Hot Friends of Hot Jupiters: The WASP-47 system
Ever since a mechanical failure caused the end of the original Kepler mission in 2013, the Kepler spacecraft has been conducting a survey of new stars, searching for planets across the ecliptic plane in its new K2 mission (https://blog.planethunters.org/2014/12/12/more-about-the-k2-campaign-0/). The K2 dataset is a goldmine of fascinating science results. One such result is the recent discovery of two new planets in the WASP-47 system.
Until a few months ago, everyone knew that hot Jupiter planets don’t have “friends”, or nearby small planets in close orbits to the host star. These other planets had been searched for extensively, through radial velocity measurements, analysis of the transit times of the hot Jupiters, and even through transits by Kepler during its original mission. All of these searches turned up nothing.
This all changed one day last July, when Hans Martin Schwengeler, a Planet Hunter who enjoys poring over Kepler and K2 data searching for new transiting planets by eye, came across the telltale signatures of two extra transiting planets in the hot Jupiter system WASP-47. WASP 47b was, by all indications, a perfectly normal hot Jupiter — in the discovery paper, Coel Hellier wrote “With an orbital period of 4.16 days, a mass of 1.14 Jupiter masses, and a radius of 1.15 Jupiter radii, WASP-47b is an entirely typical hot Jupiter”. The discovery of additional transiting planets dramatically changed the narrative.
When Hans came across the planets, he posted them to the Planet Hunters forum, where he and other citizen scientists discuss their findings. Andrew Vanderburg came across the post suggesting that a known hot Jupiter had planetary companions. Using his K2 data reduction pipeline (https://blog.planethunters.org/2015/01/08/a-recipe-for-making-a-k2-light-curve/), he analyzed the light curve and confirmed Hans’s discovery – there were additional planets in the system, a super-Earth at a 0.8 day period and a Neptune at a 9 day period!
Andrew emailed me, and at first I hardly believed that the lightcurve was real. How could a hot Jupiter have close-in planetary companions? I knew people had been looking for this type of companion for years via both photometry and transit timing variations, but the lack of discoveries indicated that they might not exist. I performed some numerical stability simulations (because it seemed at first like this system could not be dynamically stable!) and sure enough, the N-body simulations showed that the system was likely stable on timescales of 10 million years.
At that point, we formed a team with Hans, Andrew, MIT Professor Saul Rappaport, University of Michigan Professor Fred Adams (my advisor!), and me. Once this team was formed, we devoted ourselves to understanding as much about the systems as we could. Some work by Saul and Andrew confirmed that the planets were all orbiting the same star, Andrew fit the lightcurve to get the planet properties, and I ran more stability simulations. Soon enough, Fred suggested that I look at what transit timing variations (or TTVs, which happen when transits come late or early because of the gravity of other planets in the system) we would theoretically expect to see from the system – and I found that for the outer two planets, the TTVs should be observable.
I then measured the TTVs from the lightcurve, and sure enough – there was something there. After some discussion, we realized we could measure the masses of the planets from those TTVs! Though I had never done dynamical fits before, I wrote the code to utilize Kat Deck’s TTVFAST code in a Markov Chain Monte Carlo fit. With some advice from Kat and help from Fred, I eventually got the fits working and we were able to measure or put limits on the masses of each planet.
In a little less than two weeks, we had put together a paper deriving planet properties from the lightcurve, mass limits from the TTVs, and showing that you CAN detect companions to hot Jupiters using TTVs!
This result is exciting because it is the very first time a hot Jupiter has been found to have such close-in other planets. Before this discovery, it was unclear if hot Jupiter could have nearby friends, as they might destabilize the friends’ orbits during migration. This discovery opens up new questions about how these systems form – it is possible that there is more than one migration mechanism for hot Jupiters.
The paper on WASP-47 and its new companions, which was published earlier this week in ApJ Letters and is available at http://arxiv.org/abs/1508.02411, was a collaboration between myself (Juliette Becker, a graduate student at the University of Michigan), graduate student Andrew Vanderburg (Harvard CfA), Professor Fred Adams (the University of Michigan), Professor Saul Rappaport (MIT), and Hans Schwengeler (a citizen scientist).
Comets or Aliens?
Let’s deal with the big question first. Has Planet Hunters discovered aliens?
