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The Role of Planetary Migration in the Evolution of the Solar System

In the past two decades, exoplanet hunters have discovered almost 1800 planets beyond the Solar System, and there is more than twice that number of potential candidates still awaiting further confirmation. Of the known alien systems, astronomers have found a substantial number of planets travel around their parent stars in truly unusual orbits, unexplainable by any planetary formation mechanism.

The list of peculiar cases includes bodies that travel along completely different orbital planes to one another, worlds that take millennia to complete an orbit, and those that possess extreme comet-like eccentricities. Even more extreme are the rogue planets out there that orbit no star, presumably having been ejected from their solar systems altogether. However, the most inexplicable bodies are hot Jupiters, which orbit their parent stars in a matter of hours to days at a fraction of the distance that Mercury lies from the Sun. At such close proximity to the star, temperatures would simply be too high for a massive planet to retain its gaseous envelope during formation.

If these bodies cannot have formed at their current locations this may mean that planetary orbits are subject to dramatic change throughout the evolution of a system; meaning that where we observe a body now may not be where it formed, or where it will eventually end up. This reordering is referred to by scientists as planetary migration.

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Artist impression of an accretion disk. Image credit: NASA/JPL-Caltech

There are three ways in which planetary migration is understood to occur: the first describes a gas driven process in which the planetary disk effectively pushes or pulls the planet to a new position; the second arises as a result of gravitational interactions between neighbouring bodies, where a large object can scatter a smaller one and thereby create an equal and opposite resulting force back onto itself; and the third is due to another gravitational effect, tidal forces, which mainly occur between the star and the planet and tend to result in more circular orbits.

Surprising as it may seem to some, it is widely accepted that planetary migration has shaped and influenced the architecture of the Solar System quite dramatically. In fact, its dynamic past actually explains the existence and properties of several Solar System entities, and shows that our planetary system might not be as unique as once thought. So how have the planets moved since their birth?

It all began with the inward migration of the largest planet in the Solar System, Jupiter. The gas giant, weighing more than all the other planets combined, is believed to have travelled right up to the orbit of Mars, 1.5 AU from the Sun, before travelling back out to its present location almost four times as far. Luckily for Mars this occurred some 600 million years into the birth of the Solar System (around 4 billion years ago) before any of the terrestrial planets had formed and only four gas giants ruled the skies. At this time, Jupiter, Saturn, Uranus and Neptune possessed much more compact orbits and were surrounded by a dense disk of small icy objects.

Artist impression of planetary system. Image credit: Dana Berry

Artist impression of planetary system. Image credit: Dana Berry, Harold Levison, Dan Durda, SwRI.

Jupiter was drawn towards the Sun by the first type of planetary migration, gas driven, whose effects work differently depending on the mass of the planet. For low-mass planets, like the Earth, the mechanism occurs when the planet’s orbit perturbs the surrounding gas or planetesimal disk driving spiral density waves into it. An imbalance can occur between the strength of the interaction with the spirals inside and outside the planet’s orbit, causing the planet to gain or lose angular momentum. If angular momentum is lost the planet migrates inwards, and if it is gained it travels outwards. This is known as Type I migration and occurs on a short timescale relative to the lifetime of the accretion disk.

In the case of high mass planets, like Jupiter, their strong gravitational pull clears a sizeable gap in the disk which ends Type I migration and allows Type II to take over. Here the material enters the gap and in turn moves the planet and gap inwards over the accretion timescale of the disk. This migration mechanism is thought to explain why hot Jupiters are found in such close proximity to their stars in other planetary systems. The third type of gas driven migration is sometimes referred to as runaway migration, where large-scale vortices in the disk rapidly draw the planet in towards the star in a few tens of orbits.

The three types of disk migration. Image credit: Frédéric Masse.

The three types of disk migration. Image credit: Frédéric Masse.

The best understanding of how the planets have moved in throughout our system’s evolution arose from the Nice Model, proposed by an international collaboration of scientists in 2005. This model suggests that at the inner edge of the icy disk, some 35 AU from the Sun, the outermost planet began interacting with icy planetesimals, influencing the second sort of migration to occur: gravitational scattering. Comets were slingshotted from one planet to the next, which gradually caused Uranus, Neptune, Saturn and the belt to migrate outwards. Jupiter’s powerful gravity flung the icy objects that reached it into highly elliptical orbits or out of the Solar System entirely, which in order to conserve angular momentum, further propelled its journey inwards.

