By the end of September, the first science grade K2 observations from Campaign 0 should be made available to the astronomical community and the public. Stayed tuned to this space for updates on the data release and how we’re making Planet Hunters ready to accommodate the K2 observations. While we eagerly await the public release of the first full science grade data from K2, I’ve been thinking about how K2 serves as a stepping stone to TESS, which is expected to launch in 3 years from now.
Over its 2 year mission, TESS is going to monitor ~200,000 of the brightest stars across the sky for the signs of exoplanet transits by taking measurements of the stars’ brightness every 2 minutes. Most of these stars will be observed for only 27 days in total (though some patches of sky will be observed longer – see the expected sky coverage plot below) , but the worlds discovered around these bright stars, unlike most of the Kepler planet candidates and confirmed planets, will be able to be followed-up using ground-based techniques and technology as well as from the space-based James Webb Space Telescope (JWST). This will enable astronomers to probe the composition and structure of these planets’ atmospheres as well as their bulk composition.
One thing that I hadn’t appreciated from TESS was the engineering images it will take in addition to the 2 minute light curves. TESS will target a small number of bright stars at a 2 minute cadence, but every 30 minutes TESS will take the equivalent of a full frame engineering image across its roughly 2000 square degree field-of-view. These means we basically get the equivalent of Kepler observations but with blurrier vision (Kepler had pixels that covered 4 arcseconds per pixel. TESS’s are much larger covering 21 arcseconds) and 20x more area. Below is a simulation generated of what a subsection of one of these engineering images might look like from a presentation by TESS principal investigator George Ricker at NASA’s Exoplanet Exploration Program Analysis Group (ExoPAG) meeting back in January.
We know from Kepler that it is possible to detect a plethora of exoplanet transits with 30 minute observations, so there is an exciting prospect of mining the engineering images. With the science that has already been done with Kepler both in the field of exoplanets and other astrophysics, the TESS engineering images will no doubt be a treasure trove of data waiting to be tapped into.Before Kepler the only star that had been monitored to such precision and cadence was the Sun. Kepler has changed that, but TESS will take it to the next level. With the Kepler-like quality of the engineering data, it means that if you don’t like the stars the TESS team decided to target, anyone can do an exoplanet search on other stars in the TESS field among other searches and studies like looking for supernovae or cataclysmic variables. There is a wealth of science to be mined out of the TESS full frame images, and I think there is a potential for citizen science (and likely Planet Hunters) to play a role in utilizing these observations to their fullest.
If you’re interested in learning more about the TESS spacecraft , camera design, and mission goals you can check out this paper by the TESS Team which is where I got the information for this post.
Each day something new from across all the Zooniverse projects is featured on the Daily Zooniverse blog organized by the Zooniverse’s Grant Miller. Have you classified a weird light curve or participated in an interesting discussion on Talk? Now’s the chance to have that highlighted on the Daily Zooniverse. Grant and the Daily Zoonvierse team are looking for contributions from the volunteers of Zooniverse projects (including Planet Hunters) to feature. Just add the hashtag #dailyzoo to a light curve or discussion page on Talk to nominate it.
If you want to also share your nominations with the rest of the Planet Hunters community, there is a thread started on Talk where you can can list your finds for everyone to see (do make sure to include the hashtag). If you’re looking for inspiration Echo-lily-mai, one of our Planet Hunters Talk moderators, has nominated this folded light curve plot of a candidate heartbeat star made by volunteer Sean63 :
Today we have a guest post from Colette Salyk. Colette is the Leo Goldberg Postdoctoral Fellow at the National Optical Astronomy Observatory in Tucson, Arizona. She studies the evolution and chemistry of protoplanetary disks (the birthplace of planets) using a variety of ground and space-based telescopes.
Welcome to Part II of my three-part post about studying the chemistry in protoplanetary disks! (You can find Part I here.) In the last post I talked about techniques for detecting molecules. But once we detect them, what do we do with these detections? Ultimately, we want to make chemical “maps” of the protoplanetary disks, so we can understand what kinds of environments planets are forming in at different distances from their host star. In this post I’ll explain how we use the Doppler shift (yet again!) plus Kepler’s law to locate molecules in protoplanetary disks. (In Part III, I’ll discuss some of the connections between disk chemistry and the formation of planets.)