The answer is no. But that doesn’t mean that all of the press who have written about us in the last 48 hours, sending a flood of volunteers to the site, are completely misguided. Let me backtrack…
A few weeks ago we submitted the ninth planet hunters paper to the journal, and that paper is now available on the arXiv service. Led by Tabetha Boyajian at Yale, it describes a rather unusual system (what the Atlantic called the most interesting star in the Galaxy), which was identified by Planet Hunters, four of whom (Daryll, Kian, Abe, Sam) are named on the paper*. They spotted a series of transits – which is normally what signifies the presence of a planet – but these were unusual.
The star’s light dimmed for a long period of time, loosing a fifth of its brightness for days or even months at a time. More mysteriously, the duration of the dips was not always the same, so this couldn’t possibly be a planet. This behaviour is unique amongst the more than a hundred thousand stars studied by Kepler – we have a bone fide mystery on our hands.I think the team’s immediate thoughts were that it must be the star itself that’s misbehaving, but stars aren’t known to behave like this and some careful follow up reveals it to be nothing more than a normal F-type star, slightly hotter and more massive than the Sun. So it’s not the star, and we’re sure too that it’s not Kepler itself misbehaving; something is really blocking the light from this star.One option is a disk of dust around the star. It’s from such disks that planets form (see DiskDetectives.org for more on this!) and so that wouldn’t be too surprising. Yet enough dust to cause the deep eclipses we see would glow brightly in the infrared, and there’s no sign of a strong infrared source around this star.
You can read the paper to find out what else we considered, but we think the best explanation is that there is a group of exocomets in orbit around the star. Comets are an appealing scenario to invoke because they would be faint in the infrared, and because they move on elliptical orbits, accounting for the random timing of the transits and their different lengths. Such a group of comets could have come from the breakup of a larger object, leaving a cloud of smaller remnants in similar orbits behind.
Much detailed work is needed to flesh out the details of this (pleasingly outlandish!) scenario. One possibility is that the recent passage of a nearby star triggered the cometary bombardment whose effects we’re seeing. The paper is currently in the peer review process and there is – of course – the possibility that there is a perfectly sensible solution we haven’t yet considered. However, so far over 100 professional scientists have had a look at the lightcurves and not managed to come up with a working solution.
One other proposed theory is that this pattern of behaviour is due to a fleet of alien spaceships in orbit around a star, a possibility considered by Jason Wright and collaborators here. Jason and co were tipped off about our discovery by the team, and it’s included in their paper as an object with ‘a bizarre light curve consistent with a “swarm” of megastructures’, much to the excitement of much of the internet. ‘Consistent with’ isn’t the same as ‘definitely is’, of course – and personally, my money is very firmly on the comet theory with a side bet on weird stellar behaviour – but until those models are properly investigated alien spaceships remain a possibility. The Wright paper points out this star is now a supremely interesting target for SETI (the search for extraterrestrial intelligence), and we agree – I hope radio astronomers will go and listen for signals. We need more observations of transits in action, too, and will be trying to follow-up to try and work out what’s actually going on.In the meantime, who knows what else is lurking in the Kepler data? Planet Hunters is about finding planets, but this ability to identify the weird and unusual is one of the project’s great advantages. Get clicking at www.planethunters.org, and let us know through Talk if you find anything a little odd.
* – This isn’t the final version of the paper, and we have more names to mention too before we’re done.
Extracted Lightcurves From Vanderburg & Johnson (2014) Now Available at MAST
A new MAST High Level Science Product from K2 has been delivered that includes extracted lightcurves. Courtesy of Vanderburg & Johnson (2014), long-cadence targets from Campaigns 0 and 1 now have detrended, extracted lightcurves available at MAST, including 20 different photometric apertures. There’s a MAST Classic Search Interface so you can get lightcurves based on target IDs, coordinates, EPIC catalog fluxes, etc. You can also use our interactive plotter to explore the lightcurves using any of the photometric apertures before downloading the FITS files. Check out all the details here: http://archive.stsci.edu/prepds/k2sff/
A Recipe for Making a K2 Light Curve
Today we have a post by Andrew Vanderburg. Andrew is a graduate student at Harvard University who works on producing and correcting K2 light curves and searching them for planets. He recently joined the Planet Hunters team to provide K2 light curves for classification.