An extension to this theory is the ‘Grand Tack model‘, which is named after the unusual course of Jupiter’s migration towards the Sun before stopping and migrating outwards again, like a sailboat tacking about a buoy. At the distance that Mars would later coalesce, material had been swept away due to Jupiter’s presence. This resulted in the stunted growth of Mars and a material-rich region from which the Earth and Venus formed, explaining their respective sizes. The gas giant’s travels also prevented the rocky material in the asteroid belt from accreting into larger bodies due to its strong gravitational influence. Although Jupiter swapped positions with the asteroid belt twice the movements were so slow that collisions were minimal, resulting in more of a gentle displacement.

nice

The Nice Model, Gomes et al. 2005.

But why did Jupiter’s migration to the Sun’s fiery depths cease? For that it has Saturn to thank. As the two planets moved further away from each other, it was believed they became temporarily locked in a 2:1 orbital resonance. That meant that for every orbit of the Sun Saturn made, Jupiter made two. The Nice Model showed that the planetary coupling increased their orbital eccentricities and rapidly destabilised the entire system. Jupiter forced Saturn outwards, pushing Neptune and Uranus into extremely elliptical orbits where they gravitationally scattered the dense icy disk far into the inner and outer Solar System. This disruption in turn scattered almost the entire primordial disk. Some models also show Neptune to have been propelled past Uranus to become the farthest planet from the Sun as we now know it. Over time the orbits of the outermost planets settled back into the near circular paths we observe today.

The Nice Model explains the present day absence of a dense trans-Neptunian population and the positions of the Kuiper belt and Oort cloud. It also accounts for the mixture of icy and rocky objects in the asteroid belt, like water-rich dwarf planet, Ceres, which likely originated from the icy belt. The rapid scattering of icy objects, around 4 billion years ago, dates with the onset of the late heavy bombardment period, which is predominantly recorded from the Moon’s well-preserved surface.

However, there are problems with the original Nice Model, where some simulations found that the gradual 2:1 resonant coupling of Jupiter and Saturn would have resulted in an extremely unstable inner Solar System from which Mars would have been ejected. Later research has since resulted in the ‘Nice 2 Model‘, which in part suggests that the gradual scattering of planetesimals caused the two gas giants to fall into a 3:2 orbital resonance (not the originally proposed 2:1), allowing for the Nice Model to work with a stable inner Solar System.

belt

Artist impression of the dispersed belt. Image credit: JHUAPL/SwRI

The final mechanism for planetary migration occurs through tidal interactions between different celestial bodies. Unlike gas driven migration and gravitational scattering, tidal forces act over a much longer timescale of billions of years. The process begins due to the Kozai mechanism, which is suggested to pump eccentricity into a planet’s orbit. As the tidal forces correct this effect by re-circularising its orbit the planet moves closer in. Whilst the orbits of the terrestrial planets are thought to have remained fairly stable throughout the evolution of the Solar System, this gradual process is likely to have slightly altered their paths and will remain to do so.

The knowledge of how our own planetary system evolved has helped answer many questions about unusual exoplanet orbits, but there is still a lot left to uncover. One such question asks why we observe so many hot Jupiters unfathomably close to their star, as without another large body’s influence, should it not eventually be swallowed up? Perhaps planet-disk interactions decouple at such close proximities to the star and tidal forces prevail, or perhaps we are capturing a snapshot in time just before the planet meets its fate. For now only time, further observations and, most importantly, more exoplanet discoveries will tell!

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The Search for Supereccentric Jupiters

Today we have a guest post from Bekki Dawson. Bekki is a Miller postdoctoral fellow at the UC Berkeley Department of Astronomy. Her research focuses on how planetary systems form and evolve.

The origin of “hot Jupiters,” giant planets orbiting extremely to their host stars, remains a mystery. There are two major theories for how these planets “migrated” from a location like our own Jupiter’s to the close-in orbits we observe today. The first is gentle disk migration, in which the disk out of which the planet forms pushes the planet towards the star. The second theory is more violent: another body in the system perturbs the Jupiter onto a very eccentric (elliptical) orbit. Over time, tides on the planet cause dissipation that shrinks and circularizes the planet’s orbit. In 2012, Aristotle Socrates and collaborators predicted that if this secondary theory is correct, we should find half a dozen “supereccentric” Jupiters in the Kepler sample: Jupiters that are still on very elliptical orbits and have not yet tidally circularized.

To search for these supereccentric Jupiters in the Kepler sample, John Johnson and I developed an approach we call the “photoeccentric effect” to identify eccentric Jupiters from the Kepler photometry. The approach hinges on the fact that a planet transiting on an eccentric orbit will be moving at a very different speed than a planet transiting on a circular orbit with the same orbital period, leading to a different shape and duration for the transit light curve:

photo.001-1

Even taking into account various degeneracies, we can easily identity the supereccentric Jupiters from their Kepler light curves. But surprisingly, we didn’t find any, inconsistent with the prediction. Therefore hot Jupiters continue to be a puzzle! However, the expected number was based on the number of planets that transit at least three times in the Kepler data, so in the future we hope to measure the eccentricities of giant planet candidates with only two transits; if those are missing too, the evidence would be even more compelling. Unfortunately, giant planet candidates with only two transits are not automatically caught by the Kepler pipeline, so if you spot any, let me know!

