We are pleased to announce the discovery and confirmation of our second confirmed planet : PH2 b-a Jupiter-size planet in the habitable zone of a star like the Sun-by the Planet Hunter project. The paper has already been submitted to the Astrophysical Journal and has been made public via arxiv.org.
The estimated surface temperature of 46 degrees Celsius is right for there to be liquid water, but it is extremely unlikely that life exists on PH2 b because it is a gas planet like our Jupiter, and thus there is no solid surface or liquid environment for life to thrive. In order to study this interesting system, we used the HIRES seo services spectrograph and NIRC2 adaptive optics system on the Keck telescopes in Hawaii to obtain both high resolution spectrum and high spatial-resolution images. The observations help us to rule out possible scenarios for false positive detections and give us a measured confidence level of more than 99.9% that PH2 b is a bona-fide planet rather than just an illusion.
In the meantime, we also announce the discoveries of 31 long-period planet candidates with periods more than 100 days, including 15 candidates located in the habitable zones of their host stars. The candidate list is a joint effort between the volunteer Planet Hunters, and the science team. Each individual planet candidate was identified and then discussed on Talk by Planet Hunters. Several dedicated Planet Hunters collected information on candidates and carried out light curve modeling and initial vetting for false positives. The science team then decided the priority of each target on the candidate list and conducted follow-up observations.
Although most of these planets are large, like Neptune or Jupiter in our own Solar System, these discoveries increase the sample size of long-period planet candidates by more than 30% and almost double the number of known gas giant planet candidates in the habitable zone. In the future, we may find moons around these planet candidates (just like Pandora around Polyphemus in the movie Avatar) that allows life to survive and evolve under a habitable temperature.
In addition to the 31 long-period planet candidates, we announce a watch list for 9 further planet candidates which have only 2 transits observed. They do not currently meet the three-transit criteria of being a planet candidate set by the Kepler team. However, the Planet Hunters were able to pull them out and a future third transit would greatly increase the probability of them being real, allowing us to promote them into the full candidate list.
Lots of our candidates appear on a recent list published by the Kepler team (Tenenbaum et al. 2012) of possible transit signals, but it’s good to see they have now passed the additional tests to be planet candidates (not all of the Tenenbaum objects are real planet candidates; there are plenty of false positives). 6 candidates on our list were somehow missing in that list, all of which have periods of more than 240 day. This is an indication that we, the Planet Hunters, are effective in detecting long-period planet candidates. Heading into the future, we have reason to believe that more long-period planets and potentially habitable planets can be discovered by us. Go Planet Hunters, go hunting planets!
Ji is a post-doctoral associate in the department of Astronomy at the Yale University, and the lead author on the latest Planet Hunters paper. Before assuming his current position, he attended college at the University of Science and Technology of China and obtained his Ph.D. at the University of Florida. The roll of honour for planet hunters who contributed to these discoveries is here.
Today we have a guest post from Seth Redfield. Seth is an Assistant Professor at Wesleyan University in Middletown, CT. Before Wesleyan, he was a postdoc in Austin, TX, and a graduate student in Boulder, CO. He is an avid hiker, and an oboe player (with a degree from the New England Conservatory of Music in Boston), but these days spends any free time with his kids (4 and 1 years old) and sleeping.
To date, I have working on studying known exoplanets rather than finding them. Instead of a “planet hunter”, you could call me a “planet characterizer” (which doesn’t have quite the same ring to it). Perhaps the most well-studied planet, and one of my personal favorites, is HD189733b. This is because it is a transiting exoplanet, orbiting one of the brightest stars that we know has a planet. The fact that it transits, allows us to use spectroscopy of the starlight from HD189733 while the planet is transiting to look for wavelength-dependent effects that reveal interesting properties of the planet. For example, we can measure the composition, temperature, and even wind speeds in the atmosphere of the planet. The fact that HD189733b orbits a bright star, makes all these measurements “easier”, meaning that they are still incredibly difficult and require careful observations using the world’s largest and most sophisticated telescopes, but nonetheless are possible.
