Centauri Dreams

A planet that wanders through the night far from any star is a fascinating notion, one that resonates on some primal level with me because of my childhood viewing of the 1951 film The Man from Planet X. In the movie, a scientist on a remote Scottish moor observes a rogue planet as it approaches the Earth, and deals with a visitor from that world whose apparent good intentions are brought to ruin by a self-serving character intent on exploiting the situation. I doubt similar viewing of this old classic motivated many of my readers, but evidently the idea of a rogue planet does inspire thought, given how many people wrote me about new work on the idea of wandering planets.

The paper is by Dorian Schuyler Abbot and Eric Switzer (University of Chicago) and follows up studies of similar ‘dark’ planets by John Debes (Carnegie Institution) and Steinn Sigurðsson (Penn State) — more about the latter duo in a moment. For now, focus on the process. We know that planets can be thrown out of their solar systems because of their gravitational interactions with gas giants. Indeed, the idea of planetary migration, which implies accompanying ejection events at least some of the time, has become a standard in explaining system formation.

Habitability Between the Stars

The question that interests the researchers above is whether or not such a planet might become a home for life. If so, it could be a potential carrier of life everywhere it went, an example of interstellar panspermia. One possibility for habitability is a planet with an extremely high pressure hydrogen atmosphere, which could result in a greenhouse effect strong enough to maintain liquid water on the planetary surface thanks to geothermal heat. But Abbot and Switzer focus on sub-glacial liquid water on a planetary body roughly like the Earth. And let’s qualify that further: “By Earth-like, we mean specifically within an order of magnitude in mass and water complement, similar in composition of radionuclides in the mantle, and of similar age.”

Energy from an active mantle could create a habitable environment deep below the ice of this ‘lone wolf’ planet. The authors go on to comment:

We can imagine that the ice layer on top of an ocean on a Steppenwolf planet will grow until either it reaches steady-state or all available water freezes. Geothermal heat from the interior of the Steppenwolf planet will be carried through the ice layer by conduction, and potentially by convection in the lower, warmer, and less viscous portion of the ice layer. Since convection transports heat much more efficiently than conduction, the steady-state ice thickness will be much larger if convection occurs, making it harder to maintain a subglacial ocean.

To arrive at a world with an internal ocean, the scientists first calculate the thickness of the ice layer under the assumption that heat is lost solely through conduction, then go on to show that heat loss dominated by conduction would be a reasonable assumption. The micro-scale composition of the ice is the wild card here, and the authors acknowledge that the convection issue is problematic. They call the resulting world a ‘Steppenwolf’ planet because ‘any life in this strange habitat would exist like a lone wolf wandering over the galactic steppe.’ It’s an enchanting thought, this dark world moving through the deep carrying the spark of life, yet by calculating the heat flux from the core, the authors show that it is not beyond possibility.

Characterizing the Rogue Planet

So what would it take to produce the Steppenwolf planet? Assuming a world similar to Earth in water mass fraction, radionuclide composition and age, and assuming it has no frozen CO₂ layer, the world would have to be roughly 3.5 times as massive as the Earth. Supply it with ten times the amount of water or a thick, frozen CO₂ layer, and a mass only slightly larger than Earth’s is required for the liquid ocean to exist. But what really surprises me are the authors’ calculations on the potential lifetime of such a planet. Take a look:

A Steppenwolf planet’s lifetime will be limited by the decay of the geothermal heat flux, which is determined by the half-life of its stock of radioisotopes (⁴⁰ K, ²³⁸U, ²³²Th) and by the decay of its heat of formation. These decay times are ?1?5 Gyr, so its lifetime is thus comparable to planets in the traditional habitable zone of main-sequence stars.

We can imagine various ways for life to have found a home here, the most obvious being that it could originate when the planet was still within the solar system that gave it birth. But we also know that life can form around hydrothermal vents, another possibility the authors suggest. In either case, a rogue planet of this kind would be a spectacular mission destination, assuming we could find one passing close enough to our system. Abbot and Switzer calculate that a detection would be possible within roughly 1000 AU of the Sun, where ‘detection of reflected sunlight in the optical wavebands and IR follow-up present the only viable observational choices in the near term.’ What a detection it would be, and what a strange laboratory for interstellar life.

I mentioned the work of John Debes and Steinn Sigurðsson earlier because their own work suggests that internal heat could maintain an atmosphere and sustain a liquid ocean under the ice of a wandering planet. Debes went on to show through simulations that a planet with a large moon could actually survive the ejection process with the moon still in orbit, an additional factor re life because it would supply tidal energies that could cause the interior of the planet to warm. The case for life wandering the interstellar dark may not be so far fetched after all.

For more, see Abbot and Switzer, “The Steppenwolf: A proposal for a habitable planet in interstellar space” (preprint). The Debes and Sigurðsson work is “The Survival Rate of Ejected Terrestrial Planets with Moons,” Astrophysical Journal 668 (October 20, 2007), L167-L170 (abstract).

As a fan of I Love Lucy since childhood, I’ve always been pleased that this show — and not, say, Milton Berle or Sid Caesar — is the one always referred to when talking about Earth being detected by other civilizations. And when I first thought about it, the idea that there was a detectable bubble of TV transmission forging out into the galaxy since Lucy’s first show in 1951 seemed completely wondrous. I Love Lucy is 60 light years from us now, or will be with this October’s anniversary of that first show. I’ve always wondered what extraterrestrials would make of Fred Mertz.