What Do We Really Understand About Planetary Formation?

It’s fair to say that there are a lot of gaps to fill in our knowledge of exoplanetary bodies, and 2013 proved to be a good year for bizarre discoveries. From a planet found with no star in sight, to a gas giant orbiting at an unfathomable distance, to a system containing an orbital plane 45 degrees out of whack, the list seems endless. As 2014 kicks off, we can expect no slowing down of these unusual discoveries from planet hunting teams around the world, on top of the months of Kepler data still queuing up for analysis. This opens up the topic of how these extraordinary bodies came to be, and begs the question, ‘what do we really understand about planetary formation?’

First of all, we need to imagine the early disk environment around a newly born star. The protoplanetary disk contains lots of dust and gas left over from the initial collapse of the interstellar cloud from which the star forms. Both the star and the disk rotate about a common centre of gravity, and it is the rotating debris, ranging in size from an angstrom up to a centimetre, that can evolve in the disk to form planets.

Artist Impression of Planetary Formation, courtesy of NASA

Artist Impression of Planetary Formation, courtesy of NASA

There are two widely held theories for how giant gas planets can form: core accretion and disk instability. Core accretion occurs from the collision and coagulation of solid particles into gradually larger bodies until a massive enough planetary embryo is formed (10-20 Earth masses) to accrete a gaseous envelope.  Disk instability, on the other hand, describes the process by which a massive disk rapidly cools, causing it to fragment into planet-sized, self-gravitating clumps. Both theories can be used to define the presence of giant planets, but there are a few pitfalls in these explanations and a plethora of planets that neither theory alone can seem to justify. Let’s look in more detail.

The primary accepted mechanism of planet formation is our first theory, core accretion, which is best described in several stages. During the first step, material in the disk collides and aggregates to form small centimetre to metre sized clumps of matter. The clumps then grow further by smashing and sticking together, leading to the gradual coagulation of kilometre-sized planetesimals. Some of these large bodies are massive enough that runaway accretion begins, resulting in the rapid formation of planetary embryos. Here there is a distinction between the formation of terrestrial and gaseous planets. Near the star, heavier metallic elements begin to condense at hotter temperatures and violent collisions and mergers can eventually result in the production of terrestrial planets. The bodies remain relatively small due to the amount of material found in the inner disk, and explains why the terrestrial planets in our solar system lie closest to the Sun. Farther out from the star beyond the snow line, embryos form from a mix of rocky, metallic and also a considerable amount of less dense icy material. At such cool temperatures, hydrogen and helium are able to condense and build to form much larger bodies. Around 10 Earth masses, the planet then possesses enough gravitational attraction to accrete a gaseous atmosphere of hydrogen and helium, a process which continues until all the gas in the planet’s vicinity is exhausted. This describes why the planets in the outer Solar System predominantly consist of lighter elements and are able to acquire such large atmospheres.

Artist Impression of Accretion. Image Credit: Alan Brandon/Nature

Artist Impression of Accretion. Image Credit: Alan Brandon/Nature

However, this mechanism struggles to explain massive planets forming at large distances from a star. This has led to HD 106906 b, whose orbit is 650 times greater than the Earth’s orbit around the Sun, to be proposed as forming independently from the star altogether! A problem closer to home is the extremely long time-scale required for Neptune and Uranus to form a core through accretion, which is estimated to be around 10 million years. Since the gas and dust in the protoplanetary disk probably only lasted for a few million years, this poses quite an issue. Newer accretion models may be able to account for their formation within a short enough timescale, but this is still a challenging and ambiguous area. Alternatively, could our ice giants have formed via a different mechanism?

The Solar System. Image Credit: International Astronomical Union

The Solar System. Image Credit: International Astronomical Union

A different theory of giant planet formation is via disk instability; a less popular, but still plausible, explanation. This mechanism requires no direct interactions between solids whatsoever, just the condensing of gas and dust in the planetary disk. During the very early stages of a protoplanetary disk’s formation, if rapid cooling occurs in the order of an orbital timescale, material is thought to fragment into bound objects. These fragments would then condense further into the gaseous planets we observe. This theory provides an explanation of planet formation that would occur within a very short (few thousand years) timeframe, and can also be used to explain the presence of large gaseous planets near to or very far from the star. However, whether a disk could cool quickly enough to fragment on an orbital timescale is hotly debated. It could be that it is only a possibility at very large orbital radii.

