Reflections on the scientific path leading to the discovery of a young transiting planet in a debris disk

[ Here is a guest blog cross-post from my friend and colleague Dr. Jonathan Gagné about our recent work on the discovery of AU Mic, written much better than I ever could.  Original post here: https://jgagneastro.com/2020/06/26/au-mic-b-a-new-exoplanet-in-orbit-around-a-remarkable-nearby-star/ ]

AU Mic b: A New Exoplanet in Orbit Around a Remarkable Nearby Star

By Dr Jonathan Gagné

I’m a research advisor at the Planétarium Rio Tinto Alcan of Espace pour la Vie, and I’ll keep you updated with some of our original research here. In this post, I will tell you about the recent discovery of an exoplanet named AU Mic b. This discovery comes from a long collaboration with Dr. Peter Plavchan, associate professor at George Mason University, led since 2010. I joined Plavchan’s team in 2014, and helped them analyze the data that led to the discovery of this new exoplanet.

This blog post is also available in French here.

An illustration of AU Mic b, a new Neptune-sized exoplanet. Credit: NASA’s Goddard Space Flight Center/Chris Smith, USRA

The name of this exoplanet, AU Mic b, is the same as that of its host star AU Mic, except we added a small "b" next to it. This is how we name exoplanets in astrophysics, a convention endorsed by the International Astronomical Union, who are the authority on the naming of astrophysical bodies. This practice stems from binary stars nomenclature; for example, Alpha Centauri is a binary star, and we name its two stellar components Alpha Centauri A and Alpha Centauri B. We use capital letters for stars, and lowercase letters for exoplanets. If we discover a second planet around AU Mic, we'll name it AU Mic c, and so on—we reserve the letter A (capitalized) for the host star, but we usually keep it implicit rather than naming it AU Mic A.

An illustration of the star AU Mic and its debris disk. Credit: NASA’s Goddard Space Flight Center/Chris Smith, USRA

Before we talk more about the exoplanet AU Mic b, I would like to tell you more about its parent star because it is a special one. Its name, AU Microscopii if we write it down in its entirety, comes from the Microscopium constellation where it can be found. We generally name the brightest star of a constellation with the Greek letter alpha (α Mic), the second one with beta (β Mic), and so on. However, the detailed rules are more complicated than this, especially for the fainter stars of a constellation. AU Mic is far from the brightest star in the Microscopium constellation, and that's why its name doesn't begin with a Greek letter. The Microscopium constellation is located in the Southern Hemisphere, but AU Mic can still be seen from Québec with a telescope, in late August and September evenings.

AU Mic is a young star, aged only about 24 million years, compared to our Sun's 4.6 billion years. We know AU Mic's age because it is part of the "β Pictoris moving group" a giant "stream" of stars that move together in space, and were born from the same molecular cloud. I talk more about these stellar groups in my first blog post—I spend a lot of my time studying them!

A visualization of all known stars in the β Pictoris moving group, compared with the position of the Sun, on which the green circles are centered. The green circles have radii between 10 and 100 light years. After approaching the regroupment of stars and moving around it, I stopped the camera and turned on time to view where the stars are moving towards in the upcoming few million years. AU Mic is part of this group of stars, born together but not bound to each other gravitationally. A higher-resolution version of this video is available here.

AU Mic is smaller than the Sun, with about 75% of its size, and half of its mass. We sometimes call such small stars red dwarfs, because they have a cooler temperature, and that gives them a redder color. They are also sometimes called M dwarfs or M stars in technical jargon, and they are popular stars for the search of exoplanets, for a few reasons.

First, they are very common. The physics inherent to the process of stellar formation causes about half of all stars to be born with masses smaller than half that of the Sun (i.e. M dwarfs). The fact that small stars are ubiquitous means that we can hope to find a larger number of exoplanets in total if we consider them in our searches.

Second, the fact that these stars are smaller, less massive, and dimmer makes them great for the detection of exoplanets. The transit method, whereby we wait for a planet to pass in front its star and make it dimmer,  gets us a better sensitivity to small planets when the star is physically smaller. This is true because it is the ratio between the size of the planet and its parent star that dictates how much dimmer the star becomes during a transit event.

This transit method is represented in the video below, where we see the brightness changes caused by planets of different sizes. When we use this method to detect exoplanets, we don't actually see the surface of the star, because it is too far away from us: all that we see is a tiny point of light, which brightness goes down slightly during the transit.

