Transiting Exoplanets with JWST

Transiting Exoplanets with JWST
http://arxiv.org/abs/0808.1913

abstract wrote:

The era of exoplanet characterization is upon us. For a subset of exoplanets -- the transiting planets -- physical properties can be measured, including mass, radius, and atmosphere characteristics. Indeed, measuring the atmospheres of a further subset of transiting planets, the hot Jupiters, is now routine with the Spitzer Space Telescope. The James Webb Space Telescope (JWST) will continue Spitzer's legacy with its large mirror size and precise thermal stability. JWST is poised for the significant achievement of identifying habitable planets around bright M through G stars--rocky planets lacking extensive gas envelopes, with water vapor and signs of chemical disequilibrium in their atmospheres. Favorable transiting planet systems, are, however, anticipated to be rare and their atmosphere observations will require tens to hundreds of hours of JWST time per planet. We review what is known about the physical characteristics of transiting planets, summarize lessons learned from Spitzer high-contrast exoplanet measurements, and give several examples of potential JWST observations.

Some examples of James Webb Space Telescope capabilities for terrestrial transiting planets.

Example 1: Large earth-like exoplanet. 1.5 Earth-radius, orbiting Sun-like star at 1 AU, at 20 pc distance.

Quote :

JWST could achieve a 500-sigma detection with 0.25 sec exposures, 0.5 sec down time, and 3 x 10^8 photons per integration... this kind of detection can distinguish between Earth- and Venus- like atmospheres. The observations required for such a 4-sigma discrimination in this case would span 30 hours centered on a 10-hour transit.

Example 2: M3V system, P=29 d, Jmag = 8, planet is an ocean world.

Quote :

... will have a strong atmospheric water vapor signature of 40 to 60 ppm. CO_2 features may also be detectable. With a 1.4 hour transit duration, 10 transits and 28 hours of observing time are needed for a suitable SNR. This ocean planet example is an 0.5 Earth-mass, 1 Earth-radius planet. Its lower density makes its scale height more than two times higher than Earth's, making the transmission spectrum twice as easy to detect.

Example 3: Same as before, but planet is 0.1 Earth mass, 0.5 Earth radius.

Quote :

The water vapor signatures are several ppm. 54 hours of observing time would be needed for a significant detection. With the period of 29 days in both examples, scheduling to observe 10 or more transits is critical.

They emphasize that "M3V or later stars as bright as J=6 to J=8 are rare, making transiting planets even more rare."

Example 4: superEarth with hydrogen-rich atmosphere created by outgassing with a surface gravity high enough to prevent loss of all of the hydrogen. 5 Earth-mass planet. 1.5 Earth-radius. Surface gravity would be 2.2 times that of Earth.

Quote :

The transmission spectra signal will be 10 times stronger than the medium ocean planet described in example 2. Such a hydrogen-rich kind of planet would require ~3 times less observing time than the Earth-like planet in example 2, or approximately 10 hours.

Example 5: 2 Earth-radius object orbiting 0.03 AU from 20pc-distant M5V star.

Quote :

The SNR would be high enough to detect O_3 or CO_2 features... With the 100 hour observation (for ~40 transits and with the total time divided between in-eclipse and out-of-eclipse) a SNR of 10 to 15 is possible.

Example 6: Same as above, but 10-pc-distant M8V star instead and 1 Earth-radius.

Quote :

... a 200 hour observation of a 1 Earth-radius planet orbiting in the habitable zone of a 10-pc-distant star with R=50. For comparison we note that 100 hours is a bit less than half of the Hubble Deep Field observing time.

It sounds quite ambitious and self-assured. I hope it may really detect a plenty of rocky planets as they affirm.