Also follow our twitter feeds: @EOSNExSS – Tweets from the EOS Team; @danielapai – Tweets from PI Daniel Apai.
From the DistantEarths blog of Daniel Apai
After two hours of hike up on a rocky trail in the Italian Alps, finally I stand at an elevation just above 2,500 meters, staring at a breathtaking and unique mountain range, the Dolomites, that holds an exciting clue to the habitability of our planet.
With gigantic sharp white-gray peaks emerging from the lush green of Alpine meadows, these mountains rise where the African continental plate has been slamming violently into the European plate for millions of years, forcing rocks up thousands of meters — and giving birth to the geologically young Alps.
In a trip zig-zagging Europe — visiting observatories, universities, and workshops — I stopped briefly in South Tirol for a few hikes. The most picturesque of them took me up to the Three Peaks of Lavaredo (or Tre Cime di Lavaredo), three 3,000m-high peaks, one of the gems of the Alps. Dotted by rifugi (mostly little huts, but at the easier trails often with nice cafes) the trails are popular among both tourists and locals. They offer an incredible view ascending towards the peaks, before joining an old network of high-altitude Alpine hiking trails, many of which take a week to complete.
The Dolomites are a unique mountain range within the Alps: their composition and history is different from any other in the Alps. They also hold an exciting clue to the process that keeps our planet habitable. Named after a relatively rare form and unusually stable form of carbonate rocks, dolomite, the mountain range’s unique color and composition was noted long ago and, for some time, posed one of the mysteries of geology. Now we know that the majestic dolomite layers in the Dolomite mountains are — amazingly — the work of tiny organisms: it is a very thick layer of ancient coral reefs. During part of the Triassic period (about 255-199 million years ago) the region was part of a shallow sea, which was slowly pulled deeper and deeper. But corals, only capable of living in the upper photic zone of the sea (where enough light is present for photosynthesis), kept on building their reefs higher and higher, managing to always keep the top layer of the coral reef close to the sea surface. With the sea floor sinking and the coral reef growing higher, these tiny animals constructed one of the giant carbonate deposits of the Triassic period.
As most geological periods, the Triassic also did not end well: in fact, it ended with the Triassic-Jurassic mass extinction, one of the greatest extinctions known, which eradicated about 50% of the known marine species. This extinction — occuring just before the Pangea super-continent began to break apart — paved the way for dinosaurs to become the dominant land animals in the Jurassic period that followed. The giant coral reefs of the Dolomites sank further and were covered by sedimentary layers and laid in depth for the next two hundred million years.
Only recently, when the African continental plate collided again with the European plate, were the ancient coral reefs forced to resurface again. Together with other Triassic layers these rocks — once a seafloor — were now pushed up thousands of meters to become the dramatic high peaks of the new-born Alps. Once exposed to snow, ice, rain, and wind, the layers began to rapidly erode, creating the picturesque formations I was able to see today.
But the Dolomites’s story also holds a clue to why we are here: amazingly, the process that formed them and destroys part of a grander process them keeps Earth habitable. The mean temperature of Earth and its local and seasonal variations — its climate — is relatively stable: although major global changes occurred in the past and will probably occur in the future, Earth’s mean temperature mostly remained close to the current temperature and has seen much smaller changes than Mars and Venus have.
The key long-term stabilizing mechanism that keeps Earth’s climate in the habitable range (allowing liquid water on its surface) is the carbon cycle: it is the journey of carbon through the atmosphere, the ocean, the rocks, and the volcanoes of our planet. It is a journey that may take hundreds of million of years for a given carbon atom to complete, providing a slow connection between key reservoirs of carbon in Earth: CO2 in the atmosphere and carbonate rocks in the lithosphere. What makes this journey a feedback cycle is that it is both sensitive to the temperature and able to regulate it: The amount of CO2 — a powerful greenhouse gas — in the atmosphere directly impacts Earth’s temperature: the more CO2 is in the air, the more of Earth’s own emission is captured by it and re-radiated back to Earth, just like a blanket would provide additional heating to our planet (by slowing its cooling) — just as glass windows do in a greenhouse. However, the higher the temperature, the higher the humidity in the air and the more condensation occurs — and the more it rains, the more CO2 is washed out from the atmosphere forming acidic rain. The rain then interacts with silicate-rocks and forms carbonate rocks in the silicate weathering process — or, in a planet that is so filled with life as ours, tiny organisms can grab the carbon-dioxide dissolved in the ocean to build shells or coral reefs. As the Dolomites also show, vast amounts of carbon dioxide can be captured (over long periods of time) in rocks. Slowly, the carbonate rocks will be eroded and carried by rivers to the oceans, deposited to the ocean floor and, eventually, subducted along the oceanic/continental plate boundaries. There, many kilometers deep, the carbonate rocks will be exposed to very high pressures and temperatures, converting the carbonate rocks back to the silicates and expelling CO2 and water — these gases will then find their ways to the surface through explosive volcanoes near the plate subduction boundaries.
