There are several main science drivers for this telescope in various areas of stellar and planetary astronomy as well as instrumentation and engineering. Many areas of astronomy are not flux-limited but are dynamic-range and systematic-error limited. Simply collecting more photons will not solve the problem - we don't need bigger telescopes, we need better telescopes. As an example, an active area of astronomy where high dynamic range can make great advances is star and planet formation - imaging and characterizing planets around other stars and the disks that form them. These planets and disks are in many cases factors of millions to billions fainter than their host star and are difficult to detect because of the blinding light from the star. By making a telescope capable of containing and removing this star light, imaging and characterization of these planets and disks is possible.
One of the more popular goals in observational astronomy is directly detecting the light from extrasolar planets. This goal requires minimizing scattered light from the host star and maximizing the dynamic range of the detectors. Typically, the detected exoplanet is millions to billions of times fainter than the host star (with a much wider range of flux ratios depending on the specific system). The illustration on the left shows a highly stretched simulated image of such a planet visible in the diffuse scattered-light halo around the host star. As the planet moves closer to the star and the planet gets fainter, the detection becomes more difficult.
There are many exciting images of different kinds of circumstellar disks. Recently, Hubble reported the "first visible-light snapshot" of an exoplanet around Fomalhaut. The 2m Hubble telescope does not suffer from atmospheric distortion but there is still very significant scattered light obstructing the view of the exoplanet. The image in the press-release used several images at different epoch's to remove some of the scattered light. Even with the most sophisticated scattered light removal techniques, a very large radial scattered light pattern obstructs the view of the circumstellar disk in a single image. Building better telescopes to minimize scattered light will greatly enhance the utility of these telescopes.
Precision polarimetry is a very useful tool for detecting and characterizing extrasolar planets. The image on the left shows the measured variation in polarization of the system (an unresolved point source) and a simulation of the planet's orbit on the sky. The polarization variation is phased to the measured orbit of the planet. By using precision aperture polarimetry, a precision of around 0.01% was achieved. The scattered light signature was detected above this level and was used to derive basic atmospheric properties of the exoplanet.
By using a low-scattered light telescope with the contrast enhancement techniques of coronography and polarimetry, regions around extremely bright sources can be seen. This shows polarized light from a disk around a bright (7th mag) young star. The spatial scale of the disk in polarized light is around 300AU. A ring of low polarization (corresponding to less material) at 150AU is seen suggesting a proto-planet clearing a ring in the disk.
PLANETS will be also used for planetary science (solar system planets).
Though planetary missions with spacecraft are very useful to obtain new findings on solar system bodies, long-term monitoring from the ground is also necessary. For instance, we can observe atmospheric escape on Venus, on Mars, as well as on Io, a Jupiter's moon that has active volacanoes. Optical observations from the ground will help us to understand the changes happening in the solar system, including the Earth. Observations of minor constituent molecules will also be possible with PLANETS using ultra-high resolution spectroscopy. These observations have been difficult because of the presence of bright planetary disk that bothers observations of faint emission close to the mother body.