NExSS: A ‘Virtual Institute’ for Deep Space

The Nexus for Exoplanet System Science, or NExSS, is a collaborative initiative from NASA to draw on the collective talent of researchers from ten universities, three NASA centers and two research institutes. Conceived as a ‘virtual institute,’ the effort is both geographically diverse and multidisciplinary in nature, focusing not only on the search for exoplanets but the attempt to analyze planetary environments and find life. Jim Green, NASA’s Director of Planetary Science, explains the concept:

“This interdisciplinary endeavor connects top research teams and provides a synthesized approach in the search for planets with the greatest potential for signs of life. The hunt for exoplanets is not only a priority for astronomers, it’s of keen interest to planetary and climate scientists as well.”

NExSS draws on the collective expertise of its participants in the areas of Earth science, planetary science within our Solar System, heliophysics and astrophysics to create what NASA is calling a ‘system science’ approach aimed at advancing the search for life on exoplanets. The effort will be led by Natalie Batalha of NASA’s Ames Research Center, Dawn Gelino with NExScI, the NASA Exoplanet Science Institute, and Anthony del Genio of NASA’s Goddard Institute for Space Studies. This NASA news release gives a brief summary of the different teams, selected from proposals submitted to the agency, and the subject of their work.


Image: The search for life beyond our solar system requires cooperation across scientific disciplines — the way the UW-based Virtual Planetary Laboratory has been working since 2001. Now, NASA’s NExSS collaboration will take a similarly interdisciplinary approach to the search for life. Participants include those who study Earth as a life-bearing planet (lower right), those researching the diversity of solar system planets (left), and those on the new frontier, discovering worlds orbiting other stars in the galaxy (upper right). Credit: NASA.

Sixteen projects in all have been funded through NExSS, and while I won’t try to go through all of them today, I do want to look at two a little more closely. At UC Berkeley, James Graham is leading a project that will include researchers at Stanford University. The collaboration, says Graham, draws together three different approaches to exoplanet hunting:

  • Direct imaging: The Gemini Planet Imager, for which Graham is project scientist, began its exoplanet survey at the Gemini South Telescope in November of last year, taking direct images of planets in the infrared.
  • Doppler methods: UC-Berkeley’s Geoff Marcy perfected the so-called ‘wobble’ technique that measures the effect of planets on the host star to deduce information about these worlds.
  • The transit method: Planets passing in front of their star as viewed from Earth have been detected in abundance by the Kepler mission, which has found almost 2000 of them.

The goal of the Berkeley project is to study the three methods and their overlap, using not only the Gemini South instrument but also the Keck Observatories in Hawaii and, when it becomes available, the Thirty Meter Telescope planned for a site next to Keck on Mauna Kea. “It is a wonderful confluence of multiple approaches to planet-hunting that allows us to detect planets that are both near and far from the host star,” adds Marcy, a reference to the Gemini Planet Imager’s ability to find worlds in much more distant orbits than transit or Doppler methods. UC-Berkeley’s news release is here.

At the SETI Institute, researcher Hiroshi Imanaka, a specialist in the chemistry of planetary atmospheres, is part of another team selected by NExSS out of the pool of proposals. Imanaka’s focus is on habitable zones, as he explains in this SETI Institute news release:

“One major thrust of the exoplanet community has been to find worlds orbiting in the so-called habitable zone. That’s the range of distances from a star where a planet could have temperatures permitting liquid oceans. However, liquid oceans are not the only condition under which life can exist. Some of the moons of Jupiter and Saturn are examples of places that are not in the conventional habitable zone, but might be nonetheless habitable. We want to take further steps to characterize habitable environments that lie beyond the solar system.”

In keeping with the wide-range encouraged by NExSS, Imanaka’s work draws on planetary science and astrobiology to study Titan in light of the clues it can provide about planets detected by teams like those at UC-Berkeley. His team is made up of Titan specialists who study the moon’s thick, smoggy atmosphere and exotic ethane seas. Whether or not Titan has life today, Imanaka considers it a ‘pre-biotic’ world that is producing the most complex organic compounds we know of outside Earth. The team hopes to tap the expertise of the NExSS network as it considers how conditions on Titan can inform our study of smoggy exoplanet atmospheres.

