Also follow our twitter feeds: @EOSNExSS – Tweets from the EOS Team; @danielapai – Tweets from PI Daniel Apai.
Renu Malhotra – LPL Professor and EOS Team Member – has recently given an excellent TEDx talk on the Search for Planet 9. The talk is now available online – check it out!
Atmospheric characterization of transiting exoplanets
Previous posts on this blog have discussed different methods for detecting exoplanets, including my favorite, the transit method. Transiting exoplanets, which pass directly in front of their host stars as seen from Earth, are particularly exciting because we can learn a lot more about them than other exoplanets. We can directly measure their size with the transit depth, or in other words, by how much light they block. And the duration of the transit constrains the inclination of the exoplanet’s orbit which, combined with radial velocity measurements, gives us its mass. These parameters combined determine its density, which gives us a sense of whether we’re looking at a rocky or gaseous world.
But for many exoplanets, just having the bulk density isn’t enough to determine their composition. For intermediate-mass exoplanets between the size of Earth and Neptune, many different combinations of rocky, icy, and gaseous materials can explain the bulk density equally well. To pin down the nature of these worlds, we need to study their atmospheres. We can do this with the technique of transmission spectroscopy, in which we observe exoplanet transits at multiple wavelengths and look for spectral regions where the transit depth is larger due to scattering or absorption by atoms and molecules in the upper atmosphere of the exoplanet. If the transit depth is larger around the 1.4 µm water molecular absorption band, for example, that tells us that the exoplanet has water in its atmosphere that is blocking starlight from reaching us. This information about the chemical composition of the exoplanet’s atmosphere narrows the possible range of its bulk composition.
The sub-Neptune GJ 1214b
One of our best opportunities to study an exoplanet atmosphere comes from the sub-Neptune GJ 1214b. With a mass of 6.6 Earth masses and a radius of 2.7 Earth radii, GJ 1214b is squarely between the terrestrial planets and ice giants of the Solar System in terms of size and density, making it an interesting target for studying intermediate-mass exoplanets. It orbits a relatively small mid-M dwarf star (GJ 1214), which means it produces a large transit depth for its size, comparable to that of a hot Jupiter orbiting a Sun-like star. And since its host star is only 15 parsecs away—about 25 times closer than the typical exoplanet-hosting star discovered by the Kepler Mission—it is much brighter than the typical Kepler target, which enables more precise measurements of the transit depth.
Previous measurements from the Hubble Space Telescope have found a flat near-infrared transmission spectrum for GJ 1214b. Modeling efforts show that the lack of any observable molecular features is best explained by either a high-altitude haze or cloud layer in the exoplanet’s atmosphere. In effect, this opacity source prevents from studying deeper layers of the atmosphere. However, we can still learn about the exoplanet’s atmosphere with measurements at optical wavelengths, where Rayleigh scattering from the uppermost layers of the atmosphere can make the exoplanet appear larger at bluer wavelengths. This effect depends on the mean molecular mass of the scattering particle, so optical observations of the Rayleigh scattering slope can place constraints on mean molecular mass of the exoplanet’s atmosphere, which in turn can tell us about the bulk composition of the exoplanet.
Observations with Magellan/IMACS
Motivated by this, members of the EOS team set out to measure the transmission spectrum of GJ 1214b across the optical wavelength regime. Our results are published in a recent paper in the Astrophysical Journal. Even though GJ 1214 is a relatively bright star, M-dwarfs are much fainter in the optical than the near-infrared, so we needed “bigger glass” than the Hubble Space Telescope to collect enough photons for this measurement. We turned to the 6.5-meter Magellan Baade Telescope at Las Campanas Observatory in Chile. We used the IMACS multi-object spectrograph on Magellan to collect a time-series of optical (4,500–9,260 Å) spectra of the host star and a handful of comparison stars simultaneously during a transit of the exoplanet. The spectra of the comparison stars allowed us to remove the effects of systematic noise sources introduced by observing conditions in Earth’s atmosphere or the instrument itself, revealing the transit signal in the time-series of GJ 1214 spectra or “light curve.” To make more precise measurements of the transit depth as a function of wavelength, we binned our spectra from their native spectra resolution of R~250 to 14 spectral bins. We repeated this observation for a total of three transits to further boost our signal-to-noise ratio and ensure the repeatability of the measurement.
The optical transmission spectrum of GJ 1214b
The resulting transmission spectrum is very surprising. We found that the transit depth at the red end of the spectrum is roughly the same as has been measured in the near-infrared but tends to decrease at bluer wavelengths. In effect, the apparent size of the exoplanet is smaller at shorter wavelengths, which is opposite of the expected signal produced by Rayleigh scattering in the upper atmosphere. The spectrum seems to suggest that the atmosphere is less opaque in the optical and light is passing through deeper layers than in the near-infrared. Atmospheric models, however, show that whatever cloud or haze layer is producing the flat near-infrared spectrum should be similarly opaque in the optical. So, at first glance, the optical transmission spectrum of GJ 1214b seems to be inconsistent with all atmospheric models for the exoplanet.
The solution to this problem comes from realizing that the transit depth we observe depends on both the exoplanet atmosphere and the stellar photosphere. The underlying assumption with transmission spectroscopy is that the transit chord does not differ from the rest of the stellar photosphere. Therefore, any changes in transit depth should be due to the exoplanet’s atmosphere blocking photons from reaching us. However, unocculted starspots or faculae can violate this assumption. When photospheric heterogeneities such as these are present, they introduce a systematic difference between the stellar spectrum inside and outside the transit chord. Any difference between the mean stellar spectrum inside and outside the transit chord is imprinted on the transmission spectrum we observe.
