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.
Exploring the Tiny Planetary System of Kepler-90
We recently posted news of a Planet Hunters planet discovered as part of a seven-planet system. Dubbed Kepler-90 this system is a peculiar microcosm of our own Solar System, with small (probably rocky) worlds in the middle, and larger (probably gaseous) worlds on the outside. The major different being that the outermost planet in this system is as far from the star as Earth is from the Sun. The other six planets in this system were already known about, but thanks to volunteers on Planet Hunters (http://planethunters.org) we now think that there are seven worlds circling this stars, which is just a little brighter than our Sun.
To celebrate this fact I have created a model of the whole planetary system in Celestia, an awesome, cross-platform, open-source package that lets you explore space. You can download the Celestia files model directly here or watch the video below to be taken on a tour of Kepler-90 and it’s seven worlds.
In this video, I’ve given the newly discovered Planet Hunters candidate some fetching green rings – which we do not have any evidence for or against. Also keep in mind that we know very little about what most exoplanets look like, so we’ve used artistic license to give them all different appearances, often using the surface of what might be analogue worlds in our Solar System. Maybe you can spot some familiar surfaces amongst them!
This system has some great features that make it interesting. The outermost world is roughly the the size of Jupiter but orbits at almost exactly the Earth-Sun distance of 1AU. A Jupiter-like world in an Earth-like orbit has been seen before in Planet Hunters discoveries. The middle planet in this system is at the same distance from this star as Mercury is from our Sun, but is six times as large. The rest of the planets whizz around in even smaller orbits. This star is a little hotter than our Sun so they are pretty scorching places with surfaces temperatures in the hundreds of degrees – nearly a thousand for the innermost planets.
The two innermost planets are roughly Earth sized and are really cool. The innermost one is 1.02x the diameter of Earth and the next is 1.18x. We assume that they are both rocky since they are so small. They orbit the star in just 7 days and 9 days respectively and are very close together. So close in fact that if you’re living on the inner, smaller planet then every few weeks, for about a week, the second planet appears in the sky about half the size of our full Moon.
Every year I see the rumour going round that Mars is going to be as big as the full moon. It will never happen for us – but on the tiny worlds circling Kepler-90, it happens all the time.
Update: The system used to be called KOI-351 but was given the name Kepler-90 just a day after this post went live. I have updated the name of the system in the text.
[Cross-posted on Orbiting Frog]
A mysterious object no more
This post is by Tabby Boyajian, one of the Planet Hunters science team at Yale
As you all know, planethunter volunteers use archive data taken with the Kepler space telescope to classify lightcurves and identify transiting planets. Since the launch of the Planethunters citizen science program, we have contributed five scientific publications reporting on the discovery of dozens of candidate and confirmed exoplanetary systems – otherwise undiscovered by the Kepler team.
The design of the project is expanding with the opportunity for Planethunter volunteers to support astronomers interested in using Kepler data for scientific research unrelated to the main exoplanet goals of the Kepler mission. We have dubbed this as our own ‘Guest Scientist’ program. The idea is that guest scientists participate in Planethunters Talk forum and make requests for the public to collect particular light curves, such as signatures of moons or rings, pulsators, variable stars, flare stars, cataclysmic variables, or microlensing events.
We are delighted to announce that the first paper presenting results associated with the Planethunters Guest Scientist program has been accepted for publication in the Astrophysical Journal! In this paper, the lead scientists Doug Gies and Zhao Guo from Georgia State University and Steve Howell and Martin Still from NASA AMES follow up on a mysterious object in the Kepler field identified by Planethunters, later confirming it to be an unusual type of cataclysmic variable. They perform an in-depth analysis on the Kepler lightcurve as well as observations made at the Kitt Peak National Observatory 4-m Mayall telescope and RC spectrograph. The result is a newly published paper, so take a momtent to read ‘KIC 9406652: An Unusual Cataclysmic Variable in the Kepler Field of View’ or to check out the planethunters talk thread where the object was first discovered and discussed:
Thanks you all for your enthusiasm and contributions to the scientific community. We have several other projects underway so keep an eye out for updates in the future!
Assessing the Kepler Inventory of Short Period Planets
You might remember that I’ve been working on a systematic search of the Q1 light curves to examine the frequencies of large planets (> 2 R⊕ -Earth radii) on orbits less than 15 days. I’m happy to announce that my paper titled “Planet Hunters: Assessing the Kepler Inventory of Short Period Planets” has just been accepted to Astrophysical Journal. The paper is available on-line here if you’d like to read it (warning: it’s quite long coming in at 22 pages of single spaced text, 13 figures, and 8 tables!), but I’ll give the highlights below.
