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Gaps in the Data

I wanted to give a brief update on the gap question and talk a little more about what causes those gaps in the data.

You might have noticed that the gap question is no more. All of the lightcurve sections from Quarter 2 have breaks of varying sizes in them which was not the case for the Q1 data, so we removed the gap question from the interface yesterday. The gaps are caused by a few different things: Kepler went into safe mode and wasn’t taking data, the spacecraft was rotating towards the Earth, the spacecraft has executing a roll (or quarterly roll as its called) to reorient its solar panels, or the data is bad either due to a cosmic ray hit or something else.

The spacecraft rolls and safe mode tend to make of the majority of the data breaks. Kepler must rotate towards Earth to send its science data on timescales of approximately 30 days. During those monthly data downlinks Kepler must point away from the field and point its antenna towards the Earth to send the 150,000 lightcurves of data collected to the science operations center via NASA’s Deep Space Network. Every few months, the spacecraft must also reposition its solar panels toward the Sun and point Kepler’s radiator into deep space with a quarter turn, which causes an additional gap of about 1 day in the lightcurves. The reason we don’t see any gaps in the Q1 data (about 35 days) is because it encompasses one downlink of data, but since Q2 is 90 days there is both the quarterly and month rolls.

I’m off to Kitt Peak for an observing run to observe a transit in our own solar system. Dwarf planet Huamea’s moon (Nemaka) is passing in front of Haumea Friday night and I’ll be attempting to observe the drop in light caused by Nemaka on the WIYN telescope (3.5 m) while my collaborators will be observing the event from the Hale Telescope (200 inch) at Palomar Observatory.

Happy Hunting,

~Meg

PS. I also wanted to say thank you for everyone’s patience and understanding while we’ve sorted out the Q2 data upload and the Talk links.They should hopefully be done late tonight early tomorrow

A word from the Kepler Team

Dear Planet Hunters: Dr. Natalie Batalha, Deputy Science Team Lead for the Kepler Mission, asked us to post the following message:

Welcome! We are so glad you’re here!

I’m sure I speak for the entire Kepler team when I say how happy we are that Zooniverse is being applied to the Kepler data. For some time now, I’ve watched the public actively work with archived data from other missions. The folks at Unmanned Spaceflight, for example, regularly share the latest images they’ve doctored up from Solar System missions like MER and Cassini. And the SOHO mission recently hit a milestone, discovering its 2000th comet on December 26th, 2010. The discoverer was not part of any formal SOHO science team but rather an astronomy student at Jagiellonian University in Krakow, Poland. I’ve added “Citizen Scientist” to my urban dictionary and appreciate its tremendous potential.

That’s all well and fine when it comes to Martian landscapes, comets, and sunlight glinting off the surface of methane lakes millions of miles away. But how in the world could we entice the public to look at boring old lightcurves? PlanetHunters.org has done exactly that. Not only are thousands of people looking at light curves, they are getting just as hooked on their variety as we are! Welcome to the ranks of those who love light curves.

The Kepler spacecraft is a new piece of technology. Never before have humans stared at stars with such unwavering precision and patience. And whenever humanity does something new, there are sure to be surprises. One of the biggest surprises to me so far is the impact that Kepler is having on stellar astrophysics. Who knew, for example, that a star like RR Lyrae — one of the brightest and well-studied objects in the sky — would blow the dust off textbooks written on this class of star? Who knew we’d see such a symphony of variability occurring just below the noise levels typical of ground-based telescopes?

But the name of the game here is planet hunting. I’ve heard people wonder why they should bother to hunt for planets when the Kepler team has spent years designing savvy computer algorithms to do exactly that — algorithms that can tease signals out of the noise that the human eyes cannot even see. The answer is simple.

Kepler relies, in large part, on automation. We are a relatively small team. There are currently less than 15 scientists working in the Kepler Science Office here at Ames. In the early days, there were only 5 of us! Let’s say we divided up the 150,000 stars we are monitoring amongst the 15 scientists at Ames. We’d each be responsible for 10,000 stars. If we spent only 60 seconds looking at each star, it’d take us over 160 hours to finish out allotment. That’s a solid month of doing nothing else but looking at light curves. Just in time since more data comes down from the spacecraft each month and the process would have to start all over again. Such a plan would never have earned taxpayer dollars. We need our scientists doing other things — like monitoring the instrument and optimizing the software and vetting out the false positives and interpreting the results. And so we write computer software that combs through the data searching for transit-like features.