Because transiting planets are so useful, I follow with excitement all the searches of transiting planets, hoping they will find one around a bright, nearby star. However, the strategy for searching is directly at odds with finding one around a bright star. In order to find the rare planetary system that is edge-on, and therefore transits, one must observe many tens of thousands of stars. Bright stars tend to wash out large sections of our detectors and make it difficult to see the multitude of fainter stars around them. For this reason, all the searches largely avoid the bright stars.
Indeed, HD189733b was not discovered first by its transit, but by the radial velocity method of observing the host star orbit the center of mass of the system. It is for this reason, that I feel that the planets that will become household names, meaning the planets whose names will be known by school kids around the world, have yet to be discovered. These will be small, Earth-like planets, for which we can just barely detect using the radial velocity method, but which will also transit a bright host star and thereby make it possible for us to probe the characteristics of the planetary atmosphere.
So, as this young field of exoplanet research matures, I see this clear synergy between the detection of exoplanets and characterization of those exoplanets. Obviously, exoplanets must be detected in order to be characterized. The handful of exoplanet atmosphere detections to-date have uncovered a diverse collection of atmospheres that appear to be influenced by a myriad of planetary and stellar phenomena (such as planetary composition, stellar flares, etc). So, the characterization of exoplanets motivates us to find more exoplanets with new and extreme properties. I feel like we are at a similar point to astronomers 150 years ago when spectroscopic observations of stars were being made for the first time. Every discovered exoplanet is amazing, but it is likely that the planets we are talking about now will not be the planets we will be obsessing over in twenty years.
One final note, is that the brightest stars being observed by Kepler are almost as bright as HD189733, so let me take this opportunity to make a plug for searching the brightest stars in the Kepler field. Anything found to be transiting those stars will certainly be of interest to the “planet characterizers” out there.
Today we have a guest post from Nate Kaib. Nate is a postdoctoral researcher at Northwestern University. In terms of research, his main interests lie in using computers to model the orbital dynamics of exoplanets as well as the small bodies of the solar system. He’s written numerous papers on the evolution of comet orbits within our own solar system and how they can be used as a tool to constrain the solar system’s history. More recently Nate has gotten interested in the evolution of planetary systems residing in binary star systems.
In a select number of exoplanet systems, astronomers have successfully measured the inclinations between the stellar equators of parent stars and the orbital planes of the planets found around these stars. This is done by measuring something called the Rossiter-McLaughlin effect, which is a subtle change in the Doppler shift of a star’s light that occurs during a planetary transit (see wikipedia for a brief overview). Based on models of star and planetary system formation we would expect all of these inclinations to be near zero since the star and planetary system form out of the same spinning, collapsing gas cloud. Indeed, our Sun is only inclined about 7 degrees with respect to the Earth’s orbit.
Surprisingly, however, groups led by Amaury Triaud and Josh Winn have found that many planet orbits are highly inclined to the equators of their host stars. In fact, some are even retrograde, meaning the planet orbits the star in the opposite direction that the star spins. Explaining these results has presented a major problem for theorists, but several promising mechanisms have been proposed. Most of them assume that all planetary systems initially form with the star’s spin and planetary orbits aligned. However, later dynamical processes alter the planetary orbits. These include planets scattering off one another during orbital instabilities as well as Kozai resonances, which is where a distant massive perturber such as a giant planet or binary star excites both the inclination and eccentricity of an inner planet.
These previously proposed mechanisms imply that highly inclined planet have all gone through at least one major disruptive instability. This would then seem to preclude well-ordered multi-planet systems similar to our own solar system from having planetary orbits that are highly inclined with respect to their star’s equator. In a recent paper, however, we showed that an additional mechanism exists to alter planetary orbital inclinations, rigid body precession. Unlike the other mechanisms, this one will only act in well-ordered tightly packed systems of multiple planets (like our own). The other key ingredient is that there must also be a distant binary star in the system. In this configuration, the binary star’s gravity tugs slightly on the planetary system. These perturbations are very weak but add up over time. If only one planet is present in the system, then the Kozai resonance mentioned above will usually be activated. However, if more than one planet is present, then the self-gravity of the planets cause them to evolve in a coherent manner. They remain on nearly circular orbits, and their mutual inclinations all remain low. Instead, the plane of the entire planetary system begins to tip over relative to the star. (How far it tips over depends on the exact orbit of the binary star.)