For example, consider the case of unocculted starspots. Being relatively cool, starspots produce few high-energy photons and are therefore faint at blue wavelengths. Their presence makes the unocculted stellar disk fainter on average than the transit chord in the blue optical. So any transiting exoplanet would produce an artificial large transit depth in the blue by blocking the relatively bright transit chord, potentially mimicking the signature of Rayleigh scattering by the exoplanet atmosphere. Unocculted faculae, which are hotter and brighter at short wavelengths, produce the opposite effect and can mask a scattering signature.
Previous studies have relied on monitoring long-term changes in the brightness of the host star to constrain the contribution of unocculted spots to the observed transmission spectrum. Brightness changes, however, can only tell us about inhomogeneities in the stellar photosphere that vary over time. Persistent features, such as polar or latitudinal active regions, won’t contribute to periodic variability but will affect transit measurements.
Considering the contributions of the exoplanet atmosphere and stellar photosphere simultaneously, we found that the transmission spectrum of GJ 1214b is best explained by the combination of two factors: 1) a hazy exoplanet atmosphere that produces a flat transmission spectrum across the optical and near-infrared, and 2) unocculted stellar faculae that produce a decrease in the transmission spectrum at shorter wavelengths. Due to the presence of stellar faculae, it’s not possible to uniquely determine the contribution of Rayleigh scattering to the optical spectrum and therefore pin down the mass of any scattering particle.
Our findings show that more work is necessary to understand the effects of persistent photospheric features on exoplanet transmission spectra. This will be very important in the near-term future with the scheduled launch in early 2018 of the Transiting Exoplanet Survey Satellite (TESS), which will discover many interesting low-mass exoplanets around nearby stars. Building upon the Kepler Mission, TESS will be an all-sky survey of bright stars for transiting exoplanets. So unlike the Kepler exoplanets, TESS-discovered exoplanets will orbit host stars that are bright enough to allow for in-depth exoplanet characterization, including searches for potential biosignatures. To ensure stellar photospheres do not mask or mimic spectral features we interpret as biosignatures, it will be necessary to disentangle the stellar and exoplanetary contributions to transmission spectra. On the EOS team, we are currently exploring solutions to this problem, including stellar heterogeneity indicators and transit observations at high-resolution or over a wider spectral range.
The full details of this work are published in the Astrophysical Journal: Rackham et al. 2017 ApJ 834, 151.
There are many ways astronomers have developed to detect exoplanets. Mikayla Mace introduced the most popular methods—radial velocity, transit, and direct imaging—in an earlier post on this blog. Each of these has their own strengths, making them useful for detecting exoplanets with different orbital parameters. In some cases, one detection method can used to verify an exoplanet detected via another method and strengthen our confidence in the detection.
Combined, these methods have been used to uncover a bounty of exoplanets. There are more than 3,300 confirmed exoplanets known today and another 4,600+ exoplanet candidates discovered by the Kepler mission, according to the NASA Exoplanet Archive. While many exciting exoplanet discoveries are being announced daily (including our nearest neighbor, Proxima b, announced today), the field is transitioning in many respects from an era of exoplanet discovery to one of exoplanet characterization. We now know that exoplanets exist and have discovered a large enough sample to study their occurrence rates; the next step is to gain a better understanding of the nature of these worlds, including their formation and composition.
We can expand on the same techniques we use to detect exoplanets in order to begin to characterize them. Direct imaging is the most amenable to this (see, for example, Kevin Wagner et al.’s recent work on the exoplanet in the triple-star HD 131399 system). Once we’ve reduced the glare from the host star, we can isolate and record the light we receive from the exoplanet. Absorption bands in its spectrum indicate the presence of different molecules in its atmosphere, and we can compare its spectrum to models to get an idea of the exoplanet’s temperature and surface gravity. These direct studies, however, are currently limited to young and therefore hot exoplanets with masses larger than Jupiter’s and orbits larger than Pluto’s. While the future for characterization via direct imaging is bright, studying sub-Jovian exoplanets in older systems is still out of reach for today’s instruments.
The transit method, on the other hand, can be extended in some cases to study the (relatively cool) atmospheres of exoplanets smaller than Neptune with current facilities. To detect an exoplanet with the transit method, the stars must be aligned—figuratively speaking. Literally, the plane of the exoplanetary system must match our line of sight to the system. Then, the exoplanet will pass in front of its host star from our point of view during its orbit, blocking some starlight and producing a distinct, periodic decrease in the star’s light curve or amount of light we register over time.
These transiting exoplanets are astronomical gold mines in the sense that we can learn a lot more about them than other known exoplanets. The amount of starlight they block gives us a direct measure of their radii. Complimentary radial velocity measurements tell us their masses, which we can combine with their radii to calculate their bulk densities. High-precision observations from space-based observatories like Kepler or the Hubble Space Telescope can detect emission from the exoplanet as its dayside rotates in and out of view during its orbit, which astronomers can use to construct rotational brightness maps and study how energy is redistributed across the exoplanet. Studying blueshifts or redshifts introduced in the stellar spectrum during the transit (the Rossiter-McLaughlin effect) can tell us if the exoplanet’s orbit is misaligned with the star’s rotational axis, which provides a key to understanding the exoplanet’s formation and subsequent orbital evolution.
