Searching for Habitable Planets Darin Ragozzine Harvard-Smithsonian Center for Astrophysics Science A-54: Life as a Planetary Phenomenon April 8, 2010

Astronomers take the pictures. Astrophysicists explain what is happening in the pictures. Planetary Scientists focus on planets: Orbits, Origin, Evolution, Atmospheres, Surfaces, Interiors, … Jupiter, Great Red Spot, and Red Jr.Hyperion, small icy moon of Saturn

Really, we’re computer programmers…

Abstract - Review requirements for habitability (liquid water, solid surface, some protection) - Examples of the most habitable planets/system known to date, along with astrophysical principles that affect habitability: = overview of # of super-earths, smallest examples (e.g. CoRoT-7b), orbital properties = HD (3 Neptunes + asteroid belt) - life probably needs stability: solid surface with nearby liquid water, i.e., not Neptunes - asteroid belts: needs to be the right size to not have frequent sterilizing collisions (but it's nice if biodiversity enhancing collisions happen) = HD (3 Super-Earths) - without transit, hard to be clear about habitability = GJ 581 (potentially habitable) - additional data can modify planetary orbital parameters (especially in multi-planet systems) and affect estimates of habitability - dynamical interactions between planets can change orbits (Malinkovich cycles, want low eccentricity and obliquity) - planetary system dynamical interactions (SECULAR AND RESONANT; STABILITY AND CHAOS) SHOW ON BOARD) can help determine maximum masses = need to understand entire system to verify habitability = GJ 1214b - M dwarfs are great for detecting small habitable zone planets (MEarth) - tidal interactions: can despin such planets which might be bad - Are there other Earths out there? = Kepler (describe mission, data, ability to find other Earths) - Multiple transiting planets (my models) = Information rich: will teach us a lot about planetary systems Conclusion: with Kepler single and multiple-planet systems, we'll know much much more about habitable planets around other stars

Outline Review: Habitability and Super-Earths Overview of currently known Super-Earths Getting serious about habitability Anticipated future of Super-Earths (from Kepler) My Work Conclusions

The Copernican Revolution Geocentric → Heliocentric Universe  We are not the center of the Universe Completion: Other planets like ours

Consider again that dot. That's here, that's home, that's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer... every saint and sinner in the history of our species lived there – on a mote of dust suspended in a sunbeam. – Carl Sagan

Extrasolar Life: Review Life in other places either has to emerge there or it needs to be placed there (panspermia or colonization) –Panspermia between different planets in the same system is difficult but not unimaginable; extrasolar panspermia is extremely difficult (has been calculated) –Colonization is a question to return to later in the course as it implies intelligence In either case, the planet must be habitable; why planets (instead of stars) was discussed last time Habitability is defined more-or-less by where Earth-based life would be habitable (and not just for selfish reasons) –Presence of liquid water; requires specific temperature and pressure Neptunes don't have liquid water: by the time the temperature is high enough, the pressure is too high –Can't have too much H and He because this raises the pressure –Also don't have a “solid” surface/interface

Extrasolar Life: Review If we also require life to originate on a planet, which seems reasonable, then somewhat stricter conditions apply: –(Some parts of our solar system appear could be somewhat hospitable to life now, but aren't ideal for the emergence of life) –The presence of a “surface”, probably a solid surface, is important Surfaces concentrate materials; chemical reaction rates are a strong function of concentration Surfaces imply a reservoir of material and geochemical cycles –A safe and stable environment that lasts for long enough time for (proto)life to form and evolve (millions to billions of years)

Super-Earths Best candidates: Super-Earths –Solid Surfaces –Low atmospheric pressures (not big enough to hold on to H and He) These Super-Earths need to be: –In the Habitable Zone –In a stable exoplanetary system

Overview of Current Super-Earths ~20 known with masses less than 10 M_Earth (see and with the smallest minimum mass under 2 M_Earth Note that most of these planets are detected through radial velocity and thus only have minimum masses: these could generally be Neptunes (or perhaps even Jupiters) Only 2 have been detected in transit, so that M, R, and density are known: CoRoT-7b and GJ1214b Most have periods < 10 days and semi-major axes < 0.1 AU and nearly circular orbits

2 Transiting Super-Earths GJ1214 b and CoRoT-7b: same size (?)

2 Transiting Super-Earths GJ1214b and CoRoT-7b: same temp(?)

STARS: Come in a range of masses ( M sun ) Mass, Radius, Luminosity, Temperature, and Color are all strongly correlated (while stars are burning Hydrogen = “Main Sequence”)! Bigger stars have lower densities, higher luminosities, higher temperatures, and bluer colors; smaller stars have larger densities, lower luminosities, lower temperatures, and redder colors. Which property primarily determines location of HZ?

HD 69830: 3 Neptunes + Belt 3 Neptune-mass planets (or bigger) Asteroid belt just outside outer planet –Good or bad?

HD Super Earths (P: 4,10,20 days) Small planets tend to be in multiple systems Minimum masses –4, 7, 9 Earth masses

Gliese small planets (P: 3, 5, 13, 66 days) Minimum masses (2,16,5,7) Outer two planets are near/in the Habitable Zone (!) Dynamical interactions: –Secular evolution Climate –Maximum masses

Scientists Find New Earth!

Reliable Science News Best: be an expert and read the peer- reviewed journal article Next best: press releases, quotes from reputable scientists Wikipedia is usually good

Gliese 581 M dwarf: close-in Habitable Zone Easiest to find (in Doppler and Transit) Problems: –Tidal locking –Flaring –High UV,X-ray

Getting Serious about Habitability Liquid Water and Solid Surface Need to estimate: –Surface Temperature –Surface Pressure –Planetary Density –Atmospheric Composition Presence of Other Perturbing Planets Transiting Planets are the ONLY WAY

NASA's Kepler Space Telescope Ultra-precise, long-duration photometry of over stars –Able to detect 80 ppm drop due to transit of Earth-size planet

Kepler Just getting started (Probably) has discovered many Super- Earths Multiples?

My Work If/When Kepler finds multiple transiting planets, how cool will it be? –Answer: most information rich planetary systems outside our own Solar System by far Precise characteristics Learn about the entire system

Mutual Events!

Conclusions / Take Home Super-Earths are the easiest extra-solar planets to find that might have life (habitable and life emergable) Currently a small population (~20), but Kepler will revolutionize (dozens or more) Best for life: –Liquid water at a solid surface (Super-Earth) –Solar-like stars help –Stable systems: orbit of the planet only mildly perturbed by other planets in the system The best planets for life are in many ways the most difficult to find (hot, big planets around small stars) and characterize To get serious about habitability, you need transiting planets