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Poor Astronomy | A young, hot exoplanet that was first observed with optical interferometry



Exoplanet discoveries seem to occur virtually every day; We become really good at discovering alien worlds orbiting strange stars. At present, about 4,000 such discovered planets are counted, which is unbelievable since the first one was not found until 1992!

There are many different ways to discover them, but one of the most impressive is direct imaging: a picture of the planet literally makes it next to its star. That's quite a feat, since a star can easily be billion times brighter than a planet.

For some planets, however, you can play the system. For example, when the star and its planets are very young, the planets are still hot and energized by the heat left over from their formation. If you look into the infrared part of the spectrum, outside of what our eyes can see, the stars are weaker and the planets brighter, so the contrast is improved.

This technique was used to directly represent not one, but . a system of exoplanets orbiting the star HR 8799. This is a very young star that is only about 30 million years old (the sun is 4.6 billion years old) and very close, about 1

28 light-years away. The last bit also helps, because the closer a star is to us, the farther away the planets seem to be from it (just as you can easily separate two fingers when held right in front of your face, but not when the hand is is one kilometer away).

In 2008, astronomers used the Keck and Gemini telescopes to obtain images of three planets orbiting HR 8799 in the infrared. In fact, they have been so often observed that we can actually see their orbit motion!

The three planets are called HR 8799b. c and d. But wait! There is more! Literally: A fourth planet was found in 2010 and is called HR 8799e.

Recently, astronomers announced that this fourth exoplanet was first observed using a relatively new technique: optical interferometry * . 19659002] Interferometry is a highly complex technique that combines observations of telescopes in different locations to create essentially a virtual telescope as large as the distance between them. This allows the observation of extremely small details, which are much smaller than with a telescope alone. I describe the technique in an article about the current image of a black hole, if you want details.

Interferometry is easier with light with longer wavelengths, such as radio waves. At shorter wavelengths, it is much more difficult, but for many years. The Very Large Telescope in Chile actually consists of four single 8.2-meter monster telescopes, which are about 100 meters apart. Often they are used as stand-alone riflescopes, but this time astronomers observed with them HR 8799e in the infrared and separated it from the parent star, even though it is only 0.39 arc seconds away (one arc second corresponds to an angle in the sky of 1) 3600 degrees Size of a quarter coin at 5 kilometers). Even more amazing is that the star is 10,000 times brighter than the planet! So that was quite a success.

But there is more. Not only did they see the planet, but they could also get a spectrum of it and divide its light into individual colors. This is amazing. This is very difficult with weak objects.

This is crucial because the spectrum can tell a lot about the light-emitting object. For example, the astronomers discovered that the planet is indeed very hot: about 1,150 Kelvin (877 ° C). At a distance of 2.5 billion kilometers from the star – about as far as Uranus from the sun – one might expect it to be very cold, but remember that this planet is still young and glows with heat, which is left over from its creation.

They were also able to measure the mass of the planet as 10-fold Jupiter (but with an uncertainty that means it could be up to 17 or up to 6 times Jupiter's mass) and a size of about 1.17-fold Jupiter to calculate. That is, it is about 6 times more dense than Jupiter. That's pretty dense, denser than the earth. This object may be more like a brown dwarf than an exoplanet – these are half-mass objects between planets and stars and can be very dense.

But there is more: different atoms and molecules absorb different wavelengths (colors) of light, ie they can be identified in a spectrum. In studying the spectrum of HR 8799e, astronomers found evidence of carbon monoxide (CO) but none of methane. That's surprising; Given the temperature and pressure of the atmosphere, it would be expected that CO reacts with hydrogen to form methane. Something has to prevent that. The authors speculate that vertical winds separate CO from hydrogen and prevent interaction.

Imagine! Wind patterns can be derived on a hot young planet in 1,280 trillion kilometers away! The spectrum also indicates the presence of iron and silicates (rocks) as clouds in the atmosphere, which are kept in steam due to the intense heat. The vertical winds might then be due to intense convection, where hot air rises and cool air sinks, implying that the iron and silicates can cool and rain inside the planet – a situation so strange and inhospitable as possible Imagine.

That's the power of interferometry. This is the first time that an exoplanet has been observed with an optical interferometer. The spectrum was many times better than ever for HR 8799e. This technique should be quite useful in the future and can be applied to any planet that is more than 0.1 arcsec away from its star, even though the star is less than about 25,000 times brighter than the planet. I imagine we can find some candidates that will meet these restrictions.

Astronomers also expect to be able to resolve features if they have telescopes about 10 kilometers apart – 100 times farther than the VLT telescopes in the clouds of the exoplanets! That's incredible … and not science fiction.

I wonder how long it will take to get pictures of features on a planet orbiting another star. A few decades?

I hope less. That would be amazing to see.


* Technically, the telescopes used infrared light, but this is still called "optical" interferometry, to distinguish it from radio interferometry.


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