I HAVE met my nemesis many times, but one occasion sticks in my memory. I had just started a PhD in astronomy at the University of Cambridge, and was due to use their largest telescope for the first time. This instrument was capable of seeing distant nebulae or the ice caps of Mars in detail, and I was pretty excited. But before I reached the building, the sky clouded over. I saw nothing that night.
Clouds have been astronomers’ worst enemy ever since we began observing the night sky. They are why our best telescopes are on mountaintops that peek above their pillowy shapes, with users having to forsake comforts such as family, friends, convenience stores and reasonable quantities of air to breathe.
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It seems astronomers on planets circling other stars would be similarly inconvenienced. As we have spied more and more exoplanets from Earth, we have learned that they, too, have clouds. At first these alien clouds seemed an annoying barrier to our penetrating a planet’s atmosphere. But they have turned out to be a gateway to understanding a neglected factor in whether a world is amenable to life: its weather.
Astronomers suspected for centuries that other stars had their own planets, but it wasn’t until the early 90s that we unearthed evidence of them. The first exoplanet orbiting a sun-like star was found when two Swiss astronomers, Michel Mayor and Didier Queloz, monitored one called 51 Pegasi for more than a year. They noticed that its light became redder, then bluer, in a cycle lasting about four days. It meant the star was wobbling, a hint at the gravitational tug of an orbiting planet. The size of the wobble suggested the planet had a mass at least half that of Jupiter, and orbited eight times closer to its star than Mercury is from the sun. It was a new type of world, one that came to be known as a “hot Jupiter”.
That was just the start. In 1999, two PhD students, David Charbonneau and Timothy Brown, used a home-built telescope to observe a hot Jupiter known as HD 209458b revolutionised exoplanet-hunting; in the past 20 years, astronomers have used it to detect thousands of exoplanets. We now know our galaxy contains a panoply of worlds, from searing hot gas giants to super-Earths – rocky planets larger than ours but smaller than Neptune.
What are they really like, though? Shortly after the first hot Jupiter was spotted, Sara Seager, now at the Massachusetts Institute of Technology, predicted that we could by looking at their star’s light. As it passes through the edges of the planet’s atmosphere, this light subtly changes because each gas present absorbs at a particular wavelength. If we could use this to identify the gases, we could tell whether that planet has an atmosphere suitable for life, perhaps similar to Earth’s blend of nitrogen and oxygen. It might even be possible to pick up evidence of a cocktail of gases that could only have been produced by living things.
Astronomers quickly saw molecules both familiar and strange, but they were limited to observing gigantic, hot planets that probably couldn’t host anything resembling life on Earth. Smaller, rocky planets were much harder to characterise.
This became possible when a new high-precision camera was installed on the Hubble Space Telescope and was pointed at such a world, a super-Earth called GJ 1214b. Astronomers secured a whopping 60 orbits of Hubble time in 2012 and 2013 to investigate. It was the longest, most detailed ever look at a super-Earth’s atmosphere, but the spectrum that came back was flat and featureless. – a possibility that had not been at the forefront of astronomers’ minds.
The transit method works best for planets close to their star, because these block out more light. So the exoplanets we study are nearly all scorchers, often with temperatures in the hundreds of degrees Celsius. Forming clouds involves gases condensing into droplets, something that will not happen to water or methane – the sorts of molecules that make up clouds in our solar system – at those temperatures. The clouds in GJ 1214b’s atmosphere had an average temperature of about 200°C, so whatever they were made of, it must be something less familiar (see “Making exoclouds”).
These clouds were not just surprising – they were panic-inducing. Clouds should not shut out signals from an atmosphere completely. Even the thickest cloud should have some atmosphere above it that starlight can skim through on its way to our telescopes. Earth’s atmosphere is substantial, more or less, until an altitude of about 100 kilometres, but clouds rarely appear above about 25 kilometres. The best explanation for GJ 1214b’s flat spectrum was that its clouds sat almost at the top of its atmosphere.
This did not bode well. Cooler planets would be more likely to have clouds, so astronomers’ chances of getting useful data on cool, rocky worlds, even ones close to their star, did not look good. Thankfully, though, in the years since we first observed GJ 1214b, things have developed in an unexpected direction. Exoclouds have turned from foe to friend.
Peeling back the skin
First, we found a way to peer through them. Many of the exoplanets we observe are tidally locked, meaning they have a dayside that constantly faces their star. That makes clouds way more common on the much cooler nightside. Michael Line at Arizona State University and Vivien Parmentier at the University of Oxford realised that the boundary where day meets night could be an intermediate region filled with .
Inspired by this possibility, in 2016 I revisited HD 209458b, the tidally locked hot Jupiter that Charbonneau and Brown discovered. I wrote a computer program that would consider millions of possible types of atmosphere and how each would make a planet look at different wavelengths. I then compared those calculations with how HD 209458b actually looks. By finding the best match, I discovered that HD 209458b had high-altitude patchy clouds with about 57 per cent coverage. Exercises like this start to teach us just a little about the weather on exoplanets.
I could work out the quantities of various molecules in the planet’s atmosphere, but the exact numbers were not the biggest surprise in this case. What made me sit up was that our models signalled the presence of hydrogen cyanide, a molecule that is key to some origin-of-life theories. These suggest that most amino acids, the building blocks of proteins, could be produced when hydrogen cyanide is stewed in intense radiation. That is not to suggest that life could get started in the scorching hot clouds of HD 209458b, but it shows what we might spot when we look beneath exoclouds.
