Category Archives: Exoplanet Exploration

Mont Blanc, Slopes, Skiers, and the HST/WFC3 Ramp Effect

Hanging out at the Servoz Train Station in the French Alps

Hanging out at the Servoz Train Station in the French Alps

It is a beautiful, sunny, but cool day in the little village of Servoz in the French Alps: surrounded by breathtaking snow-capped mountains – among them the legendary Mont Blanc – I am sitting on a tiny railway station waiting for the little red mountain train that will carry me out of the valley. With still an hour to go before the train I am hoping that getting out will be easier than getting in: this was the first time a lecture I was about to give had to be postponed due to an avalanche.
I had spent five amazing days here, three of which at the Les Houches School for Physics, where Nicolas Iro, Francois Forget, and Valerio Lucarini organized an outstanding school on planetary circulation. This was definitely a school to remember – great lectures, lots of discussions, great food and skiing opportunities allowed by the long mid-day breaks, and fun evening discussions, all set in a picturesque Alpine setting. This was also an important meeting for me personally because we announced here the publication of our new study which, I believe, is an important step in exoplanet characterization with the Hubble Space Telescope. It may even be important for Hubble’s successor, the James Webb Space Telescope. Our study offers a solution to the infamous “ramp effect”. This vaguely understood effect has been plaguing all Hubble Space Telescope observations — among them some of the most beautiful data on exoplanet atmospheres — ever since the instrument was installed in the memorable 2009 Hubble Servicing Mission 4.
Hubble was never designed to do exoplanets: it really was built to be a cosmology machine, mankind’s most advanced telescope (for science) peering into the depths of the universe and — due to the fact that the speed of light is finite — also to the depth of time. Its sharp vision has revealed galaxies back to about one billion years after the Big Bang (or about 12.5 billion years ago). Yet, somewhat paradoxically, that sharp vision is not enough to explore planets even around the closest stars. Planets orbit their bright host stars so closely that even Hubble’s resolution and image quality are not enough to distinguish the light of the planets from the light of their host stars. But that is not the end of the story for Hubble: as a testament to human ingenuity, different teams of astronomers realized that for some systems observing brightness changes in time can help separate the light of the planet from that of the star. Soon after different techniques popped up that used this idea: from changes in the star’s brightness we are now able to deduce when a planet passes in front of its host star (planetary transit) and from precise measurements of the level of dimming in different colors we can start figuring out what the atmosphere is made of (if it contains water vapor, for example, the planet will appear larger at the wavelengths where light can be absorbed by water than at wavelengths where it can’t). For other systems, observations showed that very close in and very hot Jupiter-like planets – as they orbit their host stars – will add varying levels of extra light to the pixel where the star’s light is collected. With a very hot dayside, these planets are very bright and depending on how much of their bright dayside we see (in different parts of their orbits) the extra light will vary. This allowed astronomers to create the first crude maps of hot jupiters, a spectacular result from the last decade. This is now an unexpected but booming field of discovery and results from planets received visibility and generated excitement on par with the exciting cosmology results. And this was just a beginning – exoplanet programs more ambitious than ever are being executed on Hubble.
Even with constant illumination HST's WFC3 detector does not show a flat line but a hook-like pattern known as the "ramp effect". A major challenge for precise time-resolved observations required by exoplanet observations.

Even with constant illumination HST’s WFC3 detector does not show a flat line but a hook-like pattern known as the “ramp effect”. A major challenge for precise time-resolved observations required by exoplanet observations. From Zhou et al. 2017

But there was one problem that continued to annoy astronomers, continued to limit HST’s accuracy for these measurements. It led to both some unreliable results and to forcing astronomers to discard large amounts of precious Hubble time. The problem became known as “the hook” or the “ramp effect”. Although HST exoplanet programs rely on extracting tiny changes in time, Hubble – simply put – will see all stars change regardless of whether there are planets around them. Measuring a typical star’s brightness with Hubble – which should yield a precisely flat line – will instead result in a funny hook-shaped light curve: a shape that will be different for each star and even for each Hubble orbit observations of the same star! This effect is typically 1.5% – it is small for most other studies with Hubble, but huge for exoplanets where we are after much tinier effects. How can we then use Hubble to map clouds on other planets if even stars appear to change?
The Les Houches school had many participants who have been developing impressive models for how exotic exoplanet atmospheres should behave – and compared their model predictions to mostly data from Hubble, most of which was affected by – to different levels – by the infamous ramps. Therefore, in my lecture I decided to highlight both the problem and the solution we found for it.

