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Exoplanets are cunning little beasts.
Since they are very small, very dark, and very far away, seeing them directly is extremely difficult and rare. As such, we usually infer their presence from their effects on their host stars – meaning that when we see one directly, on occasion, it’s a cause of excitement.
And the excitement is exactly what you should be feeling with the discovery of an exoplanet called COCONUTS-2b, orbiting a star called COCONUTS-2.
COCONUTS-2b (named after the poll COol Companions ON Ultrawide orbits) is not only the closest directly imaged exoplanet to Earth to date – at a distance of only 35 light years – it’s a rarity among discoveries exoplanets: a giant relatively cold massive gas orbiting its star at a great distance.
“With a massive planet in a very wide orbit and a very cold central star, COCONUTS-2 represents a very different planetary system from our own solar system,” said astronomer Zhoujian Zhang of the Astronomical Institute of China. University of Hawaii. .
COCONUT-2b, red dot top left. (Zhang et al., The Astrophysical Journal Letters, 2021)
The most common methods of detecting an exoplanet rely on two effects that exoplanets might have on a host star. The first is called the transit method, and it relies on changes in the depth of a star’s light. When an exoplanet passes between us and its host star on its orbital path, this transit is detectable in the form of weak hollows in the star’s light.
The radial velocity (or “oscillation”) method relies on changes in the wavelength of a star’s light. When the exoplanet orbit the star, it exerts a slight gravitational influence that causes the star to wobble minutely. When it moves in a small circular motion, the wavelength of its light shifts slightly as it moves closer and further away from us.
These two methods are best able to detect massive and near-planet exoplanets – massive, because the signal will be larger and easier to discern, and close because they orbit quickly, which means that astronomers can get the multiples. signals that they need in order to confirm that they are caused by an orbiting body and not by some random passing object.
It was, however, the great distance from its host star – approximately 6,471 astronomical units, or 6,471 times the average distance between the Earth and the Sun – that made COCONUTS-2b visible in direct images. At this distance, its orbital period is about 1.1 million years (which could be a record for a known exoplanet).
“Directly detecting and studying the light of the gas giant planets around other stars is generally very difficult, as the planets we find usually have small separation orbits and are therefore buried in the brightness of the light of their host star.” University of Hawaii astronomer Michael Liu said.
What made it visible is that, although it is cool for a gaseous giant exoplanet, it is still quite warm, with a temperature of around 434 Kelvin (161 degrees Celsius, or 322 degrees Fahrenheit), despite its distance from the heat of the star. .
COCOONUTS-2b is still quite young, until only about 800 million years old; its hot temperature is the residual heat of the formation of the exoplanet, trapped inside the massive exoplanet, six times the mass of Jupiter. This heat means that the exoplanet glows faintly in infrared wavelengths – enough to be discernible in direct images.
Its enormous orbital distance will also have other benefits for future research. Perhaps this could help us better understand how gas giants are formed, a process we still don’t quite understand – and taking a closer look will help us better understand the diversity of gas giants.
“With its enormous orbital separation, COCONUTS-2b will be an excellent laboratory for studying the atmosphere and composition of a young gas giant planet,” Liu said.
The research was published in Letters from the astrophysical journal.
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