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Direct Imaging

"Direct imaging" is the technical term for "actually taking a picture of a planet."

Direct imaging probes a different type of planet than either the transit or wobble method, and the planets found with direct imaging are very different than any in our solar system. The reasons that these planets are so different are directly related to the challenges of this method. The long and the short of it is that stars are very bright, and planets are not.

Stars are incredibly bright and distant. Planets, on the other hand, generate very little light of their own and mostly reflect light from their home star. From our point of view, most stars are only a single pixel on our telescope's detectors, and the planets' light falls into that pixel as well. We can't separate the planets' light from their star's light.

However, for some nearby systems, we have a chance of detecting a planet this way because our telescopes can detect more detail. Astronomers use various image-processing techniques to remove the light of the star while leaving the light from the planets.

By analogy, if you're trying to look at a person through the glare of their flashlight, likely the first thing you'll do is to put your hand over the light. This is also how astronomers do direct imaging: they try their best to block out the light from the star, so that they can see the dim planets around it. Scientistis have developed sophisticated instruments and techniques that help them block out starlight very well. We typically look at light outside the visible spectrum, such as infrared, for this purpose.

Once you block the light of the star, what kinds of planets can you see? Planets orbiting too close to the star will be lost in the glare, even in the best of circumstances. The planets that have been directly imaged are distant from their stars and typically orbit them with periods of hundreds of years. This also means that nearby stars are better targets, because the angular separation between a star and its planets will be larger the closer it is to us.

The star shown below is only 120 light years from Earth, which is fairly close in galactic terms. At the center is some light noise that comes from blocking out the star's image. You can see four planets in this image, all of which are large gas giants at great distances from their central star. Because stars are so bright, only planets that orbit far from their star can be revealed with this method.

Brighter planets will also be easier to detect. There are two ways for a planet to shine: it can reflect the light of its host star, or it can emit its own light. Reflected starlight won't be anywhere near a match for the light of the star -- especially at large separations -- so that leaves the latter option as our best bet. Planets do emit light (energy) of their own. Some of this energy is absorbed and re-radiated starlight. Some is energy left over from the planet’s formation, which is radiated away as the planet cools down. The bigger the planet and the younger the planet, the more energy it has available to give off. Direct imaging is therefore most sensitive to young, large planets.

Astronomers therefore target nearby, young stars in their search for directly imaged planets. After removing the light from the star, they look for small sources of light that persist as the telescope set-up changes. Over time, we also check that the planets are in fact moving around their star as you would expect from an orbiting planet.

Directly imaged planets are our best opportunity to study young planets and planetary systems, which are not as readily detectable with other techniques due to the intrinsic variability of young stars. They offer a unique glimpse into the process of planet formation. At the same time, they may not be the best planets to investigate for signs of life because they are young and outside the habitable zone.

An advantage of the direct imaging method is that continuous observation can give us more detailed information about the planets' size, orbit, and more. We can also use this method to detect planets that are not in-line with their stars from our point of view. On those rare times when we can detect a planet through direct observation, we will likely be able to gather a great deal more information as time goes on.

As of this writing only 17 planets have been directly imaged. None of them are more than 500 light years from Earth.

The Future of Direct Imaging

Scientists are working on two advanced techniques that they hope will improve the sensitivity of direct imaging to Earth-like planets. The first is a “starshade,” an opaque, external screen that flies separately from a space telescope and blocks the light of the star hosting a planet we wish to observe. Although starshades were first proposed in the 1960s, recent innovations in technology and design have turned this into a realistic option. The second is an internal coronagraph, which similarly blocks the light of the star but is built into the telescope rather than being flown externally. Coronagraphs get their name from observations of the Sun: a coronagraph blocks out the bright disk of the Sun, allowing the much-fainter solar corona to be seen.

Both methods are promising, and their development is supported by NASA. The science goals of the two missions are similar: to find new planets and study their atmospheres. Super Earths are particularly interesting targets because of the hope of detecting biosignatures in their atmospheres.

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