I’ll turn you back to the NASA news release for a breakdown of the other projects, which include Debra Fischer’s team at Yale working on spectrometers of Earth-detecting precision for nearby stars, Drake Deming’s team at Yale using Kepler data to study exoplanet atmospheres, and Jason Wright’s Penn State team studying ‘hot Jupiter’ atmospheres with a high-precision technique called ‘diffuser assisted photometry.’ The news release offers links to each.

The formation of NExSS comes at a time when we’re still making discoveries from the abundant Kepler data and anticipating future missions like TESS (Transiting Exoplanet Survey Satellite), which will, unlike Kepler, home in on exoplanet possibilities around nearby stars, and the James Webb Space Telescope, which will measure the infrared spectrum of starlight passing through the atmospheres of transiting exoplanets. Both missions are to be launched within the next few years. The Wide-field Infrared Survey Telescope (WFIRST), scheduled for launch in about ten years, will offer wide-field imaging and spectroscopic surveys of the near infrared sky. NExSS exemplifies the multidisciplinary approach necessary to pull all this incoming information together.


Project Dragonfly: Design Competitions and Crowdfunding

by Andreas Hein

Centauri Dreams readers most likely know Andreas Hein as the head of Project Hyperion, an effort for Icarus Interstellar to examine the prospects for manned interstellar flight, but he has also written in these pages about the uploading of consciousness. Now working on his PhD at the Technical University of Munich, Andreas today tells us about a new Kickstarter campaign in support of Project Dragonfly. Developing under the auspices of the Initiative for Interstellar Studies (of which Andreas is deputy director), Dragonfly explores interstellar flight at the small scale, and as he explains below, leverages the advances in computing and miniaturization that designers can use to change the paradigm of deep space missions.


Humanity has existed for over 200,000 years. It is only about 200 years since we entered the age of industrialization, and in the last 50 years, we have discovered ways of going to the stars [1]. However, the approaches conceived required spaceships the size of a tanker and massive space infrastructures. Today we live in a time when we may soon have the technological capabilities to launch a spacecraft to the stars, but now the spacecraft may be no bigger than a suitcase. Project Dragonfly is the first design study for an interstellar mission based on a small, laser-propelled spacecraft. In this article, I will explain the background of Project Dragonfly and the rationale for the Project Dragonfly Design Competition and crowdfunding campaign.

Many previous approaches for going to the stars have depended on extremely large and heavy spacecraft, based on propulsion systems like nuclear fusion or antimatter. Existing concepts of fusion-propelled spacecraft are as heavy as skyscrapers. Accelerating all the fuel with the ship until it is exhausted is actually not a very efficient way to get to the stars. Project Dragonfly aims at a different approach: The basic idea is not new – it is, in fact, very old. For centuries, humans have travelled the seas using sailing ships. We also plan to use a sail. But a sail which is made of an extremely thin reflective surface. This sail would be illuminated by a laser beam from a laser power station somewhere in the Solar System [2]. The photons of the laser beam push the sail, just as the wind pushes the sail of a sailing ship. And through this push to the sail, the spacecraft slowly accelerates.

However, as the spacecraft does not use any on-board fuel, it can accelerate to very high velocities in the range of several percent of the speed of light. Furthermore, Project Dragonfly builds upon the recent trend of miniaturization of space systems. Just a few decades ago, thousands of people were involved in developing the first satellite, Sputnik. Today, a handful of university students are able to build a satellite with the same capability as Sputnik, one that is much cheaper and weighs hundreds of times less than the first satellite. Recently, NASA announced a prize for the development of interplanetary CubeSats [3].

