Searching for Life Beyond The Solar System Dr. Victoria Meadows NASA Astrobiology Institute Spitzer Science Center/California Institute of Technology

Life Beyond Our Solar System “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)

What Is Astrobiology? Astrobiology is the scientific study of life in the universe, its past, present and future. Astrobiology seeks to answer three questions: –How does life begin and develop? –Does life exist elsewhere in the universe? –What is life’s future on Earth and beyond? Astrobiology is an interdisciplinary science –combines biology, chemistry, geology, astronomy, planetary science, paleontology, oceanography, physics, and mathematics to answer these questions. Astrobiology is the scientific study of life in the universe, its past, present and future. Astrobiology seeks to answer three questions: –How does life begin and develop? –Does life exist elsewhere in the universe? –What is life’s future on Earth and beyond? Astrobiology is an interdisciplinary science –combines biology, chemistry, geology, astronomy, planetary science, paleontology, oceanography, physics, and mathematics to answer these questions.

Where would we start the search for life outside our Solar System? First, find a habitable world

What Is a Habitable World? A world that can maintain liquid water on its surface

Challenges: Separating Planet and Star In the visible, they don’t give off their own light They are VERY far away, which makes them very faint They are lost in the glare of their star

Learning About the Planet We will not be able to see details on it Everything we learn will be “disk-averaged”. The signs of life must be a global and on the surface Our interpretation is only as good as how deep we can see!

Learning About Distant Worlds Radio Infrared Visible Ultra- Violet X-Ray Gamma Rays

Greenhouse Warming T effective T surface Greenhouse Venus -43C 470C 513C Earth -17C 15C 32C Mars -55C -50C 5C After Table 9.1, Bennet, Shostak, Jakosky, 2003 Δ 37 C Δ 520 C A planet’s greenhouse effect is at least as important in determining that planet’s surface temperature as is its distance from the star!

So Many Planets… 211 planets known beyond our Solar System! But there’s ONE problem…

Too Big! These planets are mostly “giant planets” Small, rocky, Earth-like terrestrial planets around good parent stars are still very difficult to find. –A handful of M < 10 Earth masses known –Recent discovery of Gl 581c, > 5.1 M earth R. Hasler

How can we tell if a planet is inhabited? Hi! DEAFENING SILENCE! Without direct contact with an alien civilization, or travelling to the nearest solar system, our best chance for finding life in the Universe is to look for global changes in the atmosphere and surface of a terrestrial planet.

Signs of Life Astronomical Biosignatures are global-scale photometric, spectral or temporal features indicative of life. Earth shows us that life can provide global-scale modification of: –A planet’s atmosphere –A planet’s surface –A planet’s appearance over time Biosignatures must always be identified in the context of the planetary environment –e.g. Earth methane and Titan methane

CH 4 O3O3 Signs of Life: Atmosphere

Reflectivity Signs of Life: Surface

Signs of Life: Time NOAA-CMDL

A Diversity of Worlds

Star-Planet Distance Circularity A Diversity of Worlds in Space… Water ContentWetDry

…and Time Archean Modern Proterozoic

Earths Around Other Stars

CO 2 CH 3 Cl CH 4 O 3 + N2ON2O H2OH2O Earth AD Leo planet Active M Star Planets Earth-like planets around M stars with similar surface fluxes can produce simultaneous strong signatures of O 2 or O 3 and CH 4, CH 3 Cl or N 2 O. N2ON2O

Early Earth-like Planets

Modern Earth 355ppm CO 2

Proterozoic 0.1PAL O 2 100ppm CH 4 15% decrease in ozone column depth Segura, Krelove, Kasting, Sommerlatt,Meadows,Crisp,Cohen

Archean N % 2000ppm CO ppm CH 4 100ppm H 2 Karecha, Kasting, Segura, Meadows, Crisp, Cohen

High CO 2 Early Earth-like Planet

The Coevolution of Photosynthesis with the Atmosphere On Extrasolar Worlds

Why Are Plants Green?

Terrestrial Planet Finders Direct detection of planets Launch… ? Terrestrial Planet Finder NASA Darwin ESA