In the figure below, I’ve reproduced the observed water emission line that I discussed in my first post, but have converted wavelength to velocity using the Doppler shift equation, (λ−λ0)/λ0 = v/c , and centered the line at zero velocity. Note that in the first post, I focused on the shift of the entire line relative to the theoretical line center; here I am repositioning the line to account for this new center, and we’ll be discussing Doppler shifts relative to this new center.
Although molecules emit/absorb at very specific wavelengths, the water vapor emission line we observed is clearly not thin and pointy. Instead, it has a rounded shape — something we refer to as “line broadening.” This broadening occurs for all spectra, due to two reasons. One reason is that the instrument optics always blur out the signal somewhat — this is called the “instrument response function.” The other reason is that the molecules themselves are always moving, and the motion of each molecule produces a Doppler shift. Collectively, they produce emission at a range of wavelengths. In our case, the instrument response function (plotted in the figure) is much narrower than our line. Therefore, the broadening is dominated by the motion of the molecules.
The molecules are moving around due to a variety of reasons, including bouncing around due to their temperature, being kicked around by turbulence, and being in orbit around the star. The last effect dominates in our case, and I’m going to focus on that motion in this post. A simple example that may help you picture how orbital motion broadens the emission line is to consider a thin ring of molecules orbiting a star, oriented edge-on to our view. The molecules on one side of the star are moving away from us, and are redshifted; the molecules on the other side are moving towards us and are blueshifted. The amount of Doppler shift also depends on the orientation of the motion — as we examine parts of the ring that appear “closer” to the star from our point of view, we see progressively more transerve motion, and progressively less radial (and therefore Doppler shift-producing) motion. This collection of Doppler shifts turns a thin theoretical emission line into something broader, with symmetric blueshifted and redshifted components.
How fast are the molecules moving in the disk as they orbit their host star? If you’ve taken Astronomy 101, you’ve probably heard of Kepler’s laws — they are a set of relatively simple rules that dictate how the planets of the solar system orbit around the sun. Kepler’s third law relates the period (P) and semi-major axis (a) of planetary orbits, stating that P^2 ∝ a^3. Alternatively, astronomers often convert period to velocity (using v = 2πa/P), and put in the correct constants so that the law applies to stars of all masses (not just ones like the sun), to obtain: v = sqrt(GM⋆/a), where G is the gravitational constant and M⋆ is the mass of the star. This is a very powerful statement, because it means that we can directly relate velocity (v) to distance from the star (a). Since we can use the Doppler shift to measure velocity, we can therefore use the line broadening to measure the location of the molecules.
In contrast to the simple ring example I gave above, real emission lines originate from a range of disk radii, and the amount of light emitted at each radius also depends on the temperature and density of molecules. Also, the line width depends on how inclined the disk is with respect to our view. The figure below shows example emission lines originating from a disk where I’ve assumed the molecules are located between two radii, Rin and Rout, and that the disk is inclined by 30°. Have a look at the plots to see how the line shape depends on both Rin and Rout.
What I find especially cool about this technique is that it works especially well when the molecules are at small radii. For example, it’s really easy to tell the difference between molecules located at 0.1 AU vs. molecules located at 1 AU! It’s not currently possible to obtain this kind of detailed spatial information through imaging alone, and so we sometimes say that we’re achieving “super-resolution”. I think this is a neat parallel to the Kepler mission, in which the transit observations are used to obtain detailed information about the sizes and orbital radii of planets, even though we cannot directly image the planets.
Now some questions for you. Have a look at the detected water emission line in the first figure. Assuming this disk is inclined by 30°, as I assumed in my models, where do you think the molecules are located in this disk?
