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 (O2, O3, CH4) 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 Finderconcepts

• 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)

http://exoplanets.org/massradiiframe.html

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

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

• 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

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• 105 precision photometry• 0.95-m aperture capable of detecting Earths• Launched: March 5, 2009

December 2011 data release

Candidate label

Candidate size (RE)

Number of candidates

Earth-size Rp < 1.25 207

Super-Earths 1.25 < Rp < 2.0

680

Neptune-size 2.0 < Rp < 6.0

1181

Jupiter-size 6.0 < Rp < 15 203

Very-large-size

15 < Rp < 22.4

55

TOTAL 2326

• 48 of these planets are within their star’s habitable zone

Kepler-22b

• 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 3414%– 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 1/3rd 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