Exoplanet Clouds; Friend and Foe

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Different colotd pedicted of Hot Jupiters based on their temperatures and the compounds in their atmospheres.
An illustration representing how hot Jupiters of different temperatures and different cloud compositions might appear to a person flying over the day side of these planets on a spaceship, based on computer modeling.  (NASA/JPL-Caltech/University of Arizona/V. Parmentier)

Understanding the make-up and dynamics of atmospheric clouds is crucial to our interpretations of how weather and climate behave on Earth, and so it should come as no surprise that clouds are similarly essential to learning the nature and behavior of exoplanets.

On many exoplanets, thick clouds and related, though different, hazes have been impediments to learning what lies in the atmospheres and on surfaces below.  Current technologies simply can’t pierce many of these coverings, and scientists have struggled to find new approaches to the problem.

One class of exoplanets that has been a focus of cloud studies has been, perhaps unexpectedly, hot Jupiters — those massive and initially most surprising gas balls that orbit very close to their suns.

Because of their size and locations, the first exoplanets detected were hot Jupiters.  But later work by astronomers, and especially the Kepler Space Telescope, has established that they are not especially common in the cosmos.

Due to their locations close to suns,  however, they have been useful targets of study as the exoplanet community moves from largely detecting new objects to trying to characterize them, to understanding their basic features.  And clouds are a pathway to that characterization.

For some time now, scientists have understood that the night sides of the tidally-locked hot Jupiters generally do have clouds, as do the transition zones between day and night.  But more recently, some clouds on the super-hot day sides — where temperatures can reach 2400 degrees Fahrenheit –have been identified as well.

Vivien Parmentier, a Sagan Fellow at the University of Arizona, Tucson, as well as planetary scientist Jonathan Fortney of the University of California at Santa Cruz have been studying those day side hot Jupiter clouds to see what they might be made of, and how and why they behave as they do.

“Cloud composition changes with planet temperature,” said Parmentier, who used a 3D General Circulation Model (GCM) to track where clouds form in hot Jupiter atmospheres, and what impact they have on the light emitted and reflected by the planets.  “The offsetting light curves tell the tale of cloud composition. It’s super interesting, because cloud composition is very hard to get otherwise.”

The paper by Parmentier, Fortney and others was published in The Astrophysical Journal.

Artist's impression of a hot Jupiter. Image Credit: NASA
Artist’s impression of a hot Jupiter. (NASA)

Solid observational evidence of clouds on the days sides of hot Jupiters has been collected for only a short time, and is done by measuring parent starlight being reflected off the atmosphere.  Enough information has accumulated by now, Fortney said, to begin to offer theoretical explanations of the measurements being made.

“What this suggests is that the cloud behavior is quite complex — there is no ‘uniform planet-wide cloud,’ for these tidally locked planets,” he said in an email.

“The hot day side may sometimes lack clouds, compared to the cooler night side, where many clouds form.  Energy redistribution, via winds, leads to gas that is moving into “sunset” from day to night being cloud-free, but gas going into “sunrise,” moving from night to day is full our cloud material that will evaporate when the gas warms up.

The atmospheres are way too hot for water clouds. Instead, the cloud material detected has been iron and silicate rocks (well-known from brown dwarf atmospheres), and manganese sulfide (which has been suggested for brown dwarf as well.)

The different elements and compounds in the clouds give hints about the appearance of the planets, and Parmentier used the GCM model to predict what these planets would look like to the human eye.

The differences in color, said Fortney, are a function of the amount of heat coming off the planet and the stellar scattered light coming off of atmospheric gases and clouds.  “Not all clouds are the same color, which is fun.”

He also said that “this is the first in what will be a longer study to better understand the transport of cloud material around the planets.

For this first study, we only suggest that clouds will form when the temperature is right, but we didn’t track how the cloud material moves with the flow.  That is the next step for a more comprehensive and accurate model.”