We simply think further. What could we do with these technologies in 20 or 30 years? Would it be possible to build spacecraft that can go to the stars but are as small as today’s picosatellites or even smaller? It is time to explore and innovate.


Image: Swarm of laser-sail spacecraft leaving the solar system. Credit: Adrian Mann.

History of the Project

The idea behind Project Dragonfly emerged in early 2013 when I visited Professor Gregory Matloff in New York. Matloff, author of The Starflight Handbook, is one of the key figures in interstellar research with major contributions to the area of solar sails. We talked about different propulsion methods for going to the stars and realized that nobody had yet done a mission design for an interstellar laser-propelled mission. Soon after this conversation, Project Dragonfly was officially announced by the Initiative for Interstellar Studies (i4is), with Kelvin F. Long and myself as co-founders.

However, it took another year to get to the point where we were able to organize an international design competition in order to speed up our search for a feasible mission to another star, based on technologies of the near future. One of the challenges for defining proper requirements for the competition was the development of preliminary mission concepts in order to identify performance drivers and showstoppers. Once these preparations were finished, we could launch the competition. See my essay Project Dragonfly: The case for small, laser-propelled, distributed probes for an early competition announcement and scientific backgrounder.

Why a Competition?

Design competitions have a long tradition in the history of technology: The Longitude rewards for the precise determination of a ship’s longitude at sea (1714), the Daily Mail prize for crossing the English Channel by an airplane (1908), the Orteig prize for crossing the Atlantic with an airplane (1919), and more recently DARPA’s Grand Challenge, which focuses on developing autonomous vehicles (2004). These competitions defined a set of requirements — most importantly performance requirements — the technology would need to fulfill. The solutions were up to the entrants.

Competitions offer the advantage that almost anyone can participate. This opens up opportunities for new entrants and innovative solutions, with the potential to disrupt the existing technological landscape [4]. Of course, with Project Dragonfly we are not yet at the level of the “Grand Challenges”. Nevertheless, the so-called Alpha Centauri Awards were announced by Kelvin Long in 2013, offering several cash prizes for relevant areas in interstellar travel. The Project Dragonfly Design Competition is one of the areas for which the Alpha Centauri Prize will be awarded [5].

The objective of the Project Dragonfly Design Competition is to develop a feasible mission design for a small, laser-propelled interstellar mission. Four international university teams are currently working on studies for small, laser-propelled interstellar spacecraft: Cairo University, the Technical University of Munich, the University of California Santa Barbara (UCSB), and Cranfield University, working with the Skolkovo Institute of Science and Technology (Skoltech) in Moscow and the University Paul Sabatier (UPS) in Paris. The final design reports of the teams will cover all areas that are relevant to make the mission a success and to return scientific data: Instruments, communication, laser sail design, power supply, secondary structure, deceleration propulsion, etc. Furthermore, both technological as well as economic feasibility will be assessed by the teams.


Image: The Project Dragonfly design team at Cairo University, made up of aerospace and communication engineering students, is one of four working on laser sail design.

The teams will meet with members of i4is in London at the headquarters of the British Interplanetary Society this July in order to evaluate their designs. The results from the competition will serve as a basis for future technology development of such a mission.

Project Dragonfly Kickstarter Campaign

The Initiative for Interstellar Studies has now launched the first ever Kickstarter campaign for supporting an interstellar design competition. The funds raised during the campaign will allow us to support the student teams during the competition. The campaign began on the 15th of April and will run until May 14th. All supporters will receive unique and exclusive rewards for their pledges, among them T-shirts and posters with an image of the winning team’s spacecraft, painted by the grand master of space art, David A. Hardy. The campaign is further supported by renowned space artist Adrian Mann. Further awards consist of a 3D-printed version of the winning team’s starship as well as early access to team reports, a signed version of the Beyond the Boundary book edited by Kelvin F. Long, etc.

The campaign can be accessed via:

Thanks a lot in advance for your support. Let’s find a way to get to the stars in our lifetime!


Image: An evocative futurescape by space artist David A. Hardy.