As a proof-of-concept, we demonstrated in a recent paper that such rigid body precession is likely ongoing in one very well-known planetary system, 55 Cancri. This system consists of 5 planets on roughly circular orbits. In addition, there is a small star orbiting the entire system at 1000 AU. Because this star takes at least 10000 years to make one orbit, we do not know its true orbit. To characterize its potential effect, we therefore ran hundreds of computer simulations modelling its effects on the planets for many different plausible binary orbits. The final results indicate that the star causes this rigid body precession most of the time, and we found that the most likely angle between the planet orbits now and their original plane is a little over 60 degrees. Thus, in addition to flipping over planetary orbits by more violent processes, it may be possible to do it gently and wind up with planetary systems like our own that are highly inclined to their host star.
To better understand rigid body precession, below is a movie (click on the image to view the movie) made from one of our simulations. In this movie, the white orbits are the two outermost planets of 55 Cancri. (The binary star is not shown since it is 200 times further away.) The green arrow marks the initial orientation of the planetary orbits, while the red arrows marks the instantaneous orientation. This evolution shown in this particular movie takes place over 50 million years, but other simulations require billions of years for similar types of evolution to occur (depending on the exact binary orbit).
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. His hobbies include cooking, clumsy puppeteering and all things pop culture. You can read more about his research here
The discovery of PH1, together with all the other circumbinary planets identified by Kepler, provides theorists with an incredibly important clue on how planets form in binary environments. The clue is that these planets, rather than being mere sci-fi tropes, are actually detected in Nature! Their existance is all the more remarkable when we consider that all the planets detected so far (including PH1) orbit rather close to the central binary; so close, in fact, that they almost straddle the region of dynamical instability (inside which a planet would eventually be either ejected or collide with one of the stars). The fact that these planets successfully formed and survived to be observed by us indicates that the process of planet formation must either be very robust to the dynamical perturbations from the central binary (if the planet formed near its current location), or have happened in a less disturbed region (with subsequent migration to the current location).
Planet formation in tight binaries has long been the subject of a number of studies in the scientific literature; indeed, both our closest neighbor, Alpha Centauri, and the system hosting one of the first detected planets (with a tentative detection dating back to 1988!), Gamma Cephei, are examples of tight binaries where planet formation is significantly hindered. The discovery of the first circumbinary planet, Kepler-16, reignited interest in the topic, and I (together with a number of other teams) have been performing numerical simulations that model planet formation, in an attempt to better understand the physical processes at play.
Binary systems represent a stringent test for planet formation theories, since the environment can be highly disturbed by the presence of multiple stellar components. Computer simulations show that the planetesimal stage is the most vulnerable to disruption, becoming a “bottleneck”. This bottleneck is primarily caused by the interplay between the gravitational perturbations of the binary (which raise the eccentricities of planetesimals) and the aerodynamic drag from a protoplanetary gas disk (which acts to misalign the planetesimal orbits). The latter misalignment raises the velocities at which planetesimals collide, such that instead of growing into larger objects, they get destroyed. The reason why the alignment of the orbits is crucial is easy to understand: imagine the collision between two cars on two contiguous lanes, versus that of two cars colliding at an angle. The latter collision will be much more violent, as there will be an additional velocity component perpendicular to the motion of either cars. The end result is that, in the simplest models, planet formation can only proceed far enough from the central binary (in the case of Kepler-16, outside 4 AUs).
The implication is that circumbinary planets might only be able to form far enough from the central binary (how “far” depends on the properties of the central binary), and subsequently migrate to their current observed location. This picture, while attractive, might be too simplistic to capture the full complexity of planet formation in this environment. More realistic models will need to include a number of physical ingredients (such as the dynamical evolution of the gas disk), which are usually neglected for the sake of computational expediency but could potentially play a big role in determining how and where circumbinary planets were formed. For instance, my models indicate that even small perturbations from turbulence in the disk can make the disk hostile to planet formation by perturbing planetesimal alignment. Comprehensive (and computationally expensive!) simulations of planet formation in the binary environment are still ongoing: they might reveal further roadblocks, or uncover “sweet spots” in the range of physical parameters where planet formation can proceed. As the census of circumbinary planets continues to grow thanks to the efforts of scientists and volunteers, it will provide a larger sample of planets to inform and constrain our models.