It’s a challenge to design a one-size-fits-all approach to transit detection. The transit are buried in the light curves of stars with widely different properties and behaviors. You’d build one kind of tool for finding a needle in a haystack but a different kind of tool for finding a needle in a swamp. We don’t even yet know what all the possibilities are because we’ve never looked at stars with this kind of precision.

Another consideration is that the software pipeline requires 3 transits for complete modeling and pipeline generation of they key statistics that are used to vet out the false positives — astrophysical signals masquerading as planet transits. It’s certainly true that we’ve gone back and cherry-picked some of the more compelling light curves displaying less than 3 transits — especially those of the brightest stars. However, many such signals are still lurking in the archive.

So what else did our algorithms miss? Ah, let’s find out, shall we? We’re here with you, ready to help. Come stand here in the crow’s nest and experience the thrill of discovery with us. We welcome your keen eyes!

A huge thank you to the folks at planethunters.org for putting this together.

Natalie Batalha

Deputy Science Team Lead

Kepler Mission

Transits (examples)

Solar_system_scale-2

The Kepler team recently announced the detection of five stars, each with multiple transiting planets (Steffen et al, 2010). The left Figure below shows the Quarter 1 (Q1) light curve for the star SPH10102031 (Kepler ID 10723750) with two transit dips from two different planets. The transits do not repeat because the orbital periods are longer than the time baseline. The first transit dip is from a planet that is about the size of Jupiter. To highlight the typical boxy shape of a planet transit curve, we have zoomed in on the second transit event in the Figure below and on the right. The depth of the transit is about 0.25% and given the assumed radius of the star, the planet radius is about 7.6 times the radius of the Earth (larger than Neptune, but smaller than Jupiter).

SPH10102031 SPH10102031b

Light curves for two other stars in the Steffen et al 2010 paper are shown below.  The Figure on the left is the Q1 light curve for SPH10120491 (Kepler ID 8394721).  This light curve contains transit dips from three different planets! One of these is very obvious and is caused by a planet that is 6.5 times the radius of Earth that only transits once during the 35-day light curve.  However, there are two other transiting planets that are harder to see with radii of just a few times that of the Earth.  One of these planets transits every 13.5 days and the other transits every 27.4 days.

SPH10120491 SPH10017624

The Figure above and to the right shows the light curve for SPH10017624 (Kepler 5972334).  There are three transit dips from a Jupiter-sized planet that orbits every 15.4 days.  In this Figure, it is virtually impossible to see the second planet, which has a radius just twice that of the Earth and transits every 2.4 days.

To get a better look and some practice (you won’t be able to save these), pull up these amazing light curves and use the zoom tool to identify the transits.

SPH10102031

SPH10120491

SPH10017624

Variable stars (examples)

The reasons for changes in the brightness of a star can be divided into two categories: (1) orbiting companions or (2) stellar astrophysics.

(1) In principle, the variability from orbiting companions (this includes eclipsing binaries or transiting planets) should be as regular as clockwork.  In practice, the variability can deviate from clockwork regularity if stellar binaries get too close together, if there are multiple transiting planets, if there is additional background electronic noise or astrophysical noise.

(2) Brightness variations caused by physical processes internal to the star (stellar astrophysics) can arise from pulsations of the star, starspots or flares. Flares are random spikes in the light curve brightness.  Pulsations from stars (like RR Lyraes) are quasi-periodic: they can appear to be regular for a while and the cycles are relatively short (generally hours to a day or so). The Figure below shows two variable stars with short periods that might be best classified as “variable” and “pulsating.” These could be short period binary systems – this could quickly be verified with follow-up observations.

puls1 puls2

Starspots produce complex variations. As the star spins, the spots rotate in and out of view with a   periodicity of a day or two (for the most rapidly spinning stars) to several days for slowly rotating stars (the Sun has a rotation period of 25 days).  Starspots can form at different latitudes on the star.  Since some latitudes rotate faster, spots can show multi-cyclical variations. The light curves below might be best classified as variable and irregular.  However, a case could be made for classifying the light curve in the figure below (and left) as variable and regular. Even though the amplitude of the curves changes, the time from one peak to the next is about the same.

irreg1

irreg2

Eclipsing binaries vs transits

Observing at Keck, Christmas Eve 2010.

