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The Road to Characterizing PH1: Transits and Initial Modeling

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:


and plotting them all together:

Yes, we had discovered transits and at that point decided it was worth turning Keck towards this system to further characterize it.

The Road to Characterizing PH1: The Overview

Haven Giguere/Yale

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.

The Road to Characterizing PH1: Visual-band Imaging

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.

SARA telescope at Kitt Peak

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.

Crowded Kepler field with the host stars for PH1

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.

PH1 : A planet in a four-star system

A family portrait of the PH1 planetary system: The newly discovered planet is depicted in this artist's rendition transiting the larger of the two eclipsing stars it orbits. Off in the distance, well beyond the planet orbit, resides a second pair of stars bound to the planetary system. Image Credit: Haven Giguere/Yale.

Image credit: Haven Giguere/Yale

Today we’re pleased to announce the discovery of the first confirmed planet discovered by Planet Hunters, and it’s a fabulous and unusual world. Labelled ‘Planet Hunters 1’ (or PH1) in a paper released today and submitted to the Astrophysical Journal, it is the first planet in a four-star system. It is a circumbinary planet – one which orbits a double star – and our follow-up observations indicate that there is a second pair of stars approximately 90 billion miles (1000 Astronomical Units) away which are gravitationally bound to the system.

This is much closer than the nearest stars are to the Sun, so anyone viewing the sky from PH1 would have a spectacular view of all four stars. More importantly, this amazing system will help us understand how and where planets can form – producing a stable planet in a system where four different stars are moving about can’t be easy. This is the seventh circumbinary planet, and the first to be in a quadruple system.

The planet itself has a radius a little more than 6 times that of Earth, making it a little bigger than Neptune. It’s mass is harder to pin down (and being in such a complicated system didn’t help), but we have a definite limit that means it must be no more than half that of Jupiter – so this is definitely a planet.

A huge amount of work went into this discovery (as well as a fair bit of observing time on the Keck and other telescopes), but a lot of the credit should be pointed at the Planet Hunters who made the discovery. It was Kian Jek and Robert Gagliano, working together on Talk that made the initial discovery; there’s a post from them on exactly what happened up already. The paper also credits Hans Martin Schwengeler, Dr. Johann Sejpka, and Arvin Joseff Tan all of whom flagged one or more of the transits before the paper was published! This is great news for us and we’re sure there are more planets hiding in data, both at the main interface and over on Talk. For today, though, we can celebrate the arrival of Planet Hunters 1!

Chris

PS We’ve announced discoveries before, of course – as well as being the first four-star planetary system, this is the first where we’ve been able to obtain not only transit information but follow up with radial velocity measurements, detecting the wobbles of the parent stars as well as the dips in light seen when the planet moves in front of them. This is the gold standard for planet discovery, and so this is officially a planet, not just a planet candidate.

PPS The paper, of course, still has to be refereed. We’ll keep you updated here as that process goes on, but as Meg is presenting the details of the system at the annual Division of Planetary Sciences meeting right now we thought you’d want to know the news as soon as possible. There will be more posts about exactly how PH1 was tracked down later in the week, so watch this space. In the meantime, you might prefer version of the paper, which has been annotated with the ScienceWISE tool in order to help explain some of the more technical language.

Planet Hunters’s First Circumbinary Planet- A True Team Effort

Today we have a guest post by Planet Hunters Robert Gagliano and Kian Jek, the discoverers of PH1, our first confirmed planet and first circumbinary planet.

Kian Jek found an anomalous dip in APH10421275 in May 2011 which turned out to be KIC 12644769 (Kepler-16b) the Kepler team’s first circumbinary planet discovery. He documented it on Talk in his thread “Strange transit in an EB”. He subsequently started a thread in the forums called “Finally-an EB with a planet?” Meg Schwamb then added a list of all known Kepler Eclipsing Binaries (EBs) with links to the light curves to this thread in November 2011.