Latest Science Paper Accepted for Publication: The First Kepler Seven Planet Candidate System and 13 Other Planet Candidates from the Kepler Archival Data
Today we have a post from Joey Schmitt, a graduate student in the Astronomy department at Yale University, where he is working with the exoplanet group led by Debra Fischer, and in particular he has been working on the follow-up of Planet Hunters planet candidates.
We at Planet Hunters are happy to announce the acceptance of the PHVI paper to the Astronomical Journal, in which 14 new planet candidates were discovered. All of these new planet candidates are located far from their host stars. In fact, seven of them lie in their host star’s habitable zone. Unfortunately, all of these planets are too large to be Earth-like.
Two of the new planet candidates are in multiple candidate systems. One of them, the new candidate orbiting KOI-351, is the seventh planet candidate orbiting its host star. Planet Hunters actually detected three new candidates around this star when KOI-351 was only known to have three candidates, showing how great the Planet Hunters can be in discovering multiple planet systems. The planets in KOI-351 also show strong gravitational interactions between the planets, which helps to confirm them as true planets. The gravity from some planets in the system causes other planets to transit before or after what we would otherwise expect, called transit timing variations. In fact, the second-to-last planet transited a full day after we expected it would. Others in the exoplanet field have been working for over a year to determine the masses of these planets.
The new candidate in KOI-351 makes it the only star with seven known transiting planets. After our submission in October, two other teams claimed confirmation of the seven signals to various levels of certainty. Look forward to the brand new stars in the K2 campaign, changes to the Planet Hunters strategy, and new papers of the latest planets and candidates discovered by Planet Hunters.
You can read the revised accepted version of the paper here. The Planet Hunters volunteers who participated in identifying and analyzing the candidates presented in this paper are acknowledged at http://www.planethunters.org/PH6, and the contributions of the entire Planet Hunters community are individually acknowledged at http://www.planethunters.org/authors.
NASA has recently approved funding for the two-wheeled Kepler mission dubbed ‘K2.’ Field 0 was an engineering field that Kepler started monitoring before the senior review decision. The data will be science quality with Kepler monitoring about ~8000 sources, which includes open cluster M35. Observations started on March 8th and were recently completed on May 30th. You can see the proposals astronomers put in requesting targets for Kepler to monitor and the final selected target list here.
With the Senior Review decision and the funding, the K2 mission officially starts with observations of Campaign 1. On May 30th, Field 1 observations officially commenced and should last for roughly 75 days. You can find out which targets Kepler is observing in Field 1 here.
The engineering data of Field 0 should now have been downloaded to the ground and is likely undergoing processing at NASA. The preliminary data products should be ready hopefully sometime in August. With new stars there will be chances to find new undiscovered planets. The Planet Hunters team and Zooniverse team are working on ways to have the data ready and accessible on the website soon after it is released by NASA and the Kepler team to the astronomical community and the public. Stay tuned to the blog as we get closer to August.
Today we have a guest post from Stefano Meschiari. Stefano is a postdoctoral researcher at the University of Texas at Austin. He works on planet formation in binary environments and planet detection through radial velocities and transits. He developed the Systemic software as a tool for scientists and citizens to analyze radial velocity data, and he is also the creator of Super Planet Crash. His hobbies include cooking, clumsy puppeteering and all things pop culture. You can read more about his research here. Today he’s going to tell us a bit more about Super Planet Crash.
Since Newton’s law became the accepted model for gravitation, physicists, mathematicians and astronomers have been preoccupied with a simple question: is the Solar System stable? Planets weakly perturb each other through gravity, and over time, these perturbation can add up in an unpredictable (chaotic) way. This can even result in planets colliding or being expelled from the system. For exoplanetary systems, a common criterion to validate a set of orbital parameters is to check that the system is stable on scales of billions of years — if the system is unstable, we would not be observing it today!
Super Planet Crash is a simple online game that lets you design exoplanetary systems and drive them to instability. It is a digital orrery that evolves a planetary system according to Newton’s law of gravity in your browser. You can add up to 12 bodies of preset mass in initially circular orbits. Your score is calculated based on how close your system is to instability — more massive planets will yield more points, but your system may go unstable more quickly! It’s a delicate balancing act.