[1] Dyson, F. J. (1968). “Interstellar transport.” Physics Today, 21(10), 41-45.

[2] Forward, R. L. (1984). “Roundtrip interstellar travel using laser-pushed lightsails.” Journal of Spacecraft and Rockets, 21(2), 187-195.

[3] Staehle, R., Blaney, D., Hemmati, H., Lo, M., Mouroulis, P., Pingree, P., … & Svitek, T. (2011, August). “Interplanetary CubeSats: opening the solar system to a broad community at lower cost.” In CubeSat Workshop, Logan, UT, USA (pp. 6-7).

[4] Calandrelli, E. D. (2013). An evaluation of short innovation contest implementation in the federal context (Masters’ Thesis, Massachusetts Institute of Technology).

[5] Hein, A.M., Project Dragonfly Design Competition website


Voyager to a Star?


The latest imagery from New Horizons has me wondering what it must be like to be on the team for this mission. Although released a week ago, the photo at left was taken by the Ralph color imager aboard the spacecraft on April 9. The distance from Pluto and Charon in the shot is about 115 million kilometers. This is the first color image ever made of Pluto/Charon by an approaching spacecraft, one that gives us a sense of what lies ahead as the distance continues to diminish. Imagine being part of this long effort and seeing a new world and its system of moons swimming into focus, unveiling landscapes never before seen.

New Horizons takes me back to the Voyager days, and in the context of the approach to Pluto/Charon, the publication of Jim Bell’s The Interstellar Age (Dutton, 2015) is truly apropos (I’m sure the publishers had exactly this in mind). Subtitled “Inside the Forty-Year Voyager Mission,” the book lets us glimpse what it was like inside JPL when the planetary encounters occurred. Bell, now president of the Planetary Society’s board of directors, was caught up in Voyager as a young grad student at Caltech, a formative experience in his career as a planetary scientist. In describing new planets coming for the first time into detailed view, he turns to the words of an old friend:

“The sense of exploration we get with these missions is a very ‘human explorer’ kind of feeling, even though our senses are on the distant spacecraft,” my friend, planetary science colleague, and Voyager imaging team member Heidi Hammel says. “I feel like an old-fashioned mountain climber when I am making discoveries, seeing something for the first time, realizing that no human before me has ever seen what I am seeing. It takes your breath away— for just a moment you feel a pause in time as you know you are crossing a boundary into a new realm of knowledge. And then you plunge in, and you are filled with childlike joy and wonder and delight.”

Hammel goes on to say that you have to temporarily shelve that feeling if you’re a scientist working on the mission, at least while an active encounter is going on. But the wonder of the event is something you always have with you, and it’s something a good scientist wants to share. It’s interesting in the New Horizons context that Bell talks about Jon Lomberg’s One Earth: New Horizons Message project. What Lomberg has in mind for New Horizons, you’ll recall, is an uploaded message, a Voyager ‘Golden Record’ sent digitally into the spacecraft’s computer memory after its Solar System work is done (see New Horizons Message Update].

Could we do something like this for the Voyagers? It’s an intriguing notion. Lomberg pointed out to Bell that while the Golden Record will always be a part of Voyager, there is nothing on the spacecraft that tells of its remarkable passage through our Solar System, something that says ‘here’s what Voyager was and here’s what Voyager found.’ Bell would like to see us upload some of these historic photos. Imagine: The volcanoes of Io, the spectacular cliffs of Miranda, the bizarre cantaloupe terrain of Triton, all could be used to create what he calls an ‘electronic postcard’ that will complete the Voyager story for any future intelligence that finds them.

And is a trajectory change a possibility? This is interesting stuff. Right now, the Voyagers will take about 30,000 years to reach the outer edge of the Oort Cloud (the inner edge, according to current estimates, is maybe 300 years away). Add another 10,000 years and Voyager 1 passes some 100,000 AU past the red dwarf Gliese 445, which happens to be moving toward the Sun and will, by this remote date, be one of the closest stars to the Solar System.