Yet the technique I used only works when there are gaps in the cloud at the day-night boundary. Totally cloudy planets, such as GJ 1214b, need another workaround.
Caroline Morley, now at the University of Texas at Austin, had one. Her idea was to just before and after it passes behind its parent star. If you subtract the starlight from the light of star and planet combined, you are left with the thermal radiation from the planet alone (see “Diagram”). Like starlight, thermal radiation bears clues to molecules in a planet’s atmosphere. And, crucially, what we observe would be coming from the dayside of the planet, where clouds are less common.
Another idea relies on the fact that just as we see the moon changing from sliver to crescent to half, so we see different portions of an exoplanet illuminated as it orbits its star. Measuring the heat emitted during its various phases can create a temperature map of the planet, giving a crucial insight into exoweather.
Heather Knutson at the California Institute of Technology applied this to the hot Jupiter HD 189733b. This planet is known for its vibrant blue atmosphere, thought to contain lots of silicon clouds that rain glass. Knutson’s map revealed that the night and dayside temperatures only differed by about 200°C, much less than anticipated. She also found that the hottest point on the planet was some 30 degrees away from the dayside’s centre. Together, these findings indicated strong winds transporting energy around the atmosphere. That probably means that the glass rain falls almost sideways – not pleasant.
In general, strong winds could have hugely positive ramifications for a planet’s habitability. Imagine a tidally locked planet with water in its atmosphere. On the searing dayside, it would be steam, and on the nightside it would freeze, leaving no place for that key facilitator of life, liquid water. By spreading heat around, winds could create temperate areas where it could exist.
The idea is most relevant to cool, rocky planets, but it is more difficult to construct temperature maps of such worlds because they emit less heat. However, astronomers have looked at an unusually hot rocky planet, the super-Earth 55 Cancri e. A temperature map created by Brice-Olivier Demory at the University of Cambridge and his colleagues found that the hottest point was also offset, indicating something is transporting energy. And the planet’s dayside was 1400°C hotter than the nightside, a greater difference than would be expected if winds were present. Demory has suggested the puzzle could be solved if the planet has a molten dayside surface, with magma flows carrying heat around. This is supported by observations that heat emitted by the dayside varies from year to year, consistent with volcanoes spewing reflective material at intervals and cooling the atmosphere. It paints a picture of a hellish world where vaporised rock rains from the .
Weather on Earth involves not just the wind, rain, temperature and so on in the abstract, but also cycles and patterns in which those vary over time. Take the way moisture-drenched winds roll on to the Indian subcontinent during monsoon season, for example.
We are now starting to see similar patterns on exoplanets. A team led by David Armstrong at the University of Warwick, UK, examined four years of observations of the hot Jupiter HAT-P-7b and noticed that its brightest point moved from west to east roughly every hundred days or so. Armstrong reckons strong winds are carrying clouds from the nightside to the dayside, where they persist for a short time before evaporating in temperatures exceeding 2500°C. All this was detectable because, as the winds change speed, the fraction of clouds surviving on the dayside, and hence the amount of reflected light, changes. By watching clouds, we are seeing the weather patterns of an alien world.
One way to find out whether exoweather is a plus or a minus for the chances of life to arise is to build a computer model that combines everything we know about how weather works on Earth with everything we know about the conditions on a particular exoplanet. That is exactly what a recent collaboration between the UK Met Office and a team led by Ian Boutle at the University of Exeter, UK, did. They applied a weather and climate model to the atmosphere of our nearest exoplanet, Proxima Centauri b, to see whether it could host liquid water. There was a worry that, being tidally locked, the planet might have a frozen nightside and a desolate, dry dayside. But the simulations indicated that its winds ameliorate these extremes, leaving surface temperatures amenable to liquid water. If correct, this finding has profound importance for the search for life in our galaxy.
“On this hot Jupiter, glass rain probably falls almost sideways – not pleasant”
Before long, we should be able to confirm it using images of the planet captured directly by the Extremely Large Telescope, now under construction in the Atacama desert in Chile; it will be ready for action in about 2025. Let’s just hope the skies above it stay clear.
Making exoclouds
Clouds get fairly exotic in our solar system. On Saturn’s moon Titan, they are made of hydrocarbon molecules. Then there are clouds of ammonia on Jupiter and sulphuric acid on Venus. But that is nothing to the wild clouds we think are out there deeper in space.
Astronomers know some exoplanets have clouds because of the way starlight passing through their atmosphere is obscured. They can’t tell what these exoclouds are made of, but they can make educated guesses based on temperature. The searing hot planet HAT-P-7b may have clouds made from the same minerals as ruby or sapphire, for example, while other exoplanets might have clouds made of glass. That may mean that somewhere in the cosmos it is raining rubies right now.
We have little idea how clouds like these behave and what influence they have on a planet’s habitability. To start to find out, Sarah Hörst at Johns Hopkins University in Maryland has begun making exoclouds in her lab. She fills a chamber with a range of gases to simulate the atmospheres of exoplanets, and then ramps up the temperature, sometimes to more than 300°C.
So far, Hörst has measured which mixtures of gases produce the most cloud. It turns out that cool atmospheres rich in water or methane do so.
This knowledge might help us to gauge which exoplanets are worth observing in depth – watery worlds might prove too cloudy to see anything.
Soon, Hörst hopes to measure how these particles interact with light. That will be telling. The degree to which clouds trap heat is crucial to controlling the temperature on Earth, and understanding exocloud composition will provide clues to how this plays out for other worlds too.
This article appeared in print under the headline “And now for the exoweather…”