Robert Gagliano did a systematic search of the ~ 1500 known Kepler EB’s, looking for possible planets in February 2012.  He initially spotted a possible transit in Q4.1 at day 244 in SPH10052872 and subsequently a possible 2nd matching transit at day 106 in Q2.3. Interestingly, the day 106 transit had been detected previously by JKD and commented on by Kian in the thread “Potential TERNARY System“. Robert also noted a possible 3rd transit in Q5.3 at day 379 but didn’t comment on it because it was distorted and he wasn’t sure whether this was a real 3rd transit. This Q5.3 transit was subsequently predicted by an seo company and officially confirmed by Kian.

Kian decided to check the Skyview image to be sure it wasn’t contaminated from other background stars and did an analysis to determine if the transit period, depth, and duration were consistent with a planet. He detrended the light curve with a modified smoothing filter that removed the EB eclipses, leaving the suspected planetary transits in place, and then folded the curve to confirm that the profile of the transits were similar in depth and duration. His analysis was confirmatory. Meg then assembled an outstanding science team of 10 professionals to conduct extensive follow-up observations and data analysis. Eureka! KIC 4862625 was Planet Hunters’s  “Tatooine”….we bagged our first circumbinary planet!

Mind the Gap

As I write this, I’m sitting on a train from London in the middle of the English countryside bound for Oxford. I’ll be spending the next week at the Zooniverse’s Oxford headquarters visiting Chris. I’ll  be working and thinking about all things exoplanets and Planet Hunters.

Close to this time last year I visited Oxford for a weeklong visit after the AAS Division of Planetary Sciences (DPS) Meeting in Nantes, France. Chris and I were working on finalizing and interpretting the first go through of the weighting scheme and Round 2 review and planning in the short term where Planet Hunters was heading. During that week, sitting in the Royal Oak (the pub where it all started in some sense – it’s the place where the idea for Galaxy Zoo was born), Chris and I, over a pint, outlined and planned what would become my short period planets paper. The project has made alot of progress since then, and we couldn’t do it without the contributions from all of you who make it possible with your classifications on the main site and efforts on Talk. Planet Hunters has 3 scientific papers now published or soon to be pubished in astronomical journals (Chris’s Quarter 2 planet candidates paper was recently accepted for publication in the Astronomical Journal last week).

There’s lot to do this week and plan for especially with Kepler’s extended mission and the start of Kepler data being released every 3 months once the Quarter is complete come November. (More on that to come in November/December as we get closer to the extended mission.) This week, I’ll be showing Chris some of the research I’ve been doing over the summer, and we’ll plan the next few papers we aim to write. I’ve been working on improving the scheme I developed for Quarter 1 to identify transits by combining your classifications, and I’ve started applying it to Quarters 3,4, and 5. This summer also included some follow-up work on a few of our planet candidates we’ve identified in the past 6 months, though the results aren’t quite finished yet. My collaborators and I are still working hard on that, and I’ll share the results once they’re ready and we’re confident in them.  I’ll be presenting the results from this work and what Chris and I get accomplished this week in Reno, Nevada at this year’s DPS meeting in October. My abstract was accepted and I’m scheduled to give a talk on the first day of the conference.

Turning a planet candidate into a bona fide planet

Today we have a guest post by Tom Barclay, Tom is a member of the Kepler team and also a collaborator and co-author on our second Planet Hunters paper. Tom  is a research scientist supporting the work of the Kepler mission. He got his Ph.D from University College London in the UK before moving to NASA Ames Research Center in California where he spends time improving the quality of the Kepler data products, finding new planet candidates and supporting the wider astrophysics community.

The Kepler team have found several thousand exoplanet candidates. The number of targets showing transit-like signals is increasing on a nearly daily basis as we search through light curves. However, these candidates are just that, candidates. Even though the planet candidates list is thought to have a high degree of fidelity, meaning that the vast majority of candidates are indeed real planets (somewhere in the region of 90%), it requires significant amounts of time and resources to turn a planet candidate into a planet.

