Meet the scorching exoplanet that's dark as charcoal with a sapphire sky

 HAT-P-7b is an ultra-hot exoplanet that's heated to such an extent, its atmosphere is one of the richest in elements yet seen.

Exoplanet HAT-P-7b. Credit: NASA, ESA and G. Bacon (STScI)
Published: February 25, 2022 at 8:47 am
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HAT-P-7b (or Kepler-2b) is an exotic exoplanet. Discovered in 2008, it has a radius greater than that of Jupiter and an orbital plane so tilted relative to its star (108°) that it’s nearly in a polar orbit.

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But what makes the planet truly exceptional is how hot it is.

HAT-P-7b hugs its bright, A-class star so tightly that its orbit takes less than three days – it’s over 20 times closer to its sun than Earth is to ours.

Day-side temperatures on the planet are calculated to be regularly above 2,200°C – similar to many stars themselves.

You may have heard of a type of exoplanet known as a hot Jupiter. Well, fittingly, HAT-P-7b sits in a class of planet known as ‘ultra-hot Jupiters’.

Read our guide to the 9 weirdest exoplanets, or our interview with exoplanet hunter Niall Deacon.

An artist's impression of the planet HAT-P-7b, a gas giant with stormy weather. Credit: University of Warwick/Mark Garlick
An artist's impression of the planet HAT-P-7b. Credit: University of Warwick/Mark Garlick

It’s also one of the darkest planets ever observed, with an albedo (surface reflection) of less than 0.03 – roughly that of a lump of charcoal.

The planet is so black it absorbs more than 97% of visible light shining onto it.

Chemical models of its atmosphere predict that on its slightly cooler night-side the crystalline aluminium oxide mineral ‘corundum’, the same substance as rubies and sapphires, condenses as clouds.

The enormous heating has also inflated the planet’s upper atmosphere into a puffy envelope and this, combined with the brightness of its sun, means that HAT-P-7b is an ideal target for studying with transmission spectroscopy.

Transmission spectroscopy splits the starlight passing through an exoplanet's atmosphere to reveal information about that planet's atmosphere. Credit: Christine Daniloff/MIT, Julien de Wit
Transmission spectroscopy splits the starlight passing through an exoplanet's atmosphere to reveal information about its chemical composition. Credit: Christine Daniloff/MIT, Julien de Wit

In this technique, the starlight that passes through a planet’s atmosphere is analysed to reveal information about the chemicals that have become imprinted on it.

Aaron Bello-Arufe at the National Space Institute, part of the Technical University of Denmark, and his colleagues are the first team to analyse HAT-P-7b using transmission spectroscopy, and they’ve discovered even more astonishing details.

Observing HAT-P-7b passing its host star

The astronomers used the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N), a high-resolution spectrograph on the 3.6m Telescopio Nazionale Galileo at the Roque de los Muchachos Observatory on La Palma in the Canary Islands.

They observed a single transit of HAT-P-7b across its star on the night of 18 December 2020 and recorded the spectrum of light passing through its atmosphere.

The radial velocity method of exoplanet detection looks for a shift in the spectrum of star light as a star wobbles due to the gravitational pull of an exoplanet in orbit around it. Credit: ESA
The radial velocity method of exoplanet detection looks for a shift in the spectrum of star light as a star wobbles due to the gravitational pull of an exoplanet in orbit around it. Credit: ESA

They reported detecting a whole host of elements including iron, calcium, magnesium, sodium and chromium – possibly titanium too.

Bello-Arufe’s work has marked HAT-P-7b out as one of the exoplanets with the greatest number of atomic species detected in its atmosphere.

And this metal-laden air is testament to just how hot the planet is.

But Bello-Arufe’s team also discovered that the spectroscopic lines of these atmospheric atoms are significantly blue-shifted in their observations, revealing just how fast the atmosphere is churning in the heat.

They calculated wind speeds of over 2km per second – some 7,200 km/h – as the expanding day-side air rushes towards the cooler night-side.

What’s more, because different atomic species are expected at different altitudes, the team was able to construct a rough wind-speed profile.

The next step, they say, would be to use emission spectroscopy to study the chemical composition of the day-side of the planet.

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Lewis Dartnell was reading Mining the Ultra-Hot Skies of HAT-P-7b: Detection of a Profusion of Neutral and Ionized Species by Aaron Bello-Arufe et al. Read it online at: arxiv.org/abs/2112.03292.

Authors

Astrobiologist Lewis Dartnell University of Westminster
Lewis DartnellAstrobiologist

Dr. Lewis Dartnell is an astrobiologist and science author based at the University of Westminster.

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