Neptune-Sized Exomoon Found Orbiting a Jupiter-Sized Planet?

Finding planets beyond our Solar System is already tough, laborious work. But when it comes to confirmed exoplanets, an even more challenging task is determining whether or not these worlds have their own satellites – aka. “exomoons”. Nevertheless, much like the study of exoplanets themselves, the study of exomoons presents some incredible opportunities to learn more about our Universe.

Of all possible candidates, the most recent (and arguably, most likely) one was announced back in July 2017. This moon, known as Kepler-1625 b-i, orbits a gas giant roughly 4,000 light years from Earth. But according to a new study, this exomoon may actually be a Neptune-sized gas giant itself. If true, this will constitute the first instance where a gas giant has been found orbiting another gas giant.

The study, titled “The Nature of the Giant Exomoon Candidate Kepler-1625 b-i“, recently appeared in the scientific journal Astronomy and Astrophysics. The study was conducted by René Heller, an astrophysicist from the Max Planck Institute for Solar System Research, who examined lightcurves obtained by the Kepler mission to place constraints on the exomoon’s mass and determine its true nature.

An artist’s conception of a habitable exomoon orbiting a gas giant. Credit: NASA

Within the Solar System, moons tell us much about their host planet’s formation and evolution. In the same way, the study of exomoons is likely to provide insight into extra-solar planetary systems. As Dr. Heller explained to Universe Today via email, these studies could also shed light on whether or not these systems have habitable planets:

Moons have proven to be extremely helpful to study the formation and evolution of the planets in the solar system. The Earth’s Moon, for example, was key to set the initial astrophysical conditions, such as the total mass of the Earth and the Earth’s primordial spin state, for what has become our habitable environment. As another example, the Galilean moons around Jupiter have been used to study the conditions of the primordial accretion disk around Jupiter from which the planet pulled its mass 4.5 billion years ago. This accretion disk has long gone, but the moons that formed within the disk are still there. And so we can use the moons, in particular their contemporary composition and water contents, to study planet formation in the far past.”

When it comes to the Kepler-1625 star system, previous studies were able to produce estimates of the radii of both Kepler-1625 b and its possible moon, based on three observed transits it made in front of its star. The light curves produced by these three observed transits are what led to the theory that Kepler-1625 had a Neptune-size exomoon orbiting it, and at a distance of about 20 times the planet’s radius.

But as Dr. Heller indicated in his study, radial velocity measurements of the host star (Kepler-1625) were not considered, which would have produced mass estimates for both bodies. To address this, Dr. Heller considered various mass regimes in addition to the planet and moon’s apparent sizes based on their observed signatures. Beyond that, he also attempted to place the planet and moon into the context of moon formation in the Solar System.

Artist’s impression of an exomoon orbiting a gas giant (left) and a Neptune-sized exoplanet (right). Credit: NASA/JPL-Caltech

The first step, accroding to Dr. Heller, was to conduct estimates of the possible mass of the exomoon candidate and its host planet based on the properties that were shown in the transit lightcurves observed by Kepler.

“A dynamical interpretation of the data suggests that the host planet is a roughly Jupiter-sized (“size” in terms of radius) brown dwarf with a mass of almost 18 Jupiter masses,” he said. “The uncertainties, however, are very large mostly due to the noisiness of the Kepler data and due to the low number of transits (three). In fact, the host object could be a Jupiter-like planet or even be a moderate-sized brown dwarf of up to 37 Jupiter masses. The mass of the moon candidate ranges somewhere between a super-Earth of a few Earth masses and Neptune’s mass.”

Next, Dr. Heller compared the relative mass of the exomoon candidate and Kepler-1625 b and compared this value to various planets and moons of the Solar System. This step was necessary because the moons of the Solar System show two distinct populations, based the mass of the planets compared to their moon-to-planet mass ratios. These comparisons indicate that a moon’s mass is closely related to how it formed.