Because the loss of CO2 from the atmosphere is temperature sensitive (higher temperature leads to more rain and more carbonate formation) but the source of the CO2 is temperature insensitive (volcanoes do not care about the surface temperatures), the whole cycle forms a net negative feedback cycle: higher temperatures will result in cooling and lower temperatures will result in warming. The negative cycle means that it is stabilizing the temperature of Earth: because the carbonate reservoirs are vast, the effect is powerful; but because it takes hundreds of millions of years to transport carbonate rocks to subduction zones via plate tectonics, the cycle is also very small. While it has kept Earth habitable on long timescales (~100 Myr), the cycle can’t work well on short timescales (<10-30 Myr).
How would this apply to other Earth-like planets? While on present-day Earth the carbonate formation is dominantly through organic processes (various shell-forming marine organisms are happy to make use of the CO2 dissolved in the ocean), in the early Earth and, presumably, in other Earth-like planets with little or no life the same process can occur inorganically, but somewhat slower, in silicate rock weathering.
Therefore, as long as the overall composition of other Earth-like planets are the similar to ours, we would expect them to sport a carbon cycle (either organic or inorganic), also providing a stable climate for them — as long as the planets remain within the temperature range where the carbon cycle can work.
This means that carbonate deposits should be common even beyond the Solar System — and, just perhaps, a few in the Galaxy will also match the majestic beauty of the Dolomites.
We know of a lot of exoplanets. Most, though, we know of only by indirect means – typically via the planet occulting some of the starlight during an eclipse or changing the star’s velocity due to the planet’s own gravity. As of today (June 29, 2016) the exoplanet database lists 2,933 confirmed planets, with another 2,504 probable candidates, totaling to about 5,000 bona fide planet detections. Of these, only seven have been successfully imaged. Why, if we know of so many planets, do we only have images of so few?
The answer is, as you may guess, complicated. Some of the challenges are technical – for example the size of current telescopes and the distortion of the Earth’s atmosphere restrict imaging capabilities to the outer parts of alien solar systems (typically past the orbit of Saturn). Other challenges are more fundamental. The capable targets are necessarily the youngest and most massive, which translate into the brightest since planets cool as they age. These limitations are so extreme, in fact, that imaging other planets is only currently possible for worlds that are less than about 2% the age of the Earth. This leaves very few stars, on the order of a few thousand, that are close enough and of the right age that their planets may be directly imaged.
HR 8799: The first directly imaged extrasolar planetary system. Planets b, c, d, and e together consist of roughly twenty times the mass of our own solar system. (Images from Marois et al. 2010).
In suit, directly imaged exoplanets are quite rare – but the results of the few successes showcase the beautiful complexity of other planetary systems. For instance, the HR 8799 hosts four planets that are each roughly five to ten times as massive as Jupiter – accounting for more than twenty times the mass of our own solar system, all outside of the distance of Saturn’s orbit (Marois et al. 2008, 2010). Monstrous planetary systems such as this exemplify the existence of solar systems that would have been considered science fiction prior to their discovery.
Extreme Adaptive Optics Systems SPHERE (left) and GPI (right). Two of the current planet hunting instruments installed on 8-meter class telescopes. EOS scientists are conducting a search for giant planets using SPHERE, pictured left. Image credits: ESO (left) and Marshall Perrin (right).
Currently there are several ground-based instruments with unprecedented sensitivity that are beginning to discover gas-giant planets in the outer regions of nearby solar systems. EOS scientists Kevin Wagner and project lead Daniel Apai are conducting a search for planets using SPHERE – the most complex and expensive of the latest generation of planet finding instruments (stay tuned for results!). Soon, bigger ground-based telescopes will push capabilities to smaller masses (e.g. Saturn-like worlds) in orbits more like those of the planets in our own solar system. But still, the potential to image an Earth-like plant remains for the distant future.
Succession of NASA flagship space telescope primary mirrors. The proposed HDST (“High Definition Space Telescope”), if selected for mission development, will explore alien Earths for atmospheric biosignatures, and will allow the first scientific survey for life outside of Earth. Image credit: C. Godfrey (STScI).
Perhaps the best possibility for detecting truly Earth-like planets is direct imaging with a large (12-16 meter) space telescope – the successor to Hubble and JWST. Direct imaging has the added advantage of being sensitive to a planet’s intrinsic emission/reflection spectrum, which is imprinted upon by the planet’s atmospheric absorption profile, allowing for chemical studies of exoplanetary atmospheres. One of the main science drivers behind such a mission is not only the detection of Earth-like planets, but also the investigation of their atmospheres for chemical signs of life (e.g. oxygen is primary of biological origin on Earth). This opens an exciting possibility – the bridging between two classical facets of science, astronomy and biology, into the modern science of Astrobiology. There are several concepts of an Earth-like exoplanet-imaging telescope, at various stages of conceptual review, and proposals are being put forth in the coming years to begin construction. Probably by the year 2040, if all goes according to plan, we will be able to start a true scientific search for alien life.
