Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Tuesday, July 5, 2011

Pastures New

The dearth of recent posts is about to come to an end. Life, Unbounded now has a new home at Scientific American's crispy fresh blog network! This has been a while in the making and creates a whole new platform and audience for our discussions on the search for origins in the universe. Scientific American (with the incredible efforts of Bora Zivkovic) has put together an amazing network of bloggers to inform, enlighten, entertain, and amuse the world. It's rather humbling that Life, Unbounded gets to be a part of that, and it's going to be tremendous fun.

This site will remain however, as an archive and dormant outpost. The "new" blog will simply continue the content and style at a different address. Come along and see.

Saturday, June 4, 2011

Nematodes, Mars, and Moons

There's been a lot of interesting stuff going on recently, many items worthy of more detailed posts that will come in the future. The discovery of multi-cellular life in the Earth's subsurface is a huge one. Several species of nematode, including a new one adapted to extreme environments, have been identified in rock fracture water at kilometer depths in a South African gold mine. These are the same environments that are already known to host some remarkable microbial life, including the lonesome D. Audaxviator that relies on natural radioactivity to generate the chemical energy it requires. The tiny nematodes feast on such single-celled organisms and have been sealed off from other subsurface and surface environments for at least 10,000 years. Suddenly the prospects for "complex" life in planetary subsurfaces seem a whole lot better; especially on Mars.

Which brings us to an exceedingly interesting new result from the study of the formation rate of solar system planets. Mars in particular may have formed fast and early, in a mere 2 to 4 million years or so, as told by the hafnium-182 isotopic clock in martian meteorite samples. This places it into a very different category of object than the Earth. In effect it was and still is a large planetary embryo, even though we've all been calling it a small planet. Embryos have not undergone major collision with other similar bodies. By contrast, the Moon-forming impact that likely occurred on our homeworld (although modern reanalysis of lunar rock water content hints at possible complications) was part of the agglomeration process 4.5 billion years ago that puts Earth into a different section of the planetary zoo.

How fast different planets assemble is something we don't always remember to think about, but there are important implications. For example, if Mars did form this fast it would have coincided with a timespan in the young solar system where elements such as highly radioactive aluminum 26 would still be abundant (having been left behind from earlier supernova events that helped generate the very nebula out of which we formed). A young Mars could have been extra hot with internal radioactive decay, opening up the possibility of extensive interior melting and setting up a very different geophysical pathway compared to the Earth. This in turn could influence the nature of subsurface environments today, and their potential occupants.

Finally, on another related topic. An interesting paper appeared recently that employed a large ensemble of gravitational simulations to explore just how rare Earth-Moon type systems might be for planets in habitable zones around Sun-like stars. The Moon is a big satellite, and as such contributes significantly to the dynamical evolution of our planet - from its day-length to the gravitational tides that sweep across us every twelve hours. It also plays a role in the spin-axis stability of our planet (where our poles point) and therefore the long-term climate variation. It's a tricky problem to study for sure, but the results are intriguing. The kinds of giant impacts that can create Earth-Moon type systems may occur at rates of around 1 in 12 systems (with a range of possibly as many as 1 in 4, or as few as 1 in 45). This isn't hugely frequent, but in a galaxy with potentially millions of Earth-analogs that certainly adds up.

All three of these discoveries or studies are noteworthy because they help crack the door open a little further into our fast developing picture of not only our particular context as a habitable planet, but the opportunities for life elsewhere. Worms, isotopes, and collisions, it's a great mix.

Sunday, May 22, 2011


Busy week, the week that was. Catch me on NPR's All Things Considered offering up some thoughts on the current SETI situation. Also my posts have been slowed as I get into a final stretch on a new book project for a general audience. If you like astronomy, cosmology, black holes and astrobiology you're in for a treat - I hope. Hitting the stores in 2012, I'll be saying more about this over time.

As for right now, well.....