As to Voyager 2, it will pass 111,000 AU from Ross 248 in roughly the same time-frame, at which point the red dwarf will actually be the closest star to the Sun. According to Bell, Carl Sagan and the team working on the Golden Record wondered whether something could be done about the fact that neither Voyager was headed for the interior of another Solar System. Is it possible that toward the end of the Voyagers’ active lifetimes (somewhere in the 2020s), we could set up a trajectory change that would eventually lead Voyager to a star?

The idea comes out of Sagan’s Murmurs of Earth (Random House, 1978), which Bell quotes as he describes the concept:

Both [Sagan and the Golden Record team] and I wonder if it might be possible to command one final “empty-the-tank” thruster firing, just before final communication with each Voyager is lost, to “redirect the spacecraft as closely as possible so that they will make a true encounter [with these stars]. If such a maneuver can be affected then some 60,000 years from now one or two tiny hurtling messengers from the strange distant planet Earth may penetrate into their planetary systems.” If no one else does, I will try to remember to make this request to Suzy Dodd or whoever is running the Voyager Project in a decade or so, as the spacecraft power levels wind down. We have the fuel. Feel free to mention it to your congressperson.


A trajectory change would increase only infinitesimally the faint chance that one of these spacecraft would someday be intercepted by another intelligent civilization, but the message of this maneuver is really to us. There is a certain magic in the idea that these venerable machines might one day be warmed by the light of another sun. In any event, I’m much in favor of Pioneer plaques, Golden Records, and One Earth Messages, as they remind us that our spacecraft are our emissaries as well as our scientific tools. How we conceive of them through the information they carry helps us gain perspective on ourselves, and shapes the context of all our future explorations. If we can do it, I’m all for giving the Voyagers one last, hard nudge into the unknown.


Transient Listening: A Caution

by James Benford

Searching for the faintest of signals in hopes of detecting an extraterrestrial civilization demands that we understand the local environment and potential sources of spurious signals. But we’ve also got to consider how signals might be transmitted, the burden falling on SETI researchers to make sense out of the physics (and economics) that constrain distant beacon builders. James Benford, CEO of Microwave Sciences and a frequent Centauri Dreams contributor, now looks at the problem in light of recent transients and discusses how we should move forward.

Jim Benford

The recent activity on Perytons leads us to a major lesson. We have a vast microwave network all around us that can interfere with transient radio astronomy. Our cell phones, though not powerful, influence the stronger transmitters and antennas of the cell phone towers. Add to that the many Internet hubs, microwave ovens, wireless equipment and extensive communication webs. All these may have fast transients with features that are largely unreported.

Filtering out extraneous sources is very important in the broader context of radio telescope astronomy, especially for the growing field of transient radio astronomy. There are many types of possible astronomical transients: of course pulsars, but also magnetars, rotating radio transients (RRATs), gamma ray burst afterglow in radio, etc.

And there may be ETI beacons, which are likely to be transient as well. This may apply especially for the beacons I, and my coauthors Gregory and Dominic Benford, have explored in earlier papers. We argued on economic grounds (both capital costs and operating costs) that they are likely to be transient. [See SETI: Figuring Out the Beacon Builders and A Beacon-Oriented Strategy for SETI, as well as the citations listed at the end of this article].

Traditional SETI research takes the point of view of receivers, not transmitters. This neglects the implications of a simple fact: a receiver does not pay for the transmitter; the sender determines what to build.

But most radio astronomy observers are unfamiliar with the technologies and techniques of transmitters, whether it’s commercial electronic equipment, military equipment or SETI beacon builders.


Image: Magnetar SGR 0418+5729 with a magnetic loop. Magnetars, a potential source of transients, are peculiar pulsars – the spinning remnants of massive stars – that are characterized by unusually intense magnetic fields. Astronomers discovered them through their exceptional behavior at X-ray wavelengths, including sudden outbursts of radiation and occasional giant flares. Credit: ESA/ATG medialab.