Design your exoplanetary system and watch it go BOOM at http://www.stefanom.org/spc.
Last August, I wrote about the end of Kepler’s original mission as it had been operating for the past 4 years. Kepler was launched in 2009 with a goal for providing a census for planets around Sun-like stars and helping us understand the frequencies of rocky planets. Kepler stared at the same field monitoring 160,000 stars nearly continuously for those 4 years. To achieve the precision pointing to obtain precise enough measurements to detect rocky terrestrial planets, Kepler had to point with extreme precision with the stars moving very little on the camera. To do this Kepler had three reaction wheels (and one spare) that would help nudge the spacecraft slightly one way or another. Last year, Kepler suffered a second reaction wheel failure that prevents it from continuing with its mission of monitoring the Kepler field. Pointing at the Kepler field, the spacecraft moves too much, and this effectively ended the Kepler mission as is. Kepler had taken its last observations of the Kepler field.
The Kepler team devised a new way of observing with Kepler using solar irradiation to help stabilize the spacecraft and act as the third reaction wheel. They set out to test it and prove this was a viable mission (which they dubbed ‘K2‘) that would return interesting science and discoveries worthy of NASA funding. Back in December, NASA gave the go ahead for K2 to compete with other viable missions in the Senior Review. Well, what is this Senior Review? Space missions cost money. You have to pay for the engineers that keep the spacecraft happy and running, pay the project managers and support staff and scientists, have funds if there are guest observer programs, as well as it costs money to use time on the Deep Space Network to send commands to and receive the data from your favorite telescope. The NASA Senior Review is NASA’s way of prioritizing and deciding which already existing missions will continue on and receive funding from the limited amount of funds available to spend while building and launching new spacecraft. Ben Montet from Astrobites has a nice summary description of the competing missions from this year’s Senior Review. Funding is tight and although these missions and spacecraft have all produced interesting science and capable of continuing to do that, not every mission that was on the chopping block is guaranteed to get money to pay for its operating costs. There simply isn’t enough to go around.
Officially today, NASA has announced the results from the Senior Review. You can read the full report from the panel here and the response from NASA. The verdict from the panel for Kepler/K2: “This is an outstanding mission and we look forward to the results from the program. K2 uniquely addresses a range of observational goals and is expected to engage a broad community of scientists.” K2 has been recommended by the review to continue with the extended K2 mission, and NASA has agreed to provide funding. The Kepler team didn’t get all the money they asked for, but 90% of the requested budget more than enough for the K2 mission to officially start science operations in June. K2 is a go! There will be new light curves from never before seen stars coming from Kepler over the next 2 years!
Congratulations to everyone involved in the Kepler project who made this happen. They put in lots of tireless effort to find a way to use Kepler in a novel observing scheme and prove that it could deliver interesting science worthy of continuing on. The Senior Review specifically about the science goals and case for K2: “K2 will allow exoplanet surveys of all stellar classes,O-M, giants-dwarfs, and white dwarfs as well as the asteroseismology of late stars, studies of nearby open clusters for the fundamental properties of pre-main sequence (PMS) and zero age main sequence (ZAMS) stars, and explore supernovae and accretion physics in AGNs. These are but a small sample of what can be achieved with the study of precise photometric long term continuous data“ .
This is exciting times for the study of extrasolar planets, as Kepler is now primed to deliver a whole new crop of planets and other astrophysical discoveries and results. The Planet Hunters science team and the Zooniverse are working on preparing the site to be able to ingest and serve the K2 data to you all in a fast and efficient way. Stay tuned to this space as we get closer to August when the first science grade K2 data is released.
You can learn more about the K2 mission at http://keplerscience.arc.nasa.gov/K2/
In the past two decades, exoplanet hunters have discovered almost 1800 planets beyond the Solar System, and there is more than twice that number of potential candidates still awaiting further confirmation. Of the known alien systems, astronomers have found a substantial number of planets travel around their parent stars in truly unusual orbits, unexplainable by any planetary formation mechanism.