There's a certain poetry to the astronomical news that's been hitting the media about planets that may be doing precisely as their name implies (πλανήτης in case your Greek isn't as rusty as mine). Painstaking monitoring of about 50 million stars in our galaxy by the MOA and OGLE gravitational microlensing surveys have revealed a very substantial population of Jupiter-sized objects that orbit at least 10 times further from their stars than the Earth does the Sun, and may not even be bound to stars at all. Sumi et al. report these results in Nature, along with a nice commentary by Wambsganss.

Although only ten such candidates are actually detected the statistical implication (since lensing events are so incredibly rare) is that there are twice as many of these planetary bodies than normal main-sequence (hydrogen burning) stars in our main galactic terrain. That is a lot. Even more interestingly Sumi et al. claim that most of these objects, perhaps 75%, may be unbound from any parent stellar systems and are true wanderers. The basis for this argument comes from existing constraints on long orbit exoplanets. These are obtained from projects trying to directly image such worlds. Bottom line is that the imaging efforts around stars do not see as many planets as the microlensing results would imply, hence these objects have gotta be out in interstellar space.

Personally I think there is every reason to believe that there really is a huge population of free-floating planets out there. A couple of years ago various researchers came up with a framework for explaining exoplanetary orbital architectures that requires episodes of intense planet-planet gravitational interaction within a system. One consequence; lots of planets ejected away from their birth places. My colleague Kristen Menou and I dabbled in this to investigate the predictions for planet imaging surveys. It was fun. Many planetary systems may be born in configurations that are inherently dynamically unstable, chaotic. Flinging worlds to the void is a great way to 'cool' the system down.

So, apart from the wow-factor of rogue/free/unbound planets, there is real reason to chase and confirm this result. It could be a pivotal clue that tells us not only how most planetary system achieve their configurations, but also confirms that planet formation itself is an efficient process. You have to make planets a plenty around stars in order for this to all work. The alternative is also fascinating - perhaps these worlds form via a route more akin to that of stars and brown-dwarfs, another feather in the cap of gravitational accretion.

Sunday, May 15, 2011

Flippin' Planets

A year ago it started to become clear that a significant number of exoplanetary systems harbored retrograde worlds. These planets are seen on small orbits that have the opposite sense to the spins of the stars that host them. This is a peculiar and puzzling arrangement. We've all been brought up to think of planets and stars forming out of the same proto-stellar disk of material and that angular momentum is an almost sacrosanct quantity - can't fiddle around with that. Getting planets to go in the opposite direction is quite unsettling.

Clearly the universe is having none of that silly narrow-minded thinking. However, the question has been how to produce these retrograde worlds; hot Jupiters zipping the "wrong" way around their parent stars in closely circular orbits. A new paper by Naoz and colleagues presents some intriguing and thorough calculations about a couple of phenomena that could do the trick.

They consider pairs of large, gas giant sized, planets that start out in decently large prograde orbits. For example, the inner planet may orbit at 6 astronomical units (AU) from the star, the outer planet at about 60 AU. The trick is to follow what happens if those initial orbits are highly misaligned. In other words if the orbital planes of the two planets are more than 50 degrees inclined with respect to each other. In this type of configuration long term gravitational nudging between the planets - particularly the outer one on the inner one - can result in severe changes in the ellipticity or eccentricity of the inner planet's orbit, as well as its orientation. Over a span of a few hundred thousand years the orbit can not only shift from being nearly circular to being nearly "needle like" (highly elliptical) but its orientation can change dramatically. In the most extreme instances it can quite literally flip, shifting all the way through a 90 degree inclination to more than 90 degrees - in effect going retrograde.

This flip can occur during an eccentricity spike, which I think gives us some physical insight to what happens. At the far point (apastron) of a super high eccentricity orbit the planet is going to be almost hanging in space, slowing to a near standstill before plunging back star-wards. If already on a highly inclined orbit (close to that 90 degrees) then all it will take is a little tug from the outer planet in the right direction to flip the orbit over. It's a bit like that moment of indecision teetering at the top of a snowy hill with skis attached to feet. Left to take the piste, right to go to the bar.

