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Extrasolar Storms: Belts, Spots, and Waves in Brown Dwarfs

Our new paper came out today in Science, presenting evidence for bands, zones, spots, and waves in brown dwarfs and a model that explains well several until-now mysterious changes in the brightnesses of brown dwarfs.

Podcast: Learn more about our project from the Science Magazine’s podcast!


I am excited about our results because they open a new window on very fundamental processes in brown dwarfs (atmospheric circulation, heat exchanges, and cloud formation) and, at the same time, they also explain a number of past observations that puzzled brown dwarf experts. As always with brown dwarfs, the results are much more far-reaching than people often realize: brown dwarfs are excellent proxies for giant exoplanets: often what we cannot learn from giant exoplanets  we learn from brown dwarfs.

Brown dwarfs and Exoplanets: This is the decade of exoplanets, so one may wonder why are brown dwarfs important. News often describes brown dwarfs as “failed stars”, a label I find misleading: in fact, most brown dwarfs are much more similar to giant planets than to stars. What’s more, it is almost certain that the brown dwarf population contains a large number of ejected giant planets — bona fide exoplanets that were booted from their natal systems by more massive siblings. Known brown dwarfs have temperatures between 250 K to about 2,500K — completely overlapping with the temperatures of giant exoplanets; the compositions of many brown dwarfs are likely very similar or identical to many of the giant exoplanets. But most excitingly, the physical and chemical processes in brown dwarf and exoplanet atmospheres are the same; the identical processes, combined with the fact that brown dwarfs are much easier to study is the reason why we learn so much about exoplanets from brown dwarfs.

A great video summary of our results by JPL:

So, what’s new? Our study shows atmospheric circulation in brown dwarfs for the first time: it shows that brown dwarfs have bands and zones, spots, and that cloud thickness in the zones is continuously changed by atmospheric waves. We found that brown dwarfs are similar to the gas giants in the Solar System (in that they have zonal circulation) , but that they are more like Neptune and less like Jupiter (their brightness variations are driven by large-scale waves in zones rather than Great Red Spot-like storms as in Jupiter). The waves are an interesting piece of the puzzle: we see large-scale waves in the solar system planets (including Earth), but we have not yet seen waves with wavelengths similar to the entire planet — like the ones we now found in brown dwarfs.

Why is atmospheric circulation important? Atmospheric circulation — large-scale flows of air in atmospheres — is very important as it sets how heat and particles/droplets/gas are distributed in a planet. For example, in Earth atmospheric circulation (such as Hadley cells) transport heat between the warmer equatorial regions to the cool polar regions and this circulation pattern not only determines the temperature distribution, but also sets which regions on Earth are dry or rainy and how clouds form over the planet.

Morning Glory clouds in Australia are atmospheric waves rendered visible by cloud formation.

What are these waves? On a fundamental level waves are changes that propagate through a medium. For example, dropping a pebble in a lake will force the water to move away from its equilibrium  — and that change will propagate across the surface of the lake. Atmospheres have many different types of waves: for example, (gravity) waves are common and they often propagate on the interface of warm air sitting on top of cold air — these waves are invisible to us (as air is mostly transparent), but they can lead to spectacular sights when clouds highlight them. (This Berkeley meteorology class’s page gives a couple of cool examples). The two examples shown here are small waves — atmospheric circulation is driven by large-scale waves, with wavelengths that are hundreds or thousands of kilometers.

Atmospheric waves at the interface of rapidly flowing air (above) and near-stationary air (below), leading to mixing and heat transport. Photo by Benjamin Foster.

What does this tell us about exoplanets? Whatever we find in brown dwarfs should be pretty much be the same for most giant exoplanets in the galaxy — only the rare hot jupiters (very heavily irradiated by their host stars) should look different (but even for those, the underlying processes that shape their atmospheres should be the same). So, based on our results we would expect that most giant exoplanets will have zonal circulation; we should expect that their atmospheres are not homogeneous, structureless, but in fact should display large brightness variations in the infrared. We should also expect that giant waves will propagate in their atmospheres (parallel with their equators) and that these waves will change the thickness of the clouds. Our next steps will be to figure out what processes drive these waves (probably some combination of heat transport, winds, and rotation) and to improve the cloud models — the same cloud models that are used to interpret exoplanet atmospheres, too. Importantly, we also learned that the atmospheres of gaseous exoplanets should have regions with very different appearance: where the clouds are thinner or lower, we can see into the deeper, hotter atmosphere. This should be an important warning for most current studies that use one-dimensional atmospheric models: in other worlds they assume that every bit of the planet is like the rest.

I am also excited about our results because they demonstrate how much we can learn from unresolved data — basically, from a single pixel. This is crucial as this is all we are going to get for exo-earths: we will not be able to build large enough telescopes to take detailed images of the surfaces of exoplanets — but we will still be able to learn about their atmospheres (and surfaces) from time-resolved observations!

Near-infrared images show bands on Jupiter. Brighter regions have thinner cloud cover, allowing radiation to escape more easily from its hotter interior.

