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The discoveries of the exoplanets seem to be taking place almost every day now; we discover very well the extraterrestrial worlds orbiting extraterrestrial stars. At the present time, there are about 4,000 of these discovered planets, which is amazing given the fact that the first one was discovered only in 1992!
Many different methods are used to discover them, but one of the most impressive is the direct imagery: literally, take a picture of the planet next to its star. It's quite a feat, because a star can easily be a billion times brighter than a planet.
However, for some planets, you can play with the system. For example, if the star and its planets are very young, the planets continue to shine, fueled by the remaining heat of their formation. If you look in the infrared part of the spectrum, apart from what our eyes can see, the stars tend to be paler and the planets brighter, which contributes to the contrast.
This technique was used to directly image not one but a system HR 8799. It is a very young star, about 30 million years old (the Sun is 4.6 billion years old, for comparison) and very close, about 128 light-years away. This last element is also useful because the closer a star is to us, the more planets will seem distant from it (just as you can easily separate two fingers if they are held in front of your face, but not if the hand is one kilometer).
In 2008, astronomers used the Keck and Gemini telescopes to obtain images of three of the planets orbiting the HR 8799 in the infrared. In fact, they have been observed so many times that we can see their orbital motion!
The three planets call HR 8799b, c and d. But wait! There is more! Literally: a fourth planet was found in 2010 and called HR 8799e.
Astronomers have recently announced that this fourth exoplanet was observed for the first time thanks to a relatively new technique: optical interferometry.*.
Interferometry is an extremely complex technique combining observation of telescopes at different locations, essentially creating a virtual telescope the size of the distance that separates them. This allows you to observe extremely small details, much smaller than can be done with a single telescope. I describe the technique in an article on the recent image of a black hole if you want details.
Interferometry is easier with light at longer wavelengths, such as radio waves. It's a lot harder with shorter wavelengths, but it's been done for many years. The very large telescope in Chile is actually composed of four monstrous telescopes of 8.2 meters separated by about 100 meters. Often, they are used as stand-alone oscilloscopes, but this time astronomers have used them to observe the HR 8799e in the infrared, separating it from its parent star when it was only 0, 39 second arc (a second of the sky is an angle equal to 1/3600 of degree, equivalent to the size of a quarter of a coin to 5 kilometers). Even more incredible, the star is 10,000 times brighter than the planet! It was therefore quite an achievement.
But there is more. Not only did they see the planet, but they could also obtain a spectrum by dividing its light into individual colors. It's amazing! This is extremely difficult to do with pale objects.
This is of utmost importance because the spectrum can reveal a lot of information about the object emitting light. For example, astronomers have discovered that the planet is indeed very hot: about 1,150 Kelvin (877 ° C or 1,600 ° F). At a distance of 2.5 billion kilometers from the star – about the same distance as Uranus from the Sun – we can expect it to be very cold, but remember that this planet is still young and shines heat leftover from his training.
They have also been able to measure the planet's mass at 10 times that of Jupiter (with an uncertainty that could reach 17 or 6 times the mass), and calculate a size of about 1.17 times that of Jupiter. That means it's about 6 times denser than Jupiter. It's quite dense, denser than the Earth. This object may look more like a brown dwarf than an exoplanet. They are objects located halfway between planets and stars and can be very dense.
But even more: different atoms and molecules absorb different wavelengths (colors) of light, which means that they can be identified in a spectrum. In examining the spectrum of HR 8799e, astronomers detected signs of carbon monoxide (CO), but none of the methane. This is surprising; given the temperature and pressure of the atmosphere, one could expect the CO to react with hydrogen to form methane. Something must prevent that. The authors assume that vertical winds separate CO from hydrogen, preventing them from interacting.
Imagine! We can deduce models of wind on a hot young planet 1,280 trillion kilometers away! The spectrum also indicates the presence of iron and silicates (rocks) in the form of clouds in the atmosphere, held in a vapor due to intense heat. The vertical winds could then be due to intense convection, where warm air rises and cold air descends, implying that iron and silicates can cool and rain inside. planet – a situation as foreign and inhospitable as possible imagine.
Such is the power of interferometry. This is the first time that an exoplanet is observed with the help of an optical interferometer and the spectrum obtained is several times greater than that ever obtained with HR 8799e. This technique should be very useful in the future and can be applied to any planet located more than 0.1 second of its star's arc, though it is also less than 25,000 times brighter than the planet. I imagine we will be able to find candidates to meet these constraints.
Astronomers also calculate that if they can use telescopes about 10 kilometers apart, 100 times farther than the VLT telescopes, they could actually start solving the problems that occur in the exoplanet clouds! It's amazing … and not science fiction.
I wonder how long it will take before we can have images of the features of a planet orbiting another star? A few decades?
I hope less. It would really be an amazing thing to see.
*Technically, telescopes used infrared light, but there is still talk of "optical" interferometry to distinguish it from radio interferometry.
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