Hot Jupiters, exoplanets around the same size as Jupiter that orbit very closely to their stars, often have cloud or haze layers in their atmospheres. This may prevent space telescopes from detecting atmospheric water that lies beneath the clouds, according to a study in the Astrophysical Journal. (NASA/JPL-Caltech)
Hot Jupiters often have cloud or haze layers in their atmospheres. This may prevent space telescopes from detecting atmospheric water that lies beneath the clouds, according to an earlier study in the Astrophysical Journal. (NASA/JPL-Caltech)

The new insights into hot Jupiter clouds via the GCM allowed the team to draw conclusions about wind and temperature differences.

Just before the hotter planets passed behind their stars, a blip in the planet’s optical light curve revealed a “hot spot” on the planet’s eastern side. And on cooler eclipsing planets, a blip was seen just after the planet re-emerged on the other side of the star, this time on the planet’s western side.

The early blip on hotter worlds was interpreted as being powerful winds that were pushing the hottest, cloud-free part of the day side atmosphere to the east. Meanwhile, on cooler worlds, clouds could bunch up and reflect more light on the “colder,” western side of the planet, causing the post-eclipse blip.

“We’re claiming that the west side of the planet’s day side is more cloudy than the east side,” Parmentier said in a JPL release.

While the puzzling pattern has been seen before, this research was the first to study all the hot Jupiters showing this behavior.

This led to another first. By teasing out out how clouds are distributed, which is intimately tied to the planet’s overall temperature, scientists were able to determine the compositions of the clouds — likely formed as exotic vapors condense to form minerals, chemical compounds like aluminum oxide, or even metals, like iron.

The science team found that manganese sulfide clouds probably dominate on “cooler” hot Jupiters, while silicate clouds prevail at higher temperatures. On these planets, the silicates likely “rain out” into the planet’s interior, vanishing from the observable atmosphere.

So while exoplanet clouds can and do mask important information about what lies below in a planet’s atmosphere, scientists are learning ways to use the information that clouds provide to push forward on that process of characterizing the vast menagerie of exoplanets being found.

 

Analysis of data from the Kepler space telescope has shown that roughly half of the dayside of the exoplanet Kepler-7b is covered by a large cloud mass. Statistical comparison of more than 1,000 atmospheric models show that these clouds are most likely made of Enstatite, a common Earth mineral that is in vapor form at the extreme temperature on Kepler-7b. These models varied the altitude, condensation, particle size, and chemical composition of the clouds to find the right reflectivity and color properties to match the observed signal from the exoplanet. Courtesy of NASA (edited by Jose-Luis Olivares/MIT)
Analysis of data from the Kepler space telescope has shown that roughly half of the dayside of the exoplanet Kepler-7b is covered by a large cloud mass. Statistical comparison of more than 1,000 atmospheric models show that these clouds are most likely made of enstatite, a common Earth mineral that is in vapor form at the extreme temperature on Kepler-7b. These models varied the altitude, condensation, particle size, and chemical composition of the clouds to find the right reflectivity and color properties to match the observed signal from the exoplanet. (NASA, edited by Jose-Luis Olivares/MIT)

 

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Cloudy, With a Chance of Iron Rain

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Analysis of data from the Kepler space telescope has shown that roughly half of the dayside of the exoplanet Kepler-7b is covered by a large cloud mass. Statistical comparison of more than 1,000 atmospheric models show that these clouds are most likely made of Enstatite, a common Earth mineral that is in vapor form at the extreme temperature on Kepler-7b. These models varied the altitude, condensation, particle size, and chemical composition of the clouds to find the right reflectivity and color properties to match the observed signal from the exoplanet. Courtesy of NASA (edited by Jose-Luis Olivares/MIT)
Many exoplanets being discovered are covered with thick clouds, offering an opportunity to analyze their compositions but hiding the lower atmosphere and surface from measurement and view.  This artist rendering of Kepler-7b is based Kepler Space Telescope data and shows that half of the dayside of the planet is covered by a large cloud.  Statistical comparison of more than 1,000 atmospheric models show that these clouds are most likely made of Enstatite, a common Earth mineral that is in vapor form at the extreme temperature on Kepler-7b. (NASA/ edited by Jose-Luis Olivares/MIT)

 

From an Earthcentric point of view, rain of course means falling water.  We can have storms with falling dust — I experienced a few of those while a reporter in India — but rain is pretty much exclusively H2O falling from the clouds. But as the study of exoplanets moves aggressively into the realm of characterizing these distant planets after they are detected, the concepts of rain and clouds are changing rapidly.