How did we do it? We used the Spitzer Space Telescope and watched the brown dwarfs rotate. As they rotate their brightness changes: when a brighter spot rotates in the visible hemisphere, Spitzer will see the brightening. The brown dwarfs we observe take between 1.5 and 13 hours to turn around fully: we used Spitzer to observe 32 complete rotations of each brown dwarf. This allowed us not only to map the cloud distribution, but also how it changes from rotation to rotation and also over longer timescales: our observations were following the brown dwarfs for more than a year. We then used a novel computer algorithm developed by my colleague Theodora Karalidi to figure out how the brown dwarfs look and how the clouds change. Another team member, Mark Marley from NASA Ames, used a different set of models (cloud structure and light propagating through the clouds) to help figure out how high in the atmosphere are the clouds we see. Initially, we expected that the changes we see are driven by Great Red Spot-like stable features (the GRS has been seen in Jupiter for more than 300 years) — but the brightnesses of the brown dwarfs changed way too much to be explained by spots, Waves, however, worked extremely well. We then realized that the waves and bands not only explain our own data, but a humber of other puzzling findings reported by other teams. We had an excellent team of experts who all contributed different pieces to solving this puzzle.

Very large and very rapid changes in the lightcurve of 2M1324. From Apai et al. (2017, Science).

What’s next? One of our next steps is to expand this study to directly imaged giant exoplanets, which will allow us to explore how cloud properties and dynamics change with the mass of the objects — this cannot be done well with the sensitive, but low-resolution Spitzer Space Telescope. We are using the Hubble Space Telescope in our program Cloud Atlas, to prepare coarse cloud maps for about a dozen or so cool brown dwarfs and exoplanets.



The Mystery of Sedona’s Red Rocks

Cathedral Rock, Sedona

Cathedral Rock, Sedona

Just got back from majestic Sedona, Arizona, where my family and I spent Thanksgiving. Sedona is a  charming and crazy amalgam of spectacular geology, amazing Fall foliage, exciting restaurants, and an eclectic mix of new age shops and centers. Believers of aura photos, energy vortices, and natural healing flock from all over the country to the countless psychic and supernatural shops in this beautiful town. Sedona may be infamous for its fortunetellers but deservedly famous for its amazing rock formations, which provided the backdrop for many Western movies, with stars from John Wayne to Clark Gable filming there.

Sedona’s rocks are also exciting for anyone interested in Earth’s past as they provide spectacular and rare insights into the Permian period (299 to 251 million years ago) when the Pangea supercontinent converged. At the time of Pangea, all continents on Earth joined together: one could have walked from the Northern American plate to the Australian, African, or even to the Antarctic plates.

Sedona and the soft rocks of the Supai Formation (red), below the young Coconino sandstone. Daniel Apai.

Sedona and the soft rocks of the Supai Formation (red), below the young Coconino sandstone. Daniel Apai.

Today, the rock formations cut across about 2,000 feet of Permian deposits: they consist of beautifully exposed wind-deposited (eolian) and coastal deposits. The amazing dark red rock layers that surround Sedona are part of the Supai Group: these interbedded layers have been deposited in the early Permian, when the Colorado plateau has been partly covered by an inland sea and a large desert. The inland sea has extended and receded many times during the early Permian; every time it receded the desert expanded and giant sand dunes covered the region that once was occupied by the sea. The shallow sea and the dunes left deposits that are different in color and deposit grain size; these layers of alternating colors make up the Supai group (seen as the mostly reddish, layered rocks in the photos). On top of the Supai group is the whitish/grayish Coconino Formation – a  younger, thick sandstone layer, deposited in the mid-Permian, in giant wind-blown dune fields (such locations are also known as erg, Arabic for sea of sand).

The soft red sandstone Supai group is easily sculpted by wind and rain erosion; harder sections of the Coconino formation on top of the red sandstone can protect somewhat the underlying the softer rocks, leading to the characteristic columns and spires typical to Sedona.

Although I have been to Sedona many times in the past, I somehow missed the Slide Rock State Park. This is a great location where flash floods have cut across the soft Supai sandstone and the creek now hosts a fast stream with beautiful pools — in the summer crowded with bathing families, but pleasantly serene in the Fall.

Ancient coastal and wind deposits surround Sedona.

Ancient coastal and wind deposits surround Sedona.

I may have missed the opportunity to have my aura photograph taken or to learn about my future from a Sedona fortuneteller, I can certainly understand the sense of magic this spectacular and serene scenery invokes in visitor – although for me, the true magic is not the mist in the crystal ball but knowing a bit about the incredible and distant past these majestic rock formation witnessed.


[1] A Guide to the Geology of the Sedona and Oak Creek Area

[2] Ancient Landscapes of the Colorado Plateau


On to A New Year and New Exoplanets!

Grand Canyon Panorama

The 2014 year has brought much excitement in the field of extrasolar planets and 2015 is set to be at least as exciting as the past year: new powerful adaptive optics systems are searching the northern and southern skies for new exoplanets and Kepler2 should start bringing a large number of new planet candidates!

Just after Christmas my family took a break and visited the Grand Canyon, just a few hours drive from Tucson. I took the pictures from the South Rim’s Mather Point. Amazing to think how, in just about 5-10 million years, the apparently small Colorado river eroded away one vertical mile of rocks deposited over 1.8 billion years!

Back to the field, the first week of January also brings along the largest US meeting of professional astronomers, the winter meeting of the American Astronomical Society. This year astronomers are gathering in Seattle and we can take for certain that during the course of the next week there will be exciting announcements every day.

The large AAS meeting will be preceded by the open meeting of the NASA Exoplanet Analysis Group, where many in our field gather to review progress in exoplanet research and plan the next steps. The meeting will be broadcasted live, so you can watch it even if you are not in Seattle!

I wish everyone an exciting new year!

In just 5-10 million year the Colorado river eroded one vertical mile of mostly sedimentary rocks deposited over nearly two billion years.

In just 5-10 million year the Colorado river eroded one vertical mile of mostly sedimentary rocks deposited over nearly two billion years.