For instance, moons that formed through impacts – such as Earth’s Moon, and Pluto’s moon Charon – are relatively heavy, whereas moons that formed from a planet’s accretion disk are relatively light. While Jupiter’s moon Ganymede is the most massive moon in the Solar System, it is rather diminutive and tiny compared to Jupiter itself – the largest and most massive body in the Solar System.

Artist’s impression of the view from a hypothetical moon around a exoplanet orbiting a triple star system. Credit: NASA

In the end, the results Dr. Heller obtained proved to be rather interesting. Basically, they indicated that Kepler-1625 b-i cannot be definitively placed in either of these families (heavy, impact moons vs. lighter, accretion moons). As Dr. Heller explained:

“[T]]he most reasonable scenarios suggest that the moon candidate is more of the heavy kind, which suggests it should have formed through an impact. However, this exomoon, if real, is most likely gaseous. The solar system moons are all rocky/icy bodies without a significant gas envelope (Titan has a thick atmosphere but its mass is negligible). So how would a gas giant moon have formed through an impact? I don’t know. I don’t know if anybody knows.

“Alternatively, in a third scenario, Kepler-1625 b-i could have formed through capture, but this implies a very unlikely progenitor planetary binary system, from which it was pulled into a bound orbit around Kepler-1625 b, while its former planetary companion was ejected from the system.”

What was equally interesting were the mass estimates for Keple-1625 b, which Dr. Heller averaged to be 19 Jupiter masses, but could be as high as 112 Jupiter Masses. This means that the host planet could be anything from a gas giant that is just slightly larger than Saturn to a Brown Dwarf or even a Very-Low-Mass-Star (VLMS). So rather than a gas giant moon orbiting a gas giant, we could be dealing with a gas giant moon orbiting a small star, which together orbit a larger star!

An artist’s conception of a T-type brown dwarf. Credit: Tyrogthekreeper/Wikimedia Commons.

It’s the stuff science fiction is made of! And while this study cannot provide exact mass constraints on Keplder-1625 b and its possible moon, its significance cannot be denied. Beyond providing astrophysicists with the first possible example of a gas giant moon, this study is of immense significance as far as the study of exoplanet systems is concerned. If and when Kepler-1625 b-i is confirmed, it will tell us much about the conditions under which its host formed.

In the meantime, more observations are needed to confirm or rule out the existence of this moon. Fortunately, these observations will be taking place in the very near future. When Kepler-1625 b makes it next transit – on October 29th, 2017 – the Hubble Space Telescope will be watching! Based on the light curves it observes coming from the star, scientist should be able to get a better idea of whether or not this mysterious moon is real and what it looks like.

“If the moon turns out to be a ghost in the data, then most of this study would not be applicable to the Kepler-1625 system,” said Dr. Heller. “The paper would nevertheless present an example study of how to classify future exomoons and how to put them into the context of the solar system. Alternatively, if Kepler-1625 b-i turns out to be a genuine exomoon, then my study suggests that we have found a new kind of moon that has a very different formation history than the moons we know as of today. Certainly an exquisite riddle for astrophysicists to solve.”

The study of exoplanet systems is like pealing an onion, albeit in a dark room with the lights turned off. With every successive layer scientists peel back, the more mysteries they find. And with the deployment of next-generation telescopes in the near future, we are bound to learn a great deal more!

Further Reading: Astronomy and Astrophysics

Kepler Spots the First Exomoon Candidate 4000 Light Years From Earth

Ever since it was deployed in March of 2009, the Kepler mission has detected thousands of extra-solar planet candidates. In fact, between 2009 and 2012, it detected a total of 4,496 candidates, and confirmed the existence of 2,337 exoplanets. Even after two of its reaction wheels failed, the spacecraft still managed to turn up distant planets as part of its K2 mission, accounting for another 521 candidates and confirming 157.