I’ll start by being careful with my terminology. The Kepler team use two terms when deciding a candidate is a planet. Confirmation and validation. The former generally only used when we have spectroscopic radial velocity follow-up observations. These are measurements of the wobble induced on the star by the mass of the planet. The planet and star orbit a common point in space. When the planet is moving towards us the star moves away, and vice versa. When the star moves away it gets a little redder and when it moves towards us it get a little bluer. We measure these shifts and it tells us how fast the star is moving in along out line of sight.

Radial velocity measurements in combination with a transit give the planet’s mass and radius. A radial velocity detection of a planetary mass object (normally taken to be less than 13 Jupiter masses) is very unlikely to be erroneous and we are therefore happy to confirm the existence of a planet.

In order to measure a radial velocity a planet must be close enough and massive enough to have a measurable effect on the star. The best instruments currently available are sensitive to a periodic change in radial velocity of around 1 m/s and even getting this precision requires a bright star. The Earth causes a radial velocity pull on the Sun of around 10 cm/s, measuring with this precision is out of the question with currently available instruments. We therefore require another method to use another method if we want to turn small planet candidates into planet.

Validation of a planet

Validation of a planet applies when we use a statistical argument to say that it is much more likely that the transit signal is caused by a planet passing in front of the the target star (I’ll call it star A) that it is to be caused by something else.

There are 4 main ‘something else’, or false positive, scenarios we consider.

  1. A background eclipsing binary
  2. A background planetary system*
  3.  An eclipsing binary physically associated with the star A
  4.  A transiting star-planet system physically associated with star A (***There is some debate on whether a planet orbiting a star other than star A should really be considered a false positive. It is still a planet but it does contaminates our statistics on how many small planet are in the Galaxy.**)

A background eclipsing binary is a system of two stars that are appear fainter than star A, usually because they are far away (although they could be intrinsically faint stars which are, counter-intuitively, in the foreground between us and star A). The two fainter stars pass in front of one another much like a transiting planet does and cause a periodic dip in brightness. Because star A is much brighter than the eclipsing system, the eclipse depth appears to be much shallower than it really is and hence the eclipse looks similar to planet transiting star A.

A background planetary system is much the same as scenario (1) but the fainter system contains a star and a planet instead of two stars. If we think the transit is of a planet around the larger star A, we get the planet radius wrong. If we are not careful this scenario could cause us to claim a Jupiter-sized planet is Earth-sized.

Scenario (3) is what is known as a hierarchical triple. There are three stars in the system, star A and two lower mass stars which eclipse each other and orbit around the same center of mass as star A. This is more common than one would initially think guess. Around half of all stars are members of binary systems and in the region of 10% of these are triple or multiple star systems. The light from star A washes out the eclipse of the smaller stars and the eclipse looks much more shallow than it intrinsically is.

Finally, there is the case where a star-planet system orbits star A. The depth of the transit is decreased by the presence of extra light from star A and we get the planet radius wrong.

We try to obtain high resolution images using fancy techniques like adaptive optics imaging which changes the shape of one of the telescope’s mirrors to correct for the movement of the air in the atmosphere. These images allow us to see very close to the star and therefore look for other stars nearby in the image that could cause the transit-like signal. Typically if we don’t see star nearby star A we are able to say there are no stars further than 0.1 arcseconds away (0.00003 degrees) which could cause the transit-like signal. We are then able to make use of models of our Galaxy to predict the probability that there is a star in the right brightness range and within the allowed separation from star A that could mimic the transit signal. It is common for us to be able to say there is less than one in a million chance of a there being an allowed background star. When we take into account the probability that a background star is an eclipsing binary or hosts a planet the result is usually that it is very unlikely that there is a background eclipsing binary or star-planet system.

