4.1 The hot ones: HD 189733 b, HD 209458 b and WASP-12 b
The majority of the exoplanets that astronomers have managed to look at in detail fall into the class of planet called ‘hot Jupiters’. Three of the best known are: the first transiting planet discovered, HD 209458 b; its similar cousin HD 189733 b, which was found not long afterwards; and WASP-12 b, one of the hottest hot Jupiters we know of.
You’ve already taken a look at the spectrum of HD 209458 b (shown again below), so you know that water vapour, sodium and potassium were all identified in its atmosphere. The same gases have also been found in HD 189733 b – but also something even more exciting.
Figure 17 shows the measured transmission spectrum for HD 189733 b. You can see that, as in the HD 209458 b spectrum, sodium, potassium and water vapour are all identified, but their signatures look very different. The sodium and potassium signatures in particular are tiny.
Why is this? Scientists think the answer is clouds. Clouds high up in an atmosphere can stop starlight from passing through so easily, making the planet look a bit bigger, and also masking the little bumps and wiggles in the transmission spectrum caused by gases. Nobody should really have been surprised by this, as most of the Solar System’s planets have clouds, but no one had really thought much about what clouds might be made of on a 1000 °C planet!
But what do we actually mean by clouds, particularly on other planets? Clouds are made of tiny liquid or solid particles that have formed from atmospheric gases. These particles are suspended in the atmosphere to form a visible mass. On Earth, water vapour in the atmosphere condenses to form cloud particles of liquid water or ice crystals. Clouds don’t have to be made of water though. In Week 2 you saw Jupiter’s clouds, the highest of which are made from ammonia ice. You also saw Venus’ sulfuric acid clouds.
Now certainly the temperatures on hot Jupiters are much too high for water to exist in liquid or solid form. Instead, their clouds could be composed of silicate minerals, chemical compounds like aluminium oxide, or even metals, like iron. And just as clouds produce rain on Earth, clouds on hot Jupiters could produce some very exotic and unpleasant rain, perhaps even rain of molten metal.
Inspired by this result, scientists decided to see if they could measure the tiny amount of light that is reflected from the cloudy atmosphere of HD 189733 b. To do this, they looked at what happened when the planet disappeared behind the star, halfway around its orbit from the transit. Sure enough, as the planet disappeared they saw a little bit less blue light coming from the system, but the amount of red light they saw didn’t really change. This told them that the planet HD 189733 b would look blue if we could see it up close, perhaps looking like the image in Figure 18.
Our final ‘hot one’ is WASP-12 b. This is a strange planet, about 1000 °C hotter than HD 189733 b and HD 209458 b. Because it’s being ‘cooked’ so much by its star, astronomers think that it’s in the process of losing its atmosphere. The heated gas expands so much that it escapes the planet’s gravity; some of it falls onto the star and some diffuses outwards to shroud the entire planetary system in a wispy gas. This phenomenon was discovered by researchers at The Open University using the Hubble Space Telescope, by looking at how spectra of the system changed as the planet and its vastly expanded atmosphere transit its star. In effect, the star is consuming its planet.
Activity 2 Pick your favourite planet
Pick one of the hot Jupiters HD 209458 b, HD 189733 b and WASP-12 b and look it up online. There’s quite a lot of information about all three, since all have made the newspapers at some point. You can also look them up on. See if you can find out when your chosen planet was discovered, how big it is compared with Jupiter, and how hot it is. What is the fact that most surprises you about your planet?