The planets of our solar system formed out of a swirling disk of gas and dust billions of years ago. The material that accreted to become the Earth lacked water and organic material because it formed at a distance that was too close to the sun for such materials to condense and collect on the new-born planet.
Despite Earth’s rocky start, life organized, evolved, and now dominates this small, terrestrial planet orbiting 93 million miles away from the sun. How can this be?
Earth now exists within the habitable zone, which means that now liquid water can exist on the surface without freezing or boiling off into space. The water and organic ingredients for life that we have today were delivered to Earth objects after its formation and the atmospheric pressure on Earth allowed stable liquid water to exist.
Scientists understand this delivery process (and more) using computer models that simulate the formation of our solar system. “By applying these models to other planetary systems, we can investigate if exoplanets receive water and organics in the same way as Earth,” said Gijs Mulders, PhD., post-doctoral researcher in the Lunar and Planetary Laboratory at the University of Arizona.
The Genesis Database is a new tool and resource that will allow scientists to model the final stages of planet formation, according to Fred Ciesla, PhD., associate professor in the Department of the Geophysical Sciences at the University of Chicago. What makes this database so innovative is that it will run thousands of models at once—a whole order of magnitude more than the number of simulations that have previously been run, Mulders said.
More models allow for more examples of what can happen for a variety of planetary architectures. Both Ciesla and Mulders are interested in characterizing rare events. The Genesis Database will allow them to have a large set of simulations that reflect both the diversity in exoplanetary systems and some of the specific details of our solar system.
“If we change where we start each body, even just a little bit, we can get very different results. The number of planets may be different, their masses, or their compositions.” Ciesla said. “So if we want to understand the likelihood of getting something like the Earth, then we have to run a very large number of these simulations and think about the probability of getting different outcomes.”
Building a Model
To understand this, we first have to understand the basic model for solar system formation. First, disks containing the material for planets swirl around a star. Planets form like giant snowballs: Van der Waals’ interactions start to stick micrometer and millimeter-sized particles together, they then combine to form meter-sized objects, then kilometer-sized objects. These then coalesce and eventually form planetary embryos, roughly the size of Mars. In our solar system, this is roughly the largest body objects that can grow while the disk is still present. Once the disk is gone, the giant impact phase begins.
The giant impact phase is where most modeling begins. “This is an extremely important stage in planet formation because water and organics are delivered to the Earth,” Gijs Mulders said. “And we expect this to be the case in other planetary systems as well.”
Models start with many planetary embryos scattered around a star. State-of-the-art models require large amounts of computing power, and take months to simulate the process of planet growth and water delivery. “The entire evolution is determined by gravitational interactions between the growing planets and the bodies they accrete,” Ciesla said. Scientists then return to the model results and try to match them with actual, observable extrasolar systems.
The Search for Life
Planetary scientists best understand hot Jupiters—a type of giant, gaseous planet that orbit close to their stars—because they are easiest to detect. But terrestrial, Earth-like planets are where they are most likely to find life.
Unfortunately, Earth-like planets are much harder to characterize because they are so small and farther from their parent star, in the habitable zone. “We cannot directly measure their composition, so simulations are a crucial tool for understanding the water and organic content of planets in the habitable zone,” Mulders said. “The Genesis Database will become a valuable resource to study the composition of exoplanets in the habitable zone.”
By Daniel Apai
Includes interview with Nick Siegler and Shawn Domagal-Goldman
Over the weekend, at the Hilton on the San Diego Bay, a small group met to speak about the present and future of NASA’s Exoplanet Exploration program. To someone not in the field of exoplanets the talks and debates may have resembled science fiction: giant space telescopes, rockets and spacewalks, hyper-precise measurements of stellar motion, search for alien life, exploration of volcanism on exoplanets, laser-combs, starshades, and other Earths across the Galaxy were just a few of the topics that were debated. The memorable images included cows illuminated by lasers in a Nevada desert. It was a fun meeting and a timely one, too.
The field of exoplanets is hotter than ever: we learned that planets are literally everywhere and that planets with sizes similar to Earth are the most common among the known planets. Many of the stars (probably 1 in 4) harbor about-earth-sized planets with stellar heating similar to Earth. Not only did we learn about the frequency of the planets, but also about their properties. New missions and instruments are being built and planned, conferences and school galore, and amazing discoveries are made almost weakly. The enthusiasm is palpable in the field; yet. we know that reaching our grand goal of finding extraterrestrial life is going to be anything but easy.
We can only find life if it produces a signature that is detectable from vast – literally astronomical – distances. Seen from space humans, trees, elephants, or even whales are undetectable and unremarkable, yet Earth would reveal its secret to an outside observer through the surprising abundance of a highly reactive gas, molecular oxygen. Oxygen is and has been produced by advanced photosynthetic organisms, first in the ocean and then on land. About 2.3 billion years ago oxygen has saturated the planet’s surface and rapidly accumulated in vast amounts in our atmosphere, From that point on Earth’s atmosphere became a glowing indicator of life for the entire Galaxy – at least, for civilizations that are slightly better in building telescopes than we are.