Ruling out a physically associated star-planet or eclipsing binary system can be much more challenging. We can again use the high resolution imaging but it is much more likely that a companion star is very close to star A than is the case for a background star. One thing on our side is that the shape of the transit can be used to rule out a stellar eclipse: eclipses are usually much more ‘V-shaped’ than the typically ‘U-shaped’ planet transit. We can often say that we cannot fit the shape we observe with a stellar binary. It is also possible to rule out planet transits around a smaller star because the timescale of the ingress and egress (the part of a transit where the planet is moving into and out of transit) does not agree with the transit depth as both these piece of information yield the planet radius. However, we really need good signal-to-noise in order to place firm constraints on the ingress and egress durations. Even so, it always gives us some information even if it is not particularly constraining and this can be used to calculate a false positive probability.

The final step is to sum up the combined false positive probabilities from the different scenarios and compare that to the probability that the transit signal is due to a planet transit around star A. If the transit scenario is much more likely (say 1000 times more likely) than a false positive we claim the planet is validated. On other occasions we have to hold our hands up and say we can’t rule out the false positive scenario with a high enough degree of confidence and the source of the signal remains a planet candidate.

The case where stars host multiple planet candidates, such as that found by the Planet Hunter in the paper by Chris Lintott, is a particularly interesting one. This is because the probability that the a multi-planet candidate system contains a false positive is much lower than for single planet candidates system, somewhere in the region of 50 times less likely. This makes validation much easier.

Planet Hunters have already shown they can find these multi-planet system. Keep searching a more will appear, especially long period ones. There is a good chance that there is an Earth-like planet hiding somewhere in the data currently available.

Transit of Venus: Live

In June 2012 people all over the world will watch the planet Venus transit across the Sun. Planet Hunters is all about spotting planets as they move across the face of a star so we thought it would be good to share the event with everyone. Venus will pass directly between the Earth and Sun on the night of June 5th and the morning of June 6th. This historic event can be seen from many parts of the world and will not happen again for 105 years!

As the map above shows, most people will only see part of the transit. With the help of the GLORIA team, we’ll be showing a live feed of the whole event on the Planet Hunters site. The webcast is being streamed from Tromsø, Sapporo and Cairns and will feature commentary in English and Spanish during the key parts of the event.

Check out our guide to the Transit of Venus, which we’ll update as we approach the event itself. It covers a basic history of the transits, and include information on when and where to see it. It also links to other useful resources for the event, including a Transit Guide from the GLORIA group, and the NASA observers handbook links. We hope you’ll try to see the transit when it happens, but if you’re unable to for some reason, then the webcast means that you can still be a part of this last-chance astronomical event.

Awesome People: More from ZooCon1

Today we have a guest post by Jules, fellow Planet Hunter and zooite who attended the ZooCon1. Jules is a lead moderator and blogger for the Solar Stormwatch and Moon Zoo forums as well as a volunteer on the Zooniverse Advisory Board.

Just back from the very first #zoocon1 in Chicago. I attended as a volunteer on the Zooniverse Advisory Board. As Meg said it was a chance for the science teams from new projects to meet with and learn from representatives of current projects and for everybody to meet up with Zooniverse techies and developers. It made sense then for some of the “old hands” to present an overview of their own projects. Meg’s Planet Hunters talk was particularly interesting as it highlighted the value of Talk and the great collaborative work being done there by volunteers.

A brief foray into data reduction showed the kind of work necessary to make the clicks usable. For example, there are 5,508 stars with possible transits. Removing all pulsating stars, which can be mistaken for transits, reduced the number of candidates to 3,404. Further examination of these transits reduced the pool further to 77 transit candidates – a much more manageable number.

Here’s Meg in action demonstrating the light curves of different sized planets.

The discoveries Meg highlighted included a slide showing 4 planet candidates missed by Kepler one of which is being re-investigated because of the work done by Planet Hunters. Kepler 16, the circumbinary system, also got a mention as did the impressive volunteer-led analysis on cataclysmic variables and heartbeat stars.

Old Weather, Mergers and the Milky Way Project were also put in the spotlight. Afterwards someone from one of the new projects told me how amazed they were that volunteers would want to do more than just click and another told me that they found the Planet Hunters story particularly inspiring and wanted to know how Planet Hunters had attracted these “awesome people.”