Today we have a guest post from Julia Fang. Julia is an astronomer and grad student at UCLA working with Jean-Luc Margot. She works on the dynamics of multi-body systems, ranging from multi-satellite asteroids in the Solar System to extrasolar multi-planet systems. Recently she’s been using Kepler data to constrain the architecture of planetary systems. In her free time, she enjoys watching hockey, doing public outreach, and posting about planetary news on Twitter
By figuring out the architecture of planetary systems, such as the alignment or misalignment of planetary orbits, we can provide important constraints on planet formation and evolution models. For example, well-aligned planetary systems (like the Solar System) are consistent with a standard formation model of planets forming in a protoplanetary disk. Planetary systems with misaligned or inclined orbits can be indicative of past events that increased their inclinations. As a result, information on the alignment or misalignment of planetary systems can reveal clues to important planet formation and evolution processes.
Recently, Jean-Luc Margot and I used the latest planet candidate catalog released by the Kepler team to perform a statistical analysis of the inclinations of planetary systems. To do so, we created artificial planetary systems, simulated observations of them by the Kepler spacecraft, and compared their properties with the actual Kepler planet detections. Our best-fitting models showed that most inclinations of planets are less than 3 degrees — implying a high degree of coplanarity! Such alignment is also consistent with planetary orbits in the Solar System, with the exception of Mercury.
To put these inclinations into perspective, Jean-Luc made a batch of pancakes and (ignoring the first and last pancakes as outliers) measured an average thickness and an average radius of the pancakes. This corresponded to inclinations of six degrees. Crepes, on the other hand, were too thin. Consequently, the best mental image for the geometry of planetary systems (with inclinations less than 3 degrees) is somewhere between that of a crepe and that of a pancake.
Today we have a guest post from Willie Torres. Willie is an astronomer the Harvard-Smithsonian Center for Astrophysics (Cambridge, MA), and member of the Kepler team. His work for Kepler includes the statistical “validation” of transiting planet candidates that cannot be confirmed in the usual way, that is, by measuring their mass and showing that it is small enough to be a planet. He also works on determining fundamental parameters of stars in eclipsing or astrometric binaries, for testing models of stellar evolution.
Transiting circumbinary planets are interesting because they show us that planets can form in environments that are very different from our Solar System. Instead of being a single star, the central object is actually a pair of stars orbiting each other, and in these systems the planet can occasionally pass in front of one or even both stars, producing transit signals. For circumbinary planets such as those the Kepler Mission has announced (Kepler-16, Kepler-34, Kepler-35, Kepler-38, and most recently Kepler-47), the orbit of the two stars is such that they eclipse each other periodically, and these typically deep eclipses are what calls attention to them in the first place, in the light curves produced by Kepler. The neat thing about these transiting circumbinary systems is that they can also provide a wealth of information about the stars that is normally not available in regular transiting planet systems with a single host star. Two important stellar properties one can often measure are the masses and radii, from knowledge of the orbit of the stars around each other.
Masses and radii can of course also be determined in favorable eclipsing systems that don’t have planets, but when there is a transiting circumbinary planet, it’s even better. This is not hard to understand: as the planet passes in front of one or both stars, it is actually chasing a moving target because the two stars are revolving around each other. Each time the planet transits, the stars are in a different place in their orbit. This means that by measuring the precise times of these transits, we are actually mapping the orbit of the binary in a different way than would normally be done for a regular eclipsing binary. This provides extra information about the motion of the stars, and in particular it constrains the ratio of the masses between the two stars very well. It also helps to determine their sizes. Combining this with additional observations such as radial velocities measured from spectra of one or both stars, their masses and radii can be measured to high precision.