However, according to a new study conducted by a pair of researches from Columbia University and a citizen scientist, Kepler may also have also found evidence of an extra-solar moon. After sifting through data from hundreds of transits detected by the Kepler mission, the researchers found one instance where a transiting planet showed signs of having a satellite.

Their study – which recently published online under the title “HEK VI: On the Dearth of Galilean Analogs in Kepler and the Exomoon Candidate Kepler-1625b I” – was by led Alex Teachey, a graduate student at Columbia University and a Graduate Research Fellow with the National Science Foundation (NSF). He was joined by David Kipping, an Assistant Professor of Astronomy at Columbia University and the Principal Investigator of The Hunt for Exomoons with Kepler (HEK) project, and Allan Schmitt, a citizen scientist.

Artist’s impression of NASA’s Kepler spacecraft. Credit: NASA

For years, Dr. Kipping has been searching the Kepler database for evidence of exomoons, as part of the HEK. This is not surprising, considering the kinds of opportunities that exomoons present for scientific research. Within our Solar System, the study of natural satellites has revealed important things about the mechanisms that drive early and late planet formation, and moons possess interesting geological features that are commonly found on other bodies.

It is for this reason that extending that research to the hunt for exoplanets is seen as necessary. Already, exoplanet-hunting missions like Kepler have turned up a wealth of planets that challenge conventional ideas about how planet formation and what kinds of planets are possible. The most noteworthy example are gas giants that have observed orbiting very close to their stars (aka. “Hot Jupiters”).

As such, the study of exomoons could yield valuable information about what kinds of satellites are possible, and whether or not our own moons are typical. As Teachey told Universe Today via email:

“Exomoons could tell us a lot about the formation of our Solar System, and other star systems. We see moons in our Solar System, but are they common elsewhere? We tend to think so, but we can’t know for sure until we actually see them. But it’s an important question because, if we find out there aren’t very many moons out there, it suggests maybe something unusual was going on in our Solar System in the early days, and that could have major implications for how life arose on the Earth. In other words, is the history of our Solar System common across the galaxy, or do we have a very unusual origin story? And what does that say about the chances of life arising here? Exomoons stand to offer us clues to answering these questions.”

A montage of some of the potentially-habitable moons in our Solar System. From top to bottom, left to right, these include Europa, Enceladus, TItan and Ceres. Credit: NASA/JPL

What’s more, many moons in the Solar System – including Europa, Ganymede, Enceladus and Titan – are thought to be potentially habitable. This is due to the fact that these bodies have steady supplies of volatiles (such as nitrogen, water, carbon dioxide, ammonia, hydrogen, methane and sulfur dioxide) and possess internal heating mechanisms that could provide the necessary energy to power biological processes.

Here too, the study of exomoons presents interesting possibilities, such as whether or not they may be habitable or even Earth-like. For these and other reasons, astronomers want to see if the planets that have been confirmed in distant star systems have systems of moons and what conditions are like on them. But as Teachey indicated, the search for exomoons presents a number of challenges compared to exoplanet-hunting:

“Moons are difficult to find because 1) we expect them to be quite small most of the time, meaning the transit signal will be quite weak to begin with, and 2) every time a planet transits, the moon will show up in a different place. This makes them more difficult to detect in the data, and modeling the transit events is significantly more computationally expensive. But our work leverages the moons showing up in different places by taking the time-averaged signal across many different transit events, and even across many different exoplanetary systems. If the moons are there, they will in effect carve out a signal on either side of the planetary transit over time. Then it’s a matter of modeling this signal and understanding what it means in terms of moon size and occurrence rate.”

To locate signs of exomoons, Teachey and his colleagues searched through the Kepler database and analyzed the transits of 284 exoplanet candidates in front of their respective stars. These planets ranged in size from being Earth-like to Jupiter-like in diameter, and orbited their stars at a distance of between ~0.1 to 1.0 AU. They then modeled the light curve of the stars using the techniques of phase-folding and stacking.

