Is Our Civilization Detectable?

I haven’t even finished the first line of this post and I’m already in a digressive mood. The mental sidetrack comes from yesterday’s talk about the Square Kilometer Array, whose primary installations are now to be built in both South Africa and Australia. By observing an object through many instruments simultaneously, astronomers can use the technique called interferometry to combine incoming data and emulate a much larger instrument. The SKA’s sensitivity promises to be high enough to allow the detection of possible leakage radiation from another civilization, which prompted me to recall a quote I had buried in my archives:

“I know perfectly well that at this moment the whole universe is listening to us — and that every word we say echoes to the remotest star.”

The words are those of Jean Giraudoux, a French writer and diplomat whose plays, written between the two world wars, gained him an international audience (Christopher Fry was among the admirers who adapted Giraudoux’s work into English). Here the play is The Madwoman of Chaillot, a dizzying fantasy about corrupt business people who are trying to dig up Paris to retrieve its supposed oil reserves, and Countess Aurelia, the eccentric who with the help of an oddball ensemble of misfits puts these ‘wreckers of the world’s joy’ on trial. Giraudoux wrote the play in 1943 during the German occupation and died before he could see it performed.

Among the many plays, novels and essays the prolific Giraudoux wrote, this one eventually caught the eye of Carl Sagan, who used it to introduce a chapter about radio SETI in Intelligent Life in the Universe, the 1966 title he produced in collaboration with the Russian astronomer I.S. Shklovskii. I always enjoy paging through this book to see how our ideas of astrobiology and SETI have changed over the years. The SKA story invariably reminded me of Sagan’s concern over our own leakage radiation to the stars — he imagined the radio voice of Enrico Caruso traveling outward forever at the speed of light:

By now, the signal has propagated some 40 light years into space. If there is an advanced technical civilization within 20 light years of the Sun, they may have received that signal 20 years ago, correctly interpreted it as the result of another technical civilization, and immediately beamed their response to us. We should receive that signal any day now. But if the nearest technical civilization is many hundreds of light years away, we will have to wait a little longer.

And I have to add this entertaining riff, evidence that Sagan occasionally needed to lighten up:

…the characteristic signs of life on Earth which may be detectable over interstellar distances include the baleful contents of many American television programs and the mindless outpourings of rock-and-roll stations. It is a sobering thought indeed that the Beverly Hillbillies may be our only interstellar emissaries.

I’m not sure who it was who added Chuck Berry to the Voyager Golden Record, but whoever it was, good for him/her.

Whispers in the Night

People sometimes assume that stray signals would be easily snared at interstellar distances, but we’re learning that it would take a mammoth installation to make such a catch. The film Contact, made from Sagan’s novel of the same name, uses the wonderful device of a broadcast returned to us, a transmission from the 1936 Olympics in Berlin. Receiving such a signal parroted back to us would surely flag the detection of an extraterrestrial civilization and cause researchers to begin the necessary work to look for embedded information inside it.

The people behind the Square Kilometer Array talk about the ability of this instrument, once its vast telescopic resources are in place and connected to powerful computing facilities, to pick up something as weak as the extraterrestrial equivalent of an airport radar around another star. It’s a fantastic prospect, implying our ability to add a new layer to our existing SETI investigations. Is it possible that instead of scanning the skies for beacons, we might simply begin to pick up the extraneous signals of a civilization going about its daily life? The goal is energizing, but hearing claims about extraterrestrial detections always makes me uneasy.

Image: Signals from our civilization are gradually working their way into the galaxy. But would a conventional radio telescope be able to detect them at our present level of technology? Would the SKA?

Back in late 2010, James Benford discussed leakage radiation at a meeting of the Royal Society in Britain, asking whether the kind of installations we currently have on Earth could detect signals this weak if sent from a nearby star. It turns out a typical radio telescope like the Parkes instrument in Australia, if located near Alpha Centauri, would not be able to detect our TV transmissions at all. Benford pointed out that signal information is transmitted in bands on each side of the central frequency and that broadcast antennae aim their transmitted power mostly toward the surface. Signals that get into space are not coherent and are unlikely to be noted.