To explain how this works, let me tell you more about tourists in the Alps. After the school ended on Friday I stayed for the weekend in Les Houches and, following Alain Leger’s advice, I used Saturday to go up to one of the highest peaks – Aiguille du Midi (3,842m), which offered incredible views of the Chamonix valley. I took a thrilling cable car ride up to the top – a whopping 2,807-meter ascent in just about forty minutes!

Moonrise and Aiguille du Midi with its astonishing viewtower

Moonrise and Aiguille du Midi with its astonishing viewtower

On top of the peak is a crazy tower – it seems small from below (see the photo with the moonrise that I took from the village of Les Houches) but standing on top of it is a majestic and humbling experience. At the top of the tower is a viewing terrace with one of the most beautiful and panoramic views I have ever seen. Standing on the terrace on top of a ten-story-high needle-like tower carefully balancing on a 100 meter-high cliff, buffeted by strong, icy winds, and blinded by the bright, untamed sun of the high altitudes, I can all but wonder about the raw power of nature. Gigantic mountains and majestic peaks all around  – among them Mont Blanc (4,810m), Dome du Gouter (4,304m), Mont Maudit, Aiguille de Verte (4,121m). I went up on a good day but the low-level of oxygen (only 45% of that at sea level), the high wind, the sun, and the brightness of the snow offered a glimpse of what it may be like to try to climb one of these stairs (although the only climb I did was the stairs to cafeteria, one of the highest in Europe).
The peaks of Les Drus (3754m)

The peaks of Les Drus (3754m) and Aigulle Verte (4121 m)

Dozens of skiers traveled with me on the cable car from Chamonix to the top of the mountain: most equipped not only with skis but with ice axes, ropes, and climbing harness. I suspected that their goal was not the cafeteria but to ski down from the top, which must be an incredible (and seemingly dangerous) experience.
From the peak I could watch these skiers; holding on to chains and inching on top of an impossibly steep cliff to reach the slightly gentler slope that does not end in a thousand meter free fall. Then they started, one by one, their descent – following a gradient in gravitational potential energy.
Skiers in the Alps - starting from almost 4,000 meters they will ski down to Chamonix.

Skiers in the Alps – starting from almost 4,000 meters they will ski down to Chamonix.

Chamonix valley - surrounded by majestic peaks.

Chamonix valley – surrounded by majestic peaks.

Interestingly and unknown to the skiers, they follow a similar pattern – for a very similar reason — to what we have seen in the HST data.
Imagine now that you want to figure out how many people are brought up to the mountain top by the cable cars by counting the skiers that arrive back to Chamonix. You could determine that, say 50%, of the people on average will ski (while the rest of us enjoy the view and a glass of French wine); so you would know that for every person you count at the bottom there was – on average – another one that went up. That is easy. However, if you were to count the people arriving to the bottom via ski – right after the arrival of the first cable car – you would see first only a few arriving, then more and more, until the number of skiers arriving each minute reaches an approximately constant value. If you plot the number of skiers arriving in 5-minute chunks of time, you may get a curve that is similar to what HST measures when it observes a star of constant brightness. You may wonder why do you see fewer skiers first, then more skier a bit later, even though the cable cars run precisely on schedule and they are always packed to full capacity.
View from Aiguille du Midi
The solution for both skiers and for HST has to do with what happens to them after departure and before arrival. Experienced skiers can ski all the way down; they start one by one and arrive roughly the same amount of time later than they started. But some skiers – perhaps the less experienced or less lucky – will fall, often hitting obstacles covered in snow. Recovering from a fall could be easy or – if the skier hit a bad obstacle – could take a longer time. With many skiers going down, some of those hidden obstacles will be visible as skiers will be trying to stand up and recover from the fall: as long as a skier is stuck at an obstacle, other skiers will easily see and avoid those.
If you start observing the arrival of the first skiers in the morning, those that arrive first are the ones who did not fall – then you start seeing skiers arrived who started early, but fell once. Just by counting the skiers’ arrival rate you may think that cable car is not bringing up skier at a constant rate – but if you look carefully and you understand the pattern, you can figure out how many obstacles are there and what is the chance that a skier hits an obstacle.
In our paper – led by University of Arizona graduate student Yifan Zhou – we proposed a model in which HST’s detector is like a slope with hidden obstacles (traps). When the detector is illuminated — say, by a bright star — electric charges will be freed that will travel in the detector (helped by an electrical potential difference) until they are detected. However, if a charge hits an obstacle, which are most likely imperfections in the detector, they can get trapped and it will take time until they can find their way out of the traps, leading to their delayed arrival. Yifan has done a great job in translating this idea into a set of equations and then went on to show that this works perfectly for many different HST datasets! It is an exciting result that was accepted to the Astronomical Journal ( and which we are already using to revisit some of the most interesting HST exoplanet observations.
It seems to work so well that, who knows, one day we may even use it to figure out how many people ski down from the Aiguille du Midi peak.
Eventually, the little red train did come and is taking me to the next adventure – the UK Exoplanet Community meeting. Scotland, here I come!
 Daniel Apai at Aiguille du Midi

