The planet with a sapphire sky
This ultra-hot exoplanet is heated to such an extent that its atmosphere is one of the richest in elements yet seen
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. 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. Fittingly, HAT-P-7b sits in a class of planet known as ‘ultra-hot Jupiters’.
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 per cent 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. 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.
High-speed metal
They 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 planet is one of the darkest ever observed, with a surface brightness of less than 0.03 – roughly that of a lump of charcoal”
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.
Prof Lewis Dartnell is an astrobiologistat the University of Westminster.
He 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