Well that’s Citizen Science for you. Volunteers come with a great mix of interests, skills and the knack of finding treasure!

Direct Imaging of Planets

HR8799 direct imaging planet detections Credit: Marois et al (2010)

Today we have a guest blog from Sasha Hinkley talking about a different way of detecting exoplanets than the transit method we use at Planet Hunters. Sasha is a Sagan Postdoctoral Fellow at the California Institute of Technology in Pasadena, CA. Sasha received his PhD from Columbia University in New York City and has been involved in the direct imaging of exoplanets for several years.

In recent years, astronomers have identified hundreds exoplanets (as well as over 2000 new candidates from the Kepler mission), launching the new and thriving field of exoplanetary science.  The vast majority of these objects have been discovered indirectly by observing the variations induced in their host star’s light.  The Doppler surveys detect stellar “wobbles” induced by the planets, and provide valuable information about the orbital separations, eccentricities, as well as lower limits on the masses of companion planets. At the same time, observations of planets that transit their host star, creating a brief dimming of the stellar light, can provide fundamental data on planet radii and even some coarse information about the compositions and atmospheres of these extremely hot planets.  However, studying those objects out of reach to the Doppler and transit methods will reveal completely new aspects of exoplanetary science in great detail.

The direct imaging of exoplanets, i.e. actually obtaining an image of exoplanets, is a technique that is sensitive to massive planets at much larger orbital distances—larger than even the orbital distance of our Neptune. This technique is already providing a completeley new and complementary set of parameters such as luminosity, as well as detailed spectroscopic information.  This spectroscopic information will provide clues to the planets’ atmospheric chemistry, compositions, and perhaps may even shed light on non-equilibrium chemistry associated with these objects.   Moreover, the direct imaging of these exoplanets will allow astronomers to more fully characterize the architecture of planetary systems, especially at young ages where the radial velocity methods are hampered by the instrinsic stellar “jitter” of the stars. Observing the placement of these planetary mass companions at very young ages serves as a “birth snapshot”, lending support to various planet formation models.

The major obstacle to the direct detection of planetary companions to nearby stars is the overwhelming brightness of the host star.  For example, if our solar system were viewed from 70 light years (average for a nearby star), Jupiter would appear roughly a billion  times fainter than our Sun with a separation on the sky comparable to the size of a dime viewed from 5 miles away. As such, these planets are completely lost in the glare of their host star. The key requirement is the suppression of the star’s overwhelming brightness through precise starlight control.

Astronomers are currently overcoming this incredibly challening task through precise starlight control, using sophisticated instruments and observing strategies at the largest ground-based telescopes.  So far, astronomers have successfully obtained direct images of a handful of exoplanets, including around the stars HR 8799, Fomalhaut,and Beta Pictoris.  These studies have demonstrated that direct imaging of exoplanets is now a mature technique and may become routine using ground-based observatories. More so, we will soon see a new fleet of instruments dedicated to detailed spectroscopic characterization of planetary mass companions making these kinds of discoveries routine, initiating an era of comparative exoplanetary science.

One such instrument, the Gemini Planet Imager (GPI), has been built by a consortium of American and Canadian institutions and will be deployed to the 8 meter Gemini South telescope in 2013.  This instrument will survey several hundred, nearby young stars achieving sensitivities that will allow it to image planets with masses a few times that of Jupiter, and  gather information on the detected exoplanets’ spectrum and any polarized light they may emit.  The Europoean counterpart to GPI is the SPHERE project  at the Very Large Telescope.  This project, also in the Southern Hemisphere, will be a similar dedicated exoplanet imaging instrument with similar science goals as GPI. A pre-cursor project called Project 1640, on the Palomar 5m telescope is currently testing some of the techniques to be used by these projects and hopes to image exoplanets in the Northern Hemisphere.  These instruments will likely obtain images of dozens of exoplanets in the next several years, and reveal completely new aspects of planetary science that we could not yet have imagined.

Image Credit: Marois et al (2010)