It’s Christmas Eve and I’m starting a five-night observing run at the Keck Telescope using a high-resolution spectrograph (HIRES) to search for exoplanets. In the photo here, I am communicating with the telescope operator, Terry, by polycom. He is on the summit of Mauna Kea at 14,000 ft where the air is thin and I see that he has oxygen flowing. I’m glad that I’m working in comfort at Keck HQ in Waimea.

Tonight, I’m using the Doppler technique to measure the velocities of stars.  Orbiting planets tug their host stars around a common center of mass.  This reflex stellar velocity is largest for massive planets.

When small stars eclipse larger stars, the brightness dip can be virtually the same as those for transiting gas giant planets. To confirm a transit candidate as a planet, Doppler measurements are needed to determine the mass of the transiting object. The Kepler team has a massive follow-up campaign (led by Dr. Geoff Marcy at UC Berkeley) using the same setup that I’m using now.  Dr. Natalie Batalha (Deputy Scientist for the Kepler project) explains that the team is also eager to have others helping and to have Planet Hunters combing through the data. Watch for a blog post by Dr. Batalha here soon!

Some of you have asked how many consecutive low points you should see during a transit. That depends on how close the planet is to the star. Close planets orbit faster and transit in a few hours while more distant planets take several hours to transit. You should look for more than one low point.  Since the brightness measurements are taken every 30 minutes, a 3 hour transit would consist of just 6 low points. However the ingress, or first transit point, might be transitional and not reach the transit floor. Ditto for the egress, or last transit point).

The light curves for eclipsing binary stars are quite spectacular – they remind me of sketches I used to make with a “spirograph” toy I had as a kid. Some of the planet hunters have called this a shutter effect and I’ve written a quick program to demonstrate what is happening. In the Figure below, I created a theoretical light curve for a contact eclipsing binary with an orbital period of just 6 hours. If we had observations of this star every few minutes, then the light curve would look similar to a sine wave (left plot). However, if we observe this star less frequently (a slow “shutter speed”), then some interesting patterns emerge. The plot on the right in the Figure below shows an under-sampling of the light curve over 30 days. The pattern is similar to what appears in some of the eclipsing binary curves you are finding in the Kepler data.

shutter-both

Eclipsing Binaries

Figure 1.  Eclipsing Binary (detached, Algol type)

Figure 1. Eclipsing Binary (detached, Algol type)

I’m Debra Fischer, a Professor of Astronomy at Yale University. Many of you have already discovered some amazing eclipsing binary light curves, and we wanted to provide you with some information. The Figures here show examples that you have put into collections. Some great additional examples are shown in a paper from the Kepler team (Prsá et al. 2010 http://arxiv.org/abs/1006.2815).

The Kepler light curves show how the brightness of the star changes with time.  In Figure 1 (APH10135736 = KID 6449358) above, there are two stars orbiting each other.  Similar to transiting planets, these stars cross in front of each other. The light curve shows the brightness level of the star, plotted vs time in days.  Most of the time, both the larger, hotter star and the smaller cooler star yield a combined brightness measurement for the light curve. When the deep dip in brightness (the primary minimum) occurs it’s because the smaller cooler star is eclipsing the hotter star, which contributes most of the light; when the smaller dip (secondary minimum) occurs, it’s because the larger hotter star is eclipsing the smaller star, which contributes less light to the combined brightness. Stars with flat regions punctuated by relatively sharp dips (e.g. Figure 1) are known as Algol binaries.

A key indicator of eclipsing (or transiting planet) light curves is repeatability.

  • you can count the number of days between the large dips to determine the orbital period (about 5 days) of this binary star system in Figure 1
  • you can determine how long it takes the stars to cross by the duration of the transit dip (hours for Figure 1)
  • you know that one star is larger than the other if the transits don’t have equal dips

Notice that the depth of the brightness dips for an eclipsing binary star can be similar to those for a transiting planet. The transit depth tells us the ratio of the size of the transiting (or eclipsing) object relative to the size of the primary star and the smallest stars have diameters that are similar to Jupiter (stars are gas and the increased gravity from the larger mass star compresses the structure).

