Highlight: Rich Young | Nick Woolf | Alan Penny | Bill Borucki | Chas Beichman | Bob Siegel | Rajendra Vickramsingh | Ted Kostiuk | Darren Williams
Rich Young: Why does the effect of dust increase with wavelength?
Nick Woolf: As we go to longer wavelengths, we are looking at a bigger patch of sky and more dust. And as we go to longer wavelengths, radiation from the telescope becomes important source of background. Out to 20 microns dust will be the limit.
Alan Penny: Just exactly how long a wavelength we choose to work to is important. Important factor is T of planet. We need to get out to at least 20 to 25 microns in order to determine the planets surface T.
Nick Woolf: Until we get out beyond 20 microns, we dont see curvature of the continuum spectrum. And to determine abundance of water vapor, even to identify it, we need to know the T
Alan Penny: Water vapor and other molecules have rich spectra and that will make it tougher to determine the underlying continuum. There is a premium on going to longer wavelengths. Also we need to continue to study this problem, with respect to the temperature of the optics, where we might find it necessary to cool in some way in order to ensure we get the necessary information to interpret the spectrum we do see.
Bill Borucki: Concerning the measurement of the dust around other stars: I thought it was understood that there would be an attempt to measure the amount of dust and whether or not it would be a problem. Any information on this?
Nick Woolf: We have made estimates of what to expect. One system under construction, due to be working the end of this year, will begin to test the techniques. What we may discover is unpredictable; it could be a terrible problem and then wed have to revisit our plans.
Chas Beichman: NASA has recognized the problem. If get much above 10 X zodi (i.e. if the extra-solar dust cloud has 10X the density of the local zodiacal cloud), then our current concepts are badly hurt, especially if there is spatial structure in the dust cloud at the appropriate scale. But there are a variety of telescopes on line or coming on line shortly that will enable us to study this problem in some detail. Theres the Keck interferometer, LBT, and VLTI to attempt measurements of the warm dust, and theres SIRTF to measure the cold dust in the outer regions of these solar systems. So on a time scale of 3-5 years we should know the magnitude of problem.
Nick Woolf: Weve analyzed what happens if a small fraction of the systems are dust free (no more than 1 zodi) and the rest are dusty. Answer is: make larger apertures/separations, observe dust-free systems, working to larger distances to include a useful number of such systems.
Bob Siegel: There are other techniques for attacking these problems. For example: gravitational lensing has been mentioned, along with nulling. Anyway to combine techniques to increase resolution?
Nick Woolf: Gravitational lensing picks out sources at large distances, so its not much use to TPF-like searches. But there is an old technique coming back into favor: occulting disks. Football field sized occulting devices a million km away from the orbiting telescope. Needs a cooling shade (football field sized, too!). Studies are going on to see if there is some better way than what we currently plan.
Rajendra Vickramsingh : One could combine nulling with radial velocity difference between planet and star. Im not sure what effect dust would have in that case. Does it follow radial velocity of planet? Could use Doppler differential of planet going around the star. So, a spatial-spectral nulling technique. Molecule by molecule, looking at lines as they go in and out of a spectral window.
Nick Woolf: If we are looking for spectral lines of a planet against the stellar light, then we can see the lines from planet go to and fro, while the stellar lines are fixed. But the dust glows over the whole spectral range, and just produces plain noise, so this problem is not helped by this technique. Also, to get into that regime, we would need very high spectral resolution, much more than our 25 m telescopes can do. So this combined technique is not in our thinking at the moment, but maybe in the future.
Ted Kostiuk: In estimates of the limitation in sensitivity due to T of optics and background, did you take into account the shot noise from the star and background? And how does that effect the choice of resolution of the instruments you use? This problem would get better as go to higher spectral resolution.
Nick Woolf: If there is more energy to detect , then there is more photon noise. So it would be harder to detect planet. There doesnt seem to be a way to win. Best is to reduce starlight as much as possible by ANY technique available, then take the amount of planet light left and spread it over as wide a spectrum (resolution) as you can detect, given starlight and time available.
Darren Williams: A lot of planets will not be 255 K, they will be warmer. So that should help. Even if theyre not habitable, studying them will still be instructive.
Nick Woolf: We will undoubtedly observe a wide variety of planets, many hotter and brighter, but they will still be interesting.