Friday, October 10th, was "day zero" of the conference, when many formal and informal workshops are held. I attended a workshop titled "Observing Planets (wherever they may be) with James Webb Space Telescope." The James Webb Space Telescope (JWST) is the planned successor to the Hubble Space Telescope (HST). It was conceived as the next-generation space telescope after Hubble (in fact, it used to be called the NGST) and is an upgrade to HST in many ways. At the workshop, we heard updates on its capabilities and also had the chance to voice some suggestions we have about making the telescope more useful for planetary observations.
An artist's impression of the James Webb Space Telescope
will be a revolutionary tool for observing planets outside of our solar system,
or exoplanets, as it will enable us to
discover many more exoplanets orbiting around other stars. Detecting planets around another star is a technological challenge because planets do not emit light on their own, and are
thus vastly outshined by the star they orbit around. Astronomers primarily approach this challenge in three ways.
First, planets can be detected by observing the spectrum of the light emitted by the star. When an orbiting planet cause a wobble in the motion of its host star, the Doppler shift in the spectrum can be used to measure the planet's mass and its orbit's period and eccentricity. In some cases, the Doppler shift in the light emitted from the planet itself is also detectable, which can be used to glean additional information about the distant planet.
Second, when the tilt of the planet's orbit is just right, the planet can pass in front of the host star and block some light, an event called a transit. The timing of transits in a system will directly indicate the planet's orbital period (and thus its mass). The magnitude of the dimming caused by a transit will give us good ideas about the radius of the transiting planet as well.
The third and most challenging detection method is the direct imaging of an exoplanet. From the vantage point of Earth, an exoplanet looks much darker than its host star, so collecting enough photons from the planet will require a relatively long exposure time. By the time we collect more photons from the planet than occur in the background noise level, however, the star will have completely saturated the detector and buried the planet's signal. Thus, we must somehow block the star's light in order to see the planet—a technique that relies on hardware called a stellar coronagraph.
Hiding the sun behind a tree trunk -- notice that the details are much more visible when the sunlight is blocked -- kind of like coronagraphy.
JWST will improve detection of exoplanets using all three methods. Its higher sensitivity to light will enable spectral measurements that allow detection of much smaller Doppler shifts from much darker objects. Its more sensitive optics will also improve transit measurements, and it will be able to take high-resolution spectrum measurements as transits occur. Its coronagraph will enable, for example, direct imaging of a planet twice the mass of Jupiter orbiting at a 7 AU radius around a low-mass star that is 13 light years away from Earth.
JWST's ultra-high sensitivity presents many exciting opportunities to observe objects inside our own solar system as well. It will vastly improve our ability to observe Pluto and other icy objects at the outer fringe of our solar system. It will also take some of the most sensitive and highest resolution images ever achieved for Uranus and Neptune in infrared wavelengths. However, JWST's ultra-high sensitivity becomes an issue for observing things closer to our sun
In its current configuration, Jupiter and Saturn are both mostly too bright for the telescope to look at all (there are some filters that allow limited observations of Saturn in some wavelengths) —their light will completely saturate the detectors and just create a bright white dot without resolving any features. This obviously poses a problem for researchers studying Jupiter and Saturn (like me), because long-term monitoring of Jupiter and Saturn has been done using HST, and we will lose that capability when the Hubble reaches its end of mission. At the workshop, several solutions were proposed to address this issue; we will see if any of them can be realistically implemented without impacting the existing strengths of JWST.
The workshop was organized and moderated by Jonathan Lunine of the University of Arizona. Contributors to the workshop included Drake Deming and Mark Clampin of NASA's Goddard Space Flight Center, Glenn Orton of the Jet Propulsion Laboratory, John Stansberry and Brian Jackson of the University of Arizona, and Jeff Valenti of Space Telescope Science Institute.
October 17th, 2008: This article was edited by Kunio to reflect notes contributed by Glenn Orton to clarify JWST's outer planet observing capabilities. The "coronagraph" picture was also added.