºC Temperatures ~ 0ºC-100ºC

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

First Order Calculation of a Habitable Zone Star’s Luminosity Star’s Luminosity Planet’s Bond Albedo Planet’s Bond Albedo Temperature range of the habitable zone; 0ºC-100ºC Temperature range of the habitable zone; 0ºC-100ºC Neither the size nor mass of the planet is a factor Neither the size nor mass of the planet is a factor

Astrometric Signature of a Habitable Planet Increases with Star Mass

The Two SIM Key Science projects (PIs Shao, Marcy) selected 120 optimal target stars for low mass planet detection The Two SIM Key Science projects (PIs Shao, Marcy) selected 120 optimal target stars for low mass planet detection The habitable zones were calculated for all of these stars The habitable zones were calculated for all of these stars Assume that there is a planet orbiting in the middle of the habitable zone: what is the minimum detectable habitable planet mass Assume that there is a planet orbiting in the middle of the habitable zone: what is the minimum detectable habitable planet mass Given the precision, what planets could be detected??? Given the 1  as precision, what planets could be detected???

The Five Best Candidates Star Alpha Centauri A Alpha Centauri B AltairVegaProxima Centauri Spectral TypeG2VK1VA7VA0VaM5.5Ve Distance (pc) Mass (Msun) Habitable Zone, Middle (AU) Tidal Locking Radius, 4.5Gyr(AU) Lifetime (Gyr) Minimum detectable planet mass; 4  detection (Mearth)

SIM should be able to find several large habitable “earths” and dozens of habitable “Neptunes” although the likelihood of finding a habitable “Earth” is low SIM should be able to find several large habitable “earths” and dozens of habitable “Neptunes” although the likelihood of finding a habitable “Earth” is low SIM will also discover and confirm the existence of hundreds of planets not restricted to the habitable zone!! SIM will also discover and confirm the existence of hundreds of planets not restricted to the habitable zone!! Conclusion:

References California & Carnegie Planet Search, exoplanets.org California & Carnegie Planet Search, exoplanets.org Hart, M. H., "Habitable Zones about Main Sequence Stars", Icarus, vol. 37, Hart, M. H., "Habitable Zones about Main Sequence Stars", Icarus, vol. 37, Kastings, J.F., Whitmire, D.P., Reynolds, R.T. (1993) "Habitable Zones around Main Sequence Stars", Icarus 101, Kastings, J.F., Whitmire, D.P., Reynolds, R.T. (1993) "Habitable Zones around Main Sequence Stars", Icarus 101, SIM: Key Science Project: Discovery of Planetary Systems with SIM, SIM: Key Science Project: Discovery of Planetary Systems with SIM, es.html es.html

Second Order Corrections* Star’s Luminosity and Temperature over the entire lifetime of the main sequence stage Star’s Luminosity and Temperature over the entire lifetime of the main sequence stage Greenhouse Effect (GE): Warms the planet by trapping IR radiation Greenhouse Effect (GE): Warms the planet by trapping IR radiation Wavelength dependent and time varying Albedo Wavelength dependent and time varying Albedo Atmospheric Composition and Pressure: The effects on the GE, atmospheric lapse rates, and atmospheric convection rates Atmospheric Composition and Pressure: The effects on the GE, atmospheric lapse rates, and atmospheric convection rates Whether the planet started frozen and had to be thawed or whether the planet was always “warm” Whether the planet started frozen and had to be thawed or whether the planet was always “warm” Ocean Cover and Cloud formation Ocean Cover and Cloud formation *Kasting et. al 1993

Other Considerations on the HZ Tidal Locking of close planets Tidal Locking of close planets Spin Axis tilt Spin Axis tilt Companion Stars Companion Stars Carbonate-Silicate cycle as a feedback mechanism on the Greenhouse Effect (CS cycle) Carbonate-Silicate cycle as a feedback mechanism on the Greenhouse Effect (CS cycle) Organic feedback mechanisms Organic feedback mechanisms Plate Tectonics (PT): A key component of the CS cycle Plate Tectonics (PT): A key component of the CS cycle Planet’s Mass, Radius, and composition determine the length of time and magnitude of PT Planet’s Mass, Radius, and composition determine the length of time and magnitude of PT Jupiter Sized Objects: Can stunt the formation of planets. Example: Mars Jupiter Sized Objects: Can stunt the formation of planets. Example: Mars Planetary Formation Zone* vs. Habitable Zone Planetary Formation Zone* vs. Habitable Zone * Weatherill 1996