An artist’s conception of a habitable exomoon. Credit: NASA

These techniques are commonly used by astronomers who monitor stars for dips in luminosity that are caused by the transits of planets (i.e. the transit method). As Teachey explained, the process is quite similar:

“Basically we cut up the time-series data into equal pieces, each piece having one transit of the planet in the middle. And when we stack these pieces together we’re able to get a clearer picture of what the transit looks like… For the moon search we do essentially the same thing, only now we’re looking at the data outside the main planetary transit. Once we stack the data, we take the average values of all the data points within a certain time window and, if a moon is present, we ought to see some missing starlight there, which allows us to deduce its presence.”

What they found was a single candidate located in the Kepler-1625 system, a yellow star located about 4000 light years from Earth. Designated Kepler-1625B I, this moon orbits the large gas giant that is located within the star’s habitable zone, is 5.9 to 11.67 times the size of Earth, and orbits its star with a period of 287.4 days. This exomoon candidate, if it should be confirmed, will be the first exomoon ever discovered

The team’s results (which await peer review) also demonstrated that large moons to be a rare occurrence in the inner regions of star systems (within 1 AU). This was something of a surprise, though Teachey acknowledges that it is consistent with recent theoretical work. According to what some recent studies suggest, large planets like Jupiter could lose their moons as they migrate inward.

If this should prove to be the case, then what Teachey and his colleagues witnessed could be seen as evidence of that process. It could also be an indication our current exoplanet-hunting missions may not be up to the task of detecting exomoons. In the coming years, next-generations missions are expected to provide more detailed analyses of distant stars and their planetary systems.

An artist’s conception of a distance exomoon blocking out a star’s light. Credit: Dan

However, as Teachey indicated, these too could be limited in terms of what they can detect, and new strategies may ultimately be needed:

“The rarity of moons in the inner regions of these star systems suggests that individual moons will remain difficult to find in the Kepler data, and upcoming missions like TESS, which should find lots of very short period planets, will also have a difficult time finding these moons. It’s likely the moons, which we still expect to be out there somewhere, reside in the outer regions of these star systems, much as they do in our Solar System. But these regions are much more difficult to probe, so we will have to get even more clever about how we look for these worlds with present and near-future datasets.”

In the meantime, we can certainly be exited about the fact that the first exomoon appears to have been discovered. While these results await peer review, confirmation of this moon will mean additional research opportunities for Kepler-1625 system. The fact that this moon orbits within the star’s habitable zone is also an interesting feature, though its not likely the moon itself is habitable.

Still, the possibility of a habitable moon orbiting a gas giant is certainly interesting. Does that sound like something that might have come up in some science fiction movies?

Further Reading: arXiv

“Super Saturn” Has an Enormous Ring System and Maybe Even Exomoons

Astronomers watching the repeated and drawn-out dimming of a relatively nearby Sun-like star have interpreted their observations to indicate an eclipse by a gigantic exoplanet’s complex ring system, similar to Saturn’s except much, much bigger. What’s more, apparent gaps and varying densities of the rings imply the presence of at least one large exomoon, and perhaps even more in the process of formation!

J1407 is a main-sequence orange dwarf star about 434 light-years away*. Over the course of 57 days in spring of 2007 J1407 underwent a “complex series of deep eclipses,” which an international team of astronomers asserts is the result of a ring system around the massive orbiting exoplanet J1407b.

“This planet is much larger than Jupiter or Saturn, and its ring system is roughly 200 times larger than Saturn’s rings are today,” said Eric Mamajek, professor of physics and astronomy at the University of Rochester in New York. “You could think of it as kind of a super Saturn.”

The observations were made through the SuperWASP program, which uses ground-based telescopes to watch for the faint dimming of stars due to transiting exoplanets.

The first study of the eclipses and the likely presence of the ring system was published in 2012, led by Mamajek. Further analysis by the team estimates the number of main ring structures to be 37, with a large and clearly-defined gap located at about 0.4 AU (61 million km/37.9 million miles) out from the “super Saturn” that may harbor a satellite nearly as large as Earth, with an orbital period of two years.

