Exoplanet atmosphere explored by astronomers

What chemicals can we find on the planets outside our solar system?
19 September 2017

Interview with 

Ryan MacDonald, University of Cambridge


Next year marks 30 years since scientists detected the first planet outside our own solar system. These are called exoplanets and, since 1988, nearly 4000 more have been confirmed. Now astronomers are going a step further and can even uncover what chemicals are in the atmospheres around these remote worlds and work out what the conditions are like there. Izzie Clarke went to see Cambridge University space scientist Ryan Macdonald who’s been gazing at a planet 800 light years away called WASP-19B. Previously, they knew only that it was a gas giant, Jupiter-sized and sizzlingly hot...

Ryan - We didn’t really know what to expect. We thought there might be hydrogen and helium in the atmosphere but what we were trying to do is tease out the trace molecules, things that we might not even see on any other planets in our own solar system. Firstly we confirmed that there is water in the atmosphere. This had previously been seen in 2013, but what made our measurements unique was that they were of such high resolution and so precise that we were able to go beyond just seeing the water and actually find small traces of sodium in the atmosphere and crucially, for the first time, a definitive detection of a truly alien and strange molecule titanium oxide.

Izzie - Why is that so rare and does this tell us anything about this exoplanet?

Ryan - The reason why this is particularly special is that titanium oxide is a very strong absorber of ultraviolet light, much like ozone in the atmosphere of the Earth. What this means is that the very upper layers of this planet’s atmosphere will start to warm up due to the absorption of ultraviolet light. And, as you might expect on the Earth, whenever a particular area of the atmosphere warms up, it can actually drive strong winds around our own planet. It’s how things like hurricanes work on the Earth. So, the fact that there is a layer of this strange molecule strongly absorbing ultraviolet light will drive strong winds around the planet that could dramatically alter the nature of the planet’s atmosphere.

Izzie - How do you even go about finding this because, obviously, this exoplanet is not in our solar system, it’s very far away so how on earth can you do that?

Ryan - We use a clever technique called transmission spectroscopy. What we do is we stare at the light from the parent star and we wait until the planet passes in front of it; it’s almost like a shadow tracking across the face of the star. Now, when we observe this at a number of different colours of light and, in fact, we looked in blue light, green light, and red light, what you see is that the size of the planet changes depending on the colour you look at. Because, if you look at a colour where the atmosphere is completely opaque, the shadow appears to be slightly larger. Then, if you look in colours where the atmosphere’s transparent, it’s slightly smaller. So, by turning a dial where we change the colour of the light we look at and see how the size of the planet changes, we can use this to extract the chemical composition of the atmosphere.

Izzie - What sort of device are you using? Have you got something out in space that is tracking this?

Ryan - You can absolutely use telescopes in space but for these particular observations we actually use a telescope on the ground - the very large telescopes down in Chile. This is a very exciting observational breakthrough because the fact that telescope’s on the ground are now catching up, and perhaps even going beyond what we can do in space at the moment, is incredibly promising for what we’ll be able to do in the future with an entire new generation of telescopes that are being built right now.

Izzie - Do you think in the future we could actually look into even more greater detail of exoplanets and their atmosphere?

Ryan - Absolutely! At the moment we’re still in very early phases; we’re just doing preliminary investigations of these atmospheres. In the near future, when we have a new generation of telescopes in space such as the James Webb Space Telescope, and new telescopes on the ground such as the Extremely Large Telescope, that’s when we’ll be able to detect brand new molecules like oxygen and ozone for the first time. It’s detecting molecules like that in earth-like atmospheres, that we’ll be able to do perhaps in the next 5 to 10 years, that is what we’ll need to do if we really want to get a handle on whether there is life elsewhere on the universe which is our long term focus in the field.


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