High-Contrast Imaging

The planets in our Solar system are at least a billion times less luminous than the Sun. This is roughly the contrast between a firefly and the total light output of a sports stadium. Young gas giants, however, are hot, and the contrast at near-infrared wavelengths can be about a million (rather than a billion). With the aid of adaptive optics and high-contrast instruments, we can see these planets using 8-10 meter telescopes.

The movies below show how we actually look for faint planets within the glare of a bright star. In the left panel (actual data from a binary star), the telescope rotates with the Earth, and the diffraction pattern rotates along with the telescope. The orientation of the stellar companion remains fixed. In the right panel (a simulation), we see the stellar diffraction pattern expand as we step through wavelength. You might be able to spot a faint planet above and to the right of the center of this image--watch for a faint point of light that does not expand with wavelength. New instruments like CHARIS, an integral-field spectrograph we are building at Princeton, will allow us to follow the stellar diffraction pattern through a wide wavelength range and help us discover faint planets.

Finding a planet by direct imaging means that we actually receive photons from the planet. This gives us a probe of its atmosphere, and allows us to measure its temperature, gravity, and chemistry. High-contrast imaging from space will be the only way to measure the surface properties of terrestrial exoplanets, and the only way to probe the atmospheres of terrestrial planets that do not transit.