The Future of Exoplanet Research

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.

EXOPAG Meeting San Diego

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.

Launching Toward Other Earths – EOS Updates from the PI

News and updates on NASA’s Earths in Other Systems Project from PI Daniel Apai. May 10, 2015.

NASA's new Nexus for Exoplanet System Science program offers an ambitious, novel approach to study and understand habitable exoplanetary systems.

NASA’s new Nexus for Exoplanet System Science program offers an ambitious, novel approach to study and understand habitable exoplanetary systems.

Sunday early morning with a coffee in my hand, sitting next to giant blooming Saguaro cacti and citrus trees in Tucson with the spectacular Catalina mountains in the background. Two tiny hummingbirds angrily hover around each other in the air, in a surreal, high-speed aerial fight over the nectar drops in our bottlebrush flowers. A rare, quiet moment to reflect on the launch of our Earths in Other Systems project and the five years ahead of us in this exciting endeavor.


Morning Coffee with Saguaros and Catalina Mnts

After almost two years in planning and preparation, our Project EOS has finally began: an exciting meeting at NASA HQ has launched NASA’s new Nexus for Exoplanet System Science program (which is funding EOS), we published the first paper with EOS results and investigators, the first postdoctoral researchers and a program coordinator are joining our project in May, our website is also online, and we began preparations for transforming a group of offices at the Steward Observatory of The University of Arizona into the EOS “Headquarters”.

EOS Overview

Our EOS team studies the formation of planets capable for sustaining life through three closely connected questions.

Project EOS is an ambitious, exceptionally large-scale research project that combines different disciplines and research techniques to understand how Earth-like planets form. While we now know that Earth-sized planets that receive similar amount of energy from their host stars as Earth does are common in the Galaxy, we do not know how similar these worlds are to Earths: do they only have the same size, but very different compositions, or are many of these worlds truly Earth-like, each carrying in it a potential for rich and complex living systems to emerge? Consider Venus, Earth’s “evil twin”:  81% as massive as Earth and orbiting at 72% of the Earth-Sun distance, it is a world that — seen from hundreds of lightyears — could appear misleading similar to Earth. Yet, through differences in its formation and evolution Venus has become a world with a surface and atmosphere astonishingly different from Earth: entirely devoid of water, lacking plate tectonics and its ability to bury CO2 and stabilize its, Venus’s thick CO2 atmosphere traps the incoming solar radiation and heats up to about 740 K (464 C). Or consider the opposite extreme: NASA’s Kepler mission has found a new type of planets, super-Earths, to be very common in the Galaxy. Many of these super-earths may have very low densities, an evidence that they must have lot of water and light, extended atmospheres. And a “lot of water” here means hundreds or thousands of Earth oceans’s worth of water, completely covering the silicate mantle of the planets, most likely in hundreds of km-thick high-pressure water ice layers, below thick liquid oceans or high-pressure steam atmospheres. These “water worlds” may be just inhospitable to life as the hot, acidic, bone-dry desert Venus has evolved into.

