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.
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.
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.
By using a low-scattered light telescope with the contrast enhancement techniques of coronagraphy 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.
Our Solar System
PLANETS will provide significant tools for planetary science in our solar system. Though planetary missions with spacecraft are very useful to obtain new findings on solar system bodies, the spacecraft data are limited by instrumental constrains, mission lifetime, orbital motion, and instantaneous field-of-view. Complementary ground-based observations can probe atmospheric global region remotely and can provide long-term coverage. Such observations will help us to understand the changes happening in the solar system, including the Earth.
Low scattered-light capability with coronagraphy and high-resolution spectroscopy enables to visualize neutral atmosphere and plasmas escaping from icy satellites of Jupiter and Saturn as well as Io that has active volcanoes. These observations have been difficult because of the presence of bright planetary disk that bothers observations of faint emission close to the mother body. Long-term monitoring from the ground will reveal activities and dynamics on and around the solar system body.
It is essential to carry out continuous measurement of planetary atmosphere, such as the Jovian infrared aurora and the volcanoes on Jovian satellite Io, to understand its time and spatial variations. A compact and easy-to-use high resolution infrared spectrometer provide the good opportunity to investigate these objects continuously. We are developing an Echelle spectrograph called ESPRIT (Echelle Spectrograph for Planetary Research In Tohoku university). The main target of ESPRIT is to measure the Jovian H3+ fundamental line at 3.9 micron, and H2 nu=1 at 2.1 micron. This spectrograph is characterized by a long slit field-of-view of ~ 50 arcsec with a spectral resolution is over 20,000.
Observations of Mars and Venus atmospheres will be performed with PLANETS using ultra-high resolution spectroscopy with a spectral resolution of >1,000,000. It’s one of the most appropriate instrument to addresses (i) trace gases and its isotopes, and (2) dynamical/thermal structures in the upper atmosphere. Our observations reveal the planetary worlds of our solar system, and increase our knowledge what/how is habitable planetary environment and its evolution? PLANETS thereby expand the frontiers of the human race.
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