A Missing Piece to the Puzzle

A specific example: Identifying the source of Perytons at the Parkes radio telescope – the microwave ovens – is incomplete. It misses identifying a cause for the observed frequencies received following a descending curve, flattening a little at later times. That’s approximately like the ‘dispersion measure’ (DM) due to interstellar plasma. [see Perytons: A Microwave Solution].

How can that happen in a microwave oven? A jerked-open oven door cuts off the voltage V to the magnetron. The frequency emitted from a magnetron scales as V/B, with the magnetic field B fixed by a permanent magnet. Voltage is proportional to frequency (f~V), so emitted frequency falls as the voltage declines and turns off. The magnetron passes through several lower frequency modes, which explains why the intensity of the radiation varies up and down as modes wax and wane.

Electron emission in the magnetron also declines as the cathode cools. The emitted frequency thus mimics a dispersion measure. (Note that amplifiers, such as the klystrons within transmitters used in the Deep Space Network and planetary radars, won’t show such behavior because of the way they operate. All oscillators, such as magnetrons, can behave this way.)

A Parallel

Therefore there is a parallel between what has now been found about Perytons and what we found about SETI beacons: Five years ago we looked at them from the point of view of those who would build beacons, as opposed to those observers who presume that beacons are all designed around the observers’ convenience or requirements. Similarly, the searchers after Perytons didn’t understand microwave sources – such as microwave ovens – and therefore missed that possibility for some years. People who are doing radio astronomy are usually not conversant with microwave radiating technologies. Alas, in the paper the Swinburne group just published, they still say that the cause of the emissions is “obscure.” It isn’t. Knowledge of how magnetrons work would have led to understanding their “dispersion measure.”

Because of the increasing emphasis in radio astronomy on searching for transients of many varieties, the point of view has to change. Radio astronomers had best study the transient background in detail, to eliminate false positives in their search for unusual astrophysical events. And the Peryton episode is a reminder for SETI that every possible alternative has to be explored before fingers are pointed at an extraterrestrial explanation.

This could be a bit tedious, but it’s essential. And it could avoid future embarrassments. If it’s any consolation, any transmitting aliens hailing us may have considered this as well. Possible implications for our search strategies should be explored.

For more on the Benfords’ papers on beacons, see James Benford, Gregory Benford and Dominic Benford, “Messaging with Cost-Optimized Interstellar Beacons,” Astrobiology Vol. 10, No. 5 (2010), pp. 475-490 (abstract / preprint), and the same authors’ “Searching for Cost-Optimized Interstellar Beacons,” Astrobiology Vol. 10, No. 5 (2010), pp. 491-498 (abstract / preprint).


Exomoons: A Data Search for the Orbital Sampling Effect and the Scatter Peak

Exomoons continue to be elude us, though they’re under intense study. One detection strategy is called Orbital Sampling Effect, as explained in the article below. I’ll let Michael Hippke describe it, but the intriguing fact is that we can work with these methods using existing datasets to refine our techniques and actively hunt for candidates. Michael is a researcher based in Düsseldorf, Germany. With a background in econometrics, statistics and IT, he mastered data analysis at McKinsey & Company, a multinational management consulting firm. These days he puts his expertise to work in various areas of astrophysics, and most recently appeared here in our discussion of his paper on Fast Radio Bursts (see Fast Radio Bursts: SETI Implications?).

by Michael Hippke


Our own Solar System hosts 8 planets (plus Pluto and other “dwarf planets”), but 16 large moons with radii over 1,000km. And we have detected thousands of exoplanets – planets orbiting other stars – but not a single exomoon. The question of their existence is interesting, as some exomoons might in fact be habitable. Lately, there has been some speculation that, overall, there might be more habitable moons than planets in the universe. Consequently, we really want to know more about moons!