Watch an animation of the team’s analysis of the J1407/J1407b eclipse below:

The entire expanse of J1407b’s surprisingly dense rings stretches for 180 million km (112 million miles), and could contain an Earth’s worth of mass.

“If we could replace Saturn’s rings with the rings around J1407b,” said Matthew Kenworthy from Leiden Observatory in the Netherlands and lead author of the new study, “they would be easily visible at night and be many times larger than the full Moon.”

Saturn's relatively thin main rings are about 250,000 km (156,000 miles) in diameter. (Image: NASA/JPL-Caltech/SSI/J. Major)
Saturn’s relatively thin main rings are about 250,000 km (156,000 miles) in diameter. (Image: NASA/JPL-Caltech/SSI/J. Major)

These observations could be akin to a look back in time to see what Saturn and Jupiter were like as their own system of moons were first forming.

“The planetary science community has theorized for decades that planets like Jupiter and Saturn would have had, at an early stage, disks around them that then led to the formation of satellites,” according to Mamajek. “However, until we discovered this object in 2012, no one had seen such a ring system. This is the first snapshot of satellite formation on million-kilometer scales around a substellar object.”

J1407b itself is estimated to contain 10-40 times the mass of Jupiter – technically, it might even be a brown dwarf.

Further observations will be required to observe another transit of J1407b and obtain more data on its rings and other physical characteristics as its orbit is about ten Earth-years long. (Luckily 2017 isn’t that far off!)

The team’s report has been accepted for publication in the Astrophysical Journal.

Source: University of Rochester. Image credit: Ron Miller.

Note: the originally published version of this article described J1407 at 116 light-years away. It’s actually 133 parsecs, which equates to about 434 light-years. Edited above. – JM

On the Road to One Thousand Exoplanets

A quiet milestone in modern astronomy may soon come to pass.  As of today, The Extrasolar Planets Encyclopedia lists a current tally of 998 extrasolar planets across 759 planetary systems. And although various tabulations differ slightly, very soon we should be living in an era where over one thousand exoplanets are known.

The history of exoplanet discovery has paralleled the course of the modern age of astronomy. It’s strange to think that a generation has already grown up over the past two decades in a world where knowledge of extrasolar planets is a given. I remember hearing of the promise of such detections growing up in the 1970’s, as astronomers put the odds at detection of planets beyond our solar system in our lifetime at around 50%.

A "Periodic Table of Exoplanets" Credit: PHL @ UPR Arecibo.
A “Periodic Table of Exoplanets” Credit: PHL @ UPR Arecibo.

Sure, there were plenty of false positives long before the first true discovery was made. 70 Ophiuchi was the site of many claims, starting with that of W.S. Jacob of the Madras Observatory way back in 1855. The high proper motion exhibited by Barnard’s Star at six light years distant was also highly scrutinized throughout the 20th century for claims of an unseen companion causing it to wobble. Ironically, Barnard’s Star still hasn’t made it into the pantheon of stars boasting planetary worlds.

A portrait of the HR8799 planetary system as imaged by the Hale Telescope. (Credit: NASA/JPL-Caltech/Palomar Observatory).
A portrait of the HR8799 planetary system as imaged by the Hale Telescope. (Credit: NASA/JPL-Caltech/Palomar Observatory).

But the first verified claim of an exoplanetary system came from a bizarre and unexpected source: a pulsar known as PSR B1257+12, which was discovered to host two worlds in 1992. This was followed by the first discovery of a world orbiting a main sequence star, 51 Pegasi in 1994. I still remember getting my hands on the latest issue of Astronomy magazine— we got our news, often months later, from actual paper magazines in those days —announcing “Planet Discovered!” on the cover.