View from the hellish surface of Venus from the Soviet Venera probes.

View from the hellish surface of Venus from the Soviet Venera probes.

How many of the planets in the solar neighborhood are truly Earth-like — moderately rich in volatiles and organics — is an essential question to answer if we want to carry out a meaningful search for extraterrestrial life: for surveying nearby Earths for signatures of life is going to be one of the most complex and challenging endeavors in science yet.

In Project EOS twenty-five of the best experts from five disciplines will work together over the next five years to understand how the composition and volatile and organics budget of newly formed Earth-sized planets is set. In a fascinating set of projects we will look at the smallest scales and back in time, probing the mineralogy and composition of micron-sized grains in ancient meteorites using the most sophisticated microscopic techniques, to explore the history of volatiles and organics in planetary building blocks at the time when the Solar System was young. We will also use optical, radio, and infrared telescopes to study young stars and, around them, planetary systems in formation to piece together the incredible story of a dusty disk rapidly transforming itself into a planetary system that may support life. In search of new knowledge our team will travel to most continents on Earth and will use telescopes in the Sonoran Desert, the Chilean Atacama Desert and on Hawaii’s Mauna Kea; the Hubble and Spitzer Space Telescopes. We will also build powerful computer models for the planet formation process and use these to inspect the details and fill out the gaps; we will  compare the predictions of these models to the properties of exoplanetary systems: planetary orbits, masses, densities, atmospheric compositions. If we succeed, what we learn here will guide our and NASA’s search for life beyond Earth.

I am fortunate enough to work with a team of truly outstanding scientists from the diverse fields, all working toward a shared goal. Over the next five years, our team will also be joined by a dynamic group of young students and postdoctoral researchers: the team at its largest will include over forty researchers. But we will reach an and involve much larger groups: Our results will find their way to the courses we teach and we will also build up a team of Other Earths Ambassadors – citizen scientists excited by the search for life on other planets and eager to contribute.

We will share the excitement and news from the EOS project through blog updates, public talks, Twitter and Facebook posts; join us and follow the blog and twitter feeds and you will learn about our science results, discoveries, travels, and about exploring other worlds, directly from the front line.

Twitter: @EOSNExSS, @danielapai

NExSS Kick-off Meeting at NASA HQ

Two weeks ago NASA has announced its new Nexus for Exoplanet System Science, which may prove to be a major change in the way NASA will fund exoplanet science in the future. Our UA-led team was part of the first selection and I, the principal investigator of our project, joined the program’s two-day kick-off meeting at NASA HQ. The meeting was exciting, inspiring, and challenging at the same time. There have been several press releases and articles about the program in various online and printed media; what follows is my own personal perspective on the meeting.
Introduction to VPL by Victoria MeadowsView from 17th Floor in Crystal City

Questions, Problems, & Puzzles

NASA has invited the principal investigators and key members of 16 NASA-funded teams working on topics related to exoplanet habitability, as well as the directors of the new initiative to discuss and debate the best format and goals for the new program. The teams were selected from regular proposal  submissions to different NASA programs through the usual peer-review process, but invited to NExSS in addition to their selection to carry out the research they proposed.

The motivation for launching NExSS, as I understand, comes from the rapidly growing importance of extrasolar planet habitability research within many different NASA programs. The recent restructuring of NASA research grant programs (XRP, Habitable Worlds, etc.) further emphasized planetary habitability studies across many programs, which led to different aspects of habitability funded through different channels, without a good way to coordinate research between the programs. In addition, planetary  habitability-related proposals accounted for a very large fraction of the major proposals that responded to the latest opportunity to join the NASA Astrobiology Institute.

NExSS is a new approach to study extrasolar planets: the program’s idea is to combine various studies of planetary habitability funded through existing NASA programs into a new framework – one in which the teams collaborate and have influence over the broader, longer-term research directions.

Although many people at NASA have been involved in and contributed to launching NExSS, Mary Voytek, senior scientist for astrobiology, is the chief architect of the new program and program officers Christina Richey and Doug Hudgins, among others, also played important roles. Shawn Domagal-Goldman has also provided important input and advice for the new program.