Moons are, by definition, smaller than their host planets, and thus harder to detect. Various search methods have been proposed – with the HEK project (Hunt for Exomoons with Kepler), led by David Kipping, being the most prominent team. A novel, promising method has been developed by René Heller in 2014, dubbed the “orbital sampling effect” (OSE). As with exoplanet transits, this method stacks many (dozens or ideally hundreds) of planet transits, and searches for the signature of a moon in this stack. While planet transit shapes are rather simple, the moon curves turn out the be very complex.


Image: A star with a transiting planet and its moon. The angled area shows the inclination of the moon orbit. Orbit positions beyond the dashed line are not undergoing transit, and are thus not observable.

In my recent work, I have processed data from the Kepler space telescope to search for this effect. I also worked with the “scatter peak,” an exomoon detection method described by Attila Simon (Konkoly Observatory, Hungary) and team in 2012. It is based on the fact that the geometrical exomoon configuration is very likely different during every exoplanet transit: On some transits, the moon might be ahead of the planet, on other transits behind it. When stacking many transits, at a given phase folded time, one gets a flux loss in some cases, and not in others. This results in increased scatter (photometric noise) when compared to out-of-transit times.

While the sole use of the scatter peak is problematic due to stellar noise, it can be used to confirm or reject certain signals. Not surprisingly, the struggle against stellar noise, instrumental jitter and other glitches has required the development of a complex statistical framework. While the Kepler data quality is at the very limit for exomoon hunting, a few very interesting results could be achieved.

The first result is sensitivity. What moons can we detect with Kepler and the OSE? Learning the answer to this will be useful for the assessment of future time-series photometry space missions, such as TESS or PLATO 2.0. With Kepler, the limit seems to be about 0.3 — 0.4 Earth radii for a moon to be detected, which is about the size of Ganymede. In many cases, where the host stars are dimmer, or noisier, only larger moons can be detected. Despite these limitations, my work shows that the OSE is a promising method, which will one day, with better data quality and/or processing, likely succeed and find moons!


Image: The smallest radii detectable with the OSE in Kepler data are ~0.4 Earth radii. In many cases, the data and method only allows for the detection of larger moons. These are calculated limits, not real observations.

The second result is the ‘average moon’ effect. While no single moon could be detected, it is possible to “super-stack” a larger sample of planet-OSEs to estimate the average moon size in different samples. For very short-period planets with orbits shorter than about 15 days, no moons are seen. This is in agreement with stability arguments: The closer the planet to the star, the more the star “pulls” on the moon and tries to swallow it. The critical distance is not perfectly clear, but believed to be at ~15-day orbits. In my analysis, I find that the average moon signal comes up for periods over 35 days. In the sample of 35- to 80-day orbits, I find an average moon radius of about 2,000km (roughly like our moon). This estimate doesn’t tell how many planets actually have moons, or how many multiple moon systems are included in this average. It is for future studies (and telescopes) to determine this. But it is exciting that one can try.

The third result is about individual candidates. A small sample of planets shows prominent OSE-like signals justifying an in-depth analysis. It must be clearly said that, very likely, all of these will turn out to be false-positives. For some cases, it might even be possible to show that they cannot be moons, for example because some configurations are not stable over longer time frames. But this is not a bad result, for when we find false-positives, we can add the detection mechanism for these to our algorithm, and improve future searches.


Image: Planet transit (straight line), moon effect due to the OSE (dashed line) and real datapoints (dots with error bars). In this case of Kepler-264b, the data are in favour of a moon interpretation, although this cannot be considered a detection, as detailed in the paper.

Personally, I would expect that the first moon(s) that will be found will be at the long (large/massive) end of exomoon distribution, as was the case for exoplanets. This comes from a selection bias: Large things are easier to see, and will thus be detected first. It will not mean that all moons are giants, as not all planets are Hot Jupiters (which were the first planets detected). Interesting times are ahead!

For more information, the paper is Hippke, “On the detection of Exomoons: A search in Kepler data for the orbital sampling effect and the scatter peak.” It has been accepted by the Astrophysical Journal for publication. A preprint is available.