Modern Concepts for a Terrestrial Planet Finder Space Telescope James Kasting Department of Geosciences Penn State University

There are at least three concepts for a large, space-based tele- scope that could directly image Earth-size planets around other stars These missions would also look for spectroscopic biomarkers (O 2, O 3, CH 4 ) and try to infer the presence or absence of life on such planets Transit spectroscopy (e.g., from JWST) might be used to characterize an Earth around a nearby M star, but the prospects for doing this seem pretty bleak TPF-I/Darwin TPF-C TPF-O NASA’s Terrestrial Planet Finder concepts

The bad news is that none of these Terrestrial Planet Finder concepts is moving forward at the moment –Preliminary (pre-Phase A) design work for TPF-C was initiated in 2005 but abandoned after only 6 months The good news is that discoveries of exoplanets have exploded since that time..

Known extrasolar planets 704 extrasolar planets identified as of Nov. 27, 2011 –650 by radial velocity –186 transiting planets –13 microlensing –29 direct imaging –12 pulsar planets –94 multiple planet systems Few, if any, of these planets are very interesting, however, from an astrobiological standpoint –Gliese 581g (the “Goldilocks planet”) is probably not real Info from Extrasolar Planets Encyclopedia (Jean Schneider, CNRS) 704

822 stars monitored for 8 years More than 50% of solar-type (FGK) stars harbor at least one planet of any mass and with period up to 100 days Most, or all, of these planets are significantly more massive than Earth We don’t know whether any of these planets are habitable. Surface habitability requires a rocky planet within the habitable zone of its parent star

The (liquid water) habitable zone The habitable zone is the region around a star where liquid water can exist on a planet’s surface The habitable zone is relative wide because of the negative feedback provided by the carbonate-silicate cycle

Kepler Mission 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

December 2011 data release Candidate label Candidate size (R E ) Number of candidates Earth-sizeR p < Super-Earths1.25 < R p < Neptune-size2.0 < R p < Jupiter-size6.0 < R p < Very-large-size15 < R p < TOTAL of these planets are within their star’s habitable zone

Kepler-22b 600 l.y. distant 2.4 R E 290-day orbit, late G star Not sure whether this is a rocky planet or a Neptune (R Neptune = 3.9 R E ) kepler/news/kepscicon-briefing.html

 Earth The Kepler mission is designed to measure  Earth —the fraction of stars that have at least one 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

 Earth from Kepler Two different estimates of  Earth have now been published based on the Feb. (2011) Kepler data release Cantanzarite and Shao (Ap. J., in press) estimate 1-3% Traub (diagram at right) estimates 34  14% –The difference has to do with whether one assumes that the data are complete for orbital periods >42 d. (They obviously are not, so Traub’s estimate is arguably better.) This analysis should now be repeated using the 2-year dataset from the Dec. (2011) data release W. Traub, Ap. J., in press

Implications of the Kepler results for future direct imaging missions The 2005 TPF-C telescope had an 8-m long axis and an inner working angle of 4 /D, giving it an angular resolution of ~50 mas at 500 nm. This allowed it to examine half the habitable zones around the nearest 60 or so stars in a 5-yr mission, yielding an expectation value of 3 Earths –For this design study,  Earth was assumed to be equal to 0.1 If  Earth = 0.3, we only need to look at 1/3 rd as many stars, so they will be closer by a factor of 3 1/3  1.4. If we can also work at 3 /D, then we could get by with a 4-m telescope (assuming that exozodi brightness is not too great)

Conclusions Characterizing planets in the habitable zones of nearby stars requires a large, space-based direct imaging mission –Such a mission can also look for evidence of life, so it could potentially lead to paradigm-changing results RV measurements suggest that rocky planets are common around many, or most, solar-type stars Within 2 or 3 years, Kepler should provide a good estimate of  Earth.. Preliminary estimates are optimistic (as high as 34%) The larger  Earth is, and the smaller the exozodi signal, the smaller the telescope that is needed to do this mission