At the same time, an extremely elliptical orbit will bring the planet zooming in from several AU to mere tenths of an AU from the star as the planet screeches through its periastron approach. Naoz et al have computed the strength of star-planet tidal forces and the associated energy dissipation and show that the inner planet can be yanked into a tight circular orbit around the star in an incredibly brief period. A little too close to the honey pot and you get trapped. Once this happens the inner planet is totally decoupled from the outer planet, which simply cannot get its sticky gravitational mitts on the inner world any more. The end result is a close orbiting retrograde hot Jupiter.

Remarkably Naoz et al.'s models even seem to produce about the right fraction of systems in which this will happen. The only hitch is how to set up the initial configuration of planets, particularly with the outer planet on such a large orbit and with such a large mis-alignment between their orbital planes. Luckily an earlier epoch of strong planet-planet gravitational scattering from within the zone of planet formation might just do the trick.

Planetary systems are just so incredibly diverse. And, once again, we find ourselves gazing at our own system and wondering just how far we can or should take the Copernican Principle. This boring little solar system of ours may yet turn out to be a little on the special side.

Monday, May 2, 2011

55 Cancri e: Small or Big?

Detecting and characterizing exoplanets remains an extremely challenging occupation. A very good example of this has been the recent detection of the transit of a planet around a star in the nearby stellar system 55 Cancri. This is an interesting place. It contains a binary star consisting of a G-dwarf and an M-dwarf separated by at least 1000 astronomical units. At only 41 light years away the G-dwarf is visible to the naked human eye and, conveniently, harbors at least 5 planets.

These worlds had been detected using radial velocity measurements, with the first detections back in 1997. A recent reanalysis of the notoriously tricky radial velocity data (made even more so with possible mean-motion orbital resonances in the system) by Dawson & Fabrycky suggested that the innermost planet "e" was actually on a much tighter orbit than had previously been thought - with the incredibly short orbital period of 0.74 days. This also suggested it was rather lighter than thought, a super-Earth of at least 8 Earth masses.

This was intriguing because the probability of a transit increases with decreasing orbital radius. In this case it went up by a factor of three to about 33%. This has prompted a number of groups to go look for the transit again, after earlier attempts came up empty. Lo and behold the first reported transit of 55 Cancri e appeared from Winn et al. on April 27th 2011. Using the visible light telescope of the Canadian MOST mission they spotted a transit right on queue. Hopefully Dawson and Fabrycky had some champagne at the ready.

Analyzing the depth of the transit curve (a tiddly drop in starlight of about 0.02%) Winn and colleagues came up with a planetary radius of approximately 1.6 times the size of the Earth. Combined with a mass of almost 9 times that of the Earth this implies a remarkable density of about 11 grams per cubic centimeter. That's almost as dense as lead, and a clear indicator that this is an almost pure rock-iron composition.

At least that would be the case, except on May 2nd 2011 another paper appeared by Demory and colleagues that uses the Spitzer infrared telescope to detect the very same transiting planet. With this different telescope and different waveband, a transit depth is found that indicates 55 Cancri e is about 2.1 times the radius of the Earth. The statistics are good enough that this is a "3-sigma" difference from the MOST detection, indicating that this difference in measured radius would only occur by random error about 0.3% of the time.

So, with a radius of 2.1 times that of the Earth this devious planet would have to contain a significant amount of volatiles and be much less dense. Possible compositions would involve a rocky-iron Earth-like core surrounded by either 20% by mass of water or 0.1% by mass hydrogen and helium. The problem is that 55 Cancri e is so close to its parent G-dwarf star that it should have an effective temperature of about 2000 Kelvin - all over. Hardly an environment for anything that could be called a "volatile".

Its quite a conundrum. Demory et al. perform their analyses very carefully. The most likely explanation - and one that Demory and colleagues offer up - is that 55 Cancri e harbors an extended atmosphere. Perhaps carbon monoxide or carbon dioxide as a super heated "exosphere" could produce an anomalous infrared surface that makes the planet look fatter than it is. That would likely indicate some interesting compositional properties if the planet is indeed as dense as the optical data suggest. Winn et al. investigate the constraints on an atmosphere from their data. It's very interesting. The odds seem slim. The only option appears that if such an atmosphere exists it is being constantly replenished by perhaps geophysical activity on the planet itself. The current status is therefore open to question.