We already know that it rains methane on the moon Titan, sulfuric acid on Venus and ammonia, helium and, yes, water, on Jupiter and Saturn.  Some have even posited that carbon — in the form of graphite and then diamonds — falls from the “clouds” of Saturn and Jupiter, but the eye-catching view is widely disputed.

Now the clouds of exoplanets large and small are being rigorously scrutinized not only because they can potentially tell researchers a great deal about the planets below,  but also because especially thick clouds have become a major impediment to learning what many exoplanet atmospheres and even surfaces are made of.  Current telescopes and spectrometers just can’t see much through many of the thick ones.

Here’s why:  The chemical compositions of many exoplanetary clouds are so profoundly different from what is found in our solar system.  Hot gas exoplanets, for instance, tend to have clouds of irons and silicates — compounds that are in a gas form on the surface (such as it is), then rise into the atmospheres and form into grain-like solids when they get higher and colder.  For some smaller exoplanets, the composition tends to be salts such as zinc sulfide and potassium chloride.

The process of identifying the make-up of different clouds is very much a work in progress, as is an understanding of how thick or how patchy the clouds may be.

On this question of exoplanet cloud cover, scientists at the University of Arizona have just published results from the first ever direct evidence of patchy clouds surrounding a directly-imaged “super-Jupiter” planet — a technical and observational step forward of some significant importance. They did it by using the Hubble Space Telescope to detect varying brightnesses in the atmosphere around the planet, signs that the cloud cover was patchy rather than blanketing the planet.

“The images showed that the brightness changes, and that means the clouds don’t cover the whole atmosphere,” said team leader Daniel Apai.  ” We can see that as the planet rotates, it becomes less bright when the clouds face forward and more bright when they do not.  The clouds have structure, like on Earth.”

The light curve for the planet studied, which is some four times larger than Jupiter, shows differences in brightness as the planet rotates. Those differences are consistent with a patchy cloud cover rather than clouds that surround the planet completely. (NASA, Hubble Space Telescope)
The light curve for the planet studied, which is some four times larger than Jupiter, shows differences in brightness as the planet rotates. Those differences are consistent with a patchy cloud cover rather than clouds that surround the planet completely. (NASA, Hubble Space Telescope)

The first author on the paper published in The Astrophysical Journal, Yifan Zhou, said that the planet, four times the mass of Jupiter, is far from its host star (or actually, it’s a failed star known as a brown dwarf.)  As a result,  the researchers could separate the light emitted by the planet and that from the host and measure brightness changes with precision.   The observation covered almost a full rotation of the planet and found that this super-Jupiter rotates in about 10 hours.

The planet is young (10 million years) and still has an atmosphere hot enough to have “rain” clouds made of vaporized sand; silicates that are turned into gases, rise and then cool down to form tiny particles similar in size to what is found in cigarette smoke. Deeper into the atmosphere, iron droplets are forming and falling like rain, eventually evaporating as they enter the lower levels of the atmosphere.

“This rain is similar to what you might see at the Grand Canyon — it rains on top but evaporates on the way down,” Apai said. “Here we have iron and silicate clouds but in principle they are not that different from our rain clouds.  Still, we have a lot to learn about how they form, why some form into multiple layers and some do not, and how they evolve.”

And how astronomers might be able to better pierce through them.

This cloud problem is related but different from the one faced by researchers trying to read the chemical compositions of exoplanet atmospheres using transit spectroscopy.  For them, not only clouds but layers of dusty soot often block the light passing through the atmospheres and picking up signatures of what compounds are present.

 

a whole different understand of clouds and rain,
In the world of exoplanets, clouds are more often like this  — and often far more weird — than anything we are familiar with.

 

Mark Marley, a research scientists at NASA’s Ames Research Center and a co-author on the paper, has been studying clouds on Earth and beyond for years, and knows the challenges they pose– especially since they appear to be everywhere in the cosmos where there’s an atmosphere.