Sizing up the SKA

We’ll learn much more about the Square Kilometer Array as its various components come online in the late years of this decade and beyond, but the paper presenting Benford’s analysis, written with John Billingham (SETI Institute), indicates that talk about picking up airport radars and other leakage radiation may be overly optimistic. From the Benford/Billingham paper:

The assertion of Loeb and Zaldarriaga (2006) that SKA can see leakage radiation at 100 pc (316 ly) is based on the assumption that the sources are continuous, so long integration times make the leakage detectable. However, this is not true of Earth leakage. Integrating over days to months doesn’t work when the TV station you’re observing is transmitting in your direction for a time typically ~hour, before it disappears around the limb of the Earth, as stated by Sullivan. Forgan and Nichol (2010) show that, even if Loeb and Zaldarriaga were right, the probability of detection is very low.

The SKA may be too small for the task of picking up leakage radiation after all. Back in 2010, Seth Shostak (SETI Institute) wrote an essay about the matter in the Huffington Post:

Evidence of our existence has already washed over about 15,000 star systems, as the FM, television, and radar signals that were first transmitted during the late 1930s wick into space.

That isn’t news to many, of course, but maybe this is: These signals are not hard to find. If there are any aliens within a few hundred light-years, these clues to our existence could be found with an antenna the size of Chicago. For any society able to threaten us across such distances, that’s a pretty easy construction project.

Shostak was writing in the context of Stephen Hawking’s concerns about the dangers of extraterrestrial contact, and he’s doubtless right that a society advanced enough to cross the interstellar gulf to threaten the Earth would find building a Chicago-sized antenna feasible. But in our own terms (and what I’m getting at in this post is what we can detect), such an antenna would blow the budget for the entire century, being far larger than what the Square Kilometer Array can provide. Benford and Billingham went to work on the cost issues involved, assuming that Shostak was right that it would take an antenna this large:

Certainly, with ever-larger antenna area, at ever greater cost, advanced ETI can detect us. From the above analysis, we calculate that at 50-ly range, the antenna area must be ~1 km2. To assess Shostak’s claim, note that Chicago’s area is 24,800 km2 = 2.48 1010 m2. At the present value of SKA antennas, 2.4 k$/m2, the cost is 60 T$, comparable to Earth’s GNP of 70 T$. So if comparable to us, ETI would have to devote their entire science budget for a time perhaps of order a century to build Shostak’s antenna, a sobering prospect.

Sobering indeed. Obviously, a sufficiently advanced civilization would be capable of technologies and budgets that defy our analysis, but Benford and Billingham help us answer the question of whether or not our present technology — or that of the near future — would be capable of detecting our own transmissions from a nearby star. The answer is almost certainly no. Leakage radiation is going to be tough to detect even with country-spanning installations like the SKA. And we might answer Giraudoux’s Madwoman of Chaillot by saying that perhaps every word we say really is echoing to the remotest star, but the civilizations capable of hearing those words are going to be so far beyond our powers as to defy the imagination.

The Benford and Billingham paper is “Costs and Difficulties of Large-Scale ‘Messaging’, and the Need for International Debate on Potential Risks” (abstract) / full text.


Thoughts on the Square Kilometer Array

We now know that the vast collection of radio dishes and antennae that will become the Square Kilometer Array (SKA) will be built on two sites, with the majority of dishes in Phase 1 (beginning in 2016) being constructed in South Africa, and further dishes added in Australia as the project develops. The SKA is to be a radio telescope of unprecedented sensitivity capable of sky surveys at frequencies from 70 MHz to 10 GHz. A SKA news release notes that “All the dishes and the mid-frequency aperture arrays for Phase II of the SKA will be built in Southern Africa while the low-frequency aperture array antennas for Phase I and II will be built in Australia.”

Combining the signals from the project’s dishes, mid-frequency aperture arrays and low-frequency aperture arrays will offer a telescope with a collecting area equivalent to a dish with an area of one square kilometer, a truly formidable observing platform. Phase 1 construction will involve about 10 percent of the array and will involve dishes and low-frequency aperture arrays. Phase II is to begin several years after Phase I is complete, but funding for this part of the project has not been guaranteed.