As readers of this blog are probably well aware, the K2 mission is an exciting new opportunity for the Kepler spacecraft to continue searching for exoplanets, even after the failure of two reaction wheels ended the original Kepler mission. Making K2 work is in several ways more complicated than Kepler, and previous posts have already discussed how Kepler is stabilized by balancing against solar radiation and pointing itself opposite the sun in the ecliptic plane. Even with this very clever strategy for data collection, getting high quality data from K2 is not straightforward.
Once it became clear in early 2014 that Kepler would be able to continue gathering data, one of the biggest uncertainties about the K2 mission was: “How well can Kepler measure photometry in this new operating mode?” If Kepler’s worsened ability to point itself degrades the quality of its data, it may be harder for the K2 mission to accomplish its goals of finding exoplanets in new environments and around different types of stars. When the Kepler team released data from a 9 day engineering test of the new operation mode taken in February 2014, we attempted to answer that question.
After four years of being spoiled by ultra-high-quality photometry from Kepler, our first look at the K2 data came as a bit of a shock. Unlike the pristine Kepler data, K2 data (shown compared to Kepler in the first image) had wild jagged features contaminating the light curve, which made it hard to see all but the deepest planet transits. In order to continue searching for small planets in the K2 mission, something would have to be done to improve the quality of the photometry.
We started out by trying to figure out what was causing the jagged features in K2 data. Since nothing had changed with the spacecraft other than the reaction wheel failure, it was a pretty good bet that the jagged noise was due to the decreased stability of the spacecraft. We checked to see if this was the case by measuring the apparently position of stars in the images Kepler took (shown in the second image), and comparing them to the measured brightness.
The top panel shows the brightness of one particular star called EPIC 60021426 over the course of a week of the engineering test, and the bottom two panels show the horizontal and vertical position of the star, as seen by Kepler, over the same time period. It turns out that just like a cell phone video taken by a person with shaky hands, the images Kepler took were jittering back and forth. And more importantly, the jagged pattern in the location of the star in the image looked very much like the pattern seen in the brightness data.
We concluded that the additional noise in the data was caused by Kepler moving back and forth ever so slightly as it rolled due to a slight imbalance between the spacecraft and the Solar wind. Every six hours or so, Kepler’s thrusters fired to bring the telescope back to its original position. But the most important thing we concluded was that the additional noise in K2 data is very predictable. If the noise is predictable, then it’s correctable.
The third image shows the brightness of a particular star (once again, EPIC 60021426) measured by K2 on the vertical axis, and the position of the star on the horizontal axis. The blue dots indicate brightness measurements during normal K2 operations, and they form a tight relation with the image position. The jagged noise in K2 data depends only on where the image falls on the Kepler camera. With this realization, it’s simple to draw a line through the points (the orange line in the image), and divide it away. The red points are points taken while Kepler’s thrusters were firing, and don’t fit the pattern of the rest of the points. We simply throw them out.
After dividing the orange line from the data and removing points taken during thruster fires, we are left with a “corrected” light curve. The fourth image shows the result of the correction. The top light curve (blue) shows the raw, uncorrected K2 data, and the bottom, orange light curve shows the corrected K2 data. The correction substantially improves the quality of the K2 data.
This type of processing improves the precision of K2 to where it’s close to that of Kepler — within a factor of two for most stars. This makes it possible to detect small planets, even when the planetary signals are much smaller than the jagged variations removed by this process. We discovered the first K2 exoplanet, HIP 116454, using this exact technique, and the one transit we found was totally obscured in the raw K2 data (as shown in the fifth image).
Even now, however, this process is not perfect, and we’re still working to make it better. There are quite frequently glitches and other errors that affect the light curves and make it difficult for computer algorithms to pick out all of the transit signals. Trained human eyes like yours will be crucial for picking out all of the exciting exoplanets that K2 will observe.
More About the K2 Campaign 0
Now that were in the midst of the showing the first batch of science grade data from the K2 mission, I thought I’d give some more details about the K2 light curves and how K2 mission works.