“Clouds are just hard, and they’re a big deal.  We worry that with in future observations clouds will  limit how much we learn about the most interesting world.  We’ll be looking for oxygen or water or life and clouds could very well block our view.  A planet could be perfectly habitable, and we wouldn’t know it.”

Add to the likelihood that many exoplanets will be blanketed with the clouds is that cloud formation and behavior on Earth itself is incompletely understood.  Marley said that the biggest source of uncertainty in weather forecasting is the dynamics of clouds.

Clouds are also the often cursed bane of astronomers.  On Earth they can add greatly to the difficulty of seeing out through our atmosphere, and then light years away the difficulty of seeing into the inner atmospheres of exoplanets.

But not all is bleak.  Marley said that he tells astronomers that there’s a lot to be learned about exoplanet compositions by studying even the thick layers of blanketing clouds.  And instruments that will soon be available will be potentially much better at seeing through the exoplanet coverings.

Apai (and others) are looking especially to the James Webb Space Telescope when it launches in 2018 to pierce far better through clouds.  Its mirrors that see in the infrared will add a potential breakthrough capability, he said, allowing for deeper and far more extensive looks into exoplanet atmospheres.

In the conclusion to their paper, Apai and his colleagues predict “that Webb will help astronomers better determine the exoplanet’s atmospheric composition and derive detailed maps from brightness changes with the new technique demonstrated with the Hubble observations.”

Improvements for sure, but still a long way to go in terms of coming to terms with those clouds of silicate sands, iron, sulfuric acid, exotic salts and everything else that exoplanets send up into the sky and then rain back down to their surfaces and interiors.

An artist rendering of a "hot Jupiter" extrasolar planet orbiting very close to its host star. The planet designated HD 209458b, is about the size of Jupiter. Unlike Jupiter, the planet is so hot that its atmosphere is "puffed up." Starlight is heating the planet's atmosphere, causing hot gas to escape into space, like steam rising from a boiler. (NASA, ESA, and G. Bacon (STScI).
An artist rendering of a “hot Jupiter” extrasolar planet orbiting very close to its host star. The planet designated HD 209458b, is about the size of Jupiter. Unlike Jupiter, the planet is so hot that its atmosphere is “puffed up.” Starlight is heating the planet’s atmosphere, causing hot gas to escape into space, like steam rising from a boiler. (NASA, ESA, and G. Bacon (STScI).

 

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On Super-Earths, Sub-Neptunes and Some Lessons They Teach

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Part 2 of 2

The Kepler-452 system compared alongside the Kepler-186 system and our solar system. Kepler-186 is a miniature solar system that would fit entirely inside the orbit of Mercury. The size of the habitable zone of star Kepler-452, considered one of the most “Earth-like” exoplanets found so far, is nearly the same as that of our sun. “Super-Earth” Kepler-452b orbits its star once every 385 days. (NASA Ames/JPL-CalTech/R. Hurt)
The Kepler-452 system compared alongside the Kepler-186 system and our solar system. Kepler-186 is a miniature solar system that would fit entirely inside the orbit of Mercury. The size of the habitable zone of star Kepler-452, considered one of the most “Earth-like” exoplanets found so far, is nearly the same as that of our sun. “Super-Earth” Kepler-452b orbits its star once every 385 days. (NASA Ames/JPL-CalTech/R. Hurt)

 

With such a large proportion of identified exoplanets in the super-Earth to sub-Neptune class, an inescapable question arises: how conducive might they be to the origin and maintenance of life?

So little is actually know about the characteristics of these planets that are larger than Earth but smaller than Neptune (which has a radius four times greater than our planet) that few are willing to offer a strong opinion.

Nonetheless, there are some seemingly good reasons to be optimistic, about the smaller super-Earths in particular. And there are some seemingly good reasons to be pessimistic –many appear to be covered in a thick layer of hydrogen and helium gas, with a layer of sooty smog on top, and that does not sound like an hospitable environment at all.

But if twenty years of exoplanet hunting has produced any undeniable truth, it is that surprising discoveries are a constant and overturned theories the norm. As described in Tuesday’s post, it was only several years ago that results from the Kepler Space Telescope alerted scientists to the widespread presence of these super-Earths and sub-Neptunes, so the fluidity of the field is hardly surprising.

