What is an exoplanet? James Webb Space Telescope finds water in super-hot planet's atmosphere

WASP-18 is located around 400 lightyears away from Earth
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Traces of water vapour has been found in the atmosphere of a super hot gas giant exoplanet by the James Webb Space Telescope. The planet, which orbits its star in less than one Earth Day, around 23 hours, is ten times bigger than Jupiter - our solar system's largest planet. It is also much closer to its star than the nearest planet to our sun, Mercury. 

The exoplanet, named WASP-18b, is a gas giant and orbits its star just 1.9 million miles away - whereas Mercury is at a distance of 39.4 million miles. This planet is located just 400 light years away from Earth. But what is an exoplanet, and what does it mean to find traces of water vapour on a planet? This is what you need to know. 

What is an exoplanet? 

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Simply put, planets outside our solar system are known as exoplanets. The planets which are in our solar system orbit our star, that we call the sun. Exoplanets are very hard to see directly with telescopes as they are often hidden by the glare of the stars which they orbit. 

One way that scientists look for exoplanets is to look for 'wobbly' stars - which is actually a sight-of-line effect where a planet does not orbit its star around its centre, making the star look wobbly from a distance. However, this method, although popular, can only detect large planets. Another way is to launch spacecraft into space such as Kepler. 

Kepler uses the transit method, where when a planet passes in front of its star, it is called a 

transit and blocks out a little of the star's light. Then astronomers can observe how the brightness of the star changes during transit, and in return, figure out the size of the planet. By studying the time between transits, they can also find out how far away the planet is from the star. WASP-18b was discovered via the transit method. 

WASP-18 b, seen in an artist concept, is a gas giant exoplanet 10 times more massive than Jupiter that orbits its star in just 23 hours.WASP-18 b, seen in an artist concept, is a gas giant exoplanet 10 times more massive than Jupiter that orbits its star in just 23 hours.
WASP-18 b, seen in an artist concept, is a gas giant exoplanet 10 times more massive than Jupiter that orbits its star in just 23 hours.

What was found on WASP-18b's surface? 

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WASP-18b was first discovered in 2009, and is a gas giant exoplanet which orbits an F-type star - a main sequence star which is converting hydrogen into helium. This star emits a white-blue light, and is around 6,000 to 7,000 kelvin. 

NASA said scientists identified water vapour in the atmosphere and made a temperature map of the planet. The same side, known as the dayside, of WASP-18b always faces its star, like the moon's near side always faces Earth. This means that WASP-18b is also tidally locked, meaning a considerable difference in temperature exists across the planet's surface. 

Due to the Webb's measurements, scientists now can map these differences in detail, and because of its closeness, it would be expected that most water molecules break apart.

In a statement, NASA said: "The spectrum of the planet’s atmosphere clearly shows multiple small but precisely measured water features, present despite the extreme temperatures of almost 5,000 degrees Fahrenheit (2,700 degrees Celsius). It’s so hot that it would tear most water molecules apart, so still seeing its presence speaks to Webb’s extraordinary sensitivity to detect remaining water." By finding water vapours, scientists can figure out a planet's atmosphere, which in return can provide a clear picture of how exoplanets come to exist. 

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