So, starting from the only example we have, NASA’s Exoplanet Exploration program is aiming to build a telescope that will look for oxygen or other similarly odd gases in other earth-like planets atmospheres as possible signatures of life.
In a perhaps unusual consensus, the exoplanet community is united behind the most important goal, surveying nearby exo-earths for biosignatures. Few other approaches to detecting extraterrestrial life seem feasible. Although the goal is clear, possible approaches and ideas are plenty: the abundance of proposed approaches stems from the fact that no telescope that exists today (or at least, accessible to astronomers) is capable enough to directly search for biosignatures in known exo-earths. Building one that will be up for the job is not going to be easy: in fact, right now, we do not know how good exactly that telescope would need to be, what capabilities it would have to have — and we don’t know how we would build it.
Guided by the vision of finding extraterrestrial life, astronomers, astrobiologists, technologists, engineers, project managers are all working together to come up with concrete plans for such telescopes. Our goal is to create at least two different designs for life-finding telescopes by 2019. The year is important, because in 2020 the astronomical community will issue a major report, the Decadal Survey. This study will set the strategy for NASA for 2020s and beyond and will determine whether planning and construction of such a telescope can begin in a few years or we need to wait another decade.
What the best telescope design is will depend on what questions we want to answer and on the properties of planets, too: our meeting in San Diego explored these issues as well as the technology development needed to build a telescope more ambitious than anything very built. For example, one possible telescope design would use a “starshade” – a giant (think fifty meters or hundred and fifty feet) flower-petal-shaped mask. The strange mask would fly tens of thousands of miles in front of the telescope and could, if positioned precisely, cancel out the light of the host star completely, revealing the faint planets. However, nothing like this has ever been flown in space or used in ground – so a Northrop-Grumman team of engineers is testing this idea in the night in a dark Nevada desert, shining bright a light to a telescope from miles away and covering the light with a small starshade mask in between. One night however, a cow, perhaps intrigued by the strange glowing flower in the desert, wandered into the light beam and photo-bombed the experiment, thus becoming part of the history of space exploration.
The San Diego meeting was exciting and fun: a lot of progress has been made recently, but much more needs to be done in the next three years to finalize plans for a space telescope that can look for life on other Earths
At the meeting I also grabbed the opportunity to interview two experts who approach this question from different angles: Dr. Nick Siegler, who is the Chief Technology of NASA’s Exoplanet Exploration Program; and Dr. Shawn Domagal-Goldman, astrobiologist and biosignature-expert at the NASA Goddard Space Flight Center.
Several important studies of space telescope design and science questions will be carried out over the next year or two, pushing our technology and understanding toward the long-term goal. It will be exciting to see how this group of smart people figures out solutions to problems that were thought to be impossible to solve, and how it will overcome unexpected barriers, such as curious cows.
Just before 4 a.m. on June 2, Arizona’s night sky seemed to momentarily catch fire. A meteor, about 5-feet across, burned through the atmosphere at 40,200 mph, according to NASA estimates. The small debris that reached the Earth’s surface are called meteorites, and can be used to study the origins of the solar system.
Meteorites can be made out of iron, stone or rarely, both. Scientists at Project EOS are most interested in a subclass of stony meteorites called chondrites, because they congealed during the formation of the solar system and have been left virtually unchanged ever since. They are, in essence, time capsules from more than 4.5 billion years ago possibly carrying the starting materials from the early solar system to Earth, according to Maitrayee Bose, PhD., assistant research professor in the School of Molecular Sciences at Arizona State University and member of EOS.
“What we’re doing in this part of the project [EOS] is we are trying to understand the organic inventory of the early solar system and to do that, we’re studying organic compounds in meteorites,” said Tom Zega, PhD., associate professor in the Lunar and Planetary Laboratory at the University of Arizona.
Of the different kinds of chondrites, those called carbonaceous chondrites are the most interesting for EOS scientists, because they contain relic materials from the early solar system. The materials remained virtually unchanged prior to being delivered to Earth and contain ingredients for life in the form of organic compounds.
“The finding of compounds that are also part of the biosphere, such as amino acids… has led to hypotheses that the input of these compounds might have helped the origins of terrestrial life, and maybe could do so in other stellar systems,” said Sandra Pizzarello, PhD., emeritus professor and research professor in the School of Molecular Sciences at Arizona State University.
Opening the time capsule
Meteorites provide many clues about the past. Scientists can determine what a meteorite is composed of, the concentration of elements within the meteorite and where the grains from within the meteorite originated. So how is this done?