One well-respected researcher who is bullish on super-Earth biology is Harvard University astronomy professor Dimitar Sasselov. He argues that the logic of physics tells us that the “sweet spot” for planetary habitability is planets from the size of Earth to those perhaps as large as 1.4 Earth radii. Earth, he says, is actually small for a planet with life, and planets with a 1.2 Earth radii would probably be ideal.

I will return to his intriguing analysis, but first will catalog a bit of what scientists have detected or observed so far about super-Earths and sub-Neptunes. As a reminder, here’s the chart of Kepler exoplanet candidate and confirmed planets that orbit G, K and M main sequence stars put together by Mission Scientist for the Kepler Space Telescope Kepler Natalie Batalha.

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Kepler exoplanets candidates, both confirmed and unconfirmed, orbiting G, K, and M type main sequence stars, by radii and fraction of the total. (Natalie Batalha and Wendy Stenzel, NASA Ames)

Expanding a bit:

  • While very large exoplanets were the first to be found because of the kind of instruments and techniques used in the search, the consensus view now is that small planets are much more plentiful. Future instruments will doubtless reveal a vast collection of exoplanets in the Earth-radius ballpark and smaller, but super-Earths and sub-Neptunes may well still dominate.
  • There’s a growing body of evidence that when planets are larger than roughly 1.5 Earth radii, they will likely be surrounded by a hyrogen and helium envelope dating back to the formation of the planet. The pull of the larger planets keeps the gases intact and often will render the planet essentially inert.
  • Roughly 70 percent of the main sequence stars in the galaxy are in the M dwarf category, smaller and less powerful stars that are known to have many exoplanets. Often they orbit close in to their sun and are packed together in a tight habitable zone. But many M dwarf planets in their habitable zones are like our moon – tidally locked so one side always faces the sun. Whether or not that rules them out in terms of habitability is now a hotly debated topic.

It’s quite amazing what has been learned about super-Earths, but compared to our knowledge of our solar system planets, we know very, very little. And what, after enormous effort, imagination and cost we do know? Generally speaking, gross measurements of mass, size, orbit period and density. They can tell scientists a lot about super-Earths and larger exoplanets, such as whether they are rocky, gaseous, and the mixtures in between. But characterizing them – and ultimately determining if they are at all capable of supporting life — really requires at a minimum that ability to measure what elements and compounds are in its atmosphere.

There are techniques for doing this: When an exoplanet passes in front of its star, the chemical make-up of the planet’s atmosphere can be analyzed by looking at how light either passes through or is absorbed by molecules, providing a telltale spectral reading of its contents.

trans_spec
Probing potassium in the atmosphere of HD 80606b with tunable filter transit spectrophotometry from the Gran Telescopio Canarias (European Space Observatory.)

Caroline Morley, of Jonathan Fortney’s group at the University of California at Santa Cruz, is one of those working to understand those measurements. But so far, the larger super-Earth and sub-Neptune planets are not cooperating.

“No spectral features are coming through, and so we’re limited in our characterizing,” Morley said. “This is a fundamental problem.” The spectral blanks, she and others are convinced, are the result of either thick clouds (probably made up of salts like zinc sulfide and potassium chloride) or a sooty hydrocarbon smog surrounding the planets. They keep the necessary stellar light from passing through in a way that would allow the presence of an enriched atmosphere, if present, to be identified and analyzed.

Like Morley, Robert Charnay and Victoria Meadows of the Virtual Planetary Laboratory at the University of Washington have been working to understand the opaque nature of the sub-Neptune planet Gliese 1214b in particular. They used a 3D circulation model to determine that salt clouds, which would form at lower altitudes, could nonetheless rise to the upper atmosphere and block any spectral readings.

These obstacles to analyzing the atmospheres of super-Earths were an initial surprise and have been a major frustration in the field. The James Webb Space Telescope may be able to peer through the clouds via detection of thermal emissions, so the 2018 arrival of the successor to the Hubble is eagerly awaited.

Caroline Morey of the University of California, Santa Cruz
Caroline Morey of the University of California, Santa Cruz (Jennifer Burt)

Based on what astronomers, planetary scientists, astrophysicists and others have been able to learn so far about the super-Earths to sub-Neptunes, the picture for potential habitability does not appear particularly bright. But there are other ways to assess that informal class of planets and come up with very different conclusions.