Our meeting began with short talks at a NASA HQ auditorium, which included welcomes by Jim Green and Paul Hertz, the directors of the NASA Planetary Sciences and Astrophysics divisions. They expressed excitement about exoplanet research, emphasized the need for studying planets as “systems” and they strongly endorsed connecting research projects in different disciplines that address exoplanet habitability. Their enthusiastic support of NExSS was a clear demonstration of how strongly NASA is supporting the new interdisciplinary research coordination network.

We were also welcome by the three new co-directors of NExSS, Dawn Gelino, Natalie Batalha, and Anthony Del Genio, who work on various aspects of exoplanet research.

Next, lightning talks by the leads of each of the 16 teams introduced the scope of the teams; the single-slide presentations gave the first insights into the surprising breadth of NExSS. The NExSS teams have been selected from a set of projects submitted and selected for regular NASA programs (e.g., XRP, Habitable Worlds, Astrobiology, Heliophysics), so the sixteen teams brought very different expertise and perspectives to the table.

The projects also covered a broad spectrum in size, ranging from a few 1-2 investigator grants through a number of medium-sized teams to a few really large teams with multi-million dollar grants. These latter programs are our University of Arizona-led Earths in Other Solar Systems team (PI: Apai), the Arizona State University-led team (PI: Desch), a team led by NASA Goddard Institute for Space Sciences (PI: Del Genio), a team led by Berkeley (PI: Graham), and the one at Hammond University (PI: Moore). NASA’s press release and the team websites provide more information about the teams; I will instead focus on the kick-off meeting.


Working hard on putting the puzzle pieces together at the NASA NExSS Kick-Off meeting.

In contrast to the more usual top-down approach, our group’s first task is to brainstorm on its own purpose and definition. This has been an unusual responsibility; most committees are tasked to chart a course to reach a specific goal on a well-defined timescale. Defining our own goals and purpose is much more challenging; however, it also gave us the valuable opportunity to brainstorm and debate on the importance and achievability of different science goals over various timescales.

NASA has contracted a small company, KnowInnovate, to facilitate the creative process; this small team — two brothers — helped us move forward in the complex debate. Indeed, it has proven challenging for our team to converge on a set of well-defined goals in its first meeting; but by the end of the meeting we did identify our next steps and, I believe, made progress forward in surveying the questions, problems, and goals for the field.

Questions, Questions, Questions

Questions, Questions, Questions

The 2-day discussion resulted in covering most vertical surfaces of the meeting room with neon-colored sticky post-it notes, each with a question, problem, goal, or idea relevant for exoplanet studies. Arranged thematically, by importance, or by timescale, these stickies captured well the complexity and the heavily connected nature of next decade’s exoplanet research.

There discussion was productive and interesting; the number of questions and problems identified, and their complexity, is daunting, to say the least. Questions ranged from the impact of stellar hosts on the habitable planets through the importance of the formation and evolution of planetary systems to the unknowns of planetary interiors and life’s impact on the planet.DistantEarths-1029

Nevertheless, in a process that built on large quantities of coffee, snacks, and post-it notes, we identified some short-term steps and topics of immediate interests. These included establishing working groups on topics relevant for many questions (missing experimental data, cloud physics and chemistry), plans for workshops/conferences to connect to the community, blog-type snippets on new exoplanet research papers, just to name a few.DistantEarths-1028

It has been exciting to see a launch of a new program and one the exoplanet community can so actively shape. From my perspective, the NExSS group’s most important goal is interfacing and connecting: both within the group – in which we had a great start – and also with the broader community. The NExSS Executive Council will gradually change as PIs rotate in and out of the group over the next years, but I am very hopeful that the group will maintain its collaborative spirit as we put together the pieces of this exciting, but complex extrasolar puzzle.

You can follow our team’s work and results on Twitter (@EOSNExSS) or by subscribing to email announcements on our website ( ).

Grenoble and Planet Hunting with Adaptive Optics

In July I spent four exciting days at the Institut de Planetologie et d’Astrophysique de Grenoble (IPAG). Set in a picturesque green valley in the Alps where the rivers Drac and Ilsere merge, this scenic little town in Southern France is well-known for planet hunters.