With two competing theories for how the most massive planets form, we still have a lot to learn about the evolution of the different systems we observe, especially our own! It is likely that the formation mechanism is dependent on the system, and that both theories could work within different regimes. But neither of these theories seems to explain the presences of hot Jupiters; gas giants that orbit incredibly close to their host stars with periods of just a few days. It is believed that at such close proximity to the star, temperatures would simply be too high for the planet to retain its gaseous envelope during formation, which is where the idea of planetary migration really came to light. This suggests that perhaps where we observe a planet now isn’t really where it originally formed at all.

Check back soon for my next post discussing the different theories of planetary migration.

Happy hunting!

Comet ISON and the Best Studied Planetary System

This Thursday, the Solar System put on a celestial performance, and we had a front row seat to the spectacle. Long period Comet ISON made its closest approach to the Sun entering the Sun’s atmosphere. This sun-grazing comet was making its first entry into the inner Solar System after spending most of its lifetime in the outer reaches of the Solar System in the Oort Cloud (a spherical shell of icy bodies residing at ~10,000-20,000 AU and the repository of long period comets). For most of the Solar System’s history ISON has resided out in the Oort Cloud, but the gravitational tug from a chance passing star or the gravitational pull from the gravitational tides with the center of the Milky Way nudged ISON onto an orbit straight for the Sun.

Comet ISON was discovered in November 2012 and has sometimes been touted as potentially being the ‘Comet of the Century’ with some predictions that it might become so bright to be visible with the naked eye if it survived perihelion (its closest approach to the Sun). Since ISON was discovered with such warning before perihelion, astronomers were able to organize observing campaigns with ground-based and space-based telescopes to study how the comet changes as it got ever closer and closer to the Sun. Planetary scientists and astronomers will be pouring over the data for months and likely many years to come.

On Thursday the spacecraft monitoring the Sun including Solar Dynamics Observatory (SDO) and Solar and Heliospheric Observatory (SOHO) were poised to capture ISON as it made it’s closest approach. Comets are a conglomeration of ice, rock, dust, and frozen gases, and many don’t survive the fiery perihelion passage; the nucleus disintegrating with only dust and a rocky rubble pile remaining. It wasn’t sure if ISON would survive. It looks like something has indedeed survived perihelion passage, but ISON is providing more questions than answers. It appears to have completely  lost its coma and tail which were blasted away as it skimmed the Sun’s million-degree corona. As you’ve probably seen the contradictory statements that ISON was dead and then alive (ISON is not behaving like any sungrazing comet seen before and if you were following twitter and the news reports you were seeing science in progress. Conclusions were changing as more data came down in live time). The current word on the street is that likely a small chunk of ISON’s nucleus made it through perihelion, but it’s still not 100% clear what survived.  ISON appears to be behaving like a comet albeit a very small and dusty comet, but time will tell. Future observations over the coming days and weeks will confirm whether gas is being produced which would be the tell-tale sign that there is ice and frozen gases in from some part of the nucleus still around. If there is no gas, then it’s just a rubble of rock and dust left in orbit that will slowly dissipate. Chances are that at this point ISON won’t be naked eye visible but either way, ISON has left us with more learned about comets in the Oort Cloud and added many many many questions for astronomers and planetary scientists to solve while putting on a spectacular show for us.

Snapshots from SOHO’s C2 chronograph monitoring of the Sun showing Comet ISON after its closest approach with the Sun (left) and before (right) on its way to perihelion Image Credit:ESA/NASA/SOHO/Jhelioviewer

For full coverage and the latest on Comet ISON and how it is doing after it’s fiery encounter with the Sun, check out NASA Comet ISON Observing Campaign blog  run by Karl Battams and the Planetary Society’s Comet ISON Live Blog by Emily Lakdawalla and Bruce Betts.

What am I doing talking about a comet on the Planet Hunters blog? I do have having a soft spot in my heart for icy outer Solar System bodies, that’s the area of research I worked on for my thesis, but Comet ISON is a good reminder that we should think about these exoplanets we’re finding with Kepler and Planet Hunters as members of a larger planetary system. The transiting planets are the ones we detect, but there is much more there that we don’t see. We know in some transiting exoplanet systems there are unseen (‘invisible’) non-transiting planets, perturbing the orbits of the transiting planets changing the timings and durations of the observed transits. There is also likely the building blocks of planet formation left over as debris hanging around in many of the  planetary systems found in the Kepler field. Perhaps some of the material is trapped in belts like the asteroid belt and Kuiper belt in our own Solar System. Many of them likely have Oort Clouds (distant repository of icy bodies of their own). Debris disks likely the equivalent to our own Kuiper belt have been spotted around stars. One of the famous examples is Fomalhaut shown below.

heic0821c

Image credit: NASA/Spitzer

With all the stunning images from SOHO and the confounding behavior of Comet ISON, it reminds us that our Solar System remains the best studied planetary system and that we have still so much more to learn and understand about the Solar System’s formation and evolution. What happened to ISON is probably happening to many other icy bodies around other stars in our Milky Way. We just happened to have a front row seat,  but what a show did ISON put on for us! So the next time you look for transits on http://www.planethunters.org take a moment and think about the planetestimals, dwarf planets, and minor planets likely residing around the planet host stars too.

