The Strange Clouds Of Alien Worlds
The Strange Clouds Of Alien Worlds
The James Webb is uncovering bizarre weather patterns around distant planets
Cloud watching on alien worlds may seem an unusual activity, but it is one growing in popularity among astronomers. In recent weeks they’ve announced the discovery of clouds of shining titanium around one planet; crystals of quartz floating high above another, and a third where nightfall brings showers of sapphires and rubies.
These are strange planets, make no mistake, but they are symbolic of the surprising diversity of weather found across the galaxy. They are also a testament to a new generation of space telescopes. For the first time, these have the power to scope out planets around distant stars and to divine the weather patterns rolling across their skies.
Many of the oddest worlds explored so far have been “Hot Jupiters”, giant planets that swing close to their parent stars. They suffer from extreme conditions, where temperatures soar past the melting point of even the toughest metals; where winds blow at extreme speeds, and where chemicals mix in unforeseen ways.
Such planets are interesting to examine, but they are also easy to spot. They are big, which makes them a good target, and they fly rapidly around their stars, giving telescopes plenty of time to scan them. As a result, much of the attention of advanced telescopes has focused on exploring them.
Harder to see, but no less interesting, are planets closer in size to Earth. Hundreds have been found scattered across the galaxy, and many are potentially habitable. But in many cases we know little about them, basing our judgements of “habitable” solely on their size and possible temperature.
As only a glance at Venus will tell you, this can be sorely misleading. In mass and size it is a twin of the Earth, and it sits far enough from the Sun for water to theoretically flow on its surface. Unlike Earth, however, Venus is an awful hellhole, choked by boiling temperatures and sulphurous clouds.
From a distance, and based on their orbits alone, an alien astronomer may assume both Venus and Earth are good candidates for life. Only with a more powerful telescope - something akin to the James Webb - would they release their mistake. Venus, they would see, has a thick, toxic atmosphere. Earth, by contrast, has a rather unusual one, dominated by nitrogen and rich in unstable oxygen. Not yet proof of life, perhaps, but a definite sign that something unusual is going on.
Such a discovery may now be looming. Telescopes have already started to pick out the chemicals and clouds floating around faraway planets. Should they find one with the right mix of gases - a healthy dose of oxygen say, or perhaps of methane - then this could be the first solid hint of life beyond Earth. That would be an enormous prize for those who watch alien clouds.
I. Watching Alien Skies
In a 2002 paper, David Charbonneau reported the first sign of an atmosphere around an exoplanet. In the years before, he had looked at the light coming from a nearby star. As the planet - named HD 209458 - passed in front of the star, as seen from Earth, its light subtly dimmed as expected.
But more intriguingly its colour also changed, shifting as if the element of sodium were suddenly present. After the planet moved away the colour change reversed, as if the sodium had been removed.
This, Charbonneau concluded, pointed to sodium lingering in the atmosphere of HD 209458. As the star’s light had filtered through the planet’s air, the sodium had absorbed certain wavelengths of it, creating a detectable change in colour. He even speculated about clouds, positing that the sodium he found was concentrated in the upper atmosphere.
Vital to this search was a device called a spectrograph, which splits light into its constituent wavelengths. Starlight, when passed through a spectrograph, produces a certain pattern based on the atomic elements present in the star. When a planet passes in front, that pattern changes slightly, filtered by the presence of chemicals in the planet’s atmosphere.
The effect is small. Though telescopes like Hubble could perform spectrography of stars with known planets, they could only pick out the major gases in their atmospheres. With careful study, however, this could sometimes be enough to pick out hints of dynamic weather systems around other worlds.
In one case, observations of a hot Jupiter revealed signs of water floating in its atmosphere. During daylight hours that water was heated to thousands of degrees, enough to break the chemical bonds in the water molecules. Later, as night fell, temperature would fall and the bonds would reform, once more creating water molecules.
With the James Webb, astronomers have an instrument capable of much more detailed observations. The telescope’s scientists wasted no time in testing out this ability. The first set of images from the James Webb included the spectrograph of WASP-96b, a hot Jupiter more than a thousand light years away.
Clearly visible in the spectrograph was the signature of water, which had never before been detected in the planet’s atmosphere. Since then the telescope has gone on to study dozens of other planets in detail, from hot Jupiters to rocky planets similar in size to Earth.