A charming little town in the French Alps, a European center of adaptive optics development.  Image source:

A charming little town in the French Alps, a European center of adaptive optics development. Image source

The Grenoble group I visited is one of the world leaders in direct imaging of extrasolar planets: over the past decades Jean-Luc Beuzit and his group have developed more and more sophisticated instruments to enhance the sharpness of the images that can be obtained with the largest telescopes. Anne-Marie Lagrange and her group have used these adaptive optics systems to painstakingly search nearby stars (typically within a distance of 250 lightyears) for exoplanets. The group’s outstanding work has led to several exciting and now-classic discoveries: 2MASS1207b, AB Pictoris b, or the planet in the spectacular Beta Pictoris system, a project on which we worked together. During my stay in Grenoble I also enjoyed a memorable dinner hosted by Anne-Marie’s very friendly research group – everyone pinched in in the preparations and soon we enjoyed an amazing set of French dishes. Cheese, red wine, and exoplanets – what a pleasant way to spend an evening!

Planet around a Brown Dwarf?
Anne-Marie and her group made their discoveries with the European Very Large Telescope, a system of four 8m telescopes based at the Paranal Observatory in the Chilean Atacama desert. This telescope and its NACO instrument has discovered more directly imaged exoplanetary systems and planetary mass objects than any other telescope in the world.

Why do we need adaptive optics systems to image planets?

There are three factors that make it extremely difficult – really, almost impossible – to directly image planets around other stars:
1) Planets are inherently faint: unlike stars, planets have no stable internal heat sources apart from the decay of some radioactive isotopes. Therefore, planets are much cooler than stars – and being also much smaller, planets are typically ten million to a billion times fainter than their host stars. This means that the images must be very sensitive to detect them: very large telescopes must be used that can collect a lot of light.

2) By definition, each planet must orbit a star – and because stars are very distant, the planets appear to be extremely close to their host stars, which are always much brighter. Because of the large contrast between the star and its planets, we can only directly image planets that appear the farthest from their stars: those that are on very long orbits. For comparison, a planet at 5.4 AU separation (5.4 times the Earth-Sun separation or 1 times the Sun-Jupiter separation) will appear 5.4/10 = 0.54 arc second from its host star, the equivalent of the apparent size of a penny seen from about 5 miles. The great brightness difference between the star and its planet and the fact that they appear very close mean that an image showing exoplanets must have an extremely high contrast. And, in practice, the highest-contrast images are those that are taken with the highest quality and best-designed telescopes with near-perfect optics.

3) All ground-based telescopes must observe through the terrestrial atmosphere. As we are used to look through the atmosphere at first this may not seem to be a real problem, but it is: Just imagine trying to read time from a wall clock of a swimming pool from underwater – all you see are blurry images. Similarly, images taken even from the highest mountains are blurred by the turbulence of our atmosphere. Although observatories are built at sites that typically have clear skies and relatively calm air layers, the only ways to get truly sharp images is go above the atmosphere (think Hubble Space Telescope) or to build highly complicated adaptive optics systems that correct for the atmospheric turbulence (and even most imperfections of the telescope itself).

The astronomer slang for Adaptive Optics is AO. Although ten years ago astronomical AO systems were still a novelty, nowadays most major telescopes have them. But only the most powerful are capable of providing the image quality needed to hunt for planets.

The most capable systems can correct for changes in the atmosphere and telescope several hundred to a thousand times a second, providing incredibly sharp and stable images.

SPHERE AO system in Grenoble

SPHERE AO system in Grenoble

Perhaps the coolest thing I have seen in Grenoble was perhaps also the least exciting-looking. Tucked in a very large ground-floor laboratory was a room-sized pile of instrument boxes, all connected, and covered with green plastic covers and attached to large tubes and a thick string of cables. The tubes were blowing cold air under the covers to cool what is arguably the most complex adaptive optics system in the world: SPHERE. This highly complex instrument belongs to the next level in adaptive optics design: an Extreme AO system. Extreme AO systems (often called ExAOs) have been in the making for over a decade now with the first experimental one operating at the Palomar 5m telescope (Project 1640). The driver behind ExAO systems is to direct image extrasolar planets: to find them and to study their atmospheres. The European community has been building SPHERE for over 11 years, while the US astronomers have been working on their own ExAO system, the Gemini Planet Imager. Both systems are technological marvels – they are incredibly capable and will soon allow our telescopes to see and study dozens and dozens of new extrasolar planets. Both systems will likely deliver first science early next year and with that a friendly competition will begin for finding new worlds – and will bring us exciting new images and many surprising discoveries.