First, Bose receives a sample of a meteorite in her lab from Johnson Space Center. The sample is only tens of micrometers thick—less than half the width of a human hair. She surveys the meteorite’s surface using a secondary ion mass spectrometer capable of measuring at the nano-scale (NanoSIMS). This tool allows her to map the meteorite for carbon, hydrogen and nitrogen isotopes within the sample and determine the ratios of each isotope. Isotopes are different forms of the same kind of element which contain an equal number of protons but a different number of neutrons.
The mapping reveals tiny, less than 1 micrometer-sized locations in the meteorite whose isotopic compositions can differ throughout the meteorite as a whole. The meteorite’s composition can then be compared against the isotopic composition measurements taken from other meteorites within our solar system and also with astronomical structures outside of our solar system including material in the interstellar medium (ISM), stellar nebulae, ancient stars or dust shells around other stars using radio telescopes. From this information, Bose can confirm the meteorite’s origin.
Zega then takes the sample and focuses on an area of interest, such as an area with an isotopic composition suggesting extra solar origins. He uses a focused ion beam scanning electron microscope, or FIB, to make a cross section, and then uses a transmission electron microscope to investigate the atomic and molecular structure within the meteorite. Knowing how the atoms are arranged in space can tell him about the functional properties of the material or how the different atoms are bound to each other.
To hear an audio interview with Tome Zega about what meteorites can tell us about the origins of the ingredients for life, listen below:
Besides amino acids, “There’s a whole host of other organic compounds that occur in meteorites that are probably also a part of the building blocks [of life], or biocritical ingredients,” Zega said.
Phosphorus for example is an essential component of DNA. But scientists are still uncovering how this element was delivered to our own solar system, if it occurs in molecular clouds in space and if it chemically reacts in those clouds.
Water is another biocritical ingredient scientists are trying to learn the history of. Astronomers can directly observe water vapor in some exoplanetary atmospheres, but are still unable to directly observe liquid water on exoplanetary surfaces. But scientists have evidence to suggest that it was present as water vapor while the solar system formed.
“We know that there are certain materials in meteorites that formed around ancient stars and we think, based on their thermodynamic properties, required water vapor in their stellar atmospheres,” Zega said.
Iron oxide compounds have been found, for example, inside of meteorites. Based on the isotopic composition, their origins have been traced back to ancient stars. Based on our understanding of the way these minerals form, water was required for it to oxidize, giving scientists indirect evidence that water was present.
What meteorites mean for exoplanets
“Ours is the only solar system that we can study in such detail,” Bose said. What we learn about our own solar system can build our understanding of the potentially habitable worlds astronomers continue to discover and perhaps trace the molecular origins of life.
The Sun’s planetary system, like the many other systems discovered in the last 2 decades, formed out of a protoplanetary disk. These disks are a natural by-product of star formation (almost every young star has one) and consist of a lot of gas (mostly hydrogen) and a small amount of solid dust particles. On timescales of hundreds of thousands of years, these dust grains collide and stick together, forming larger and larger bodies that eventually become asteroids, comets, and planetary embryos.
While the Sun’s protoplanetary disk is now long gone (disks typically dissipate on a timescale of a few million years), the planets hold clues to the disk’s properties. For example, the fact that the planets all lie in the same plane indicates that the disk was relatively flat, and the observation that the inner terrestrial planets are small and rocky while the outer planets are massive gas giants indicates that the snowline (inside of which water ice could not survive) was located somewhere between Mars and Jupiter. It is clear then that the properties of planets reflect, on some level, the environment in which they formed, and that understanding the physical and chemical structure of protoplanetary disks is a crucial step to understanding the ubiquity and diversity of planetary systems.
After moving to Chicago and becoming part of the EOS team in October 2015, I have been working on one question in particular: the vertical distribution of dust grains in planet-forming disks. It is usually assumed that, while small dust grains are well-mixed vertically, large grains are concentrated in the disk’s midplane. This picture comes from comparing the processes of gravitational settling (which acts to bring solids to the midplane) and turbulent diffusion (which tries to mix solids back up to the disk atmosphere). For large grains, settling is more efficient. For small grains, mixing is thought to win.
However, this approach neglects collisions between dust particles which might limit the mobility of the grains. In a way, it is like constructing an approach for crossing a busy road without taking into account collisions with passing cars (please don’t!). Continuing along these lines, a more accurate depiction of what happens when collisions are included is the classic video game Frogger (readers born in the 90’s or later are encouraged to play a few rounds before reading further). In the game, the player tries to navigate a frog (or, a dust grain) from one side of the road (the disk midplane) to the other (the disk atmosphere), while trying to avoid cars (other dust grains). Experienced players can tell you that successfully crossing the road becomes harder when the number of cars is increased, or when the frog moves much slower than the cars.