One of the most exciting ways we can extend the transit method is by transmission spectroscopy. With this technique, we record spectra of the host star during an exoplanet transit. The host star should appear to dim at all wavelengths, since the exoplanet is blocking some starlight. However, at some wavelengths, the star will dim even more because atoms and molecules in the upper atmosphere of the exoplanet are absorbing or scattering that light. In effect, the “shadow” the exoplanet casts is larger due to its atmosphere, and the spectral shape of the shadow records the composition of the atmosphere.
As young as the field of exoplanets is, the study of transmission spectroscopy is even younger, with the first detection of an exoplanet atmosphere in 2002. Since then, dozens of exoplanet atmospheres have been studied with this technique, though systematic studies of multiple exoplanets using the same instruments and observational designs have been lacking.
To address the need for systematic studies, within Project EOS we have launched ACCESS (PIs: Mercedes López-Morales, Daniel Apai, Andrés Jordán), a collaborative survey between EOS members at the University of Arizona, the Harvard-Smithsonian Center for Astrophysics, the Pontificia Universidad Católica de Chile, and the Carnegie Institution for Science. The aim of ACCESS is to measure transmission spectra from a representative sample of transiting exoplanets. Our targets include 30 exoplanets that are excellent targets for transmission spectroscopy, with sizes ranging from sub-Neptune to super-Jupiter and effective temperatures between 600 and 2,800 K. We utilize the IMACS spectrograph on the 6.5-meter Magellan Telescope to gather the spectra.
Through ACCESS, we are compiling a library of exoplanet transmission spectra, which will ultimately enable us and the wider exoplanet atmosphere community to study trends in the atmospheric properties of exoplanets as they relate to the exoplanets’ masses, radii, and orbital parameters. While the long-term future of studying exoplanet atmospheres surely rests in the next generation of telescopes and instruments that will enable us to directly image smaller and cooler exoplanets, transiting exoplanets like the ACCESS targets are the cosmic lighthouses that are providing the first insights into the natures of other worlds.
I sat down on my third flight of the day, and the last that I would be taking to the big island of Hawaii on my way to the Mauna Kea observatories. The passenger with the seat adjacent to mine followed and sat down. My step-mom was a flight attendant, so flying is strictly routine to me by now. I typically slouch into my headphones when I sit down, not to arise until the plane is back on the ground. However, I usually spend at least a few minutes without the headphones as the passenger next to me sits down, in case they are in the mood to exchange a few words. I like to be polite and at least acknowledge people, and I’ve met some very interesting people by sitting next to them on flights.
Usually the exchange is simple and cordial, and I’m back in my music before the plane is out of the gate. Today, I had a more interesting interaction on my forty-five minute flight between the Hawaiian Islands. The passenger next to me was a native Hawaiian – his name is Jon. The typical questions:
“How’s it going?… First time in Hawaii?… Here by yourself? Visiting family?…
“Oh, you’re here for work. What do you do for work?
I love my job, so I’m usually enthusiastic about telling people that I’m an astronomer. Most people are excited by science and amazed at space – something I can easily utilize to initiate a fun and deep conversation. In Hawaii, though, many of the natives have recently been protesting the development of a new telescope, the Thirty Meter Telescope (TMT), and relations have gone to the point of violence and theft of observatory vehicles in a few cases. With these tensions, I was unusually uncomfortable giving away my career, but I did it anyways, hoping for no hard feelings. Jon seemed nice enough anyways.
“Oh, you’re an astronomer… It’s interesting that we find ourselves sitting next to each other. I’m actually one of the main individuals fighting the development of the TMT in court.
Of all the people that I could be sitting next to… However, Jon remained friendly and interested to talk to me about the issues. In fact, he’s also a professor of History at University of Hawaii, specializing in Hawaiian culture and history. No wonder he’s on the side of the protestors, whose leading vocal outrage is at the use of their sacred and religious mountain for the big mechanical industry of astronomy.
But is that what it really is? Is astronomy really the same facet of human civilization that has enveloped some of the most beautiful places on our planet with concrete jungles – forever muddling the landscape with whatever structure we envision is more economical?
The Hawaiian Islands are a perfect example of this. As we flew over the islands from Oahu (where I previously had a connecting flight in Honolulu) to the big island of Hawaii, where the volcano and observatories are located, I asked Jon about the islands. The first was Molokai, as he pointed out, an island with an almost all-native Hawaiian population. Their rural communities and expansive farms were visible from the air, with pristine volcanic beaches with clear water on all sides. The natives work hard to keep it that way, a vestige of what Hawaii was, and not the tourist trap that we think of here on the mainland. They even had something to do with the disbanding of the ferry, which would have transported cars and people between the islands. Instead, people have to fly (or catch a ride on fishing vessels), and the cars are for the most part landlocked.
Next was Maui – in stark contrast to Molokai, as Jon put it “Maui seems to have accepted everything that modern Western culture has to offer.” Hotels line the beaches. Harbors abound. Cruise ships dock and deliver their passengers to their island ‘paradise’.