The list of peculiar cases includes bodies that travel along completely different orbital planes to one another, worlds that take millennia to complete an orbit, and those that possess extreme comet-like eccentricities. Even more extreme are the rogue planets out there that orbit no star, presumably having been ejected from their solar systems altogether. However, the most inexplicable bodies are hot Jupiters, which orbit their parent stars in a matter of hours to days at a fraction of the distance that Mercury lies from the Sun. At such close proximity to the star, temperatures would simply be too high for a massive planet to retain its gaseous envelope during formation.
If these bodies cannot have formed at their current locations this may mean that planetary orbits are subject to dramatic change throughout the evolution of a system; meaning that where we observe a body now may not be where it formed, or where it will eventually end up. This reordering is referred to by scientists as planetary migration.
There are three ways in which planetary migration is understood to occur: the first describes a gas driven process in which the planetary disk effectively pushes or pulls the planet to a new position; the second arises as a result of gravitational interactions between neighbouring bodies, where a large object can scatter a smaller one and thereby create an equal and opposite resulting force back onto itself; and the third is due to another gravitational effect, tidal forces, which mainly occur between the star and the planet and tend to result in more circular orbits.
Surprising as it may seem to some, it is widely accepted that planetary migration has shaped and influenced the architecture of the Solar System quite dramatically. In fact, its dynamic past actually explains the existence and properties of several Solar System entities, and shows that our planetary system might not be as unique as once thought. So how have the planets moved since their birth?
It all began with the inward migration of the largest planet in the Solar System, Jupiter. The gas giant, weighing more than all the other planets combined, is believed to have travelled right up to the orbit of Mars, 1.5 AU from the Sun, before travelling back out to its present location almost four times as far. Luckily for Mars this occurred some 600 million years into the birth of the Solar System (around 4 billion years ago) before any of the terrestrial planets had formed and only four gas giants ruled the skies. At this time, Jupiter, Saturn, Uranus and Neptune possessed much more compact orbits and were surrounded by a dense disk of small icy objects.
Jupiter was drawn towards the Sun by the first type of planetary migration, gas driven, whose effects work differently depending on the mass of the planet. For low-mass planets, like the Earth, the mechanism occurs when the planet’s orbit perturbs the surrounding gas or planetesimal disk driving spiral density waves into it. An imbalance can occur between the strength of the interaction with the spirals inside and outside the planet’s orbit, causing the planet to gain or lose angular momentum. If angular momentum is lost the planet migrates inwards, and if it is gained it travels outwards. This is known as Type I migration and occurs on a short timescale relative to the lifetime of the accretion disk.
In the case of high mass planets, like Jupiter, their strong gravitational pull clears a sizeable gap in the disk which ends Type I migration and allows Type II to take over. Here the material enters the gap and in turn moves the planet and gap inwards over the accretion timescale of the disk. This migration mechanism is thought to explain why hot Jupiters are found in such close proximity to their stars in other planetary systems. The third type of gas driven migration is sometimes referred to as runaway migration, where large-scale vortices in the disk rapidly draw the planet in towards the star in a few tens of orbits.
The best understanding of how the planets have moved in throughout our system’s evolution arose from the Nice Model, proposed by an international collaboration of scientists in 2005. This model suggests that at the inner edge of the icy disk, some 35 AU from the Sun, the outermost planet began interacting with icy planetesimals, influencing the second sort of migration to occur: gravitational scattering. Comets were slingshotted from one planet to the next, which gradually caused Uranus, Neptune, Saturn and the belt to migrate outwards. Jupiter’s powerful gravity flung the icy objects that reached it into highly elliptical orbits or out of the Solar System entirely, which in order to conserve angular momentum, further propelled its journey inwards.
An extension to this theory is the ‘Grand Tack model‘, which is named after the unusual course of Jupiter’s migration towards the Sun before stopping and migrating outwards again, like a sailboat tacking about a buoy. At the distance that Mars would later coalesce, material had been swept away due to Jupiter’s presence. This resulted in the stunted growth of Mars and a material-rich region from which the Earth and Venus formed, explaining their respective sizes. The gas giant’s travels also prevented the rocky material in the asteroid belt from accreting into larger bodies due to its strong gravitational influence. Although Jupiter swapped positions with the asteroid belt twice the movements were so slow that collisions were minimal, resulting in more of a gentle displacement.