Exploring the Tiny Planetary System of Kepler-90

KOI-351

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.

New PH Planet

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 inner system of KOI-351

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]

Spotting RR Lyrae Stars

RR Lyrae stars are a special type of variable star that changes in brightness due to radial pulsations that increase and decrease the radius of the star . Over the past 3 years, Planet Hunters volunteers on Talk have keenly spotted several previously unknown RR Lyrae stars in the Kepler field, that were nearby neighbors on the CCDs to Kepler targets and were contaminating the photometric aperture used to produce the light curve of the real Kepler target star. You read more about some of these discoveries here. These discoveries have been passed on to collaborators in the Kepler Cepheid & RR Lyrae Working Group who have continued to study those stars including sometimes having the contaminating RR Lyrae added to the Kepler list of targets to get its full light curve.

Katrien Kolenberg who is a member of the Kepler Cepheid & RR Lyrae Working Group, recently wrote a chapter for the conference proceedings of the ’40 Years of Variable Stars: A Celebration of Contributions by Horace A. Smith’ Conference’, and she presented a similarly titled talk at the conference this past May.  In the chapter, she gives a summary of the science from the now over 55 RR Lyrae stars known in the Kepler field. She includes a shout out to  Planet Hunters  to acknowledge the project’s contribution to discovery. Congratulations to all involved in the RR Lyrae discoveries. You can read the chapter from the conference proceedings here.

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.

This schematic, by Planet Hunter Daryll LaCourse, shows off our new discovery.

This schematic, by Planet Hunter Daryll LaCourse, shows off our new discovery. (Click to make larger)

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!

A Planet Hunters Review of the Kepler Transit Candidate List

The Kepler team uses automated routines, specifically the Transiting Planet Search (TPS) algorithm, to search for transit signals in the Kepler light curves. TPS triggers on many repeated transit-like features in the light curves dubbed Threshold Crossing Events or TCEs. TPS generates many many TCEs, much more than the number of real extrasolar planets. The majority are false detections, but a few are real transits due to orbiting exoplanets. A subset of the Kepler team examine the TCE list and whittles it down to make the KOI (Kepler Object of Interest) list. A handful of Kepler scientists review each TCE and data validation report, results from a series of checks and test to help rule out astrophysical false positives that might produce a transit-like signal such as blended background eclipsing binary. It takes many many months for this process. The current Kepler planet candidate list released in January was using Q1-8, but there are many more Quarters of Kepler data available.

The Kepler team has made all of their data products publicly available in the extended mission. In December, the Kepler team  released the list of 18,406 TCEs found during a search of Quarters(Q) 1-12 data and the resulting reports produced by their data validation pipeline. These Q1-12 TCEs have yet to fully searched by the Kepler team, meaning there are likely discoveries waiting to be found.

For the past few months I’ve been working with Chris and Arfon to set up a Planet Hunters review of the TCE list. Today the review site is live, and we need your help to review these potential transit candidates and identify the ones that are likely due to real planets. We’re using a version of the round 2 review interface, we used before to vet planet candidates for my short period planets paper. For each TCE, you’ll be presented with a light curve (from the data validation report) that has been zoomed-in and folded on the period determined by TPS so that the repeat transits all line up on top of each other.

Folded light curve that has been zoomed-in and folded on the period determined by TPS so that the repeat transits all line up on top of each other. The black data points all the actual measured Kepler data folded on the period identified by TPS. The blue diamonds represent an average of the folded data. The red line plotted is a model of the transit that TPS identified. If this is the correct period and the detection is real, you should see what looks like a symmetric transit centered in the plot. If TPS identified the event correctly the red curve should match the transit.

A good TCE candidate – The black data points all the actual measured Kepler data folded on the period identified by TPS. The blue diamonds represent an average of the folded data. The red line plotted is a model of the transit that TPS identified.

We are asking you to confirm that there is a visible transit in the light curve identified by TPS (“Is there a transit?”) and determine whether the red line matches the light curve (“Does the red line fit the data?”).