After an American-led overthrowing of the Kingdom of Hawaii and subsequent illegal annexation of the Republic of Hawaii (which the US formally apologized for under the Bill Clinton administration), many natives are weary of big government and industry from the mainland. As one of the telescope operators described to me, the telescope protests are largely an issue of sovereignty. Jon, as many other natives, sees the development of the TMT, and all of astronomy in Hawaii, in exactly the same light – big industry from the mainland occupying their natural resources.
But as a scientist, that’s not how I see what we’re doing. Although it bears the similar appearance of the bulldozer and giant sheet metal, as I described to my new acquaintance, the goal is not to make money or to provide jobs, industry, etc. The goal is learning about the universe, and using modern technology to open a deeper understanding of where we are and where we come from – fundamental questions of the human condition.
“That’s a very interesting way of putting it.”
He was also succeeding in making me question the merit and methods of what I’m making a career and life obsession out of. Is it worth leveling the tops of mountains and installing gigantic metal structures, continuing to play into our self-assigned god-like role of shaping the planet for our own interests? And of the mountain’s sacred origin to the native Hawaiians, is it worth intruding on their most sacred places, essentially taking over their most holy landmark, simply because it is the best astronomical site on Earth? Is it possible for us as a global community to decide that the benefits outweigh the costs on an island so distant from most of us? Does that make it okay?
Although, not all Hawaiians are in opposition – in fact protests have been ongoing on both sides. Some Hawaiians, including the University of Hawaii, are in support of the new telescope. For the ancient Hawaiians, the necessity to navigate between the islands meant that they were among the most astronomically advanced civilizations before the development of the telescope. With that history, and with the best skies on Earth, it seems fitting that Hawaii should remain on the cutting edge of astronomy – necessitating big telescopes in the era of extremely large optical telescopes.
I have to settle in favor of astronomy. Although I could easily seek another career, higher paying than academia no doubt, I have stronger reasons for what I’m doing. I think that most of astronomy has benevolent motivations. My own research as part of Project EOS is aimed at unraveling the questions of how planets form and what types of planetary systems are out there. In the near future, this groundwork that many others and I are working on will guide our search for simple forms of alien life on other planets, and possibly even for other planets that humans could travel to and inhabit – something that may be necessary for our continued survival. Without big telescopes in the sites with the best atmospheric conditions on the ground, this research simply isn’t feasible.
“Oh, you’re interested in finding out how planets form! Well, it’s really difficult for me to tell you I don’t want you to do that.”
Our conversation seemed to be wrapping up, and so was our flight. We landed in Hilo, and I drove to the summit of Mauna Kea where I would be staying for a few days centered on my allotted time on NASA’s Infrared Telescope Facility.
I had a built in buffer day in case something went wrong with travel, and to get acclimated to the high altitude before actually having to observe. Since things went smoothly, I went out exploring the first day. Across from the astronomers housing are a few hills (that turned out to be individual volcanic caldera) with trails leading up them, and rock piles visible on their summits. Being used to hiking desert trails, I know a cairn when I see one. This was the biggest cairn I had seen, and I headed up the steep and lightly beaten trail up to it.
When I got there, I found something that I didn’t expect. Instead of a pile of rocks to mark the trail, or a cool viewpoint, I found a shrine and a tomb. There were small tributes among the rock pile – native grasses strewn into a necklace, particularly aesthetic rocks, even a seashell filled with rolled up human hair. The latter was admittedly strange to me, but in another light I see it was a beautiful tribute by someone who deeply loved the person who was buried beneath the rocks.
I continued on through seemingly endless rock piles. They literally covered the mountainside. The whole place was entirely peaceful, though sullen. Wildflowers peppered the sides of the trail, and an ominous fog descended on the mountain. Feeling like a stranger here, I decided it was time to leave.
When I returned to my room, I researched and affirmed my suspicions that the piles of rocks are native Hawaiian graves. I felt a little remorseful for having entered this realm, but I was respectful and admiring of their culture, and glad that I had a chance to glimpse into their world. I understood why the natives have such a high respect for the mountain and feel the need to protect their heritage and identity.
While reading about the rock graves, I came across a description of the sacred nature of the mountain. In Hawaiian religion, the mountain summits are “places of gods for men”. Their summits are where the people would pilgrimage to seek a connection with the heavens to inform their earthly lives. In the same light, I see astronomy as the pursuit that seeks lessons gained from the heavens to inform our lives and actions on Earth. Though I can’t claim that Mauna Kea is “my mountain” in any right, I think that astronomers are utilizing the mountain in its most sacred sense – a connecting point between heaven and Earth, where observers seek and telescopes collect the lessons of the heavens.
The current estimate for the number of stars with Earth-sized planets orbiting in the habitable zone is about one in four, according to Dr. Daniel Apai Principal Investigator for Project EOS. Other researchers estimates range from as few as 5 percent to more than 100 percent, which means that more than one exist per star.
Despite even the most optimistic statistics, the only life found in the universe is that which is found on Earth. The life on this earth, in the form of human scientist, are compelled to search for evidence of other-life in the universe for many reasons. First of all, we want to know if it can happen. Moreover, we want to know what it is like compared to life on Earth. But ultimately, everything learned from this kind of research, will illuminate what life is and how it arose on this tiny speck of dust.
How might scientist first find life?
There are different kinds of life, and each kind will have different ways to signal their presence. The first kind of life is very simple. This kind of life is usually single celled and hasn’t had time yet to greatly impact the planet it inhabits. This kind of life would be undetectable.