How to Get Your Own Exoplanet?

If you have been reading about exoplanets, you know that they all have boring names, such as GJ 876b, 51 Peg b, or WASP-19b (not to speak about the likes of KOI-762.02). Up to a few days ago the official names of exoplanets had to be the catalog identifier of their host star plus a letter assigned in the order the planet was discovered in the given system. Yes, while most sci-fi writers used exotic names for their planets, astronomers shied away from anything more exciting than GJ 436b. These were the rules – even if you discovered a new planet, you could not give it a proper name!
But this has all changed with a new decision by the International Astronomical Union, the entity that represents astronomers worldwide.

The names of exoplanets are currently a combination of the host star name and a letter in the order the planet was discovered in a given system. The first planets are assigned 'b', the second 'c', etc.

The names of exoplanets are currently a combination of the host star name and a letter in the order the planet was discovered in a given system. The first planets are assigned ‘b’, the second ‘c’, etc.

To understand the changes let me tell you more about the background of naming celestial objects and, in particular, extrasolar planets.

Let’s start with stars. All bright stars on the sky have names, often derived from their Arabic names (such as Altair or Deneb). Nowadays these names
are often used by amateur astronomers, but professional astronomers tend to use a simpler scheme: in each constellation the brightest star is named Alpha, the second brightest is Beta, etc. leading to names like alpha Persei (the brightest star of the constellation Perseus). However, most stars that we study are too faint for this system – there are not enough letters in the Greek alphabet to name the 13,234th brightest star in Orion!. We now simply use an identifier from one of the all-sky catalogs of stars. These names look like HD 172555, which is the 172,555th star in the Henry-Draper Catalog, a list of stars and celestial positions compiled in 1924. Similarly, a name like GJ 436 is the 436th star in the catalog of nearby stars compiled by German astronomers Wilhelm Gliese and Hartmut Jahreiss. Because the catalogs have the coordinates of each star, astronomers do not need to know anymore in which constellation it belongs to.

Sounds simple, right? But as astronomers began to study in detail the stars in the catalogs, many of the apparently single stars turned out to be two stars orbiting around each other (more precisely: they really orbit around the center of mass of the system). Of course, once you spent 15 years cataloging around 200,000 stars and numbered them, you don’t want to renumber all of them just because the third star turned out to be a binary, right?

Instead, astronomers decided to make a logical change to the system: if a star turns out to be a binary star, we keep the catalog number, but call the two stellar components as A and B. Some systems even turned out to have four components, leading to letters A through D.

Then came planets and with their discovery another change was needed. You could envision to add numbers for each planet, but astronomers decided that the simplest solution is to use lower-case letters for the planets: GJ436 b, for example, is the first planet discovered around the 436th star of the Gliese-Jahreiss catalog. Why is not planet “a” the first planet? Astronomers thought that “a” could lead to confusion with the star itself, so the planets start from b.

This system was simple and worked well. But as the number of planets rapidly grew and they became frequent subjects of the news in all media, there was more and more pressure to introduce more interesting names. The interest was so high that many companies decided to sell planet names – just as some companies used to sell land on the moon.
Eventually, the IAU decided to allow the public to propose planet names.

This is a welcome decision: Allowing everyone to propose names for new worlds means sharing the excitement of discovery.

So, how do you name your own planet?

Understandably, the IAU wants to proceed carefully and wants to avoid names that are controversial (xkcd collected an interesting set of possibilities). So, there is a somewhat complex submission and approval process, but the key points are that you need to identify a suitable name and gather enough support. The name should be, of course, not offensive and should not aim to lead to any financial or political advantages. The next step is to convince a large number of people to support the proposal and then submit it to the IAU for approval. The process is overall very similar to that used to name minor planets in the Solar System.