Representation of an exoplanet transit. Larger planets, or smaller stars, will cause a more easily detected change in brightness. Credit: NASA's Goddard Space Flight Center

It is not always possible to detect exoplanets with the transit method, because their orbit needs to be aligned correctly for us to see the planets pass in front of their stars. The transit method also has benefits, however: it allows us to detect exoplanets at very far distances from us. This is why NASA's Kepler mission was so prolific in discovering exoplanets. The Kepler space telescope stared at one patch of the sky for more than 3 years and discovered over 2,300 exoplanets, more than half of all confirmed exoplanets !

A visualization of all exoplanet systems discovered by the Kepler space telescope, over time, and compared with the Solar System planets. Orbital periods and the sizes of exoplanet orbits are to scale, but the sizes of exoplanets are not displayed to scale, otherwise Earth-sized planets would be too small to see them. The exoplanet colors are representative of their expected temperatures, as indicated by the legend. As you can see above, Kepler identified many exoplanets in tight orbit around their star with short orbital periods. This is just because they are easier to find as they transit more often, and it does not mean that all exoplanets have orbits tighter than the Solar system on average. Credit: NASA, Ethan Kruse, music by Kevin MacLeod

Another method often used to detect exoplanets is called the radial velocity method. When an exoplanet orbits around its star, it pulls on the star and causes the star to wobble around a little bit. We can observe that from Earth with our instruments, because the Doppler effect causes the light to become just slightly bluer when the star moves towards us, and slightly redder when it moves away from us. This is analogous to the sound Doppler effect, causing a change of pitch in the sound of a car that moves towards or away from us.

The radial velocity method is what allowed us to discover the first exoplanets around “normal” stars (some exoplanet candidates were found around pulsars a bit before that). Paul Butler, my office neighbor and fellow coffee nerd during my first postdoc at Carnegie Institution for Science in Washington, D.C., is one of the pioneers of the radial velocity method. He likes to recount how his team used a device called an iodine gas cell at the telescope to calibrate their instrument while searching for exoplanets back in the 1990s, and how they were wary that breaking the gas cell would be extremely dangerous.

An illustration of the radial velocity method to detect exoplanets. Credit: NASA, “5 Ways to Find a Planet”

Once again, smaller stars are good for exoplanet detection with the radial velocity method. The intensity of the star's wobble, and therefore the color shift of its light, depends on the ratio between the mass of the planet and that of its star. Therefore, using the same instruments and telescope, we will be able to detect planets of a smaller mass around stars of a smaller mass.

There is, however, one problem that muddies the benefits of exoplanet detection around small stars: small stars are often "hyperactive", which means that they often flare, have lots of stellar spots, and magnetic storms that cause large and random flashes of visible light, UV and X-ray at their surface. This really complicates the detection of exoplanets, because these random events "pollute" the signal of potential exoplanets, and make our data a lot noisier.

The younger stars are even worse in this regard: for reasons we will not get into this time, young stars tend to rotate faster, and that causes them to have stronger magnetic fields. Magnetic fields are what drive stellar spots, mass ejections and violent storms at their surface, so younger stars with stronger magnetic fields will be even more active. AU Mic is one of the worst possible case scenarios in this regard: it is both small and young. Peter Plavchan likes to call it "Speedy Mic" for this reason, because it is a particularly unstable star that likes to flare a lot.

A visualization of what the star AU Mic A might look like with its large clusters of stellar spots and magnetic storms causing bright flashes in visible light, as well as UV and X-ray. Credit: NASA’s Goddard Space Flight Center/Chris Smith, USRA

AU Mic is also really cool (and problematic) for yet another reason: it has a massive disk of debris around it. These debris are the result of planetary embryos, also called protoplanets, smashing into each other and getting blown to smithereens. This disk was discovered in 2005, and has been studied extensively since then. Researchers even found that clumps are moving away from the AU Mic at about 40,000 kilometers per hour in its debris disk! It is still unclear to this day what causes this.

Hubble and VLT images of the AU Mic disk. These images show the debris disk at distances up to about 60 astronomical units from the star (one astronomical unit is the Sun-Earth distance), and how its structure changes over time. Credits: NASA, ESA, ESO, and A. Boccaletti (Paris Observatory)

A study led by Cail Dailey using the Atacama Large Millimeter Array Observatory (ALMA) also looked at AU Mic's debris disk much closer in, and saw that some debris are being stirred, causing the disk to be thicker at separations from the star between 20 and 40 astronomical units. They think this is caused by either large planetary embryos, or a hidden exoplanet with less than about twice the mass of the Earth.