Figure 2. Contact eclipsing binary stars

Figure 2. Contact eclipsing binary stars

Sometimes binary stars are so close that the surfaces are distorted into an elliptical shape and the light curve between the eclipses is rounded, as in the left image of Figure 2 (APH10039007 = KID 9357275), where the orbital period is a little more than one day. You can see both the primary and secondary transit dip in this light curve. The most bizarre eclipsing binary light curves are those where the stars are even closer together, called over-contact binaries. An example of this is shown in the right image of Figure 2 (APH10102932 = KID 4633285). These stars can be so close together that they share a common envelope. The eclipse depth is variable, the light curve looks irregular, and there can be mass transfer between the stars.

Transiting Planets

The effects of 3 different types of transiting planets on a Kepler light curve. (Illustration: H. & M. Giguere)

Hi I’m Matt, a graduate student at Yale University and a member of the Science Team. We’re really impressed with the turnout so far on planethunters.org and users have already pointed out some really amazing objects! Quite a few people have asked for some clarification on what transits look like, so I’ll address that in this post.

In the figure above, we’ve taken a Kepler light curve from a star that’s about the same size as the Sun and have simulated what the effects would be if a few different types of planets were to transit.

The white dots show the amount of light from the star measured with Kepler with no planets transiting. The blue points show what we would see if a planet just like Jupiter orbiting this star were to transit. This Jupiter-size planet, at about 11.2 times the size of the Earth and one tenth the size of the star, is shown to scale transiting its parent star in the top left blue box.

The green dots show what a planet just like Neptune would look like transiting. Since it is much further away from the star than Jupiter, it would have a slower orbital speed so it would take longer to transit the disk of its parent star, which is what explains the longer duration, or wider width, of the transit event. With Neptune’s much smaller size than Jupiter, at 3.9 times the radius of the Earth, it doesn’t block out as much light, which is why the depth is much shallower.

Both of these events are very noticeable, compared to the effects of an Earth-size planet. The tiny speck on the star in the far right red box shows, to scale, what a transiting Earth-size planet would look like if we could see it. Now you get an idea of how difficult finding Earth-size planets is going to be! If that transiting planet had an orbital period of 1 year just like the Earth, then the dip in light observed from the parent star as the planet transits would be similar to the red points in the light curve. Since the Earth is much closer to the star, it has a much faster orbital speed, which then makes the duration of transit much shorter than the duration of either Jupiter or Neptune. Because the Earth-size planet is much smaller than either Jupiter or Neptune, it also blocks out less light making the dip in light we receive here on Earth barely discernible from no transit at all.

We don’t expect people to see these events all the time, so don’t worry about missing them. That’s why we’ve introduced fake planets into the mix. The fake, or synthetic, planets will help us determine the completeness of Planet Hunters, or how likely we are to detect planets of different sizes and with different orbital periods if they exist.

Stellar Variability

Greetings from Kevin Schawinski and Meg Schwamb, postdoctoral fellows at Yale and members of the Science Team.

Wow, we’ve been blown away by how enthusiastic everyone has been about the project. In this post, we wanted to talk more about another goal of Planet Hunters, which is to study and better understand stellar variability.  The public release Kepler data set is unprecedented, both in observing cadence and in the photometric precision. The lightcurves reveal subtle variability that has never before been documented.

The Kepler lightcurves are complex  many exhibiting significant structure including multiple oscillations imposed on top of each other as well as short-lived variations. Most of this variability is due by starspots or stellar pulsations.With Planet Hunters we will not only be looking for stars harboring planets outside of our solar system, but we will be able to study and classify stellar variability in ways that automated routines cannot. Unlike a machine learning approach, human classifiers recognize the unusual and have a remarkable ability to recognize archetypes and assemble groups of similar objects.

Users have the ability to identify strange or unusual lightcurves as well as tag similar curves and come up with their own classes or  ”collections”  of variability with  Planet Hunters Talk. You can add a comment and  use the #hashtag like in Twitter to mark an interesting lightcurve and alert others including the science team. Every light curve, or collection of curves has a short-message thread (140 characters) associated with it for general comments. You also can start discussions if you want to chat in a more in-depth fashion.