For a long time, life on Earth would have been undetectable. It wasn’t until cyanobacteria, or blue-green algae, evolved chloroplasts that oxygen started to build up in significant amounts in Earth’s early atmosphere as a byproduct of photosynthesis. Oxygen was poisonous to the life that previously inhabited the Earth, so they died out. Today, all of the plants on Earth turn carbon dioxide into oxygen for energy (photosynthesis). Oxygen is a very unstable and reactive gas, so unless life is present to be constantly creating it, it won’t stick around long. So hypothetically, if an alien was observing Earth using spectroscopy and remote sensing, the alien might note the oxygen in the atmosphere and conclude that Earth hosts life.
Detectable life, like blue-green algae for example, doesn’t necessarily mean complex life. Complex life, like plants and animals could be even rarer.
Rarer still is the existence of intelligent life. Civilization and technology are still very new things when thought of in the timescale of the universe. The SETI Institute primarily looks for evidence of this kind of life through technological signals such as radio waves.
“I think we will most likely we will be surprised which is I think probably the most interesting aspect,” Apai said. Despite all we know about life, the signal may not even be recognizable the first time. But once life is detected for the first time, according to Apai, then finding it should become easier as time goes on.
Special and alone? Most likely not.
Some argue that maybe Earth is a fluke, and we are the one in a trillion change that life has evolved. However, Apai said, “We have yet to find any property of Earth that is truly unusual as far as we know it.” Meaning that it’s likely that these conditions could be repeated throughout the galaxy. Another reason is that we have evidence that life began very early on in the Earth’s history, suggesting that life might emerge quickly and easily.
Science fiction as motivation.
Apai discussed his motivations for taking part in the search for life, which started early in life. Apai grew up in Hungary, part of the Eastern Bloc at the time. Science fiction was one of the genres that were not censored because the government was trying to promote the idea of a brighter future through science and technology.
Apai found an interesting paradox: “Most [science fiction] express a belief that based on rational thinking and ideas you can improve society, civilization, and the life of people, so I like that, and that’s probably part of my motivation.”
What life out there means for life down here.
Later in life, once going to school for physics he became motivated by the idea of changing how people perceive the world, and wanting to answer the most fundamental questions.
Even though it’s hard to see the everyday benefits of the search for life in the universe or such similar scientific endeavors, these pursuits actually have some of the most profound impacts on society.
The ramifications that this research has is both technological, cultural and psychological. For example, studying the motion of the planets not only led to uproar in areas in the world where the church was the most influential, but people also died, science took precedence, knowledge in other fields was gained and people’s everyday thinking was transformed.
The promise of the search for life through endeavors such as Project EOS could mean many things. New technologies could be developed and used across scientific fields, our understanding of the definition of life could improve, and the understanding of our climate and our relationship to it could be strengthened. These are all things that could happen even if we don’t find life.
Imagine what could happen if we did.
A closer look at dust particles
Young planet-forming disks contain trillions of tiny microscopic dust particles. Even in the tenuous protoplanetary disk, these particles bump into each other every now and then, sticking together and growing larger and larger with every collision, eventually forming the building blocks of planets. Historically, dust grains have been treated as being spherical and compact, but in recent years the evolution of the internal structure (or, porosity) of these particles has received more and more attention – and for good reason.
For dust particles smaller than ~1 millimeter, typical collision velocities in protoplanetary disks are below 1 cm/s. At these low velocities, collisions are in the so-called “hit-and-stick” regime: particles stick together upon contact without any restructuring taking place (the sticking is made possible by Van der Waals forces, the same forces that allow geckos to climb walls). A good analog for hit-and-stick growth in 2 dimensions is Tetris.
Figure 1 shows three outcomes of Tetris games where I was trying to mimic random impacts by playing the game with my eyes closed. The results show that undirected hit-and-stick growth naturally leads to a porous structure, filled with voids and gaps of various sizes. The average porosity (calculated as the fraction of space not taken up by solids) is very close to 50% for all three panels. This 50% is by no means a universal number and depends for example on the properties of the particles that we are adding from the top. In classic Tetris, all building blocks are composed of 4 squares (hence the name), but if we were to repeat the experiment with single squares raining down, the final porosity would be effectively 0%.
Porosity evolution in protoplanetary disks
Dust aggregates growing in protoplanetary disks follow similar rules: hit-and-stick growth leads to porous (or “fluffy”) aggregates, with the degree of porosity depending on the sizes and shapes of the collision partners. Figure 2 shows two dust particles grown in 3D computer simulations using different assumptions about the distribution of collision partners. Both aggregates have a final mass of ~0.1 microgram, but their sizes, structures, and (average) porosities are very different.
As aggregates grow larger still, collisions become increasingly energetic and instead of hitting-and-sticking they can result in significant compaction (i.e., the crushing of voids, or “de-fluffification”) or even (partial) disruption of the aggregates. The collisional outcomes and porosity evolution in this more energetic regime are very complex and understanding both requires an intimate knowledge of how different aggregates respond to mechanical stresses. Laboratory experiments and numerical simulations agree to some extent (for example, both are finding that aggregates consisting of ice-coated grains are harder to compress and destroy than rocky ones), but large regions of parameter space are still unexplored: simulations are restricted to relatively small particle sizes and experiments have difficulty probing the highly-porous regime.
