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…

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 www.exoplanet.eu and www.planetary.org/exoplanets) 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 (0.1-100 Msun)

• 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 40307

• 3 Super Earths (P: 4,10,20 days)

• Small planets tend to be in multiple systems

• Minimum masses– 4, 7, 9 Earth masses

Gliese 581

• 4 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 100000 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