Image: The proposed Square Kilometre Array (SKA) radio telescope. Credit: AAP.

Astronomy Now‘s Keith Cooper looks at the recent announcement in South Africa and Australia Share SKA Spoils, noting that both the MeerKAT (Karoo Radio Telescope, in South Africa) and ASKAP (Australian SKA Pathfinder) precursor telescopes have already been built, prompting the advisory committee to choose inclusivity as the best option. The idea is to maximize existing investments in both countries, but I’m still surprised by the result. From Cooper’s article:

What makes their decision slightly controversial is that the organisation’s own Site Advisory Committee had ruled that while both bids were excellent, South Africa had the edge overall after considering criteria such as levels of radio interference and long term sustainability of a radio quiet zone, the physical characteristics of the site, long distance data network connectivity, operating and infrastructure costs as well as the political and working environment.

And from an article in South Africa’s Times Live:

The eagerly awaited decision now means that engineers can connect antennas at Australia’s core site at Mileura station, about 100 kilometres (60 miles) west of Meekathara in western Australia. Other antennae are distributed across Australia and New Zealand.

South Africa’s site in the arid Karoo region will now also be connected by a remote link to a network of dishes stretching across southern and eastern Africa and as far away as Ghana.

What the SKA Can Do

The SKA is one of the primary global science projects of the early 21st Century, with a charter to study the earliest eras of the universe, between the epoch of recombination, when charged electrons and protons first became bound (from which came the Cosmic Microwave Background we can study today) and the emergence of the first galaxies. But its proposed areas of investigation range widely, from the large-scale structure of the universe as affected by dark energy to the workings of pulsars and black holes. For our purposes on Centauri Dreams, it’s useful to focus on the role the SKA has to play in exoplanet studies, while its potential in the area of SETI is impossible to ignore.

Combining signals from widely separated antennae allows us to achieve high resolution at radio wavelengths, making the SKA a potent tool for investigating the habitable zones of Sun-like stars in their infancy. The developing array will be able to image the thermal emission from dust in the habitable zone and chart the flow of the small particles that eventually go into making planets. Imaging features in protoplanetary disks will help us track the formation of giant planets as they open up gaps in the dust, and should offer a look at both the core accretion model of planet formation — the slow growth of dust grains into rocks and planetesimals — and the gravitational instability model, in which planets grow out of disruptions in the surrounding disk.

As for what other kinds of signals the SKA might detect, the project’s planners seem most enthusiastic about SETI. The SKA website claims that its vast network will be sensitive enough to detect an airport radar on a planet 50 light years away. Indeed, a fully fleshed out SKA should be sufficiently sensitive to detect signals comparable to our own television transmitters operating on planets around the stars closest to the Sun. While traditional SETI has proceeded largely through a search for directed beacons, the SKA will allow a search for leakage radiation from nearby stars, while expanding the range of our search for beacons by a factor of 1000.

Given our own brief window of visibility at television wavelengths as we increasingly go to cable and satellite technologies, the prospect of detecting an alien television signal from the handful of stars near enough to make it possible is inconceivably remote. On the other hand, this is the first time we can make a good case that a civilization not much more advanced than our own could indeed pick out signs of intelligent life from all those old Milton Berle broadcasts we’ve sent.

Meanwhile, it’s fascinating to consider other surprises that the SKA might produce. One possibility is the detection of so-called ‘rogue planets’ that wander the interstellar deep without a star. Last year we looked at the work of Heikki Vanhamaki (Finnish Meteorological Institute), whose calculations showed that a gas giant rogue planet with a large moon could produce auroral effects detectable out to a range of 185 light years by an array as formidable as the Square Kilometer Array. Given the number of rogue planets that may be out there (Vanhamaki thinks there could be as many as 2800 within that 185 light year range), detection of at least a few may be a possibility (see Finding an Interstellar Wanderer for more on this work).