Astronomers care about the masses and radii of stars because these measurements allow them to test their models of how stars form and evolve. Theorists have come up with a fairly detailed prescription for how a star of a given mass and chemical composition changes its properties (radius, temperature, luminosity, etc.) as time goes by. But without real observations against which to check those predictions, we can’t be sure they’re right. This is important because astronomers often use those same models to infer properties of single stars that are much more difficult to measure directly. Or they may be interested in knowing the age of a star, which also relies on theoretical models. As it turns out, observations have shown that models for low mass stars (such as the cool M dwarfs) are not quite right: real stars tend to be a little bit larger and cooler than the models predict.
Circumbinary planets in which the eclipsing binary at the center contains an M dwarf are particularly interesting, because they allow us to test theory in this problematic low-mass regime. That happens to be the case for the recent exciting Planet Hunters discovery of KIC 4862625. The primary component in the eclipsing binary is an F star of about 1.3 solar masses, and the secondary is an mid M dwarf a little under 0.4 solar masses. They orbit each other every 20 days. The circumbinary planet goes around every 138 days. With other colleagues I’ve been working on determining the stellar properties of both stars as accurately as possible, and comparing them with several sets of stellar evolution models (since models are not all created equal). For getting the stellar properties we use not only high-quality spectra taken with the 10-meter Keck telescope in Hawaii, but also results from a very sophisticated modeling of the Kepler light curve that can reproduce all the binary eclipses as well as the transits of the circumbinary planet nearly perfectly. This tells us that we at least understand the dynamics of the system pretty well (i.e., how all the objects move).
But there are always complications. In this case, we took a high-resolution image of the system and discovered that there’s another star right next to eclipsing binary that (we realize now) is introducing contamination in the Kepler light curve. It’s only about 0.7 arcsec away from the eclipsing binary, and we believe it is physically associated. But wait, there’s more! The images show that this companion is actually a close binary itself! At the time of this writing we are still trying to figure out exactly how much extra light these new objects are contributing to the Kepler photometry, so that we can take that into account in order not to bias the measured properties of the eclipsing binary stars, or of the circumbinary planet.
Today we have a blog post from Dirk Terrell. Dirk is an astrophysicist and the manager of the Astronomy and Computer Systems section at the Southwest Research Institute in Boulder, CO. His hobbies include coaching football, racing motorcycles and space art. He is a Fellow and former President of the International Association of Astronomical Artists.
The discovery of PH1, a planet orbiting an eclipsing binary with another pair of stars about 1000 AU away, was exciting for everyone involved. As someone who has worked with amateur astronomers for over twenty years, I was particularly happy to see that the initial discovery of the transits was made by two amateur scientists, Kian Jek and Robert Gagliano. My involvement came shortly thereafter when Meg Schwamb was visiting my institution , Southwest Research Institute in Boulder, Colorado, last spring to give a talk on another topic. She was meeting with Hal Levision, a dynamicist here at SwRI, and was describing the rather complicated Kepler data on the system. That’s when Hal gave me a call.
I get calls from Hal all the time, almost always on some computer-related topic. But this time he said “Do you have time to talk science?” My area of expertise is eclipsing binary stars, and here at SwRI I am surrounded mostly by scientists who do research on topics in planetary science, so I don’t often get phone calls from people here to talk about science. Curious, I walked down the hall to Hal’s office.
When I walked in, he introduced me to Meg and she began to describe this eclipsing binary system that had been observed by the Kepler satellite. She pulled up the light curve on the Planet Hunters web site. Having been doing the analysis of eclipsing binary light curves for nearly a quarter century, I immediately began to develop an idea of what the system looked like. It had an eccentric orbit and the stars were reasonably small compared to their separation, so they would not be very distorted by each other’s gravity. It was also clear that there were regular variations in the light curve outside the eclipses of the stars, probably pulsations or spots on one of the stars. It was a very busy looking light curve! Then she showed me the dips that Kian and Robert had noticed. With all of the activity in the light curve, it wasn’t quite as obvious as it is in simpler systems that we had transits by a planet. She asked me if I could help clear things up enough to warrant turning very expensive big telescopes like Keck on the system to get complementary data. I told her I’d be happy to look at it.