With the folded light curves we can see smaller planets, the rocky ones that are so hard for most of us to see in the regular light curves we show on the Planet Hunters website. There are other teams who are using the TCE list in their research and as targets for follow-up observations, but where I think we have an advantage is that we have the ability to review the entire TCE list, not just the rocky planet transits but also the Jupiter-sized and in between.

We’re not in a race with the Kepler team who am I sure are also vetting the current released list, but I believe what is unique to this project and Planet Hunters is the ability to review uniformly all the ~18,000 potential transit signals identified by TPS. The current versions of the Kepler KOI list currently has not gone back and reanalyzed all the previous planet candidates detected in previous TPS runs with the longer observational baseline. So we’ll have the first independent vetting of the Kepler Quarters 1-12 TCE catalog providing a uniform selected sample of planet candidates.

Each TCE will require 10 independent review before being retired. Once we’ve gotten through identifying what looks to be real transit candidates from the non-detections, I’ll apply some additional cuts based on the output from the Kepler validation pipeline (like how the consistent is the depth of the odd and even transits to rule out eclipsing binaries and pixel offsets in and out of transit that might indicate a blended background eclipsing binary is producing the signal) to come up with our very own Planet Hunters planet candidate list from the TCEs.

I think this project will result in a very interesting paper looking at the frequencies of super-Earth to Neptunes to Jupiter-sized planets in the Kepler field, and also serve as an efficiency estimate for the Kepler KOI vetting process. If it goes well, we may consider making this a more permanent fixture on Planet Hunters for future releases of the Kepler TCE list. My goal is to have the first results from the TCE review to show in a poster at Protostars and Planets VI conference in Heidelberg, Germany in July.

If you are interested in participating and helping out with this project, you can go to  http://tcereview.planethunters.org/ where you can join in and characterize the TCEs. Please do read  through the tutorial on the front page. It will guide you on what you should be doing, as well as show you some examples of non-detections and good TCE transit detections.

Thanks in advance,

~Meg

PH1 Paper Offically Accepted for Publication

Last October we announced the discovery of PH1 – a four star planetary system hosting a circumbinary planet (PH1b). The transits were spotted by volunteers Robert Gagliano and Kian Jek  on Talk. I’m thrilled to announce that our paper “Planet Hunters: A Transiting Circumbinary Planet in a Quadruple Star System” has been officially accepted to Astrophysical Journal. Congratulations to all involved.

Now that the paper has been accepted and is in press, you can find the accepted manuscript online and added to the Zooniverse publications page (which has a total of 4 Planet Hunters in press/published papers based on your clicks). The official journal version will be published sometime in May.

PH1b is our first confirmed exoplanet discovery, a milestone for Planet Hunters. The 6.18 Earth radii planet orbits outside the 20-day orbit of an eclipsing binary consisting of an F dwarf ( 1.734 x the Radius of the Sun) and M dwarf ( 0.378 x the Radius of the Sun). For the planet, we find an upper mass limit of 169 Earth masses (0.531 Jupiter masses) at the 99.7% confidence level. With a radius and mass less than that of Jupiter, PH1b is a bona fide planet. Not all planet candidates can be confirmed as we could with PH1b. Since PH1b is orbiting an eclipsing binary, we could use the fact that there are no changes in the timing of the stellar eclipses due to the planet to constrain PH1b’s mass.

With the acceptance of the paper, we have asked that PH1b be added to the NASA Exoplanet Archive (NExSci)’s  list of confirmed exoplanets . NExSci has taken on the role of being the keeper of the list of confirmed exoplanet discoveries. In addition, PH1b has bestowed the Kepler # that was saved for us in October. PH1b has been given officially a Kepler designation of Kepler-64b and added to the list of planets in the Kepler field. You can find out more about what the criteria for obtaining a Kepler # is here.

In the list of confirmed planets, the planet is referred to as PH1b (you might notice an extra space – that should be revised in an update to the NASA Exoplanet Archive). I like to think of the Kepler # as icing on the cake. We’ll still refer to the planet as PH1b. Kepler-64b will be an alternate designation and used in the catalog of planets in the Kepler field (PH1b will be listed as an alternative designation). The full data page for PH1b on the NASA Exoplanet Archive can be found here

For those who are wondering what the NASA Exoplanet Archive  is, Rachel Akeson, Deputy Director of NexSci and Project Scientist for the NASA Exoplanet Archive, explains below:

The NASA Exoplanet Archive is an online astronomical exoplanet and stellar catalog and data service provided to the astronomical community to assist in the search for and characterization of exoplanets and their host stars.