But why did Jupiter’s migration to the Sun’s fiery depths cease? For that it has Saturn to thank. As the two planets moved further away from each other, it was believed they became temporarily locked in a 2:1 orbital resonance. That meant that for every orbit of the Sun Saturn made, Jupiter made two. The Nice Model showed that the planetary coupling increased their orbital eccentricities and rapidly destabilised the entire system. Jupiter forced Saturn outwards, pushing Neptune and Uranus into extremely elliptical orbits where they gravitationally scattered the dense icy disk far into the inner and outer Solar System. This disruption in turn scattered almost the entire primordial disk. Some models also show Neptune to have been propelled past Uranus to become the farthest planet from the Sun as we now know it. Over time the orbits of the outermost planets settled back into the near circular paths we observe today.
The Nice Model explains the present day absence of a dense trans-Neptunian population and the positions of the Kuiper belt and Oort cloud. It also accounts for the mixture of icy and rocky objects in the asteroid belt, like water-rich dwarf planet, Ceres, which likely originated from the icy belt. The rapid scattering of icy objects, around 4 billion years ago, dates with the onset of the late heavy bombardment period, which is predominantly recorded from the Moon’s well-preserved surface.
However, there are problems with the original Nice Model, where some simulations found that the gradual 2:1 resonant coupling of Jupiter and Saturn would have resulted in an extremely unstable inner Solar System from which Mars would have been ejected. Later research has since resulted in the ‘Nice 2 Model‘, which in part suggests that the gradual scattering of planetesimals caused the two gas giants to fall into a 3:2 orbital resonance (not the originally proposed 2:1), allowing for the Nice Model to work with a stable inner Solar System.
The final mechanism for planetary migration occurs through tidal interactions between different celestial bodies. Unlike gas driven migration and gravitational scattering, tidal forces act over a much longer timescale of billions of years. The process begins due to the Kozai mechanism, which is suggested to pump eccentricity into a planet’s orbit. As the tidal forces correct this effect by re-circularising its orbit the planet moves closer in. Whilst the orbits of the terrestrial planets are thought to have remained fairly stable throughout the evolution of the Solar System, this gradual process is likely to have slightly altered their paths and will remain to do so.
The knowledge of how our own planetary system evolved has helped answer many questions about unusual exoplanet orbits, but there is still a lot left to uncover. One such question asks why we observe so many hot Jupiters unfathomably close to their star, as without another large body’s influence, should it not eventually be swallowed up? Perhaps planet-disk interactions decouple at such close proximities to the star and tidal forces prevail, or perhaps we are capturing a snapshot in time just before the planet meets its fate. For now only time, further observations and, most importantly, more exoplanet discoveries will tell!
Taichi Kato of Kyoto University and Yoji Osaki of the University of Tokyo recently published a paper on an unusual dwarf nova spotted by Planet Hunters’ volunteers that was contaminating the photometric aperture of a Kepler target star. A dwarf nova is a binary star system where one of the pair is a normal star and the other is a white dwarf. The objects orbit so closely that material from the star is falling onto the white dwarf with an accretion disk of material around the white dwarf. The light from the system is dominated by the accretion disk. Thus changes in brightness reflect the temperature and state of the accretion disk. This is the 2nd Planet Hunters dwarf nova/cataclysmic variable find to be published in the astronomical literature. Congratulations to the volunteers involved. The first Planet Hunters discovery paper was published in the Fall of last year, and you can read more about that object here.
Drs. Kato and Osaki found the discussions about this light curve from a volunteer curated blog that highlights interesting finds from Talk and the Talk thread about this interesting source . They went on to follow-up the find and further investigate the dwarf nova combining ground based, space-based telescope data, and the Kepler light curve. They found that this dwarf nova exhibited unusual features in the light curve (brightness of the accretion disk) for having a very short orbital period of the companion star.