When working with data, I like to start as closely as possible to what the instruments produce and then do what’s needed to get the data into the form needed for an analysis. That way I know exactly what’s been done. Kepler data, like those from any instrument, have various effects that have nothing to do with the object you are observing. For example, the spacecraft has to make periodic maneuvers to keep its solar panels properly aligned. These rolls show up clearly in the data because the stars being observed are in different places on the imaging detectors before and after the rolls. Kepler, as you probably know, measures the brightness of stars very, very precisely with CCD arrays, the same device inside your digital camera. The pixels that make up the CCDs are not all uniformly sensitive, so when a star is moved from one set of pixels to another, the response to the same brightness level from the star will be a little different. But even a relatively small 1% difference in sensitivity will show up as a big jump with detectors as precise as those on Kepler. To give you an idea of what these raw data look like, here is a plot of the raw Kepler data for PH1 from quarter four:
You can see five deep eclipses of the hotter star, the higher frequency pulsations or spot modulation, as well as large instrumental effects and gaps. And, as we later showed, there is indeed a transit by the planet in there too, but it certainly doesn’t jump out at you because of everything else that is going on. So, my job was to clean up the light curve so that we could isolate the potential transit signatures. While we will later be very much interested in the stellar eclipses and the spots/pulsations, at this stage of the game we just needed to get rid of them. My approach was pretty simple: break the light curve up into parts to which I could fit a combination of linear and periodic terms, and then subtract those fits to get the residuals (i.e. what was left). Since spot/pulsation modulations happened at a much higher frequency (period of about 2.6 days respectively) than the suspected transits (period of about 138 days), removing the high frequency terms would leave the transits and the stellar eclipses (20 day period) intact and the linear term would remove most of the instrumental effects. Then I could fit the stellar eclipses with the Wilson-Devinney light curve program and remove the eclipse signatures leaving only the transits. This approach worked well enough to answer our big question at the time: are there transits? Lurking in the residuals were things like this:
Today we have a blog post from Dirk Terrell. Dirk is an astrophysicist and the manager of the Astronomy and Computer Systems section at the Southwest Research Institute in Boulder, CO. His hobbies include coaching football, racing motorcycles and space art. He is a Fellow and former President of the International Association of Astronomical Artists.
Dirk is also a coauthor on the PH1 paper, and you’ll hear more about how he helped confirm and validate PH1 in the next blog post. Inspired by the discovery Dirk made paintings depicting PH1, and we wanted to share with all of you. These images and more can be found on his website. Dirk is kindly letting us share his artwork. Please contact him for all usage, private or commercial, at terrell at boulder.swri.edu.
It’s been an exciting week for exoplanets with the discovery of PH1 and the discovery of an Earth-mass planet around the closest star to Earth (Alpa Centauri B). In the coming days and weeks, you’ll hear more about the specifics of how we came out confirming and validating the discovery of PH1 and solidifying that it was a planet orbiting the two central stars in a four star system, but I wanted to give a brief summary of the data and results.
This effort has taken months and months from obtaining the telescope observations, to modeling the light curves, combining Kepler data with radial velocity observations, and applying stellar evolution models. Robert Gagliano and Kian Jek get enormous credit for the discovery and starting this process off and recognizing and spotting the planet transits.
Many collaborators have worked hard to confirm PH1 and study its properties, especially: Jerry Orosz, Josh Carter, Willie Torres, and Debra Fischer who have put tremendous effort (particularly in the past few weeks) to get us from so we found some transits to bona fide planet.
Everyone in this collaboration involved in the paper (including Kian and Robert who were coauthors on the discovery paper) are listed below:
To summarize PH1’s confirmation story, I thought I’d share my press talk slides:
Robert and Kian identified three transits in the light curve of an eclipsing binary . In the binary, there are two stars, one larger and hotter and one smaller and cooler. When the smaller cooler star crosses the face of the larger hotter star, you get some of the larger star’s blocked out and we call that a primary eclipse. When the smaller cooler star crosses behind the larger hotter star, we get a secondary eclipse where the light from the smaller cooler star is blocked out. So we see this big dip+ small dip pattern. Robert and Kian noticed three transits separated by ~137 days in Quarters 1-5 data superimposed on the light curve indicating a third small body in the system, and notified the science team of a possible circumbinary planet.