A Longer, Heavier Bombardment

We know that the early Earth was a violent place, but just how violent? The so-called Late Heavy Bombardment is thought to have occurred from 4.1 billion to 3.8 billion years ago, likely the result of asteroids being destabilized in their orbits by shifts in the orbits of the outer planets. That model is self-limiting, with the unstable asteroids being depleted over time and the Late Heavy Bombardment winding down, and it matches the dating of rocks from the lunar basins that show vivid evidence of the battering both Earth and Moon took.

But as I mentioned last week, the question of the length of the Late Heavy Bombardment is in play, with two papers in Nature suggesting that heavy impacts may have continued for a much longer time, perhaps half of the Earth’s history. William Bottke (Southwest Research Institute) and team are suggesting that during this early period, the inner edge of the asteroid belt was just 1.7 AU from the Sun — in a region called the E-belt, a largely extinct portion of the asteroid belt between 1.7 and 2.1 AU — rather than at today’s main belt inner edge distance of 2.1 AU. Asteroids dislodged from orbits here would have been ten times more likely to strike the Earth than asteroids from the region of today’s main belt. From the paper:

The main asteroid belt’s inner boundary is currently set by the v6 secular resonance at 2.1AU (one astronomical unit is approximately the Earth-Sun distance); objects entering this resonance have their eccentricities pumped up to planet-crossing values in less than a million years. Before the LHB, the giant planets and their associated secular resonances were in different locations, with the only remaining natural inner boundary being the Mars-crossing zone. Accordingly, the main asteroid belt may have once stretched into the E-belt zone as far as 1.7AU.

Destabilizing the E-belt asteroids, then, caused nearly all of them to be driven onto planet-crossing orbits over the next four billion years, with a small percentage of survivors winding up in the ‘Hungaria’ population of high-inclination asteroids. The record of major impacts is preserved in the melted droplets of debris called ‘impact spherules,’ which would have been scattered around the planet in the case of major events like the one causing the Chicxulub crater in Mexico, thought to have occurred 65 million years ago and potentially the event that caused the demise of the dinosaurs. You’ll recall it was Chicxulub that Tetsuya Hara (Kyoto Sangyo University) and colleagues used in their study of impact debris, as discussed here on Friday.

Image: Researchers are learning details about asteroid impacts going back to the Earth’s early history by using a new method for extracting precise information from tiny “spherules” embedded in layers of rock. The spherules were created when asteroids crashed into Earth, vaporizing rock that expanded as a giant vapor plume. Small droplets of molten rock in the plume condensed and solidified, falling back to the surface as a thin layer. This sample was found in Western Australia and formed 2.63 billion years ago in the aftermath of a large impact. Credit: Oberlin College/Bruce M. Simonson.

A period of bombardment emerges from the study of spherules that continued long past the time older theories assumed it had ended, making impact events a continuing player in the evolution of life. Brandon Johnson and Jay Melosh (Purdue University) add to this picture in the same issue of Nature by looking at similar spherules in rock formed between 2 billion and 3.5 billion years ago, finding evidence that the theory of an extended Late Heavy Bombardment makes sense. Johnson told Nature‘s Helen Thompson in a review of his work that it “… shows that a lot more big asteroids — meaning dinosaur-killer or larger — were hitting Earth well after the current idea of when it ended.” The paper supports Bottke’s analysis, with impacts continuing to be prolific in the Archaean eon, when photosynthesizing cyanobacteria were on the rise.

Are major impacts always killer events, or do they spur evolution by continually shaping the environment, introducing organics and other key materials as life is taking hold? From Nature‘s coverage of the story:

How life would have responded to a sustained barrage throughout this period is unclear. A giant impact would have come as a severe blow for some forms of early life, but it need not have been all bad news, says Steve Mojzsis, a geologist at the University of Colorado Boulder. That is because the energy deposited by the ongoing impacts could have created hot zones like those found near hydrothermal vents today. “These are great places for microbes,” says Mojzsis, who notes that some phylogenetic evidence suggests that the last common ancestors of all present-day life were heat-loving organisms.