One thing appears clear though: dust aggregates are unlikely to be spherical, compact, and of a uniform density. With porosity profoundly influencing an individual particle’s optical, mechanical, and aerodynamical properties, developing a more realistic model for the particle’s structure is very important for astronomers studying the appearance and evolution of dust in planet-forming disks, and remains an area of ongoing research.
“Usually the first thing you find in astronomy are the freaks,” said Dr. Travis Barman, Project EOS co-investigator and associate professor at the University of Arizona. “And the freaks tell you about the exceptions not the rule.”
Exoplanets are illusive objects. They are difficult to detect using even the most powerful telescopes because they are dim and cool relative to objects such as galaxies and stars, according to Dr. Daniel Apai, principal investigator for Project EOS. Only in the early 1990s did humanity discover evidence that planets exist outside of our solar system. Since then, two decades of research and technological development have revealed incredible insights to the fantastic worlds that exist within the Milky Way Galaxy, but astronomers are itching to learn more.
The first exoplanet discovered around a main sequence star was found using a technique called radial velocity. Astronomers observed a star (52 Pegasi) in the constellation Pegasus and noticed it was wobbling slightly. This wobbling motion suggested that a very massive planet was orbiting the star and pulling on it due to the force of gravity.
This technique uncovered hundreds of exoplanets in our galactic neighborhood and it is still used today. But because of the nature of the radial velocity method, the kinds of planets that can be detected are limited, resulting in a skewed understanding of the galaxy’s planet population. Radial velocity is most sensitive to planets that are very massive and close to their parent star. This is because large planets that are very close to their star and are very massive have a stronger gravitational interaction with that star. Interestingly, this technique isn’t as reliable for very young stars because the amount that it’s wobbling could be less than the variation of the bubbling surface of the star by nuclear fusion.
Today, radial velocity is often used to confirm the presence of planets detected using other methods. But between the years 1995 and 2009, hundreds of exoplanets were found using this technique.
In 2009, when the Kepler Space Telescope began operations, the transit method became the most efficient way to hunt for planets. The transit method works by measuring starlight for a period of time and looking for regularly occurring dimming caused by planets orbiting the star and blocking light as it passes between the star and the Earth.
The Kepler Space Telescope has found thousands of planets using this technique, but it has only been looking at a tiny fraction of the sky. Despite this, there are currently 2327 confirmed planets, according to NASA.
While the transit method is an efficient way to detect planets, it doesn’t really tell astronomers much about the planet’s characteristics. However, the clues it provides allows inferences to be drawn. For example, knowing of the distance from the star can determine if it’s in the habitable zone.
Transits combined with radial velocity data can be used to determine the density of planets. Radial velocity provides information on mass, and if astronomers know the mass and the size of the planet, they can determine the density, according to Apai. A planet that has a high density could be mostly composed of iron, a lower density could signal an abundance of water.
While these are inferences that can be made using these indirect methods, what astronomers are striving for is to directly image planets.
Directly imaging planets is the most ideal method. Direct imaging reveals the most about an exoplanet’s composition because astronomers can actually collect light from the planet (instead of a star’s movement, or diminishing starlight).
Astronomers can determine the composition of the atmosphere and estimate the temperature. These pieces of information interests Project EOS because they are critical in the search for life.
Atmospheres of exoplanets are complex and very hard to tease out of the data collected. However, they’re important: “With every planet outside of the solar system, every photon passes through the atmosphere of that object so we can’t really pretend to understand exoplanets without some knowledge of the atmosphere,” Barman said.
By observing the brightness of light at different intensities, astronomers can determine the composition of atmospheres. Oxygen and methane are two prominent signatures of life. However, seeing the presence of these compounds alone doesn’t guarantee life. Further investigation is needed.
However, this method is also the most difficult to use, and without the other methods to confirm exoplanets found using direct imaging, we would not have the abundance of data that we have today.
There are multiple techniques that planet hunters use, but they can only provide a limited piece of the puzzle. Through finely tuning theories of how planets form and advancing technology, astronomers can continue to understand the mysterious nature of planets in the galaxy, bringing us one step closer to understanding life in the cosmos.
How do planets form?
This is a question that scientists have asked themselves for centuries. Kant (in 1755) and Laplace (in 1796) postulated the nebular hypothesis, which states that the solar system planets formed from a rotating disk of material. Over the years planet formation theory has been greatly expanded and improved with the detailed knowledge gained from observing and exploring the solar system and studying the meteoritic record. The discoveries of exo-planetary systems starting around 1995 prompted the search for new directions in planet formation. Exoplanets discovered were often much unlike the solar system with planets the size of Jupiter in very close orbits, and planet migration gained ground as one of the key elements in planet formation theory. The idea of planet migration in turn affected our ideas of how the solar system formed: the ‘grand tack’ formation model postulates that Jupiter underwent such migration as well (see PLANETPLANET, for a blog post on this topic by Sean Raymond).
In recent years, a new revolution has taken place in the world of exoplanets.
The Kepler space telescope has discovered thousands of exoplanets and planetary systems, and it has become clear that the majority of stars have planets. Again, these planetary systems are often much unlike the solar system, with super-Earths and mini-Neptunes orbiting closer to the star than Mercury does to the sun. It is clear that planet formation theory needs to be expanded with new mechanisms to explain the exoplanet population discovered by Kepler. At the same time, these new planet formation mechanisms will affect our ideas of how the solar system formed.