There’s a small chance that we’re seeing a false positive, where on the sky our Kepler eclipsing binary is aligned with a faint background eclipsing binary giving rise to the transit-like signal. If the transiting body is truly orbiting both stars, we have a way of checking. A body in a circumbinary orbit (orbiting around both stars in a stellar binary), is orbiting a moving target, so the positions and velocities of the two stars are different each transit. So this means that at each transit, there are slightly different gravitational tugs on the planet causing the timing and the duration of the transit to change. We see these changes and this gives us high confidence the planet is really orbiting the Kepler eclipsing binary and not some background source. If you look below at the 7 planet transits across the larger bigger star in the binary, you can convince yourself that the widths of the transits are changing. So bingo, planet!
We got radial velocity observations from the Keck telescopes on Mauna Kea. The radial velocity observations measure the wobble of the larger star in the binary that the planet orbits. With the precision of the observations and time duration we have on the observations, we cannot measure the wobble caused by the gravitational pull of the planet. What we are measuring is the wobble due to the gravitational tug of the smaller star as it orbits the larger star in the binary.
To our surprise we found two velocity contributions in the radial velocity observations. One is from the larger star in the eclipsing binary (solid points) with the model fit for the velocity observations shown in red. The 2nd component is stationary over the roughly 5 months we were taking radial velocity observations. This 2nd component is coming from a source that is providing some light in the spectra slit we place across the Kepler target when observing on Keck. It has the same value as the average or systemic velocity of the binary. So if you take the average value of the black points that’s the velocity of the eclipsing binary (and planet host stars) moving towards or away form us. This second component has the same velocity and random field stars have velocities in the galaxy ranging from ~20-60 km/s so to have a source that has to be very close to the eclipsing binary on the sky that we see it in the Keck observations and have the same average velocity as the eclipsing binary tells us that this source is bound to the eclipsing binary.
We used adaptive optics observations from the Keck II telescope to zoom in and look around our eclipsing binary for other stars that would be contamination the Kepler photometric aperture. We also used deep optical imaging to look for slightly further contamination stars as Bill Keel described in his blog post. As Bill discussed in his post, if there are stars providing extra light to the aperture that is summed up to make the Kepler light curve, it will give us the wrong planet radius. This is because the extra light will decrease the observed transit dips causing us to underestimate the size of the planet. We go these adaptive optics observations while we getting the radial velocity observations.
In the adaptive optics observations there is a source 0.7” away (or about 1000 AU) from the eclipsing binary, and we knew about its existence when we were analyzing the Keck radial velocity observations. There is was the ah-ha moment, where we went oh, this source in the adaptive optics observations must be that second velocity source we see in the radial velocity observations, that we think is gravitationally bound to the eclipsing binary orbiting well outside the planet’s orbit. To our surprise the adaptive optics observations revealed that this source is elongated in one direction (which you can see in the slide below). What we think this means is that we’re just barely resolving the source as a visual binary (2 stars!). So that we have a pair of stars, getting 2 for the price of 1 – (getting us to four stars in the system!) orbiting the eclipsing binary. Our best guess from the Keck observations is that the 2 stars in the distant binary are separated by no more than 40-50 AU.
Combining all of these observations, we went after obtaining the properties of the planet and the stars in the PH1 system. Here below are the properties that come out of the combined photometric-dynamical model that uses the radial velocity observations and Kepler light curve for both PH1 and the stars,. The age we estimate for the entire system from spectral modeling. We estimate the mass of the planet by the fact that it not massive enough to pull on its parents stars sufficiently for us to see slight changes in the timings of when the stars eclipse each other (so when the smaller cooler star crosses the face of the larger hotter star and when the smaller cooler star crosses behind the larger hotter star).
We confirmed with modeling the eclipsing binary properties with just the Kepler light curve, and we’re confident in our estimation for the planet and host star properties. The planet is a gas giant, a bit bigger than Neptune and slightly smaller than Saturn. PH1 orbits its parent stars at a distance between Mercury and Venus in our own Solar System. The planetary system is stable. The planet happily orbits the eclipsing binary every ~138 days not really noticing there’s a second pair of stars out at 1000 AU .
