Searching for Truly Habitable Planets

Darin Ragozzine

Harvard-Smithsonian Center for Astrophysics

SPU 30:Life as a Planetary Phenomenon

April 19, 2011

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

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

SETI Plus• Other ways to detect the presence of extra-terrestrial intelligence than radio

or optical signals

– As with microfossils and biomarkers, look for any phenomena that cannot be explained (based on our current understanding) by “natural” phenomena

– Search for ExtraTerrestrial Artefacts

– Spectral signatures of large-scale asteroid mining

– Searches turn up nothing “unnatural

At best, weak evidence for microbial life elsewhere and zero evidence for

extra-terrestrial intelligence.

Fermi's Paradox: “Where are they?”

• Remember the astrophysical sense of time

• With no evidence, seems like Drake's Equation must evaluate to ~1 or all other civilizations are “hiding” (maybe)

• Possible bottlenecks

– [Number of Potentially Habitable Planets is low]

– Gap between “potentially habitable” as we know it and life-emergable

– Probability of life arising is low (Earth is an exception)

– Probability of complex life arising is low (Earth is an exception)

– Probability of intelligent life arising is low (Earth is an example?)

– Probability of communicating in a way we would recognize is low (maybe)

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) has been discussed earlier

• 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– Possibly low atmospheric pressures (never

accreted or not big enough to hold on to H)

• Ideally, these Super-Earths need to be:– Near the Habitable Zone– In a stable exoplanetary system

Overview of 2010 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

• M dwarf: close-in Habitable Zone

• Easiest to find (in Doppler and Transit)

• 5-6 planets, include one in HZ

• Problems:– Tidal locking– Flaring– High UV,X-ray

Scientists Find New Earth!

Reliable Science News

• Best: be an expert and read the peer-reviewed journal article

– But Gliese 581g “100% sure of biology”

• Next best: press releases, quotes from reputable scientists

• Wikipedia is usually okay

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

Multiples: Information-Rich

• Orbital architecture– Planet formation and evolution

• Planet-planet interactions (dynamics!)– Short-term interactions can be detected– Long-term interactions must be stable

• Comparative Planetology

Multi-Transiting Systems?

• Transiting Planets– Allow for physical characterization

– Radius, Density, Atmosphere

– (Interior, Composition, Habitability)

• Multiplanet systems– Determine orbital architecture

– Bring tools of dynamicists to bear

Multi-transiting systems are the most information-rich exoplanetary systems, combining the value of physical characterization with orbital architecture.

Ragozzine & Holman 2010

Feb 2011 Kepler Data Release

• All data from first 126 days

• Light curves generate candidates

– Could be “false positives”

– Most are likely (~90%) planets

• Search for your own candidates at planethunters.org

• Everything in today's talk is public

Over 1200 Exoplanet Candidates!

Habitable Zone Candidates

!!

Multiple Candidate Systems!!!

Numbers of multiplanets:

115 doubles, 45 triples, 8 quads,

1 & 1 of five and sixBorucki et al. 2011Lissauer, Ragozzine, Fabrycky et al. 2011b

Detrend, Renormalize

Kepler, the Multi-Transiting Machine

Lissuaer, Ragozzine, et al. 2011b

Fold on Observed Period

Transit Timing Variations (TTVs)

Conclusions / Take Home

• Fermi's Paradox requires an explanation

• Super-Earths are the easiest extra-solar planets to find that might have life (habitable and life emergable)

• Kepler has discovered hundreds of super-Earths

– Rough estimate is that the average number of planets per star is ~0.2, about 5% of these may be habitable

– Likely Tens to Hundreds of Millions of Potentially Habitable Planets

• Systems with multiple transiting planets are especially useful for studying habitable planets

– Measure mass through transit timing variations

• To get serious about habitability, you need transiting planets