‘Hot Jupiters’ are scorching hot during the day and cooler at night, study finds


Scientists have begun to unravel the mysteries of ‘hot Jupiters’, including how the exoplanets are scorching hot during the day and cooler at night.

Hot Jupiters are gas giants that orbit close to their star, typically in less than 10 days.

Because of this proximity, the irradiation from the star heats the planet to several hundred to a few thousand degrees Celsius.

In one of the largest ever surveys of exoplanet atmospheres ever undertaken, UCL researchers discovered that the night and day sides of hot Jupiters are very different.

During the day, they scorch in temperatures between 1500K (2240°F) and 3000K (4940°F) — hot enough to vaporise most metals, including titanium — but at night temperatures plunge by hundreds of degrees Fahrenheit. 

On average the researchers found a 1000K difference in temperature between night and day. 

Scientists have begun to unravel the mysteries of ‘hot Jupiters’, including how the exoplanets are scorching hot during the day and cooler at night. They analysed the atmospheres of 25 hot Jupiters using data from about 1,000 hours of telescope observations

WHAT ARE HOT JUPITERS?

Hot Jupiters are Jupiter-like giant gas planets on close orbits around their parent stars, separated by only a few stellar diameters. 

Due to their proximity, the irradiation from the star heats the planet to several hundred to a few thousand degrees Celsius.

Of the almost 5,000 known exoplanets, more than 300 are such hot Jupiters.  

Source: Max Planck Institute for Astronomy 

While there are no ‘hot Jupiters’ in our own solar system, they are a commonly observed type of planet outside it. 

Of the almost 5,000 known exoplanets, more than 300 are such hot Jupiters.  

In one of the largest ever surveys of exoplanet atmospheres ever undertaken, UCL researchers were able to answer five long-standing questions about these hot Jupiters.

By using a large sample of exoplanets and analysing an extremely large amount of data, the researchers said the were able to determine trends and resolve questions that smaller studies have been unable to conclusively answer over many years. 

As well as the extreme changes in temperature, they also found that many hot Jupiters had thermally inverted atmospheres, also known as stratospheres — that is, their upper atmospheres has temperatures that increase with altitude. 

This appeared to be caused by the presence of metallic elements – titanium oxide, vanadium oxide and iron hydride – which the researchers said absorbed the star’s light and thus heated up the atmosphere. 

This is a similar phenomenon as occurs on Earth via the ozone layer.

The researchers also found that some planets had less water than expected, suggesting they formed in a different way to the more water-abundant planets, while they detected more metals than predicted by models, meaning that those planets likely formed differently from what was previously thought.

The researchers said a better understanding of exoplanets will help to resolve questions about the evolution of our own solar system.

An artist's impression of a hot Jupter is pictured above. These planets are similar in mass to Jupiter, but can reach temperatures of 2,000 degrees Fahrenheit, meaning any water in the atmosphere would be in the form of vapour

An artist’s impression of a hot Jupter is pictured above. These planets are similar in mass to Jupiter, but can reach temperatures of 2,000 degrees Fahrenheit, meaning any water in the atmosphere would be in the form of vapour

Lead author Dr Quentin Changeat said: ‘Many issues such as the origins of water on Earth, the formation of the moon, and the different evolutionary histories of Earth and Mars, are still unsolved despite our ability to obtain in-situ measurements. 

‘Large exoplanet population studies, such as the one we present here, aim at understanding those general processes.’

Co-lead author Dr Billy Edwards, also of UCL, said: ‘Our paper marks a turning point for the field. 

‘We are now moving from the characterisation of individual exoplanet atmospheres to the characterisation of atmospheric populations.’

The researchers analysed the atmospheres of 25 hot Jupiters using data from about 1,000 hours of telescope observations.

This included 600 hours of observations from the NASA/ESA Hubble Space Telescope, and 400 hours from the Spitzer Space Telescope. 

They combined two techniques – studying information from transits (where the planet passes in front of its star) and eclipses (when the planet passes behind its star). 

Co-author Dr Ahmed Al-Refaie, of UCL, said: ‘The need for state-of the art tools and supercomputing resources are paramount in making these types of large-scale analysis possible, especially as the field of exo-atmospheres is moving to the era of the James Webb Space Telescope and ESA’s Ariel space mission.’

The study has been published in The Astrophysical Journal Supplement Series.

Scientists study the atmosphere of distant exoplanets using enormous space satellites like Hubble

Distant stars and their orbiting planets often have conditions unlike anything we see in our atmosphere. 

To understand these new world’s, and what they are made of, scientists need to be able to detect what their atmospheres consist of.  

They often do this by using a telescope similar to Nasa’s Hubble Telescope.

These enormous satellites scan the sky and lock on to exoplanets that Nasa think may be of interest. 

Here, the sensors on board perform different forms of analysis. 

One of the most important and useful is called absorption spectroscopy. 

This form of analysis measures the light that is coming out of a planet’s atmosphere. 

Every gas absorbs a slightly different wavelength of light, and when this happens a black line appears on a complete spectrum. 

These lines correspond to a very specific molecule, which indicates it’s presence on the planet. 

They are often called Fraunhofer lines after the German astronomer and physicist that first discovered them in 1814.

By combining all the different wavelengths of lights, scientists can determine all the chemicals that make up the atmosphere of a planet. 

The key is that what is missing, provides the clues to find out what is present.  

It is vitally important that this is done by space telescopes, as the atmosphere of Earth would then interfere. 

Absorption from chemicals in our atmosphere would skew the sample, which is why it is important to study the light before it has had chance to reach Earth. 

This is often used to look for helium, sodium and even oxygen in alien atmospheres.  

This diagram shows how light passing from a star and through the atmosphere of an exoplanet produces Fraunhofer lines indicating the presence of key compounds such as sodium or helium 

This diagram shows how light passing from a star and through the atmosphere of an exoplanet produces Fraunhofer lines indicating the presence of key compounds such as sodium or helium 

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