The film Contact mines the theme of stray transmissions from Earth, although in the case of Sagan’s story, it’s the transmissions from the 1936 Olympics in Berlin that trigger the detection and subsequent transmissions to Earth. A writer and music critic who I’ve known over the years once asked me about the expanding wavefront of Earthly transmissions, pondering how marvelous it would be to somehow get out in front of it and reacquire for the ‘first’ time some of the legendary performances of Arturo Toscanini with the NBC Symphony Orchestra, hopefully receiving them in better condition than the noisy kinescope versions that were used to preserve them.

Our Signals from Afar

Of course, getting out in front of the wavefront presents a bit of a problem — you’d have to travel faster than light. So let’s talk about something more realistic, which is the actual status of those interesting signals from the dawn of television. Here I’m drawing on James Benford’s presentation to the Royal Society meeting “Towards a Scientific and Societal Agenda on Extraterrestrial Life,” which convened last October in Britain and included a debate on extraterrestrial messaging that was sent to me in DVD form by Astronomy Now editor Keith Cooper. Benford looks at what an extraterrestrial civilization would be able to detect from Earth.

Remember, now, we’re talking about accidental signals, so-called ‘leakage’ radiation that was never intended as a directed signal. Benford goes to work on the math to ask whether installations like those we have on Earth would be able, if located around a nearby star, to pick up what we have been sending. The answer is no. A typical large radio telescope like the Parkes instrument in Australia could not, from a vantage near Alpha Centauri, see video footage from Earth. I’ll send you to the paper for the math (and I’ll post the link as soon as it’s available), but here’s his conclusion:

Picking up signals from commercial radio and television broadcasts is difficult. Because they are not intended to broadcast into space; broadcast antennas aim most of their transmitted power toward the surface. Most signal information is transmitted in bands on each side of the central frequency. What little detectable power reaches space is from many sources, not at the exact same frequencies, but in bands constrained by regulation by governments. Therefore, they are not coherent, so phase differences cause them to cancel each other out at great range.

What about over-the-horizon radars built during the Cold War? Much of their power was indeed radiated into space, but they have been replaced by frequency-hopping spread spectrum broadband radars that would likewise be undetectable by any technology like ours. The highest power emissions, it turns out, are those from interplanetary radars used for asteroid searches. But these signals are not directed at nearby stars, and Benford quantifies the issue using the specs of the Arecibo radar telescope. Again, I will hold off on the math, but the conclusion is that ‘there is a negligible chance of ETI noticing our asteroid search radars.’

Sending Earth’s ‘Wow’ Signal

So what would it take for an extraterrestrial civilization to notice us? Seth Shostak is on the record as saying that within a few hundred light years, clues to our existence could be picked up with an antenna the size of Chicago. Benford’s analysis shows that building such an antenna, given what we know of the present value of building an installation like the Square Kilometer Array, would run up a cost comparable to the entire GNP of planet Earth. If ETI were at our level of development, then, its entire science budget would be consumed by the project.

What about the future? Some proposed activities might flag our presence, but they would be hard to pin down:

We should be mindful also of the future possibilities for increased leakage from Earth due to beaming power for space industrial purposes, such as power transfer. Examples are transferring energy from Earth-to-space, space-to-Earth, and space-to-space using high power microwave beams… Microwave beams have been studied for propelling spacecraft for launch to orbit, orbit-raising, and launch from orbit into interplanetary and interstellar space. The power levels are ~GW with high directivity, so that isotropic radiated power W~10¹⁷ watts, would dwarf anything yet emitted into space. Observing such activities would appear to ETI as transient events.

Reception of Directed Transmissions

Moreover, the same techniques of quantitative analysis show that even directed messages like the Cosmic Call message from the Evpatoria site in the Ukraine would be detectable (and only at a low data rate of 100 bit/sec) out to just 19 years even if observed with a facility the size of the Square Kilometer Array. Detectability, Benford notes, depends on the bandwidth of the transmission. Low data rates can show that the signal is artificial but also carry little information, while high data rates require high bandwidth and suffer greatly from noise.

To detect a low-bit-rate signal, a number of additional factors must swing into play, including a predisposition to be looking at our system in the first place so that ETI would concentrate resources on that small patch of sky where our Sun is located. ETI would also have to guess the bit rate of the message, and would have to figure out that the message used binary frequency-shift keying instead of any other modulation method. All in all, these are tough requirements, though such a message could serve to flag a technological society:

The content of [Alexander] Zaitsev’s messages [from Evpatoria] will not be recoverable as messages by ETI if their radio telescopes are comparable to ours. To be observable, the receiving area must be greater than the SKA we’re contemplating building, and then only at low data rates.

Extending the messages by repeating, so they last hours, allows ETI to integrate the signal, and detect its presence at ranges of 100’s of light years. But that obliterates the message content, producing a recognizable pulse of energy. That could be taken by ETI as an undifferentiated energy source that could be artificial. But it cannot be characterized as a message.

We can’t know what technologies more advanced civilizations might bring to bear, but it’s helpful to get some constraints on the radiation leakage issue as they pertain to our own technology. A civilization anywhere near our own level of development should not, by Benford’s figures, be aware of our existence despite our television, radar and intentional messaging activities. That’s an interesting thought as we ponder our own failure, thus far, to find any trace of an extraterrestrial presence through SETI. The SETI search is clearly going to be long and arduous.

Benford goes on to argue that working out the parameters of future broadcasts and developing a database and top-level summary of Earth’s radio and laser signature should become standard practice, allowing us to calculate the possibilities for reception. All of this has implications for SETI and METI. About these, and the question of what kind of civilization might receive our signals, expect more in the coming week.

Addendum: The Benford and Billingham paper, “Costs and Difficulties of Large-Scale ‘Messaging’, and the Need for International Debate on Potential Risks,” is now available online.