Modern Concepts for a Terrestrial Planet Finder Space

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

TPF-C TPF-I/Darwin • There at least three concepts for a large, space-based telescope 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-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 • 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 581 g (the “Goldilocks planet”) is probably not real http: //exoplanets. org/massradiiframe. html Info from Extrasolar Planets Encyclopedia (Jean Schneider, CNRS)

• 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 http: //www. dlr. de/en/desktopdefault. aspx/tabid-5170/8702_read-15322/8702_page-2/

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 Earthsized (and other) planets • 10 5 precision photometry • 0. 95 -m aperture capable of detecting Earths • Launched: March 5, 2009 http: //www. nmm. ac. uk/uploads/jpg/kepler. jpg

December 2011 data release Candidate label Earth-size Super-Earths Neptune-size Jupiter-size Very-large-size TOTAL Candidate size (RE) Rp < 1. 25 < Rp < 2. 0 Number of candidates 207 680 2. 0 < Rp < 6. 0 < Rp < 15 15 < Rp < 22. 4 1181 203 55 2326 • 48 of these planets are within their star’s habitable zone

Kepler-22 b • 600 l. y. distant • 2. 4 RE • 290 -day orbit, late G star • Not sure whether this is a rocky planet or a Neptune (RNeptune = 3. 9 RE) http: //www. nasa. gov/mission_pages/ 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 31/3 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