Congratulations to all involved in this intriguing find. You can read about the study in detail with the preprint of the paper available here.
Today we have a guest post from Colette Salyk. Colette is the Leo Goldberg Postdoctoral Fellow at the National Optical Astronomy Observatory in Tucson, Arizona. She studies the evolution and chemistry of protoplanetary disks (the birthplace of planets) using a variety of ground and space-based telescopes.
One of the most interesting results to emerge from planet-hunting surveys is that planets and planetary systems are really diverse. I am trying to understand this diversity by studying the birthplace of planets – disks of gas and dust around young stars that we call “protoplanetary disks”. In particular, I study the chemistry in protoplanetary disks. In this post, I’m going to explain some of the techniques we use to detect and study molecules in protoplanetary disks using ground-based telescopes. In particular, I’m going to discuss the importance of the Doppler shift.
To detect molecules, we look for their unique spectral fingerprints. So we use spectrographs, usually on a large telescope like Keck or Gemini Observatory, or the Very Large Telescope. These observations require a lot of photons! But one challenge for these types of observations is that, if we’re observing simple molecules like water, carbon monoxide or methane, for example, these same molecules sit in the earth’s atmosphere and preferentially absorb the very photons we’re trying to detect.
This is where Doppler shifts come to the rescue. You may be familiar with Doppler shifts in the context of radial velocity searches for planets, in which the periodic Doppler shift in stellar absorption lines is produced by the gravitational pull of an orbiting planet. Recall that Doppler shifts are shifts in wavelength that are produced by relative motions between a source emitting photons, and an observer. If the source and observer are moving towards each other, the source looks blueshifted — its spectrum moves towards shorter wavelengths; if they are moving away from each other, the spectrum looks redshifted, like it has moved to longer wavelengths.
In our case, because both the protoplanetary disks and the earth are moving in space, the light emitted by molecules in the disk are seen at earth to be shifted in wavelength. Therefore, the wavelength of light we’re trying to detect is no longer exactly where molecules in our atmosphere want to absorb light.
The figure below shows an example of this. The red line shows the percent transmission of light through the earth’s atmosphere as a function of wavelength, as observed at the top of Mauna Kea. Note that at some wavelengths, the transmission is significantly less than 100%, meaning that the atmosphere absorbs a significant fraction of the light it receives from space. These regions are where water vapor molecules in the earth’s atmosphere are sucking up photons. In black is a spectrum of emission from a protoplanetary disk, obtained with the TEXES spectrograph on the Gemini North telescope. The peak in this spectrum was emitted by water vapor molecules in a protoplanetary disk. Note that it’s shifted relative to the sky absorption line due to the Doppler shift. In this case, the shift of the source line relative to the earth is consistent with a relative velocity of 18 km/s (∼40,000 miles/hour).
This Doppler shift wasn’t just obtained by chance. Because it’s the relative motion of the earth and the disk that determines the observed Doppler shift, this shift actually changes throughout the year, as the earth orbits around the sun. The diagram below is a schematic representing a top-down view of the earth’s orbit, with the location of the Earth at four hypothetical dates, as well as a possible location on the celestial sphere of a protoplanetary disk. Note that while the Earth orbits the sun at a nearly constant speed, the direction of its velocity (represented by the arrows) changes. So there are times of the year when the spectrum of this protoplanetary disk is shifted towards longer wavelengths, other times when it is shifted towards shorter wavelengths, and times when it is not shifted at all.
Assuming the geometry in this schematic, what time(s) of year might you expect the Doppler shift shown in the first figure? When do you think would be the ideal time(s) of year to plan observations of molecules in this disk? When would be the worst times of year?
Once we detect the molecules, what do we learn from them about planet formation? I’ll discuss this in more detail in a future post. But here’s some food for thought. The architecture of the solar system has a very clear division between terrestrial planets (at 1.5 AU and within) and giant planets (at 5 AU and beyond). What might have caused this dichotomy, and should we expect to see it in exo-planetary systems as well?