We wanted to see for Q1 light curves, how well we could find planets and what might be left remaining there to be found compared to the known Kepler sample of planets. I think this important because Planet Hunters can serve as a separate estimate of the planet abundance and Kepler detection efficiency. I decided first to concentrate the search of planets with periods less than 15 days so that I was certain there would be at least two transits visible in the Q1 light curve. I thought it might be harder for us to identify transits if there was only one dip, so I thought it would be a good idea to start where there were at least transits.
To figure out which of the light curves had transits, I developed an algorithm to combine the multiple classifications for each light curve (for Q1 on average 10 people classified each ~33 day Kepler light curve) by developing a weighting scheme based on the majority vote. What the weights are doing is really just helping me pay a bit more attention to those that are a bit more sensitive at finding transits when combining the results from everyone who classified that light curve. The weighting scheme makes us more sensitive to transits than if I just took the majority vote for each light curve and helps to decrease the false positives. Below is the distribution of user weights for Q1 classifiers.
Using the user weights, I am able to give each light curve a ‘transit’ score (the sum of the user weights who marked a transit box divided by the sum of the user weights for everyone who classified the light curve). To narrow the list from 150,000 light curves, I picked those light curves that had ‘transit’ scores greater than 0.5 as my initial list of candidates. I applied several additional cuts to widdle down the list (you can read all about those details in the paper). That left about 3000 light curves and approximately 4000 simulations to go through. So to identify those that had at least two transits in them, we turned to a second round of review where light curves were presented in a separate interface and volunteers were asked whether they could see at least two transits (ignoring the depths being the same or not) in the light curve and asked to answer either asked to answer ‘yes’, ‘no’ or ‘maybe’ to the question. Those light curves where the majority of classifiers said ‘yes’ were moved on to review by the science team. A big thank you to everyone helped out with the Round 2 review; your efforts are acknowledged here. As always we acknowledge all those who contribute to Planet Hunters science on our authors page.
At the end of the search after removing all the known planet candidates and transit false positives known before February 2012, there were 7 light curves that have transit-like events but not on the original Kepler candidates list published back in 2011 that used only Quarters 1 and 2. I show example transits from each of these 7 light curves in the Figure below. One of these light curves turns out to be one of the candidates from our first paper and another one was part of our co-discoveries with the Kepler team. Even those these 7 light curves weren’t found in the first Kepler candidate releases, they now have been found in the latest iteration of the Kepler candidate list released earlier this year, where they’ve used an updated and improved versions of their detection and data validation pipelines. So what that shows is that the Kepler detection and validation processes has indeed gotten better, but there’s more that we can say.
Now that we know what new things we found, and that there wasn’t anything more than the 7 candidates that are now KOIs on the latest Kepler candidate list, we can look at what that says for the completeness of the short period planet inventory. Using the simulations that you’ve helped classify, I was able to look at how good Planet Hunters is at detecting planets of different sizes on orbits less than 15 days. I randomly selected about 7000 light curves that at the time weren’t known to have transiting planets or were not eclipsing binaries and inject synthetic transits into them for varying planet radii (ranging from 2- 15 R⊕) and periods less than 15 days. The simulations are really important because one completed I could see what which of the simulations made it to the end of my candiate pipeline and which ones didn’t. Having the results from those classifications really made the heart of the paper, because we could show independent of the Kepler planet candidates and detection and validation processes, what we were sensitive to.
What was striking to me, was our detection efficiency is basically independent of orbital period and that whether there were 2 or 15 transits in the light curve, they were just as easily identified. I think this bodes well for us being just as sensitive to single transit events (I’m starting to work on testing that now). Although performance drops rapidly for smaller radii, ≥ 4 R⊕ Planet Hunters is ≥ 85% efficient at identifying transit signals for planets with periods less than 15 days for the Kepler sample of target stars. For 2-3 R⊕ planets, the recovery rate for < 15 day orbits drops to 40%. I compared to the Kepler planet candidates and found similar results (which is a good check).
Our high recovery rate of both ≥4 R⊕ simulations and Kepler planet candidates and the lack of additional candidates not recovered by the improved Kepler detection and data validation routines and procedures suggests the Kepler inventory of ≥4 R⊕ short period planets is nearly complete!