Optical and X-ray (cut-out) image of the Alpha Centauri binary stars (Optical: Zdenek Bardon; X-ray: NASA/CXC/Univ. of Colorado/T. Ayres et al.)

There is something terribly exciting about actually seeing an exoplanet. While we have discovered over 4,000 planets outside the solar system, the majority of these worlds have been identified through their influence on their star, either via a dimming of the star’s light as the planet transits across its surface, or the wobble of the star from the planet’s gravitational pull. These are incredibly powerful techniques for planet hunting, but neither allow us to actually lay eyes on the planet itself.

The method to actually see a planet is known as “direct imaging” and it is a tricky process, as the star’s light can easily overwhelm any radiation coming from the smaller, cooler planet. Exoplanet imagining has therefore focused on young Jupiter-sized worlds orbiting far from the powerful lighthouse of the star. These planets are large and their recent formation has left them packed with heat, with temperatures around 1340°F (727°C). Such hot houses emit thermal radiation at wavelengths around 5 microns, so most of the instruments dedicated to capturing planet pictures operate around this wavelength range.

Direct imaging of exoplanets is difficult, and so far has been mainly restricted to young, massive planets. This amazing animation of four planets more massive than Jupiter orbiting the young star HR 8799 includes images taken over seven years at the W.M. Keck observatory in Hawaii. (Jason Wang and Christian Marois)

However, these wavelengths are a bad choice if you want to try imaging an Earth-like world. As an evolved planet on a temperate orbit, thermal emission from a planet like our own is longer at about 10 – 20 microns. This is an awkward wavelength for observations from the Earth, as the Earth’s own thermal emission can swamp the distant signal of the planet.

Yet, being able to directly image temperate planets is an important technique for studying possible habitable worlds. As you move away from the star, the chances of the planet’s orbit transiting across the star’s surface from our view from Earth decreases. For a planet on a similar orbit to the Earth around a sun-like star, the probability is less than 0.5%. The only way to study many of these worlds may be if we can see them directly, and space-based observatories have been generally seen as the path to this kind of imaging.… Read more