This is precisely what Fred Ciesla and myself found when simulating vertical diffusion/settling in the presence of collisions: efficient vertical mixing of small grains is not possible when the dust content is high, or when the turbulence is weak. This trapping of small grains could have important consequences for certain astro-chemical processes. For example, the surfaces of small grains play an important role in the formation and transport of water and other volatiles. Collaborating with Ted Bergin (University of Michigan), we are now beginning to couple dust evolution models to astro-chemical models in order to study how dust evolution influences the distribution of water vapor and water ice in planet-forming environments. Ultimately, this will contribute to our understanding of the formation of water-rich, possibly habitable planets. Stay tuned!
Astronomers using NASA’s Hubble Space Telescope have measured the rotation rate of an extreme exoplanet by observing the varied brightness in its atmosphere. This is the first measurement of the rotation of a massive exoplanet using direct imaging.
“The result is very exciting,” said Daniel Apai of the University of Arizona in Tucson, leader of the Hubble investigation. “It gives us a unique technique to explore the atmospheres of exoplanets and to measure their rotation rates.”
The planet, called 2M1207b, is about four times more massive than Jupiter and is dubbed a “super-Jupiter.” It is a companion to a failed star known as a brown dwarf, orbiting the object at a distance of 5 billion miles. By contrast, Jupiter is approximately 500 million miles from the sun. The brown dwarf is known as 2M1207. The system resides 170 light-years away from Earth.
Hubble’s image stability, high resolution, and high-contrast imaging capabilities allowed astronomers to precisely measure the planet’s brightness changes as it spins. The researchers attribute the brightness variation to complex clouds patterns in the planet’s atmosphere. The new Hubble measurements not only verify the presence of these clouds, but also show that the cloud layers are patchy and colorless.
Astronomers first observed the massive exoplanet 10 years ago with Hubble. The observations revealed that the exoplanet’s atmosphere is hot enough to have “rain” clouds made of silicates: vaporized rock that cools down to form tiny particles with sizes similar to those 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.
“So at higher altitudes it rains glass, and at lower altitudes it rains iron,” said Yifan Zhou of the University of Arizona, lead author on the research paper. “The atmospheric temperatures are between about 2,200 to 2,600 degrees Fahrenheit.”
The super-Jupiter is so hot that it appears brightest in infrared light. Astronomers used Hubble’s Wide Field Camera 3 to analyze the exoplanet in infrared light to explore the object’s cloud cover and measure its rotation rate. The planet is hot because it is only about 10 million years old and is still contracting and cooling. For comparison, Jupiter in our solar system is about 4.5 billion years old.
The planet, however, will not maintain these sizzling temperatures. Over the next few billion years, the object will cool and fade dramatically. As its temperature decreases, the iron and silicate clouds will also form lower and lower in the atmosphere and will eventually disappear from view.
Zhou and his team have also determined that the super-Jupiter completes one rotation approximately every 10 hours, spinning at about the same fast rate as Jupiter.
This super-Jupiter is only about five to seven times less massive than its brown-dwarf host. By contrast, our sun is about 1,000 times more massive than Jupiter. “So this is a very good clue that the 2M1207 system we studied formed differently than our own solar system,” Zhou explained. The planets orbiting our sun formed inside a circumstellar disk through accretion. But the super-Jupiter and its companion may have formed throughout the gravitational collapse of a pair of separate disks.
“Our study demonstrates that Hubble and its successor, NASA’s James Webb Space Telescope, will be able to derive cloud maps for exoplanets, based on the light we receive from them,” Apai said. Indeed, this super-Jupiter is an ideal target for the Webb telescope, an infrared space observatory scheduled to launch in 2018. 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.
Results from this study will appear in the Feb. 18, 2016, edition of The Astrophysical Journal.
For other articles published by Project EOS, check our Publications link.
The curious case of KIC 8462852 by Theodora Karalidi
Discovering the first planet, other than our Earth, that hosts life is one of the holy grails of astronomy. This is a difficult task since planets that could host life are pretty small compared to their parent star, and they are so far away from us that all the information on possible life, clouds, oceans or continents on the planet is hidden in one pixel. Theorists like myself are developing codes that model the signal of different worlds and try to match it with the signals we observe from exoplanets. Trying to get as much information as possible from one pixel is a pretty challenging, but also fun work!
Recently, amateur astronomers inspecting Kepler observations noticed something weird was happening around star KIC 8462852: the observations showed lots of dips in the star light, like one or more objects were passing in front of the stellar disk. However, the dips did not match one or more planets eclipsing their parent star. Professional astronomers took over and observed KIC 8462852 with different telescopes and instruments to try to figure out what was going on. Did it have a big disk of material around it that was blocking the starlight (like the one Kevin showed on a previous post, but edge-on)? Were these weird dips the aftermath of a huge collision of two planets, like the one that we think created our Earth-Moon system? Was it a family of comets or, more intriguingly, was it signs of an alien civilization? Could it be that the dips were due to some form of alien mega-structures passing in front of the stellar disk?