The workshop I attended (see below) focused on new developments in planet formation theory. Planet formation is a complex process that involves processes at multiple scales: from the initial growth of micron-sized dust grains; to the final stages of gravitational interaction between proto-planets; and many steps in between. In this blog post I will discuss a recent idea that gained a lot of attention during the workshop and that I think will play a prominent role in future planet formation theories: Pebble accretion.
Pebbles are dust grains of a size that provides them with special aerodynamic properties. They are large enough that they can move through the nebular gas and easily encounter growing planets. But pebbles are also small enough that friction with the gas surrounding growing planets makes them spiral in. The high mobility and efficient accretion makes pebbles ideal for growing planets. Pebble accretion is a likely solution to a long-standing problem in planet formation: the growth of gas giant planets like Jupiter. With gravity alone it is hard to form a ten earth-mass core fast enough to accrete the nebular gas that comprises most of Jupiter’s mass. Pebble accretion speeds up the growth process and makes sure there is still nebular gas around after Jupiter’s core has formed.
Pebble accretion may also have played a crucial role in the formation of earth and the terrestrial planets. Jupiter’s quick formation and immense gravity could have prevented many pebbles from reaching the inner solar system, preventing the formation of super-earths and mini-Neptunes that we do see around other stars. A single planet formation mechanism that explains both the solar system and exoplanetary systems is very appealing to theorist.
But how do we observationally prove that pebble accretion is indeed the mechanism that forms planets and exoplanets?
The answer to this question may lie in the chemical composition of planet atmospheres. Jupiter’s atmosphere contains a higher concentration of heavy elements than the sun, indicating that it accreted not only gas but also some solid material. This solid material could be in the form of pebbles or as larger objects called planetesimals. This size difference between the two leads to a distinct chemical composition. Pebbles easily lose their water and other volatiles when they get heated, while planetesimals are more likely to retain their water. This difference in water content will be reflected in the composition of the exoplanet atmospheres that accreted them. Using the composition of exoplanet atmospheres to trace the early steps of planet formation is an exciting prospect for our understanding of the planet formation process.
I’m excited to see where this new direction in planet formation will take us in the coming years. Will pebble accretion also become the dominant paradigm in the formation of exoplanets? What new insights into the solar system can it provide? And can we find a “smoking gun” for pebble accretion, perhaps in the composition of exoplanet atmospheres?
Footnote: I attended the workshop New Directions in Planet Formation that took place in Leiden, Netherlands from 11 Jul 2016 through 15 Jul 2016. People tweeted from the conference under the hashtag #NDIPF . The workshop was organized by Ravit Helled (Tel-Aviv, Israel), Anders Johansen (Lund, Sweden), and Chris Ormel (Amsterdam, The Netherlands). This article is inspired by talks and discussions that took place during the workshop.
A strange and new extrasolar system was discovered by graduate research fellow and Project EOS collaborator Kevin Wagner, principal investigator for Project EOS Daniel Apai, and assistant professor of astronomy at the University of Arizona Kaitlin Kratter, announced July 7, 2016 in a paper published in the journal Science.
The system contains a total of three stars. Two stars, one sun-like in character and the other less massive, orbit around one another while also circling a much larger star many times more massive than our sun. Tucked in between this chaotic dance of stars is a giant gas planet. The planet, HD 131399Ab, was discovered using direct imaging techniques by the Spectro-Polarimetric High-Contrast Exoplanet Research (SPHERE) instrument on the Very Large Telescope in Chile.
Hunting for exoplanets is no easy task. Quite a bit of detective work goes into teasing layers of evidence out of data collected on an extrasolar system. Different hunting techniques work better for some systems than others. When possible, a combination of techniques is used to collect as much information as possible. HD 131399Ab was discovered using direct imaging techniques.
What is direct imaging and what are the many ways it can be used?
Astronomers begin the search by calling in the star detectives. “We need a variety of expertise in this game, people who know how to find young stars, they are crucial,” said Dr. Travis Barman, Project EOS co-investigator and associate professor at the University of Arizona.
Astronomers are most interested in young stars because they host young planets, and young planets are easier for astronomers to detect, according to Dr. Daniel Apai, principal investigator for Project EOS and co-discoverer of HD 131399Ab. This is because young planets recently formed from accretions of gas and dust from the protoplanetary disk, making them hotter than older planets.
Hotter objects glow brightly in infrared light, light just below the visible range, making these objects easier to detect than cooler objects like billion-year-old-Jupiter. Exoplanets are too hard to see using visible light because stars greatly outshine them.
Young stars are much rarer in the galaxy than old stars, so looking around these stars helps to narrow down the sample size.
Direct imaging is exactly what it sounds like: Pictures of planets and their stars are taken using extremely sensitive cameras on telescopes. The best candidates for direct imaging are stars that are near to Earth, planets that are planets, that are big and bright, and that have large orbits around stars that are not too far from the Sun. That’s why HD 131399Ab was so surprising. It’s one of the coolest and one of the least massive planets to be discovered using direct imaging.
Fine-tuning the light
Adaptive optics (AO) systems are a powerful tool used for directly imaging planets. The twinkling of stars is caused by rapidly changing currents in the atmosphere. AO systems remove the twinkling halo of starlight and focusing it back into the star. Computers manipulate small mirror actuators that act like pistons which mimic and cancel out movement in the atmosphere that cause the twinkling. What’s produced is a clearer image.