There is hope however. Because the star 55 Cancri shines so brightly in our skies it provides a marvelous flood of photons and the potential for extremely high quality astronomical observations. Unlike many of the mysterious new worlds we merely catch glimpses of, this one may well reveal its secrets to us in due course. Exoplanetary science continues to grow in richness.

As I write this 55 Cancri is up in the western sky of the Eastern seaboard of the United States, amongst the constellation of Cancer, the Crab. Perhaps you should run out in case you can spot it - you never know, you might just catch a transit.

Thursday, April 28, 2011

SETI Lost and Found

The announcement that the primary instrument in the Search for Extraterrestrial Intelligence (SETI), the Allen Array, is going offline for lack of money is a sobering reminder of the challenges faced by high risk science. It seems to be the result of a confluence of cuts and declines in both federal funding of the observatory site from the National Science Foundation and the drastic belt-tightening of a virtually insolvent state of California.

Others have written about all the great science that the Allen Array can be used for in addition to hunting for artificial signals. Paul Gilster at Centauri Dreams gives a pitch perfect discussion of this. I think it's also important to remember that SETI has helped pioneer voluntary distributed computing with their SETI@Home project. This began in 1999, the veritable stone age of what we might now call "crowdsourcing". Next time you fire up your iPhone or Android for information on local hot-dog stands you should think about that. By most standards SETI has produced very significant spin-offs along the way since it began in modern form in the early 1960's.

What about the root motivation for SETI though? Is there simply insufficient enthusiasm among scientists and the public to sustain an effort like this, even if it's to the tune of a just few million dollars a year? [A note to cynical misers; American Idol rakes in over $1 billion each year for its various interested parties. You could finish building the Allen Array and fund SETI in perpetuity for significantly less than that].

I feel that SETI has always been a hard sell. Spend an hour talking to one of its key proponents like Jill Tarter and one comes away utterly convinced that this is a vital thing for humanity to pursue. But over the following days and weeks then in all honesty doubt does tend to creep back in. This doubt is multi-faceted. There is the high-risk nature of the endeavor, the risk being the perception that as remarkable as a detection would be, the odds of actually getting it are so very small. Then there is the problem that there are so many unconstrained parameters involved, which just exacerbates the risk assessment question.

Things are changing though. Twenty years ago there was essentially no evidence that planets existed around any star other than our Sun. Sure, it would have been pretty bizarre and unsettling if ours were the only planetary system in a galaxy of 200 billion stars, but we just didn't know. Now we are in the happy situation of being able to argue about exactly how many small rocky worlds there should be, and even how many of them might be terrestrial analogs. The general answer is "lots". This really does change a big piece of the SETI equation as it is employed. Suddenly there is not only a way to improve the estimates of the number of potential targets, but to actually identify those targets. Hence the plans that were in place to use the Allen Array to monitor Kepler planets.

The more difficult questions are those that revolve around some of the implicit (and even explicit) assumptions in SETI. These are to do with the nature of "intelligent" or "technological" life, and the presumed or hoped for motivations of other worlds, other species. It's a quagmire. The problem is that we just don't know, not even a teeny bit, whether the example of humanity is a reasonable template or not. Even arguing about the equally sophisticated but non-technological evolution of dolphins or ants ignores the fact that all of life on Earth is the product of intimate co-evolution across 4 billion years. Similarly fraught are arguments about the longevity and placement of civilizations in cosmic time. The risks for success or failure with SETI are almost impossible to compute, one way or the other.

No. I think the real argument for SETI is that no-one can say whether or not there is another species in our galaxy sending out recognizable signals, and that is precisely why we should be listening.

Long before we had modern astrophysics humans looked out into the universe eyes wide open. There was certainly motivation to understand those objects or phenomena that we had already discovered. But there was also motivation to simply gather knowledge by finding, well, by finding whatever was out there. Modern SETI as performed by the Allen Array is well set up to capture all manner of natural signals as well as those of artificial origin. The transient phenomena of the universe represent one of the next great challenges for astronomy. Projects like the $400 million Large Scale Synoptic Telescope are aiming for precisely this regime. Part of the motivation is to simply discover things that we could not have found before.