Most methods and techniques used to discover exoplanets rely on either radial velocity or dips in the light output of a star from a transiting world. Both have their utility and drawbacks. Radial velocity looks for shifts in the star’s spectra as an unseen companion tugs it around a common center of mass. Though effective, it can only place a lower limit on the planet’s mass… and it’s biased towards worlds in short orbits. This is one reason that “hot Jupiters” have dominated the early exoplanet catalog: we hadn’t been looking for all that long.

Another method famously employed by surveys such as the Kepler space telescope is the transit detection method. This allows a much more refined estimate of a planet’s mass and orbit, assuming it transits the disk of its host star as seen from our Earthly vantage point in the first place, which most don’t.

A size comparision of exoplanets versus composition. (Credit: Marc Kuchner/NASA/GSFC).
A size comparision of exoplanets versus composition. (Credit: Marc Kuchner/NASA/GSFC).

Direct detection via occulting the host star is also coming of age. One of the first exoplanets directly imaged was Fomalhaut b, which can be seen changing positions in its orbit from 2004 to 2006.

Gravitational microlensing has also bared planetary fruit, with surveys such as MOA (Microlensing Observations in Astrophysics) and OGLE (the Optical Gravitational Lensing Experiment) catching brief lensing events as an unseen body passes in front of a background star. Distant free-ranging rogue planets can only be detected via this method.

More exotic techniques also exist, such as relativistic beaming (sounding like something out of Star Trek). Other methods include searches for tiny light variations as an illuminated planet orbits its host star, deformities caused by ellipsoidal variations as massive planets orbit a star, and infrared detections of circumstellar disks. We’re always amazed at the wealth of data that can be teased out of a few dim photons of light.

A scatter plot of exoplanet discoveries as of 2010 mass versus semi-major axis. Select exoplanets are labeled. A majority were detected via radial velocity (blue) and the transiting method (green). The remainder were detected by other methods (click here for a full discription). Graph in the Public Domain.
A scatter plot of exoplanet discoveries as of 2010 displaying mass versus semi-major axis. Select exoplanets are labeled. A majority were detected via radial velocity (blue) and the transiting method (green). The remainder were detected by other methods (click here for a full description). Graph in the Public Domain.

Universe Today has grown up with exoplanet science, from reporting on the hottest, fastest, and other notable “firsts”. A bizarre menagerie of worlds are now known, many of which defy the imagination of science fiction writers of yore. Want a world made of diamond, or one where it rains glass? There’s now an “exoplanet for that”.

Exoplanet news has almost gone from the incredible to the routine, as Tatooine-like worlds orbiting binary stars and systems with worlds in bizarre resonances are announced with increasing frequency.

Exoplanet surveys also have a capacity to peg down that key fp factor in the famous Drake equation, which asks us “what fraction of stars have planets”. It’s been long suspected that stars with planets are the rule rather than the exception, and we’re just now getting hard data to back that assertion up.

Missions, such as NASA’s Kepler space telescope and CNES/ESA CoRoT space telescope have swollen the ranks of extrasolar worlds. Kepler recently ended its career staring off in the direction of the constellations Cygnus, Hercules and Lyra and still has over 3,200 detections awaiting confirmation.

Exoplanet discoveries by year as of October 2013, color coded by method. Blue=radial velocity, Green=transiting, Yellow=timing, Red=direct imaging, Orange=microlensing
Exoplanet discoveries by year as of October 2013, color coded by method. Blue=radial velocity, Green=transiting, Yellow=timing, Red=direct imaging, Orange=microlensing

But is a given world Earthlike, or just Earth-sized? That’s the Holy Grail of modern exoplanet detection: an Earth-sized world orbiting in a star’s habitable zone. We’re cautious every time the latest “Earth-twin” makes its way into the headlines. From the perspective of an intergalactic astronomer, Venus in our own solar system might appear to fit the bill, though I wouldn’t bank the construction of an interstellar ark on it and head there just yet.

Exoplanet science has definitely come of age, allowing us to finally begin characterization of solar systems and give us some insight into solar system formation.