PH1 is the 7th circumbinary planet, and the 6th circumbinary system. Below are the orbits of all the circumbianry Kepler planets and PH1 (not depicting the second distant pair of stars).So why do we care? Well, circumbinary planets are the extremes of planet formation, and we need to understand how they form if we are really understand how we form planets in our own solar system. Planet formation models need to be able to explain both environments, and each of the systems detected gives us another puzzle piece to this picture.
Today we have a guest post from Bill Keel. Bill is a member of the science team for Galaxy Zoo, and is more accustomed to dealing with stars by the billion than one at a time. He is a University of Alabama astronomer, weekend trombonist, and occasional photographer, being gradually trained by two cats with names out of Tolkien. Both his Twitter stream and his posts on the Galaxy Zoo forum can be found under the name NGC3314, and his other professional exploits may be found at http://astronomy.ua.edu/keel.
Kepler is sometimes most effective when properly backed up by other instruments, since its design was tightly optimized for precision in measuring bright stars at the expense of other things (such as angular resolution). Here’s a case showing how interpretation of Kepler results on planetary transits can be assisted by fairly routine ground-based measurements.In late June, I got an email request from Meg Schwamb:
“We’ve found a planet with ~130 days orbit going around a eclipsing binary. The eclipsing binary has a 20 day orbit so the planet is circumbinary and there’s a third star in the binary+planet system orbiting out at ~1000 AU with a period of 10E4-10E5 years. We’ve been following up the system with Keck observations.” [We didn’t yet know at the time that this third star would itself turn out to be a binary star].
The region around this star from our perspective is very busy (like the whole Kepler field), and the Kepler measurement includes light from additional faint stars. One, in particular, appears about 3 arcseconds away from the star of interest, well within the 6-acsecond radius of a Kepler measurement. Knowing its brightness would help narrow down the planet’s properties, making sure we have the right starting points in brightness for the Kepler target star my itself.
My institution is a partner in the SARA consortium, which operates telescopes in Arizona and Chile remotely. As a result, I have fairly regular nights scheduled, and indeed there were a couple of nights I could use at our northern telescope, a 0.9m instrument on Kitt Peak, Arizona, in July (just before shutdown for the monsoon season). After a couple of tries when the weather didn’t quite cooperate, including one night that was clear but the air to turbulent for this project, I got an hour’s worth of images on the evening of July 17. The image quality (seeing, in astronomical jargon) was 1.5-1.8 arcseconds, meaning that these values give the diameter across which a stars image drops to half its peak intensity due to atmospheric turbulence. That makes separating stars 3 arcseconds apart tractable. The timing worked out well during the night – the field was within 15 degrees of the zenith, minimizing atmospheric and tracking problems.
Trying to get precision measurements of bright and faint stars simultaneously takes some care – good data on the faint star isn’t much help if the bright star is hopelessly saturated in the data. So instead of one long exposure, I took 60 1-minute observations, using a red filter to roughly match the midpoint of the very broad spectral band used by Kepler. For further analysis, that gave both the grand average of all 60, and I also used averages of subsets of 10 to help estimate certain sources of error in the processing.
Even though the fainter interfering star was clearly separated from the bright one in these images, there was enough spillover to need correction.I tried several procedures or this – the most successful took as a reference point a similarly bright star with no companion in that direction, subtracting variously scaled versions of its image to eliminate as much of the bright star’s light as possible (the subtracted images looked a little odd in the middle – much later I realized that might come from the very close companion star seen in other data).
To make sure we understood how our brightness measurements relate to the Kepler data, I checked published magnitudes for Kepler stars in this neighborhood. This gave me some bad moments until I realized that the published values were often based on short exposures with a telescope no bigger than I was using – bright stars are OK, faint stars become quickly much less accurate. Phew. Now I know this, so that if it comes up again, I’m ready.
The result? That fainter star has magnitude R=18.73, making it only 1.02% as bright as the Kepler target with planet. Other contaminating stars are still fainter, down to 0.03% of the target star’s red-light intensity.