The papers are Bottke et al., “An Archaean heavy bombardment from a destabilized extension of the asteroid belt,” Nature 485 (03 May 2012), 78-81 (abstract) and Johnson and Melosh, “Impact spherules as a record of an ancient heavy bombardment of Earth,” Nature 485 (03 May 2012), 75-77 (abstract). Be aware, too, of the Impact Earth! calculator, available online from Purdue to allow calculations of asteroid or comet impact damage based on the object’s mass. The new work should allow its engine to be fine-tuned.


Impacts Spreading Life through the Cosmos?

Still catching up after the recent series on antimatter propulsion, I want to move into some intriguing work on panspermia, the idea that life may spread throughout a Solar System, and perhaps from star to star, because of massive impacts on a planetary surface. Catching up with older stories means leaving some things unsaid about antimatter — in particular, I want to return to the question of antimatter storage, which in my mind is far more significant a problem even than antimatter production. But there’s time for that next week, and as I said yesterday, interesting stories keep accumulating and deserve our attention.

Planetary Ejecta and Trapped Microorganisms

What Tetsuya Hara (Kyoto Sangyo University) and colleagues put forth in a recent paper are their calculations about the ejection of life-bearing rocks and water into space from events like the possible ‘dinosaur killer’ asteroid impact some 65 million years ago, which involved an asteroid 10 kilometers in diameter. It’s a remarkable fact that materials can be knocked off one planetary surface and wind up on another, and in some quantity. Consider, for example, the 100 or so meteorites identified by their isotopic composition as being from Mars. They show marked similarity in chemical composition to Viking’s analysis of Martian surface rocks in 1976, and trapped gases in some closely resemble the Martian atmosphere.

So planets in the same system can exchange materials, and of course the Allan Hills meteorite found in Antarctica (ALH 84001), thought to have been ejected from Mars about 16 million years ago, caused quite a stir back in 1996 when scientists thought they had found evidence for microscopic fossils within it, an analysis that remains controversial. But whether or not ALH 84001 contained life, the discovery of various kinds of extremophiles here on Earth and the possibility that they could survive for long periods trapped in rocky debris leads to the idea that one world can seed another, and as we’ve seen in earlier posts on the topic, the idea goes back as far as the Greek philosopher Anaxagoras, with a revival of interest in the 19th Century.

Image: Would an asteroid impact like this drive life-bearing materials to other planets? Possibly so, but the bigger question is, would microorganisms be able to survive the trip? Credit: NASA.

Fred Hoyle and Chandra Wickramasinghe, who were proponents of continuing panspermia and the idea that life entering Earth’s atmosphere from outside could be a driver for evolution, would doubtless find Hara and team’s work fascinating. While the latter argue that solar storms could eject microbes from the upper atmosphere into space, they concede that bolide impacts would be the major driver:

Naturally, those meteors, asteroids or comets which strike with the strong force, would eject the most material into space. Thus it could be predicted that the asteroid or meteor which struck this planet 65 million years ago, and which created the Chicxulub crater (Alvarez. et al. 1979) would have ejected substantial amount of rock, soil, and water into space, some of which would have fallen onto other planets and moons, including stellar bodies outside our stellar system, Kuiper belt objects, Oort cloud objects, and possibly extrasolar planets…

And there’s the issue — just how much material would actually be transferred not just into the outer Solar System, but to nearby stars? Hara uses the Chicxulub crater event as the model for the kind of collisions that drive Earth materials into space, estimating that about the same amount of mass would be ejected from Earth as arrived in the original asteroid impactor.

Long Journey into the Dark

Remarkably, almost as much of the ejected materials make the journey to Europa as to our own Moon, an interesting outcome explained by Jupiter’s deep gravitational well, which in this case takes possibly biologically-laden material to a moon that contains an under-ice ocean. Another place of high astrobiological interest is Saturn’s moon Enceladus, with an internal body of water of its own, as evidenced by the geysers Cassini continues to monitor around its south pole. Here the numbers drop considerably but as many as 500-2000 Earth rocks may have reached Enceladus. Even distant Eris in the Kuiper Belt scarfs up 4 x 107 Earthly objects in one scenario.