With this opportunity open, we will surely see interesting and original planet names popping up in large numbers!

The Wildest Clouds in The Universe

Flying on a Delta MD90 jet on my way back from Munich, Germany to Tucson among gorgeous towering clouds glowing in exotic shades of yellow, orange, and purple. Amazing view – especially interesting is the all the different clouds we see are made of water.

How would clouds on exotic other planets look?

Clouds seen from a jet on planet Earth - somewhere about Texas.

Clouds seen from a jet on planet Earth – somewhere about Texas.

Earth is special (in the Solar System, but not in the Galaxy) that it hosts three phases of water simultaneously: liquid, gas, and solid (ice). This allows some fun physics to take place and lead to the hydrological cycle we all know. In the terrestrial atmosphere most water clouds form as the rising moist air cools down and water condensates to condensation nuclei and forms droplets; this process forms the low clouds (~1-2 km). Higher up in the atmosphere, where temperatures are very low, ice crystals will form clouds (>6km).

But we don’t need to go far to find exotic clouds: Earth’s hostile twin, Venus, has a thick carbon dioxide atmosphere (with a crushing surface pressure as high as 900 meters underwater on Earth!). As Venus has lost all of its water in the past, it does not have water clouds – but still has very thick clouds. And these clouds are made of hot droplets of sulphuric acid (H_2SO_4)! The cloud layer is so thick that no sunlight reaches the surface directly. The thick yellowish cloud layer has blocked the views of surface from the early space probes.

This Cassini image of Titan shows the formation of clouds close to the south pole as the winter begins. The overall yellow color of Titan is given by a thick organic haze.

This Cassini image of Titan shows the formation of clouds close to the south pole as the winter begins. The overall yellow color of Titan is given by a thick organic haze.

Or think about the amazing Titan, Saturn’s large icy moon (larger than planet Mercury!). Titan is the only moon in the Solar System that has a significant atmosphere – in fact its atmospheric pressure is higher than that of Earth. But the surface temperature of Titan is so cold (only ~93 K) that it allows the three phases of methane on its surface. The Cassini orbiter, which keeps on returning amazing high-quality images from the Saturnian system, has photographed the formation and evolution of clouds in Titan’s atmosphere. The clouds of Titan are not well understood and remain an exciting field of research.

These planetary bodies are not exceptions: in fact, every Solar System planet that has an atmosphere has clouds, too. Depending on the temperature and the pressure (often called the “P-T profile”) and the composition of their atmospheres Solar System planets have different clouds. The very thin atmosphere of Mars allows the formation of tenuous water ice and carbon-dioxide clouds; Jupiter and Saturn sport clouds made of ammonia (NH_3), ammoniahydrosulfide (NH4SH), and water; while the upper atmospheres of the even colder ice giants Uranus and Neptune also have methane (CH_4) and SH_2 clouds.

This Voyager image from 1989 shows the great dark spot and surrounding whiter clouds on Neptune. Neptune's upper atmosphere is about 70 K and at these low temperatures most clouds are composed of methane.  Image Source.

This Voyager image from 1989 shows the great dark spot and surrounding whiter clouds on Neptune. Neptune’s upper atmosphere is about 70 K and at these low temperatures most clouds are composed of methane.
Image Source.

This bonanza of clouds raises the question: which are the craziest, most exotic clouds in the universe?
This is a difficult one, of course, as there are many candidates. My favorites are probably the clouds in hot super-Jupiters and brown dwarfs (see post on Substellar zoo). These gaseous planets and brown dwarfs have no solid surfaces but have extremely high pressures in their interiors. In their uppermost atmospheres, the layers visible to us, temperatures can exceed 1,800 K. In this class of objects iron can exist in two phases: in the hotter layer deeper down as gas and in the slightly cooler, lower pressure upper layers it will form droplets. Just as water droplets form massive clouds on Earth that can pour down heavy rain, hot super-Jupiters will have iron clouds that will drive heavy rain of hot molten iron droplets… Gives a whole new perspective on bad weather!

Back on the airplane, somewhere above New Mexico I miss another opportunity to get free pretzels, but still recall the beauty of our simple water clouds.