Current data content and tools include:

  • Interactive tables of confirmed planets, Kepler Objects of Interest (which includes the planet candidates), Kepler Threshold Crossing Events, stellar parameters for all Kepler targets in Q1-12 and a list of Kepler confirmed planet names and aliases.
  • Overview pages with all available data for each confirmed planet and Kepler Object of Interest
  • Tools to view, normalize, phase and calculate periodograms for light curves, particularly those from Kepler and CoRoT
  • Transit predictions for all known transiting planets and Kepler Objects of Interest
  • URL-based access to all table data

The archive is available at http://exoplanetarchive.ipac.caltech.edu/index.html and includes links to documentation for all these services.

More about the Discovery of PH2-b

The project’s second confirmed planet, PH2-b (a Jupiter-sized gas giant planet orbiting a Sun-like star), was discovered by several members of the PH community who classified the light curve and then posted the candidate on Talk. A volunteer-organized effort took this from a possible repeat of transits to a likely candidate that was then passed to the Science Team and subsequently validated as a real bona fided planet. Volunteer rafcioo28 who was the first person to mark a transit in Q4. Mike  Chopin was the second and the one to first  post on the Talk page about the transit in February of last year. Hans Martin Schwengeler went to look at the rest of the publicly released Kepler data months later spotting the other transits. Together rafcioo28, Mike, and Hans with the help of Abe Hoekstra, Tom Jacobs, Kian Jek, Daryll LaCourse  have discovered Planet Hunters’ 2nd confirmed planet PH2-b.  I’ve asked Mike and Hans (rafcioo28 we haven’t been able to contact thus far) write a bit about their thoughts on the discovery.

PH2_moon

Artistic rendition is a hybrid photo-illustration, showing a sunset view
from the perspective of an imagined Earthlike moon orbiting the giant planet, PH2 b. Image Credit: H. Giguere, M. Giguere/Yale University

Mike Chopin

At school, at the age of fourteen, I did a project on atomic (particle) physics which gained me a grade 1 CSE. The following year I studied and passed my Physics exam which was interesting for my school since that was a subject not on the school curriculum. After leaving school, I studied OND Engineering at Kingston Polytechnic although I only completed my first year since I longed to go travelling. My wanderlust got the better of me and I joined a shipping line as a Navigating Cadet Officer. I suppose it’s easy to see why astronomy has fascinated me since knowing about stars was part of my navigation syllabus.

My childhood hero was, and still is, Captain James Cook a man I consider to be the greatest explorer of all time. I consider myself fortunate to have visited many places this great navigator charted. In 2012 his observation of the transit of Venus in 1769 was commemorated at Venus Point in Tahiti. Although I wasn’t there for 2012, I did get to Venus Point a couple of years earlier. Like Cook, I spent some time in the Navy and have a passion for boats especially under sail. I have two complete circumnavigations under my belt; the first by sea (unfortunately via the Panama Canal and not Cape Horn) the second was by air, island hopping my way across the Pacific. I have now visited ninety six countries and hope that it won’t be too long before I join the Travellers’ Century Club.

Latterly, I was employed by Lloyds TSB (Registrars) as a project officer with my principal role as the sole technical writer writing context sensitive help for software, on-line documentation, trouble-shooting guides for the IT department and interactive eLearning modules. Following redundancy, I went freelance as a writer and have had a couple of small contracts both as a writer and as a data manager.

I am delighted to have been involved with the discovery of an exoplanet, a planet orbiting a distant sun. From the outset, I enjoyed the thrill of analysing the light signals recorded and posted on the planethunters.org website. This website invites ordinary people to take part in analysis of vast amounts of data. Often called ‘Citizen Science’ this excellent website provides clear tutorials to enable the amateur to partake in this worthwhile research project.

In its simplest form, when an exoplanet passes between our line of sight and its sun, there is a reduction in the amount of light that we receive. This effect can be seen if we plot the light output from this star against time. While trying to analyse the data, I would try to imagine the planet transiting its sun, if it was a large planet and close to its sun would it cut more light than if it had been a small planet and a giant sun? Does it have a high reflectivity (albedo) and is it inclined to its ecliptic and if so, would it add or reduce the amount of light recorded. If distant suns had multiple planets with systems similar to our own solar system, then would it be possible to identify additional planets. It was with all these ideas in mind that I began my quest for the exoplanets.

Sometimes, the pattern appeared to be too random to be able to distinguish a planet and at others, beautiful patterns could be seen as if generated by an oscilloscope, these it would seem were possible candidates for a binary star and so these were recorded also. Now and again, a pattern would emerge which would make you sit up and take notice. Using the sliders on the screen, I would drag out the ‘x’ scale to magnify a section of the screen where I was certain a transit was occurring and then I would check to see whether there was a second transit which may indicate its periodicity. It was during such an event that I found, what is recently been called, PH2-b. With, what at time was simply a planetary candidate; I posted a note to see if any of my fellow planet hunters had seen what I had seen.

