In astronomy, direct observation of new objects was once the gold standard for confirming their existence. For example, the existence of the planet Neptune was deduced from orbital perturbations of Uranus, but its official discovery date is the day Johann Gottfried Galle actually laid eyes on it through a telescope. Yet when it comes to planets outside our solar system (extrasolar planets or exoplanets), indirect observation has been the only significant evidence, because significant technical challenges hamper direct imaging of these distant objects.
Difficulties of Imaging Exoplanets
Exoplanets, like the planets in our solar system, do not produce their own light but rather reflect the light of their parent stars. For each planet, this reflected light is estimated to be from a million to ten billion times dimmer than that of the parent star. Extreme sensitivity is needed to pick out the pale speck of the planet from the intense glare of its star.
On the scale of astronomical distances, planets are located in close proximity to their stars. Very fine resolution is needed to distinguish the relatively small world from the nearby sun it orbits.
Overcoming the Challenges
A type of telescope called coronagraph is one way to overcome the difficulties of producing exoplanet images. A coronagraph blocks the light from the stellar disk, permitting observation of the corona and surrounding objects like planets. The incoming light of a coronagraph is subject to diffraction, an optical artifact, but the diffraction can be manipulated to permit detail features to be detected.
Another way to image exoplanets is through the use of an interferometer, which takes advantage of the wave nature of light to use multiple small mirrors to obtain images with high angular resolution. Interferometry may thus make it possible to distinguish a planet close to its star.
A few images have been taken of exoplanet candidates. Some have not been confirmed as planets, but may actually be images of brown dwarf companion stars. However, two notable images stand out.
The first is an image of 2M1207b (red object in the lower left corner), an object orbiting the brown dwarf 2M1207, about 172 light years away. The object has been confirmed to be a planet because its mass is insufficient to it to be even a small star, so the image, taken in 2004, is the first verified image of an extrasolar planet.
A brown dwarf is very different from the Sun, a yellow star. Another star named 1RXS J160929.1-210524, which is very similar to the Sun except in age (it is about 1/1000 the age of the Sun), has a planet (small circled object, upper left) whose image was first obtained in 2008. This image is notable because of the similarities between our Sun and this star. The Gemini North telescope at the Mauna Kea observatory in Hawaii made the images of the system, which is about 500 light years away in the constellation Scorpius.
References & Credits
“How to Take Snapshots of Distant Worlds” (NASA/JPL): https://planetquest.jpl.nasa.gov/technology/planet_imaging.cfm
2M1207b image and information: European Southern Observatory
1RXS J160929.1-210524 image and information: Gemini Observatory