SETI, the Search for Extra Terrestrial Intelligence project, observed KIC 8462852 and tried to look for signs of an advanced civilization around the star. SETI looked for radio signals, laser pulses and visible signs of that civilization, but their efforts did not give any positive results. The case of KIC 8462852 became even more curious when an astronomer reported that after inspecting old observations spanning 100 years, he discovered that the star appeared to be dimming in an unprecedented rate. The rate at which the star was dimming could not be matched by any astrophysical mechanism, leading people to think that maybe there were aliens living around that star after all! Unfortunately for the people trying to find life on other worlds, another team of astronomers inspected the same observations again, and found that the reported dimming of KIC 8462852 was seen in other stars as well. So the dimming was not hinting to a special property of the star, but rather an inconsistent calibration of data over a century of observations.
Currently, the most plausible scenario is that the observed dips of KIC 8462852 are due to a family of comets orbiting the star. Astronomers tried to fit a model of a comet family to the last 100 days of the observations and found a good match! Creating such a large family of comets that could produce the series of dips we observe would need a large body breaking into pieces to become comets. The idea these astronomers proposed was that maybe there is another planet around KIC 8462852 that “pushed” the broken pieces into the comet orbits we see today. However, while they could reproduce the last 100 days of dips pretty well, they did not try to do the same with the previous dips. Approximately 700 days before the dips they chose to fit, there is an especially interesting dip. If it belongs to a comet it could mean that its tail is ahead of the nucleus, and that is not easy to explain! Aliens, comets, or something else? The mystery of KIC 8462852 is still not solved, and it looks like it will keep astronomers busy for some time to come. But that is the nature of any research. We keep on questioning our conclusions until we are absolutely sure they are true!
Exoplanets around Red Dwarf Stars by Gijs Mulders
Since the discovery in 1995 of the first exoplanet over two-thousand exoplanets have been discovered. Most exoplanets, such as those discovered by the Kepler spacecraft, orbit stars similar in mass to the sun. Directly imaged planets, such as those discussed by Kevin Wagner in a previous blog post, are mainly discovered around stars more massive than the sun (A or F stars). However, most stars in the galaxy, around 75%, are lower mass stars called red dwarfs, or M stars (See Figure 1).
These stars burn slowly through what little fuel they have. These stars are faint and red, like the smoldering ashes leftover then the flames of a campfire have died out. They are so faint that they are invisible to the naked eye, and famous red dwarfs like Kapteyn’s star can only be seen with binoculars. The closest star to the sun, proxima centauri, a companion of alpha centauri, is also a red dwarf.
It seems crazy to look for planets around these faint red dwarfs, yet they have a few advantages. Because the stars are much smaller in size than the sun, a transiting planet covers a larger fraction of its surface, and is therefore easier to detect. When the earth moves in front of the sun it blocks about 0.01% of its light. M stars can be up to 10 times smaller, which means a transiting earth-like planet can block up to 1% of the stellar light: a signal that is much easier to detect. Because the stars are also faint, planets can be located close to the star and still have a temperature that supports liquid water on the surface. Cool, small planets are therefore easier to find around red dwarfs, and to date, most of the planets discovered in the “habitable zone” of a star have been found around stars much lower in mass than the sun.
I started working on planets around low-mass stars back in 2013. I had just finished my PhD in Amsterdam working on protoplanetary disks, the birth-sites of planets, and I was ready for a new endeavor. I looked across the pond, and was hired as a postdoc in Arizona to work on formation of planets around lower mass stars. The chair of my PhD-thesis committee joked about Arizona: “The good news about Arizona it that it’s only 45 degrees (110 F) in the shade. The bad news is that there is no shade.” Excited by the prospect of working on exoplanets, I was ready to abandon my Dutch lifestyle of cycling around and getting rained on frequently. As it turns out, I still cycle around in Tucson and what being rained on lacks in frequency, it makes up for in intensity. The Sonoran desert and its monsoon storms are truly impressive (Figure 2).
We (myself and EOS members Ilaria Pascucci, Fred Ciesla, and Daniel Apai) were planning to run simulations on planet formation around low-mass stars. So we thought it would be a good idea to look at the Kepler planets. Kepler (Figure 3) observed over 100.000, mainly sun-like stars, but also a few thousand red dwarfs. A paper showing what protoplanetary disks masses were needed to form the Kepler planets around sun-like stars just appeared online. It made sense to investigate what disk masses where needed to form the planets around red dwarfs. So I started a small research project investigating what the Kepler planet population looked like around stars of different masses.
As often in academia, a research project is like an exploration voyage. You set out with a goal in mind, but often discover things you never expected to find. We know that protoplanetary disks around red dwarfs are lower in mass, so we expected them to form fewer or smaller planets. The opposite turned out to be the case: red dwarfs have more planets than sun-like stars! How can a lower mass disk form more planets? A likely explanation is that planets migrate during formation, and that red dwarfs are extremely efficient at moving their planets close to the star where Kepler can observe them (Figure 4). Why they do this is still not clear, and I’m looking forward to investigating this problem further with EOS in the future.