But perhaps what will be the most enduring legacy is what the discovery of extrasolar planets tells us about ourselves. How common (or rare) is the Earth? How typical is the story of our solar system? If the “first 1,000” are any indication, we strongly suspect that terrestrial planets come in enough distinct varieties or ”flavors” to make Baskin Robbins envious.

And the future of exoplanet science looks bright indeed. One proposed mission, known as the Fast INfrared Exoplanet Spectroscopy Survey Explorer, or FINESSE, would target exoplanet atmospheres, if given the go ahead for a 2017 launch. Another proposal, known as the Wide Field Infrared Survey Telescope, or WFIRST, would search for microlensing events starting in 2023. A mission that scientists would love to fly that always seems to be shelved is known as the Terrestrial Planet Finder.

But the exoplanet hunting mission that’s closest to launch is the Transiting Exoplanet Survey Satellite, or TESS. Unlike Kepler, which stares at a single patch of sky, TESS will be an all-sky survey looking at a half million stars.

We’re also just approaching an era where spectroscopy may allow us to detect exomoons and the chemistry taking place on these far off exoworlds. An example of an exciting discovery would be the detection of a chemical such as chlorophyll, a chemical that we know on Earth only exists as the result of life. But what a tantalizing discovery a blip on a graph would be, when what we humans really want to see is the vista of those far-flung alien forests!

Such is the exciting era we live in. Congratulations, humanity, on detecting 1,000 exoplanets… here’s to a thousand more!

Exomoons? Kepler‘s On The Hunt

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Recently, I posted an article on the feasibility of detecting moons around extrasolar planets. It was determined that exceptionally large moons (roughly Earth mass moons or more), may well be detectable with current technology. Taking up that challenge, a team of astronomers led by David Kipping from the Harvard-Smithsonian Center for Astrophysics has announced they will search publicly available Kepler data to determine if the planet-finding mission may have detected such objects.

The team has titled the project “The Hunt of Exomoons with Kepler” or HEK for short. This project searches for moons through two main methods: the transits such moons may cause and the subtle tugs they may have on previously detected planets.

Of course, the possibility of finding such a large moon requires that one be present in the first place. Within our own solar system, there are no examples of moons of the necessary size for detection with present equipment. The only objects we could detect of that size exist independently as planets. But should such objects exist as moons?

Astronomers best simulations of how solar systems form and develop don’t rule it out. Earth sized objects may migrate within forming solar systems only to be captured by a gas giant. If that happens, some of the new “moons” would not survive; their orbits would be unstable, crashing them into the planet or would be ejected again after a short time. But estimates suggest that around 50% of captured moons would survive, and their orbits circularized due to tidal forces. Thus, the potential for such large moons does exist.

The transit method is the most direct for detecting the exomoons. Just as Kepler detects planets passing in front of the disc of the parent star, causing a temporary drop in brightness, so too could it spot a transit of a sufficiently large moon.

The trickier method is finding the more subtle effect of the moon tugging the planet, changing when the transit begins and ends. This method is often known as Timing Transit Variation (TTV) and has also been used to infer the presence of other planets in the system creating similar tugs. Additionally, the same tugs exerted while the planet is crossing the disk of the star will change the duration of the transit. This effect is known as Timing Duration Variations (TDV). The combination of these two variations has the potential to give a great deal of information about potential moons including the moon’s mass, the distance from the planet, and potentially the direction the moon orbits.

Currently, the team is working on coming up with a list of planet systems that Kepler has discovered that they wish to search first. Their criteria are that the systems have sufficient data taken, that it be of high quality, and that the planets be sufficiently large to capture such large moons.

As the team notes

As the HEK project progresses, we hope to answer the question as to whether large moons, possibly even Earth-like habitable moons, are common in the Galaxy or not. Enabled by the equisite photometry of Kepler, exomoons may soon move from theoretical musings to objects of empirical investigation.