Regardless of what it finds, or doesn't, the Allen Array needs to keep operating or else we lose out on the beautiful mysteries waiting for us out there in the cosmos. Let's go help it.

Tuesday, April 19, 2011

Three Billion Years B.C.

The Earth is still forming. Every year our planet accumulates another 40 million kilograms of material, mostly in the form of microscopic interplanetary dust. More sporadically the planet is also hit by larger bodies. Hundred meter diameter asteroids or cometary lumps arrive on average every thousand years, kilometer-sized civilization manglers arrive roughly every million years. This had been going on since the Earth coagulated from the material of the proto-planetary disk around a baby Sun 4.54 billion years ago.

As we turn back the cosmic clock the rate of accumulation of material increases. The pockmarked lunar surface has served as a proxy for reconstructing the history of asteroidal and cometary impact on the Earth. Without an atmosphere or significant geophysical activity the Moon has an excellent memory of impacts, while the Earth had eroded and resurfaced itself in continual reinvention. This record has indicated that during a period between about 4.1 and 3.8 billion years ago the Earth must have been subject to a particularly brutal pummeling. A substantial fraction of the outer shell of our planet could have been laid down during what has become known as the Late Heavy Bombardment.
It's a fascinating time in the history of our world. The first indications that microbial life might have been at work come not so very long after this quite cataclysmic episode ended.

The reason for this infall of material seems likely to be connected to a period of dynamical evolution in the outer planets. Models suggest that both Neptune and Uranus could have migrated outwards and dug into a rich belt of outer, Kuiper or trans-Neptunian objects. Many of those distant small bodies would have been pushed into orbital paths that would eventually lead to passage through the inner solar system and collision with the Earth. At the same time, Jupiter and Saturn would have migrated inwards and could have scattered material from the asteroid belt onto inbound trajectories. Once the dynamical reorganization of the giant planets was finished the Late Heavy Bombardment would have tailed off. A settling planet Earth then gave rise to the tentative steps of biochemistry and single-celled organisms.

Or so we thought. New evidence is emerging from the terrestrial rock record that the Earth actually continued to be pounded by very significant impacts from 3.8 billion years ago all the way up to around 2.5 billion years ago. "Life Killer" type asteroid impacts seem to have happened roughly every 40 million years during this timespan, rather than every 500 million years as had previously been thought. 

So what gives? Where did these chunks of material come from? W. Bottke and colleagues have studied the gravitational dynamics of the teenage solar system and suggest that a now-depleted inner belt of material between Mars and Jupiter could have been scattered onto an inclined set of orbits - out of the plane of the planets. This population would then slowly "leak" into Earth-crossing paths, thereby greatly extending the tail of the Late Heavy Bombardment over another billion years or so. The leftovers of these bodies are still there, known as the Hungaria asteroids.

It all looks to fit rather well. The dynamics are believable, and provide a mechanism for the impacts that littered the planet with the molten globs of rock that geologists find in layers of ancient strata. There's just one teensy question. What are the implications for the evolution of life on Earth? While evidence of microbe-built structures like stromatolites from 3.5 to 3.8 billion years ago remain a little controversial, the presence of a diverse planet-wide biosphere is pretty incontrovertible in the 3 to 2.5 billion year ago span. Apparently microbial life not only dealt with continual destructive asteroid impacts but really did rather well for itself.

This raises another intriguing issue. As W. Bottke and colleagues point out, this prolonged period of heavy impacts does effectively stop around 2.5 billion years ago. That is suspiciously coincident with the first signs of a rising oxygen content in the Earth's atmosphere (the "Great Oxidation Event"), and the eventual emergence of multi-cellular life somewhere around 1.6 to 2 billion years ago. Is there a connection? Could the continual accumulation of planetary material have held back the full-on evolutionary party of early life? It's highly speculative, but one is tempted to think that this might be further evidence for the incredible resilience of life and its near-relentless nature once it becomes entrenched on a planet.