II. The Hot Jupiters
The discovery of the first hot Jupiters came as something of a surprise. Astronomers had thought most solar systems would look like our own, and had scaled their expectations accordingly. Instead they started finding giant planets, some much larger than Jupiter, orbiting closer to their stars than Mercury does to the Sun.
These were extreme worlds, orbiting so tightly and quickly that a year could last a matter of hours. All were heated to extraordinary temperatures, baked under the fierce glare of a nearby star. Life on such planets would be impossible - and their weather, as we have seen, promised to be truly bizarre.
That hot Jupiters at first appeared to be so common turned out to be an accident of technique. Such giant planets happened to be the easiest to find. Over time, as technology improved, smaller and less weird planets were found in greater numbers. Today hot Jupiters are not thought to be especially common among exoplanets.
Still, the same features that made hot Jupiters easy to find also makes them easy to study. Much of the early work on exoplanet weather has thus focused on them and their unique conditions. Since most are gas giants, they have thick atmospheres and, thanks to their high temperatures, show dynamic weather systems.
Many hot Jupiters are tidally locked, meaning that they always show the same face to their stars. One face of the planet is thus heated to soaring temperatures while the other, staring out into space, is far cooler. Between the two sides is a transition zone, locked into a perpetual twilight.
The daylight sides of these planets can reach incredible temperatures, hot enough for metals like iron and silicon to exist as a gas. Those temperatures drive fierce winds, blowing gases around the surface of the planet towards the cooler night sides. As the gases cross the twilight zone they cool, forming clouds and raining metal down on the surface.
On some planets mixtures of metals have been found that could add up to rains of rubies and sapphires, or of gritty sand. These, since they would come with fierce winds, would be deeply unpleasant to experience, if we somehow made the decision to visit in person.
III. A Hint of Life?
Hot Jupiters are, of course, extreme places. Most planets in the galaxy are far more sedate places. Some will look like Venus, wrapped in thick but toxic atmospheres. Many others will resemble Mercury or Mars, with thin atmospheres incapable of supporting life. A handful, however, may look like Earth, with its dynamic atmosphere of unstable chemicals.
Early studies by the James Webb have focused on the TRAPPIST star system, a collection of seven rocky planets orbiting a faint red star. Several of these planets are potentially habitable - at least, based on their probable surface temperatures. To find out more the telescope has started examining each of the planets for signs of an atmosphere.
Unfortunately the first two planets it looked at seem to be barren places more akin to Mercury than Earth. Still, there are plenty of other possibly inhabited planets out there, and the James Webb is likely to spend time examining many of them. Some may have thick atmospheres - and, researchers hope, chemical signs hospitable to life.
Our atmosphere is rich in oxygen, an element created by photosynthesis in plants. By itself oxygen is rather unstable, and would - in the absence of life - disappear quickly from the atmosphere. Spotting an oxygen rich atmosphere around an Earth-like planet will, then, almost certainly be taken as a sign of possible life below.
Some caution, however, that oxygen could be created by non-biological processes. And, indeed, not all life will necessarily follow the pattern it has here on Earth. Methane, another gas, is also unstable, and would soon disappear from Earth’s atmosphere without life (and without our pollution). Should it turn up in a distant atmosphere it would certainly be an interesting sign, even more so if it were coupled with oxygen.
Researchers have also speculated about the prospects for life on more alien planets. One idea speaks of “hycean” worlds, rich in water and hydrogen, that could support life. Recently a team claimed to have found one plausible candidate. More intriguingly, they also said they had spotted signs of an unusual chemical in its atmosphere.
If the chemical - dimethyl sulphide - is really present there, it is a tantalising hint of life. But other researchers have cautioned that the claimed signal is weak, and that the gas may not actually be there at all. Indeed, not everyone is convinced that the James Webb is capable of picking out the trace gases that would point to living creatures. For that, we may need to wait for more powerful telescopes.
All in all, however, the James Webb has opened up an active new area of research. Hot Jupiters, with their bizarre weather systems, will certainly keep astronomers busy for a while. The prospect of discovering alien life - or at least a hint of it - is, of course, a tantalising one. It may not be long until a serious claim of having done so is made.