An image of the inner debris disk of AU Mic taken by ALMA. This image is zoomed in a lot more compared to the previous ones obtained with Hubble and VLT/SPHERE. Credit: Joint ALMA Observatory, ESO, AUI/NRAO, NAOJ, and Cail Daley (Van Vleck Observatory)

Now, this is a really cool star (in all senses of the word), but it is very hard to search for exoplanets around it because it is super active. We needed something to get past these problems of stellar activity if we hoped to open up the study of exoplanets around small stars in general with the radial velocity and transit methods. One strategy to do this is to stick with the radial velocity method, but look at the star's infrared light rather than its visible light. The signature of stellar activity does not impact infrared light as much as visible light, and the signature of an exoplanet is still apparent in the infrared. However, infrared detectors are a younger technology, making them a lot more expensive and less precise. It is also a lot harder to get a good calibration for these instruments, and this is required to precisely measure a star's color shift as it wobbles around.

This is where Peter Plavchan and his team come in: since about 2010, Peter started collaborating with NASA's Jet Propulsion Laboratory to develop such calibration devices. They built a device called a methane isotopologue gas cell, analogous to the iodine gas cell Paul Butler used, except that it is designed for infrared light (and it is also way less dangerous, thankfully). Peter's team also worked on developing a device even more precise, but way more expensive, called a near-infrared laser comb.

The methane gas cell we used at the Infrared Telescope Facility to calibrate our measurements. The gas cell itself is the transparent cylinder at the center of the photo; it is attached in front of our camera, and will therefore sit between the telescope and the camera. The light we receive from a star will pass through this transparent device, and get imprinted with the unique signature of the methane molecules contained in the gas cell, before making its way to the camera. These molecular fingerprints will serve as a calibration for our scientific measurements.

As soon as Peter got his hands on his methane isotopologue gas cell, he placed it on an instrument called CSHELL at the Infrared Telescope Facility (IRTF) in Hawaii. He began observing some stars near the Earth with his team, focusing on those which exoplanets were inaccessible to visible-light instruments: small stars, and in particular young ones. As you can imagine, AU Mic was one of the first stars they pointed the telescope at, back in 2010.

The IRTF telescope, on the Mauna Kea volcano in Hawaii. The observatory literally sits above the clouds. Credit: NASA, JPL

It's only in 2014 that I joined Peter Plavchan's efforts in this project. I was 4 years into my Ph.D., working on the detection of brown dwarfs in young groups of stars, like the β Pictoris moving group with René Doyon and David Lafrenière at Université de Montréal. I applied to an online contest for a 6-months scientific exchange with researchers at the Infrared Processing and Analysis Center (IPAC) at Caltech, and to my delight, I obtained an assignment with Peter Plavchan to work on the detection of exoplanets. So I moved to Pasadena, and excitingly started working with Peter and his team during that next summer. As part of this project, I went to IRTF a few times and observed many stars, AU Mic part of those, with CSHELL and Peter's methane isotopologue gas cell.

Apart from helping with the telescope observing, I spent much of my time in Pasadena writing computer codes to transform the raw data the CSHELL camera gave us into measurements we can use and interpret. This may sound surprising when you are used to taking pictures with a commercial camera, but images that come out from scientific cameras require a lot of work before we can hope to obtain a final product. In this particular case, the final product we obtain for every star, and every night spent at the telescope, is a single measurement of how fast the star is moving towards us, or away from us (this is what a radial velocity measurement is). Pasadena is also where I discovered Copa Vida's single-origin espresso shots, which began a whole other journey for me!

Peter Plavchan and I, staring at the Sun with the Keck telescopes in the background. This is the view we had when arriving at the IRTF telescope, located behind the camera. Photos by myself and Peter Plavchan

When I came back to Montreal after this delightful experience in Pasadena, the work was far from being done—research projects like this one routinely require decades of work, especially when new instrument technologies are involved. I kept my collaboration going with Peter Plavchan, and continued analyzing CSHELL data, and observing remotely with CSHELL, as one of my several ongoing projects. In 2016, I led the writing of a scientific paper that described our first results with this project. Our instruments were not yet precise enough to detect AU Mic b, but we ruled out the presence of a planet more massive than Jupiter in tight orbit around AU Mic. Peter Gao, also part of Peter Plavchan's team during my stay in Pasadena and now a close friend, led another scientific paper in 2016 where he described the algorithm he built to take in my calibrated CSHELL data, and transform it into a radial velocity measurement (yes, this science is so complicated that it required two computer codes, mine and Peter Gao's).