To hear Dr. Travis Barman explain adaptive optics listen here:
Watch this YouTube video for an example of adaptive optics in action!
Making the stars do the work
Proper motion is one important part of the direct imaging technique. Imagine “driving down the road, the fence posts are going like mad but the mountains out in the distance are barely moving” Barman said in an exaggerated example. The fence has a high relative proper motion and the mountains—low. Stars that are closer to Earth, like the fence, will have a high proper motion, which is useful for finding planets.
First, astronomers take a picture of a star and a companion faint dot they suspect to be a planet. Then years, sometimes months, later they take second picture. If the faint dot has stayed still, and the star moved, then the dot was probably a background star, not a planet. If the dot moved along with the star then they’ve found a planet!
Sifting through the noise
Images taken using telescopes with adaptive optics can be improved even further when hundreds, and even thousands, of images are combined and cleaned up using statistical tricks, according to Apai.
Raw images taken with long exposures are stacked one on top of each other. It’s extremely difficult to distinguish an extrasolar planet from the glaring starlight, called speckles.
To tease out the information that’s important, astronomers average out the glare from the stars, called speckles, and then subtract it from the images. This creates a clearer picture allowing dimmer planets to be seen. This technique is called angular differentiation imaging (ADI).
There’s more to learn
Direct imaging is ideal because it can help uncover information about the planet (something that will be explored more in the next article), but the candidates are few, so to date, HD 131399Ab is only one of a few planets have been directly imaged. “One very special aspect of direct imaging is that it is currently the best, most direct, way to learn about giant planet formation, because we are studying planets at their very youngest,” Barman said.
Follow up studies of HD 131399Ab can tell us about its atmospheric properties. The discovery of this planet has pushed the boundaries of direct imaging. So far, it’s the coldest, and least massive planet found using this technique.
Listen here to find out more on why giant telescopes are needed to investigate extrasolar planets from Dr. Travis Barman.
Discovering Earth 2.0, another planet like our Earth that could host life on its surface, requires us to characterize the atmosphere of the planet. An important feature we need to study is the clouds in that atmosphere. What are they made of? How are they positioned across the globe and what is their vertical extent? The answers to these questions will help us determine if the climate of the planet is hospitable to life.
But how could we learn anything about the clouds of another planet when all we have is a single pixel on an image, and there is no telescope that can resolve the planetary disk? The solution comes from observing the giant planets of our solar system, like Jupiter, with “the worst camera possible”, one that brings the whole of Jupiter down to a single pixel. As Jupiter rotates around its axis, we can see the amount of light we receive from Jupiter changing due to clouds in the atmosphere. When, for example, the Great Red Spot is visible we receive more light from Jupiter, and when the spot is on the non-visible side we receive less light. The total amount of light received as a function of the planetary rotation, the so-called light curve, gives us thus information about the cloud coverage of the planetary atmosphere.
Here at the Steward Observatory, we have created a mapping technique that can use the light curve of an observed target to create the map of its atmosphere. We have tested our code on the Jovian light curve and could reproduce the non-rotationally symmetric features of Jupiter in our maps. Knowing that our mapping technique works, we have already applied it to a handful of light curves of brown dwarfs to create maps of some first exo-atmospheres.
Brown dwarfs are the link between the lowest mass stars (M stars) and (exo)planets. Brown dwarfs don’t have enough mass to sustain nuclear fusion and they spend their lives cooling down, like planets. Brown dwarfs offer ideal targets for us to map, since they cover a large range of temperatures and masses; have a wealth of clouds in their atmospheres; and usually lack a parent star that blinds us, making their mapping easier than that of exoplanets.
Luhman 16 is a brown dwarf binary consisting of two brown dwarfs (A and B) that both show light curve variability due to clouds in their atmospheres. Using observations of Luhman 16A and B with the Hubble Space Telescope, we have mapped the atmosphere of B in one day in 2013 and one day in 2014, and of A in one day in 2014. Our maps show large cloud patches in the atmospheres of A and B that cover between 20% and 40% of their atmospheres (see, for example, the movie here). The atmosphere of Luhman 16B is so dynamic that the atmosphere, and our map, changed within one day on Luhman 16B (which only takes 5 hours!). Using our maps we retrieved an upper limit for the wind speeds on Luhman 16B’s atmosphere to be of the order of 1800 mph! These wind speeds are 7 times larger than the maximum wind speed ever recorded on Earth, and 3 to 5 times larger than the wind speeds met in the giant planets of our Solar System (~340mph for Jupiter, or ~670mph for Neptune)!
Even though the atmosphere of Luhman 16B changes within one 16B day, a careful inspection of a number of published light curves showed that there is one feature that reappears in the atmosphere after hundreds, or even thousands of 16B days. We excluded the possibility that this is related to an exoplanet, and concluded that it is an atmospheric feature of 16B. Could it be a feature similar to Neptune’s Great Dark Spots that appear, live their lives and disappear; only to be replaced by another Dark Spot later? Future observations of Luhman 16B over multiple rotations could help us understand the nature of this intriguing feature.
Currently we apply our mapping technique to observations of more brown dwarfs in an effort to better understand their cloudy, dynamic atmospheres. The experience we get in the process will be valuable for mapping imaged giant exoplanets, like the ones Kevin presented, currently and, hopefully in the not-so-far future, Earth 2.0!