Dimitar Sasselov looks at astronomical problems from a more theoretical perspective, though he was a co-investigator for the Kepler telescope too. As Sasselov sees it, the basic physics of super-Earths, especially the smaller ones, actually favor life. The reason why is that it favors stability.

“There is no particular reason why a bigger planet might not be habitable,” he said. “When planets go bigger they get more stable, though certainly other problems will can and will arise. But when looking at some of the super-Earths, I contend they are as good for life as Earth, if not better.”

The additional stability comes in various forms. First is a less variable slant to the spin axis – its obliquity. Planets can get into trouble when that slant is highly changeable, as Sasselov says, pointing to Mars as an example. The planet’s steep and sometimes chaotic changes in the angle of its spin are believed to have caused dramatic climate changes.

Then there is the greater gravity that comes with a more massive planet. That increased gravity can have the effect of keeping an atmosphere from evaporating, a process that, among other things, exposes the planet to the charged particles coming from a parent star.

Dimitar Sallelov, professor of Astronomy at Harvard University and director of the Harvard Origins of Life Initiative
Dimitar Sasselov, professor of Astronomy at Harvard University and director of the Harvard Origins of Life Initiative (NASA)

And there is the increased likelihood of some kind of recycling of material from the planet surface back into the mantle, where it gets chemically enriched. Plate tectonics allows for that kind of chemical redistribution on Earth, but smaller planets are not known to have parallel processes.

With a more stable atmosphere and planetary slant, “the geochemistry of a planet has plenty of time to leap to biochemistry, and then to adapt,” he said. “Life is rooted in geochemistry and has to play by the rules of chemistry and physics.   There are many aspects to stability, and all benefit from a planet being Earth-sized or larger.”

As for Earth itself, he sees our planet as being uncomfortably close to that low edge of what makes a planet not very stable – not especially habitable.

There are, of course, limits to finding habitable conditions on most large planets. Sasselov agrees that planets bigger than 1.5 earth radii or so will tend to keep their primordial envelope of hydrogen and helium, which can have the effect of freezing everything on the planet in place and making life impossible. They will also tend to be fully gaseous rather than rocky.

But there will always be outliers, he said, and he has spent a lot of time working on one of the – the “Mega-Earth” Kepler 10c. It is a huge super-Earth with a surprisingly high ratio of rock, orbiting its sun in 45 days and breaking all the rules – such as they are now – about planet formation.

An artist concept shows the Kepler-10 system, home to two rocky planets. In the foreground is Kepler-10c, a planet that weighs 17 times as much as Earth and is more than twice as large in size. This discovery has planet formation theorists challenged to explain how such a world could have formed. (Harvard-Smithsonian Center for Astrophysics/David Aguilar)
An artist concept shows the Kepler-10 system, home to two rocky planets. In the foreground is Kepler-10c, a planet that weighs 17 times as much as Earth and is more than twice as large in size. This discovery has planet formation theorists challenged to explain how such a world could have formed. (Harvard-Smithsonian Center for Astrophysics/David Aguilar)

“We were very surprised to discover it,” said Sasselov, who was part of the Harvard-Smithsonian Center for Astrophysics team that first detected and analyzed it. “It was so large, so close to its sun, and much more massive than we would have predicted.”

It is a very hot planet and so not at all habitable, but it is a clear reminder that nature has ways of producing results that don’t appear at all plausible.

Sasselov said those anomalies may have to do with planet migration – that a large and gaseous Kepler 10c moved from the outer solar system into the close environs of its sun, and then lost some or all of its gas envelope from the heat and other solar activity. But the however it evolved – and since the system is more than 10 billion years old, it had a lot of time to evolve – it ended up a planet with 2.2 Earth radii with no apparent gas barriers around it and close to its sun. That, says Sasselov, makes it perfect to study with spectroscopy.

When NASA announced the huge mass of the planet in 2014, Kepler mission Batalha said: “Just when you think you’ve got it all figured out, nature gives you a huge surprise—in this case, literally.”

 

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