Shoveling snow was part of the job as we got caught in a snowstorm during one of our 2017 observing runs at the IRTF telescope (seen here in the photo). Photo by Peter Plavchan

By the time we published both our 2016 papers, a new and improved infrared camera came up at IRTF. This one, called iSHELL, is similar to CSHELL in what it does, except it is so much better, and in particular more precise. We immediately started using it, and soon after the team started gathering enough data, I flew to Missouri state where Peter Plavchan was now a faculty member at Missouri State University. We sat down with his new student Bryson Cale, and spent the week developing new computer codes for the iSHELL camera in a hack-a-thon style week. They both later came back to visit me in Washington, D.C. during my first postdoc, where we worked more on the computer codes.

Once we started gathering iSHELL data, it did not take long before Peter Plavchan noticed something weird about AU Mic. He emailed us in fall 2016 to tell us that he thought there might well be first signs of an exoplanet hiding in our data. However, this would have been a dangerous claim to make publicly, especially given that we were venturing in an unexplored territory with applying the infrared radial velocity method to an exceptionally active star. We needed to be absolutely certain that what we were seeing was an exoplanet, so we needed to keep observing it as often as possible.

A view of the IRTF telescope's primary mirror from inside the dome. Photo by Peter Plavchan

As we gathered more data, the signal became gradually clearer that there was an exoplanet similar to Neptune around AU Mic. But then, NASA launched the TESS space telescope, and it too observed AU Mic! The TESS mission stared at many stars close to the Sun, hoping to detect exoplanet transit events, like those that we described earlier. As is often the case with NASA missions, TESS data is available to everyone soon after it is observed, and it becomes a gold mine for any researcher to dig in. Obviously, Peter started looking for our new candidate exoplanet AU Mic b in the data that TESS gathered for the star AU Mic, and he found exactly what he was looking for: two transit events seemingly caused by a Neptune-sized exoplanet!

The transit signal also indicated that AU Mic b orbits its star every 8.5 days. This is an extremely short orbital period! Imagine if the Earth orbited around the Sun that fast: our year would only last 8.5 days. AU Mic b orbits so fast because it is very close to its star, at 0.07 astronomical units, only 7% of the distance between the Earth and the Sun. The radial velocity data we have in hand also indicates that the mass of AU Mic b is smaller than 3.4 times that of Neptune, and TESS data indicates that its size is almost exactly the same as Neptune. Interestingly, TESS data also shows hints of a possible second planet, at 0.13 astronomical units from the star (almost twice as far from its star as AU Mic b), and twice the size of the Earth. However, we will need more data to be sure, especially when working with a star as noisy and unstable as AU Mic.

This is a really cool discovery, not only because it opens the door to more research for exoplanets around young and small stars. The fact that AU Mic is so young means that we are catching the exoplanet AU Mic b in the first moments after it formed. Some researchers think it may have formed a bit further from its star, at about 10 astronomical units, and then migrated inwards. If that is true, it means that it must have migrated fast, because it already landed as its current location when the star is still only about 24 million years old.

A video based on a computer simulation, showing how a planet may form inside the disk of a star, causing spiral waves in the disk, and later migrate inwards closer to the star. Credit: Ximena S. Ramos and Pablo Benítez-Llambay

This discovery also gives us a complete picture of what a debris disk looks like in a system that just completed forming an exoplanet like AU Mic b. We know that exoplanets form inside disks around young stars, so we can now look at AU Mic's disk and wonder what imprints AU Mic b might have left on the disk. We can also hope to observe soon exactly how the intense UV and X-ray radiation of AU Mic, as well as its stellar wind, are affecting the atmosphere of AU Mic b. We think that these phenomena could cause the atmosphere to be eroded away. This is an important question because it will determine how hostile to life stellar activity may be for rocky exoplanets around small stars (we think AU Mic b is made of gas, like Neptune).