These numbers vary according to which of two models the authors use, but in either case their figures show significant movement of Earth materials into the outer Solar System. Extending the model to Gliese 581 becomes a fascinating exercise because we have reason to believe that the ‘super-Earth’ Gl 581d may orbit in the outer edges of the habitable zone there. The result: As many as 1000 rocks may have made the million year journey to fall upon a planet in the Gl 581 system. Thus we have the possibility, however remote, of dormant microorganisms moving between one stellar system and another, to fall upon a planet that conceivably can support life.

Hara and team acknowledge the uncertainties in their calculations but insist that “…the probability of rocks originated from Earth to reach nearby star system is not so small.” If this is the case, their conclusion points to the possibility that life did not originate on Earth at all:

We estimate the transfer velocity of the microorganisms among the stellar systems. Under some assumptions, it could be estimated that if origin of life has begun 1010 years ago in one stellar system as estimated by Joseph and Schild (2010a, b), it could propagate throughout our Galaxy by 1010 years, and could certainly have reached Earth by 4.6 billion years ago (Joseph 2009), thereby explaining the origin of life on Earth.

This assumes that there are 25 sites where life began 1010 years ago, with biological materials spread through the galaxy by the same kind of impact events that caused the Chicxulub crater. Add to this recent work by Brandon Johnson (Purdue) and colleagues. They’ve been investigating layers of rock droplets called spherules, which may tell us better than craters about ancient impacts, including the size and velocity of the impacting object. An initial reading of their work shows that the Late Heavy Bombardment, thought to have occurred from 4.1 billion to 3.8 billion years ago as huge numbers of asteroids and comets hit the Earth, may have lasted longer than we have previously believed.

Was Chicxulub relatively minor compared to the size of some of these impacts? Evidently so, which would account for even more materials from our planet being pushed out into nearby space. The wild card in all this is the ability of microorganisms to survive not just the impact but the journey, and when it comes to interstellar panspermia, my own credulity is pushed to the breaking point. Although I’m running out of time this morning, I want to return to two papers in Nature that examine the Late Heavy Bombardment and the history of later impacts on our planet. We’ll home in on evidence for a longer bombardment era when Centauri Dreams returns next week.

The Hara paper is “Transfer of Life-Bearing Meteorites from Earth to Other Planets,” Journal of Cosmology 7 (2010), p. 1731 (preprint). Thanks to John Kilis for the original pointer to Hara and an update on the Nature work.


Disruptive Planets and their Consequences

One of the joys of writing a site like Centauri Dreams is that I can choose my own topics and devote as much or as little time as I want to each. The downside is that when I’m covering something in greater depth, as with the four articles on antimatter that ran in the last six days, I invariably fall behind on other interesting work. That means a couple of days of catch-up, which is what we’ll now see, starting with some thoughts on a possible planet beyond Neptune, a full-sized world as opposed to an ice dwarf like Pluto or Eris. This story is actually making the rounds right now, but it triggered thoughts on older exoplanet work I’ll describe in a minute.

It’s inevitable that we call such a world Planet X, in my case because of my love for the wonderful Edgar Ulmer film The Man from Planet X (1951), in which a planet from the deeps wanders into the Solar System and all manner of trouble — including the landing of an extraterrestrial on a foggy Scottish moor — breaks out. Of course, Planet X was also the name for the world Percival Lowell was searching for in the 1920s, a hunt that resulted in Clyde Tombaugh’s discovery of Pluto, although the latter occurred more or less by chance since Pluto/Charon isn’t big enough to cause the gravitational effects Lowell was examining.

So is there a real Planet X? Rodney Gomes (National Observatory of Brazil) has run simulations on the ‘scattered disk’ beyond Neptune and, factoring in oddities like the highly elliptical orbit of Sedna and other data points on these distant objects, Gomes believes a Neptune-class planet about four times as massive as Earth may be lurking in the outer system. Sedna, you may recall, has a perihelion of 76 AU but an aphelion fully 975 AU out — it’s on a 12,000 year orbital period! As for Gomes, his team has been looking at what they call ‘true inner Oort cloud objects’ for some time, seeing objects like Sedna as markers for the existence of a planet.