Mining the Kepler data set will inevitably lead to unexpected discoveries, showcased by the successes of Galaxy Zoo. The prime examples are the discoveries of  ”Hanny’s Voorwerp” and the ”green peas” by Galaxy Zoo users. Hanny’s Voorwerp is a cloud of ionized gas in the Sloan Digital Sky Survey image of the nearby galaxy IC 2497. Unlike an automatic classification routine, citizen scientist Hanny van Arkel spotted a blue smudge next to IC 2497, recognized it as unusual, and alerted the Galaxy Zoo team and the other users. Since then, Hanny’s Voorwerp has been identified as a light echo from a recent quasar phase in IC 2497, making it the Rosetta Stone of quasars. The Galaxy Zoo participants started noticing a very rare class of objects of point sources showed as green in the SDSS color scheme. Dubbing them the ”green peas,” the citizen scientists scoured the SDSS database, and assembled a list of these ”pea galaxies.”  The ”peas” were revealed to be ultra-compact, powerful starburst galaxies whose properties are highly unusual in the present day universe, but resemble those of primordial galaxies in the early universe. The citizen scientists found veritable fossils living in the present-day universe.

With so many eyes looking at the lightcurves, we are bound to find new variability types! We’re hoping that Planet Hunters, like Galaxy Zoo, will yield exciting new results that we can’t even attempt to speculate or imagine! We can’t wait to see what turns up.

Planet Hunters Introduction

Planet&SunSmall-2

Hi, I’m Meg Schwamb a postdoctoral fellow at Yale University and member of the Planet Hunters Team. Welcome to Planet Hunters! We’ve been working hard, and we are excited to finally show you the finished product!

In the last decade, we have seen an explosion in the number of known planets orbiting stars beyond our own solar system. With ground based transit searches, stellar radial-velocity observations, and microlensing detections, over 500 extrasolar planets (exoplanets) have been discovered to date. Studying the physical and dynamical properties of each of these new worlds has revolutionized our understanding of planetary formation and the evolution of planetary systems. But we have just barely scratched the surface in understanding the diversity of planetary systems and planet formation pathways.The current inventory of known exoplanets has been limited to mostly Jupiter-sized or larger gas-rich planets, most orbiting extremely close to their parent stars. The current inventory of known exoplanets has been limited to mostly Jupiter-sized or larger gas-rich planets, most orbiting extremely close to their parent stars. While these planets have provided great insight into the formation of giant planets, beyond Mercury, Venus, Earth, and Mars, in our own solar system, little is known about the formation and prevalence of rocky terrestrial planets in the universe.

Finding Earth-size planets is a difficult task because the transit-signals, the dimming of the star’s light caused be a planet moving in front of the star, are so shallow. For a Jupiter-size planet, the transit depth is ~1% of the star’s brightness. For an Earth-size planet transiting a Sun-like star the decrease in brightness is less than .001%. Ground-based surveys have not reached the sensitivity to detect such planets around stars similar to our Sun, but with NASA’s space-based Kepler mission, launched in March 2009, astronomers are primed to start a new era in the study of exoplanets. Even with the exceptional data from the Kepler telescope, finding these Earth-sized planets will be extremely difficult, but in the age of Kepler, the first rocky planets will likely be detected including the potential to find Earth-like planets residing in the habitable zone, warm enough to harbor liquid water and potentially life on their surfaces.

NASA’s Kepler spacecraft is one of the most powerful tools in the hunt for extrasolar planets. The Kepler data set is unprecedented, both in observing cadence and in the photometric precision. Before Kepler, the only star monitored this precisely was our own Sun. The lightcurves reveal subtle variability that has never before been documented. The Kepler data set is a unique reservoir waiting to be tapped. Kepler lightcurves are now publicly available with the first data release this past June and the next release scheduled for February 2011.

The Kepler Team computers are sifting through the data, but we at Planet Hunters are betting that there will be transit signals which can only be found via the remarkable human ability for pattern recognition. Computers are only good at finding what they’ve been taught to look for. Whereas the human brain has the uncanny ability to recognize patterns and immediately pick out what is strange or unique, far beyond what we can teach machines to do. With Planet Hunters we are looking for the needle in the haystack, and ask you to help us search for planets.

This is a gamble, a bet, if you will, on the ability of humans to beat machines just occasionally. It may be that no new planets are found or that computers have the job down to a fine art. That’s ok. For science to progress sometimes we have to do experiments, and although it may not seem like it at the time negative results are as valuable as positive ones. Most of the lightcurves will be flat devoid of transit signals but yet, 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 a try?