By Kevin Wagner
Searching for planets outside of our own solar system is one of the great challenges in modern astronomy. The problem consists of measuring the brightness of a planet that is more than a million times fainter than its host star, while being so close to the star that even to the world’s largest telescopes the two appear as one due to the blurring of the Earth’s atmosphere – akin to trying to detect a firefly next to a lighthouse from over 1,000 miles away. The observational techniques and technology are finally at a point where we are beginning to overcome these challenges and have been able to take astonishing photographs of alien worlds. The first few planets to have been discovered are of course the easiest observational targets: giant gaseous planets, 2-10 times as massive as Jupiter, at large separation from their stars. While the discovery and characterization of these worlds is fueling current exoplanet science, the instrumentation development trend continues to push toward the more challenging prospect of imaging Earth-like planets that are yet fainter and closer to their stars.
The spring before I started in the Astronomy Ph.D. program at University of Arizona, my advisor, Dr. Daniel Apai, flew me from my hometown in Kentucky to Tucson in order to begin working as soon as possible on a very exciting project. He had secured about a week of time on the 8.2 meter Very Large Telescope in the Atacama Desert of Chile, using a new instrument called SPHERE (the Spectro- Polarimetric- High-Contrast- Exoplanet- Research instrument), which was designed specifically to image planets around other stars. The instrument addresses the problems of contrast and atmospheric blurring by 1) using a coronagraph, which is essentially a mask designed to block out as much of the starlight as possible, and 2) with the use of adaptive optics (AO), i.e. a rapidly deformable mirror, to very accurately correct for the blurring due to the Earth’s atmosphere so that the starlight and planet light may be separately resolved. SPHERE isn’t the first AO/coronagraphic facility on an 8-m class telescope, but it is of the first generation of “extreme-AO” systems, which use much faster and more precisely deformable optics than the previous instruments, and are anticipated to discover dozens of new worlds down to Jupiter mass planets over the coming years. Our team in particular is looking at around 100 young stars near the constellation of Scorpius, with the goal of searching a very young population of stars (less than 1% as old as our sun) to get around the fact that planets grow fainter as they get older and cool down.
Unlike Ben and Min (see previous post), a large percentage of modern astronomy is done in “queue-mode” – where the scenario plays out as follows:
- The astronomer writes a proposal to an observatory detailing a science case and observing strategy
- If allocated time, the staff at the observatory carry out the observations
- The data is sent back to the astronomer, who then proceeds as normal with data reduction and analysis.
Almost everybody agrees that this way is less fun (who doesn’t love changing spectrograph gratings on their own), but the queue-mode ensures that important observations are carried out in good conditions, whereas classical observing is at the mercy of the weather on the scheduled nights of telescope time. It also permits the astronomer the surreal experience of exploring outer space from their basement, which is how I made the discovery that was the first surprising result of the survey.
Many young stars are encircled by disks of gas and dust – primordial building blocks of planet formation that are left-over from the birth of the star. Imaging these disks allows astronomers to investigate by eye for signs of planet formation – to provide tests, constraints, and perhaps new insights to how planets form. As I described above, our science goal was originally to search for these planets themselves, so it came as a surprise to everybody on the team when our images showed the presence of a beautiful spiral disk around the young star HD 100453. The disk was actually already known to reside around the star from the amount of infrared light it emits. But with many imaging campaigns succeeding at imaging only a small fraction of known disks, including failed previous attempts around this same star, the reality of having these beautiful spiral arms show up unexpected is an example of the universe turning out to be more wonderful than we had imagined – something that observational astronomers are continuously witness to.
Through an excellent agreement with dynamical computer modeling, a subsequent study at UC Berkeley found that the spiral arms are driven by the 0.3 solar mass dwarf star at 120 AU. The spirals in this disk are the third of their kind to be observed in young stellar disks, yet the only case where the perturbing companion has successfully been imaged – suggesting that the other two systems are caused by smaller, planetary mass companions, which are current targets for the extreme-AO imaging systems. In addition to the spiral arms, the disk shows compelling evidence for a forming planetary system within, through the presence of large gap in the surface density, seen as the central red region in the image shown here. These gaps have long been regarded as evidence of gravitational sculpting by forming planets, and astronomers have just recently started to find the planets that were expected to be hiding within (see the discovery of two protoplanets in the disk gap in LkCa 15). The gap in HD 100453 extends just barely outside of the coronagraph on SPHERE, which excitingly suggests the presence of forming planets that will just have to wait for future missions that will be able to look in close enough to find them.
For more about this work, please see our HD 100453 Spiral Discovery Paper: Wagner et al. 2015, Astrophysical Journal Letters, http://arxiv.org/abs/1510.02212
And our paper detailing the discovery of an edge-on debris disk in HD 110058: Kasper et al. 2015, Astrophysical Journal Letters, http://arxiv.org/abs/1510.02210
More results from the rest of the survey coming soon!