Planet transits are small changes in the star’s light, a Jupiter-sized object produces only a 1% drop in the brightness of a Sun-like star and Earth-sized planets generate even smaller dips at the 0.01 % level. Kepler needs the stars to be precisely positioned on its imaging plane in order to achieve the photometric accuracy required to detect these drops in light. To do this the stars have been positioned and kept at the same location with millipixel precision. Kepler was able to achieve this during it’s primary mission and the first half of its extended mission To do this Kepler used three reaction wheels (one for x, y, and z directions) with one backup spare to finely nudge the spacecraft to keep the target stars positioned during a Quarter. Kepler suffered two reaction wheel failures and can no longer operate in this mode. This effectively ended the monitoring of the Kepler field, that Kepler was staring at for 4 years. The drift of the spacecraft was too large that the photometric precision was sufficient enough for a transiting planet search.
This is where K2 comes in. The K2 mission repurposes the Kepler spacecraft. Kepler has thrusters but they are used for coarser pointing corrections, they can’t be used to be the fine adjustments that used to be achieved with the third reaction wheel, but you can use the Sun in a way to be that reaction wheel. This is how K2 works. If Kepler is pointed observing fields that are along the plane of the Solar System, than the two working reaction wheels are used to maintain the x-y locations of the stars on the focal plane with the Sun and thrusters taking care of the rest. Kepler is positioned such that the irradiation of the Sun is balanced which basically keeps the spacecraft from rotating. This is a quasi-stable and every 6 hours or so the spacecraft will start to roll. The thrusters can then be used to roll the spacecraft back to it’s original orientation. (You can see this in the raw light curves just plotted. You can see a Nike check-like feature that dips slowly and rapidly goes up.The light curve processing Andrew does tries to remove as many of those artifacts and others as possible. It does a pretty good job, though occasionally there may be an artifact that remains. ) This scheme works pretty well at keeping the stars on Kepler’s focal view located on the same pixels and achieves photometric precision about 3x time worse than what the original Kepler mission was achieving. With this, we can still find planets around other stars, especially smaller cooler stars.
The K2 light curves we’re currently showing on Planet Hunters come from Campaign 0. Campaign 0 is the first full science grade test field data for the K2 mission. Kepler was staring at a field centered around see a region of the sky plotted in the star chart below. The observations commenced on March 12 and the campaign was completed on March 27th of this year. Campaign 0 serves as a full shake down of the performance of the spacecraft in this new mode of operating. The specific targets Kepler monitored in the Campaign 0 were community driven with astronomer putting proposals for what they wanted to be observed, and were decided by a Time Allocation Committee (TAC) organized by the Kepler team. You can learn more about the observing proposals and selected targets for Campaign 0 here.
On the site we’re only showing roughly 30 days worth of data, that’s because the light curves derived from the second half of Campaign 0 are more indicative of what the rest of the K2 mission will be like, so we’re only looking at that data. The observations at the start of the Campaign 0 were taken with Kepler not in fine pointing mode with a guide star and thus the positional consistency of the target stars over time on the imager is lower, causing a decrease in the photometric accuracy. Therefore we’re focusing on the better quality second half data. Future K2 Campaigns will have the full ~75 days worth of data in fine point mode, and we plan on showing all of the observations on the Planet Hunters website in the future.
The first science data from the new Kepler K2 mission is up on Planet Hunters just waiting to be looked at for new planets, eclipsing binaries, and whatever else lies in the data. This is a set of completely new stars! (Check out the K2 page for more information about the K2 mission.)
This data may go fast, so get classifying now! But don’t worry, there will be more K2 data when the next quarter is released. And when each K2 quarter is finished, keep classifying stars from the four-year Kepler mission to help solve one of the biggest mysteries in astronomy: how common are planets?
A Brand New Planet Hunters
On December 16, 2010, the Zooniverse launched Planet Hunters to enlist the public’s help to search for extrasolar planets (exoplanets) in the data from NASA’s Kepler spacecraft. Back then we didn’t know what we would find. It may have been the case that no new planets were discovered and that computers had the job down to a fine art. The project was a gamble on the ability of human pattern recognition to beat machines just occasionally and spot the telltale dip in a star’s brightness due to a transiting planet that was missed by automated routines looking for repeating patterns.
Nearly four years later, Planet Hunters has become a success beyond anyone’s expectation. To date 8 published scientific papers have resulted from the efforts of nearly 300,000 volunteers worldwide. Planet Hunters has discovered 9 planet candidate co-discoveries with the Kepler effort, over 30 unknown planet candidates not previously identified by the Kepler team, a confirmed transiting circumbinary planet in a quadruple star system (PH1b), a confirmed Jupiter-sized planet in the habitable zone of a Sun-like star (PH2b), and identified the 7th planet candidate of a 7 planet star system.
