A feature of the planet WASP-104b discovered by astronomers at the University of Keele (Great Britain) is that its dense atmosphere absorbs almost all light entering it. Researchers compare it to coal and say that it is one of the darkest planets ever discovered.
The object belongs to the class of hot Jupiters, which are huge gas giants, similar in mass to Jupiter, but located very close to their stars, due to which they have a very high temperature. Typically, such planets have an orbital period of less than 10 days.
Representatives of this class are not uncommon, but they often exhibit features that make them very interesting for research. For example, one such feature is that hot Jupiters are much darker than normal planets.
Most of the hot Jupiters discovered to date tend to absorb about 40 percent of the light. However, in 2008, astronomers discovered the planet WASP-12b, which turned out to be much darker than its counterparts. Scientists have found that the planet is capable of absorbing at least 94 percent of the light entering it. The detected WASP-104b, in turn, turned out to be even darker. Astronomers estimate that this object is capable of absorbing 97 to 99 percent of its star's light.
“Among all the darkest planets that are written about in the textbooks, I can easily put WASP-104b in the top five of the darkest. No, not even in the top three,”says the head of the study, astrophysicist at Keele University Theo Moknik.
One of the possible explanations for its "dark nature" may be the distance at which the planet is located from its star - a yellow dwarf, located about 466 light-years from us in the constellation Leo. The distance between WASP-104b and its home star is only about 4.3 million kilometers. Because of this, its orbital rotation period is only 1.77 days.
Like most hot Jupiters, WASP-104b is tidal (one of its sides is always turned towards the star). In other words, there is an eternal day on one side of the planet, and an eternal night on the other. As a result, the daytime side of the planet is so hot that clouds cannot form above it, which would increase its reflectivity, and ice cannot form on the nighttime side, since the planet's temperature is generally very high.
Instead, WASP-104b has a very thick clouded atmosphere, most likely high in sodium and potassium atomic particles that absorb light in the visible spectrum and make the planet very dark even on the day side, which is always illuminated by a star. On the night side of the planet, clouds are very likely to form, but since light never gets there, these clouds have nothing to reflect.
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Despite the fact that hot Jupiters are usually darker than normal planets, they are no more difficult to detect. All of them are very far away from us to be seen directly or seen against the background of the much brighter stars around which they are located.
The only available method for detecting these planets is by observing the change in brightness of the stars themselves, which usually decreases as the planet passes in front of the star itself and the observer. Such a search is called a transit detection method, and it is the main and very effective way to search for new exoplanets, for example, space telescopes such as Kepler.
Since hot Jupiters are very large planets, they can also be detected using the Doppler method, which is a spectrometric measurement of the radial velocity of a star. A star with a planetary system will move in its own small orbit in response to the planet's pull. This in turn will result in a change in the speed at which the star is moving toward and away from Earth (that is, a change in the star's radial speed with respect to the Earth).
In addition, such planets cannot be called black, like, say, coal, tar or the same Vantablack (the darkest material on Earth). They are referred to as dark to determine their reflective rather than radiated feature. Since these planets are usually very hot, they can have deep bluish, purple, or dull reddish glow.
The darkest hot Jupiter ever discovered is TrES-2b, which reflects just 0.1 percent of the light it hits.
Nikolay Khizhnyak
