1. Lecture 22: The Search for Life, Part 1: Kepler update Meteo 466

2. What is life? If we are going to search for life on other planets, we first need to decide what we are looking for One definition: “Life is a self- sustained chemical system capable of undergoing Darwinian evolution” --Jerry Joyce This definition, however, is better suited to looking for life in a laboratory experiment than for searching remotely on planets around other stars Jerry Joyce, Salk Institute

3. First requirement for life: a liquid or solid surface It is difficult, or impossible, to imagine how life could get started on a gas giant planet –Need a liquid or solid surface to provide a stable P/T environment This requirement is arguably universal

4. Second requirement for life: carbon Carbon is unique among the elements in forming long, complex chains Something like 95% of known chemical compounds are composed of organic carbon Silicon, which is located right beneath carbon in the Periodic Table, forms strong bonds with oxygen, creating rocks, not life Proteins

5. Third requirement for life (as we know it) : Liquid water Life on Earth is carbon- based (DNA, RNA, and proteins) and requires liquid water So, our first choice is to look for other planets like Earth Subsurface water is not relevant for remote life detection because it is unlikely that a subsurface biota could modify a planetary atmosphere in a way that could be observed (at modest spectral resolution)

6. The Goldilocks paradox Why is Venus too hot, Mars too cold, while Earth is just right? Lynn Margulis The obvious answer concerns their relative distances from the Sun However, this is only part of the story because the surface temperature of a planet is also determined by its greenhouse effect

7. Revised ZAMS habitable zone Kasting et al., PNAS, submitted (Figure by Sonny Harman) All of this leads to the concept of the habitable zone, which we have just discussed at some length

8. Definition of  Earth  Earth —the fraction of stars that have at least one rocky planet in their habitable zone –This is what we need to know in order to design a space telescope to look for such planets around nearby stars –We should be conservative when calculating  Earth for this purpose, because we don’t want to undersize the telescope

9. Kepler Mission http://www.nmm.ac.uk/uploads/jpg/kepler.jpg This space-based telescope will point at a patch of the Milky Way and monitor the brightness of ~160,000 stars, looking for transits of Earth- sized (and other) planets 10  5 precision photometry 0.95-m aperture  capable of detecting Earths Launched: March 5, 2009 Died (mostly): April, 2013

10. Kepler target field: The stars in this field range from a few hundred to a few thousand light years in distance

11. Probability of transits i = inclination of planet’s orbit to the plane of the sky  o = angle of planet’s orbit with respect to the observer (= 90 o – i) a = planet’s semi-major axis R s = stellar radius Then, the probability that a planet will transit is given by

12. Number of Earths to be detected Monitor ~160,000 stars Assume orbit at 1 AU around a G star Probability of transit: R Sun /1 AU = 7×10 5 km/1.5×10 8 km = 5×10 -3 (i.e., 0.5%) Expected number of Earths: N = 5×10 -3 (10 5 )   Earth = 500   Earth where  Earth is the expected frequency of Earth-like planets Actual numbers are slightly lower than this because not all stars being monitored are solar type

13. December 2011 Kepler data release Candidate labelCandidate size (R E ) Number of candidates Earth-sizeR p < 1.25207 Super-Earths1.25 < R p < 2.0680 Neptune-size2.0 < R p < 6.01181 Jupiter-size6.0 < R p < 15203 Very-large-size15 < R p < 22.455 TOTAL2326 Planets bigger than about 2 Earth radii (~10 Earth masses) are expected to capture gas during their formation and turn into gas or ice giants  The Earth’s and super-Earths are potentially habitable

14. Source: Christopher Burke, AAS presentation, Long Beach, CA, Jan. 7, 2013

16. Papers deriving  Earth estimates for Sun-like stars are being submitted as we speak Estimates of  Earth for late-K and M stars have already been published – These planets are easier to find because the orbital periods are shorter and the planet is bigger relative to the size of its parent star, making the transit depth deeper

17. Summary of  Earth for M/late-K stars  Earth size (0.5 – 1.4 R Earth ): 0.15 +0.13 Dressing & Charbonneau (2013) *  Earth size (0.5 – 1.4 R Earth ): 0.48 +0.12 (Conservative) 0.53 +0.08 (Optimistic) K opparapu (2013)  Earth size (0.5 – 2.0 R Earth ): 0.51 +0.10 (Conservative) 0.61 +0.07 (Optimistic)  Earth size (0.8-2.0 R Earth ): 0.46 (Optimistic) Gaidos (2013)  Bonfils et al.(2011) radial velocity optimistic estimate: 0.41 +0.54 -0.24 -0.17 -0.20 -0.15 -0.13 -0.06 +0.18 -0.15 * Dressing and Charbonneau used the old “1 st CO 2 condensation” limit for the outer edge, though, and so their estimate was arguably too low

18.  Earth for late-G and K stars Recently, Petigura et al. published an estimate of  Earth for K stars and (one) late-G star They got 0.22, but they assumed a HZ of 0.5-2.0 AU, which is too wide If planets are spaced geometrically, as in our Solar System, and if the real inner edge is closer to 0.95 AU, then their estimate for  Earth could be reduced by a factor of ~2, yielding ~0.1 Petigura et al., PNAS (2013) AU 0.51.02.0

19.  Earth for late-G and K stars Let’s look, though, at what Petigura et al. really did They took the 8 planets in the left-hand box and extrapolated out to the right-hand box, because they realized that the data were badly incomplete beyond 1 AU Petigura et al., PNAS (2013) AU 0.51.02.0

20.  Earth for late-G and K stars But, even the data in their left-hand box are not evenly distributed ⁻Only the upper half of this box is well populated (with ~7 planets, as opposed to 1 planet in the lower part) So, maybe one should extrapolate from the upper left-hand box This would increase their estimate of  Earth to 0.22*(14/8)  0.4 Petigura et al., PNAS (2013) AU 0.51.02.0

21.  Earth for late-G and K stars This estimate, though, may be slightly optimistic The conservative HZ is arguably farther out and at lower planetary mass, as shown at the right If one takes the data from the red box and extrapolates (linearly in log space) to the blue box, one gets  Earth = 0.3 Petigura et al., PNAS (2013) AU 0.51.02.0

22. Conclusions Habitable zones around F, G, and early K stars are relatively wide, regardless of whose limits one uses –The presence of liquid water on a planet’s surface continues to be a reasonable requirement for remote life detection –The CO 2 -climate feedback caused by the carbonate-silicate cycle is the key to climate stability Earth-like planets may be common –They do appear to be common around M-dwarfs, based on Kepler data (0.4   Earth  0.6) –  Earth for Sun-like stars appears to be somewhat lower, between 0.1 and 0.3, but the Kepler data are not complete in this mass range Direct imaging of planets in either the visible or thermal-IR (TPF or Darwin) should eventually allow us to characterize other Earth-like planets spectroscopically and to look for evidence of life