Direct imaging of exoplanets can seem to be the most simple system of all the different methods that exist, but, in fact, it is the most complicated from a technical point of view. For this reason, the most used methods for detecting extrasolar planets are indirect detection methods, such as exoplanetary transits and radial velocity.
The two main problems that has to face this technique are: the small size of the planet that wants to be photographed in comparison to its star, and, most importantly, the enormous difference in brightness between the two of them.
The light that arrives us from a planet is the light that it reflects from its star added to its intrinsic emission (which is infrared light, emitted by the interior heat of the planet), and this is the main obstacle for the direct imaging of exoplanets. In the case of similar planets to the Earth, the light that emits the star is about 10 billion times more intense than the one of the planet. On the other hand, planets the size of Jupiter or Neptune are 1 billion times fainter than the stars.
Fortunately, the technological innovations incorporated during the last years to terrestrial and space telescopes have allowed much progress in this field to take place. There are two techniques that are essential, with which it is possible to annul the blinding light from the star, and be able to directly observe the planets that orbit it.
The first consists of blocking the light from the star, so that only the light from the planets orbiting it can reach us. This is done by using a coronagraph, a device invented in the 1930s by Bernard Lyot with the goal of observing the Sun’s corona (from here comes its name).
Nowadays are being developed different types of coronagraphs to be able to study extrasolar planets. Their basic functioning consists in using a circular mask that enables us to block the light coming from the star. A good analogy is when a solar eclipse takes place. When this happens, the Moon blocks the light that comes from the Sun, which allows us to observe its corona.
This second direct method to detect exoplanets consists in observing the light of a star in order to nullify the brightness of the central star by using the wavy properties of light. By doing this the light of the planets that orbit it remains untouched, for which it is possible to analyse it.
The results of direct imaging
From the more than 5,000 discovered planets until now, only 50 have been directly observed, due to the difficulties that involve these two techniques. The first planet that was found with this method was GQ Lupi b in 2004. This planet orbits a star 495 light years away from the Earth, and it has a mass between 1 and 36 times the one of Jupiter.
In addition, it is important to mention that, when we say we have seen a planet, we refer to, that with the best telescopes, we are capable of perceiving a faint point of infrared light of the planet (either its own light or reflected). From here, and with information obtained using other methods, we can deduce the mass of the planet, as well as its volume, its density, the temperature of its surface, and the composition of its atmosphere in case it has one.
However, all the images we are used to seeing about newly discovered planets, that show how their surface is (if it has mountains, oceans, ice blocks, clouds…) are only artistic representations of what we can imagine there is, and they don’t represent reality.