Today in collaboration with JPL’s PlanetQuest, the Planet Hunters science team and the Zooniverse are proud to announce the launch of Planet Hunters version 2.0. We’ve taken your feedback and the lessons learned over the past 3.5 years to build a fast new interface that we think will take the project to the next stage. Using the Zooniverse’s latest technology, Planet Hunters 2.0 is built specifically with the next generation of transiting exoplanet surveys in mind, including the new K2 mission, which repurposes the Kepler spacecraft.
Kepler had been monitoring ~170,000 stars for the signatures of transiting exoplanets over the past 4 years in the Kepler field located in the constellations of Cygnus and Lyra. The new-two wheel Kepler mission dubbed ‘K2‘ will have Kepler observing brand new sets of 10,000-20,000 stars every 75 days. These stars are different from the sources that Kepler had been monitoring in the past. Your eyes will be one of the first to gaze upon these observations. Most of the K2 target stars will have never before been searched for planets, providing a new opportunity to find distant worlds. K2 observations will be made available by NASA and the Kepler team to the entire astronomical community and the public shortly after being transmitted to Earth and processed. We aim to get them on Planet Hunters 2.0 as fast as we can.
We think that Planet Hunters 2.0 will play a key role for finding extrasolar planets in the age of K2, and we have built a site we think can deliver the best science and find interesting planets with your help. We aim for rapid identification and dissemination of planet candidates discovered by Planet Hunters in the K2 era. You’ll hear more about additional new features and tools built into Planet Hunters 2.0 for analyzing K2 light curves closer to the release of the first K2 engineering observations sometime this month.
We also know there is much interesting and valuable science left to do with the Kepler field data. Much of the four years of Kepler field data has not been searched by the original Planet Hunters, and there may very likely be planets lurking in the light curves missed by the computers waiting for you discover. The new Planet Hunters will start by focusing all 17 quarters of observations on a subset of the Kepler field stars starting with cool M dwarf stars, the most common star in the Galaxy. We’ll use the classifications from these select set of stars from the original Kepler mission as well the new K2 observations to study the variety of planetary systems and their frequencies.
You’ll hear more about the science goals of Planet Hunters 2.0 and new functionality, tools, and guides built into the website in the coming days and weeks. We’re excited about this new phase of the project, and we hope you are as well. We don’t know what we’ll find, but with your help, we can’t wait to find out! Whether you’re new to the project or a seasoned veteran, with the new and improved Planet Hunters you can search for planets around other stars like never before.
It’s just possible that you might be the first to know that a star somewhere out there in the Milky Way has a companion, just as our Sun does.
Fancy giving it’s a try?
Yesterday marks the start of a new era for the Kepler spacecraft with the public release of the first observations from K2, the two-wheeled Kepler mission.
After four years of staring at the same field and the failure of 2 reaction wheels on the Kepler spacecraft, Kepler is now observing ever changing fields on the ecliptic, plane of the Solar System, for periods of ~75 days. From March to May of this year, Kepler stared at the same patch of sky monitoring stars nearly continuously for planet transits, supernovae, among other reasons. You can find more details about Campaign 0 here and the K2 mission here. Now there’s a new set of stars never before looked at, that may be harboring unknown and undiscovered planets. The Planet Hunters science team and Zooniverse team are working hard to getting the K2 data prepared and ready for showing on the Planet Hunters website.
There are some new challenges to overcome in order to get the K2 data ready, but we’re working on making it possible in the near future to view K2 data Thanks to funding from JPL PlanetQuest, we’ve been able to rebuild the Planet Hunters website to make Planet Hunters 2.0. These past many months the Zooniverse development team and the science team have been working to make Planet Hunters 2 easier to use as well as faster and more efficient for searching for exoplanet transits in Kepler field data and especially with the K2 mission in mind. We’ve incorporated much of the feedback we’ve gotten from you over the past 3 years into the rebuild. The site is not quite ready from prime time, but will be very soon. Stay tuned to this space for more updates on Planet Hunters 2 and the K2 data. In the meantime if you have questions about the rebuild we’ll try to answer them on Talk here.
The Kepler in TESS
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.
K2 keeps on rolling – the start of Campaign 1 observations
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.