Gomes ran through the results of his simulations at an American Astronomical Society meeting in Oregon in May, keeping the Planet X hunt alive, and it’s worth noting that a Jupiter-class planet at about 5000 AU may also fit the bill (see Finding the Real Planet X). For that matter, the orbits of scattered disk objects may have another explanation besides an undiscovered planet. But thinking about Gomes’ work brought me around to Jason Steffen and team, whose new paper goes to work on a much different kind of gravitational effect, the disruption caused by a ‘hot Jupiter’ as it moves through a young Solar System and scatters smaller planets.

Realm of the Wandering Planets

Steffen (Fermilab Center for Particle Astrophysics) is digging into exactly what makes ‘hot Jupiters’ take up such extreme orbits. These are planets of Jupiter’s size and larger that whip around their stars in periods of just a few days. The question is how they got to their present position, for the assumption is that planets of this size had to form much further out in their system and then move inward. There are two mechanisms that could make this happen, one of which — a slow migration through a gas disk that would allow low-mass planets to likewise migrate inward, where they can be captured into mean-motion resonance with the gas giant — seems benign. These models suggest the presence of smaller worlds near the hot Jupiter.

Image: Artist’s concept of a hot Jupiter, likely a disrupter of any planets that encounter it. Credit: NASA.

The other model is lethal to the inner system. Here, the giant planet’s migration is caused by gravitational interactions with another gas giant that result in one of the worlds being flung into interstellar space, while the other migrates inward and disrupts the orbits of any inner-system worlds. This scenario is what the Steffen paper is suggesting, for the team’s analysis of 63 Kepler planets around solar-type stars in orbits of 6.3 days or less shows no evidence at all for nearby planets. If such worlds were there, they ought to be detectable through transit timing variations (TTV) unless they are smaller than the Earth, or much further out in the system.

To compare and contrast environments, the researchers took another sample of 31 Kepler planets with ‘warm Jupiters’ — planets of Jupiter size around the same kind of star, but with longer orbital periods of between 6.3 and 15.8 days. They also checked 222 Kepler ‘hot Neptunes.’ The result: Three of the 31 ‘warm Jupiter’ systems showed companion planets in the inner system, and fully one-third of the hot Neptune systems showed the presence of inner system planets. Finally, the team looked at 52 ‘hot Earths’ in the Kepler data for TTVs, testing whether hot Jupiters and smaller worlds like these might co-exist in mutually inclined orbits. They found no evidence for high-mass companions on inclined orbits in this scenario.

The authors see this as a boost to the ‘scattering’ model, the study suggesting that hot Jupiters are migrating worlds on initially highly elliptical orbits that scattered other planets out of the inner system before their orbits became circularized close to their stars. Short period, low-mass planets would seem to have a different formation history than hot Jupiters. From the paper:

Hot Jupiter systems where planet-planet scattering is important are unlikely to form or maintain terrestrial planets interior to or within the habitable zone of their parent star. Thus, theories that predict the formation or existence of such planets (Raymond et al. 2006; Mandell et al. 2007) can only apply to a small fraction of systems. Future population studies of planet candidates, such as this, that are enabled by the Kepler mission will yield valuable refinements to planet formation theories — giving important insights into the range of probable contemporary planetary system architectures and the possible existence of habitable planets within them.

If hot Jupiter systems have a different dynamical history than other planetary systems, as this work suggests, then we have a useful filter to apply to exoplanet studies. If it can be firmly established that the presence of a hot Jupiter means no planets in the habitable zone, we know our resources are best focused elsewhere when it comes to looking for terrestrial worlds. It’s too early to make that call now, but the evidence is mounting that in most cases hot Jupiters are killer worlds when it comes to young planets in the warm regions where life may occur.

The paper is Steffen at al., “Kepler constraints on planets near hot Jupiters,” Proceedings of the National Academy of Sciences 109 (21) 7982-7987 (2012). Abstract available.