Another reason why this discovery is so cool is that AU Mic is really not that far from us: it is only 32 light years away, what we can safely call our galactic backyard. It is indeed one of the closest stars to us in the whole β Pictoris moving group, as can be seen below:

A visualization of all known stars in the β Pictoris moving group, compared with the position of the Sun, on which the green circles are centered. The green circles have radii between 10 and 100 light years. The camera moves around the β Pictoris moving group, and then I gradually increase the brightness of AU Mic to show where it is located: it is one of the β Pictoris moving group stars nearest to us. A higher-resolution version of this video is available here.

This system is so close to us that AU Mic b is one of the closest exoplanets we currently know! In fact, only 37 of the currently confirmed 4,171 planetary systems compiled on the NASA exoplanet archive are closer to us than AU Mic. But yet again, an image is worth a thousand words, and a video is worth a thousand images:

A visualization of the AU Mic system location compared with the position of the Sun, on which the green circles are centered. The green circles have radii between 10 and 100 light years. After a few seconds I turn on the locations of all 2,979 currently known and confirmed exoplanet systems for which we know the precise distance from the Sun (required to determine their 3D position). Notice how some of the exoplanet systems form square patterns in the sky when the camera is close to the Sun, at the seventh second of the video near the center of the frame: those are the shapes of the Kepler detectors, back when it always stared in the same direction during its main mission. When we move away from the Sun, we can better see how known exoplanets are located either relatively close to the Sun, or in the direction where Kepler stared. A higher-resolution version of this video is available here.

Peter started the long process of writing up and submitting a scientific paper to the journal Nature, which was finally published this June 24th. This discovery caused quite the media sensation, and even trended on CNN!

Now the science does not end here: Peter got busy teaming up with scientists around the world before publishing his Nature paper, to begin further investigations of this new planetary system. This led to five(!) additional scientific papers, all timed to also become public on June 24th. Four of these teams measured the obliquity of AU Mic b—the tilt between its rotation axis and the axis of its orbital motion, see this video for more explanations—and they all found that the planet's spin axis is aligned with its orbital axis. This is evidence that it indeed formed from AU Mic's protoplanetary disk, and likely did not suffer a cataclysmic collision with other planetary embryos.

There are still many unknowns about how planets form, so discovering young systems like AU Mic b is important to challenge our expectations and refine our understanding of this process. Each one of these discoveries provides us with a "photography" of what a given system may look like at a given age. Remember that one million years is very short at the astrophysical scale, so we cannot wait to see how AU Mic b will evolve, and we need to instead find another similar but slightly older system if we hope to constrain exactly what will happen next in the life of AU Mic b.

One of the five teams mentioned above, featuring other members of the Institute for Research on Exoplanets at Université de Montréal here in Québec, also measured the strength of the magnetic field of the star AU Mic, and how it varies with time. They used this to better understand how the star's activity affected the radial velocity measurements.

Another scientific team tried to measure the atmospheric composition of AU Mic b with a method called transit spectroscopy, but the star was so active that it prevented them from being able to measure a useful signal. NASA's upcoming James Webb Space Telescope (JWST) will probably be able to do this successfully.

Finally, another one of the scientific teams built computer simulations to better understand how AU Mic's stellar wind may affect the atmosphere of its exoplanet. They found that stronger winds will tend to compress the planet's atmosphere, and make it harder to measure its composition with our instruments. They also made predictions about what the transit would look like at a specific wavelength of light (i.e. a color of light). Measuring AU Mic b's planetary transit with an instrument designed to see this specific color only will shed more light on how strong the stellar winds of AU Mic are, and whether AU Mic b's atmosphere is getting eroded or not by the star's violent UV and X-ray emissions.

NASA got so excited about this discovery that, on top of all the nice visualizations they made for AU Mic and its planet, they also created this awesome poster:

A promotional poster of the exoplanet AU Mic b. Credit: NASA-JPL/Caltech

I'm super proud to have helped Peter and his team with this discovery, but I also want to emphasize how this is the result of such a long haul, sustained commitment on his part. Peter not only put together the technology to do this and taught several students how to use it along the way (including myself back in 2014), he also led the decades-long observations of AU Mic, and jumped through many hoops to publish this discovery, across many moves, position changes, and obviously, a world pandemic. I sure hope he will line his walls with that poster!

I would like to thank Peter Plavchan, Marc Jobin, André Grandchamps and Marie-Eve Naud for their comments.