Carl Sagan spoke of the ‘Pale Blue Dot’, the Earth as seen from Voyager 1 in the distant reaches of space, how exciting would it be if spectral analysis revealed this planet to have water and an atmosphere, another ‘Pale Blue dot’, now that would be truly remarkable.

Hans Martin Schwengeler

I’m a regular user (zoo3hans) on PH, more or less from the beginning two years ago. My name is Hans Martin Schwengeler and I live near Basel in Switzerland. I’m 54 years old, I’m married and we have two children. I’m a mathematician and work as a computer professional. I like to advance Science in general and Astronomy in particular. I did work a few years at the Astronomical Institute of the University of Basel (before it got closed because they decided to save some money…), mainly on Cepheids and the Hubble Constant (together with Prof. G.A. Tammann). Nowadays I’m very interested in exoplanets and spend every free minute on PH.

I’ve always been interested in stars, planets and the universe in general. So when I studied Mathematics at the ETH in Zurich it was natural to choose Astronomy as a second discipline. After working a few years on a Statistics research program (based on the Kalman Filter) I managed to get a job at the Astronomical Institute of the University of Basel (Switzerland) as a system manager. There I could work part time on research programs, mainly on Cepheids to determine the Hubble Constant (together with G.A.Tammann and Allan Sandage). I did this with the image processing software ESO-MIDAS, where we analyzed images taken by the ESO New Technology Telescope (NTT) or the Hubble Space Telescope (HST). I also used a program (written in Fortran-77) called superperiod to find the periods of the variable stars found in the galaxy images and see if they could be cepheids with periods between 2 and 100 days. With the Cepheid period-luminosity relationship we were then able to determine the distance of the Cepheid and the host-galaxy.

As soon as Michel Mayor and Didier Queloz detected the planet around 51 Pegasi, I was drawn in into exoplanets. I followed every single announcement of the detection of a new exoplanet on Exoplanet.eu and arXiv.org and elsewhere. So when I first took notice of the Planet Hunters project, I joined immediately. In the meantime the Astronomical Institute has been closed down (on monetary reasons) and I was working as a systems engineer at the Federal Office for Information Technologies and Telecommunications in Bern. I did not have anymore the tools needed to analyze light curves and so on. I also had to realize that to detect a planet transit in a Kepler light curve is not so easy as I first thought (except for the very big Jupiter-like ones). The learning curve was rather steep. Fortunately some fellow hunters had already gathered some very good insight and also some useful tools. So after some months I think I accumulated enough experience to do some real work here on PH.

So when I got the light curve for KID 12735740 I thought it looks very nice and might be a real planetary transit. Kian Jek had already commented on it favorably. The transit shape is more like an “U” instead of a “V”, the transit depth and duration is compatible with a 1.1 R_Jupiter planet with a period around 282.6 days. We can check this with Kian’s very good Planetary Calculator. The first thing I then usually do, is to have a look at the sky view  and then post this image to the PH Talk pages for others to have a look too My second step is then to download the FITS files from MAST (using the very good tools from http://www.kianjin.com/kepler/detrend.tar.gz ), detrend the curve roughly and view it by eye first (often using the program ggobi for this purpose). I upload the light curve also to PH if it looks interesting. Thirdly I may do a periodogram to find the period if a good period seems to be present (and upload it as well of course).

In the case of KID 12735740 I think all looks very good for a real planet candidate. Not much would be possible without the help of others, especially Kian Jek (aka kianjin) is invaluable here at PH. He compiles very good lists of “good candidates” or EB lists. I also find the other lists of “good Q2 candidates (non Kepler favorites)” (or Q3, Q4, etc. lists) very helpful in finding candidates and discuss them in more detail. It’s otherwise rather difficult to keep track of all the interesting cases on PH Talk. Kian does also the best detrending jobs, contamination vector determination, fitting of transit parameters, and more. nighthawk_black does perfect Keppix analysis, troyw has his amazing AKO service, capella, JKD, ajebson, gccgg, Tom128 and many other are very helpful too.So very often we work together here at PH as a good team.

In order to discriminate between real transits and instrumental or processing artifacts, I add comments to the “consolidated list of glitches” in the Science section on the PH Talk site. I collected a few bright and quiet and constant stars over the last few months / years exactly for this purpose. When I see a dip on one light curve and the same feature is also present on the other light curves, then it’s very likely a glitch.

I think the PH project is a great contribution to Science. I’d like to thank all fellow PH hunters for their help and also to Meg.

Kind regards,
Hans Martin Schwengeler (aka zoo3hans)

In addition to Mike, Hans, and rafcioo28, several others get a tip of the hat for marking transits in the discovery light curve for PH2: Sean Flanagan, Anand, and Jaroslav Pešek. Congratulations to you as well.