In my last blog I mentioned that I recently went to see "John Carter," the latest cinematic version of Edgar Rice Burroughs' early 20th century series of novels describing the adventures of John Carter on Mars (Barsoom). I pointed out that science fiction is fun to read and sci-fi movies are fun to watch and that when they are well made they entertain us and encourage us to reflect on what we've read or seen. The advertising and trailers for this film started that process for me, even before seeing the film.

When creative people, artists, writers, and scientists, consider extraterrestrial life they also, quite correctly, consider the habitat. Would life on a low-gravity terrestrial planet be the same as life on a high gravity terrestrial planet? How would it compare to life on a planet with an atmosphere but no solid surface? How believable is the life on Burroughs' Barsoom?

Notable in the movie is that there are two species of intelligent Barsoomian life, humanoids looking very, very human and the thark, very tall two-legged, green-skinned creatures with four arms and a really long saber tooth extending from each side of their faces. All the other animals have six limbs, including the friendly six-legged dog-like companion Woola and the giant (and I mean whale-size) hairy, white, 4+2 apes. All of the six-limbers have four digits on their hands (if they have hands) and big or many teeth!

These "designs" for alien life seem reasonable. As a human, I can hardly complain about the humanoids, and the thark's saber teeth and extra set of arms don't distract from their intelligence. There are a number of intelligent species on Earth, some of which use tools, so having two on Barsoom is not a flaw.

On Earth we see creatures with no limbs (worms, snakes, and legless lizards), four limbs (most mammals: arms and legs or just legs), five limbs (star fish), six limbs (insects), eight limbs (spiders, octopi), and many limbs (squid and related sea dwellers, centipedes, millipedes, etc.). So the number of limbs is not a concern with the depiction of life on Barsoom. But do all the species portrayed match the habitat?

The sizes of some of the species portrayed are worth discussing, but not because of Mars' gravity. Consider life on Earth, again. The largest animal to ever live here is alive today: the blue whale (Balaenoptera musculus). Look back 100 million years or so and the largest land animals, dinosaurs, were shaking the ground as they walked around. Swimming in an ocean is about as weightless as one can feel on our planet (which is why astronauts train for space walks in large pools of water). It's perhaps no surprise that the blue whale is as large as it is, given that it lives where food is abundant and it can graze over enormous areas.

But Earth's gravity has not changed from the days of the dinosaurs to the present, so it can't be gravity that makes a difference in size. It might be the atmosphere, though. Paleo-ecologists are trying to understand the environment that let dinosaurs grow to the sizes they reached. We know that Earth was warmer during their reign than now, but the long polar nights where some dinosaurs lived tell us that the situation was quite complex. So does "John Carter's" Barsoom have enough oxygen in its atmosphere, and a thick enough atmosphere, for its inhabitants to breathe, grow, and stay warm? Yes, by definition, but real Mars does not for life as we know it on Earth.

In fact, as I noted in the first blog on "John Carter", the ecosystem in the movie is non-existent. We hear that the thark eggs that didn't hatch should be destroyed so late hatchers won't be eaten by the white apes. Aside from that, it's not clear what any of the creatures on Barsoom eat. There was enough action in this movie to keep me entertained. But it would have been more fulfilling if there was a little more care given to making a believable habitat.

Science fiction challenges us to think, whether it is done well or not. When you hear about the latest Earth-mass or Earth-size exoplanet in a star's habitable zone, think about what "habitable" would really mean in that solar system and who/what might live there.

Cover of Big Little Book, 'John Carter of Mars,' published by Dell in1940. (Source: Wikimedia Commons)

I recently went to see "John Carter," the latest cinematic version of Edgar Rice Burroughs' early 20th century series of novels describing the adventures of John Carter on Mars. After he is instantaneously transported to Mars (called Barsoom) from Earth, Carter's involvement with the local inhabitants includes a love story and all sorts of mayhem. Of course I had to suspend some disbelief, but what might surprise you was that it was easier in some scenes that were less scientifically correct than for others that at first brush would seem scientifically accurate.

Instantaneous transfer between planets? OK. Not physically plausible by today's science but tolerable for the story, especially since many of us have seen and accepted some version of it in all of the productions of the "Star Trek" and "Star Wars" series.

John Carter leaping tall buildings in a single bound? Why not: Mars has less gravity holding things down, 38 percent of what's holding you in your chair at the moment (unless you happen to be on the International Space Station reading this). Superman has been leaping tall buildings for decades. Well... an Olympic high jumper in a pressurized dome on Mars would be able to jump over a bar 21.13 feet (6.45 meters) high. On the moon, an Olympic high jumper could jump 6x higher in a pressurized stadium there, but that is only 48.18 feet (14.70 meters). Human musculature in Barsoomian gravity is just not enough. Carter was jumping way farther, unbelievably far for me (though it made the story work).

Mars has a thin atmosphere, you say, so couldn't that have made the jumps easier? Yes, a little. The atmosphere on Mars, at its densest, is about the same as what you'd experience at 50,000 feet (15,000 meter) above sea level on Earth (that's 1.7x higher than Mount Everest) so there's much less air resistance. But then... what are the inhabitants breathing on Barsoom? Edgar Rice Burroughs didn't know that Mars' atmosphere is thin and mostly carbon dioxide; free oxygen is not mentioned as even a minor component.

That thin air doesn't hold much heat, either. John Carter and many of the human-like Barsoomian inhabitants were running around with a lot of skin showing, with nary a shiver. That Mars is desert-like was accepted more than a century ago. Nowadays we understand that that means dry like cold Antarctica's dry valleys, not hot and dry like Death Valley. The filmmakers used beautiful desert scenery in the film. But I'm left wondering: What did everyone eat and where did it grow?

One gripe: Mars has two small moons, Phobos and Deimos. Both move rapidly around Mars, Phobos being larger, closer, and faster than Deimos in the Martian sky. Neither of them is spherical. Both would appear smaller than presented in "John Carter's" Barsoomian sky, which I can live with, but seeing them always in the same phase and same relative position near each other in the movie was more than I could accept.

I guess sometimes I can't suspend enough disbelief but, as you can tell, it certainly got me thinking and it was a pretty interesting story with an unexpected ending.

When I consider exoplanets, I include everything I know about planets and how they "work," life on Earth, the potential for life in the Solar System, and the variations that nature has already demonstrated to us during the search for exoplanets. "John Carter" did that, and I have to say, the challenges that you readers offer in your comments really crank up my thinking. Thank you!

The only blue aliens I'm aware of lived on a moon called Pandora in a popular movie released in 2009. The foundation of your question is the more general question of why we observe a wide variety of colors "used" by life on Earth. Those colors are "used" by their organisms in many different ways. And there are a variety of mechanisms that generate the colors.

The colors of plants and animals have a variety of goals. For plants, the green of their leaves comes from the chlorophyll that absorbs violet-blue and yellow-orange-red light for photosynthesis. Some plants (like Japanese plum) have additional pigments for protection from ultraviolet light and appear dark red. Flowers have colors specifically to attract pollinators, but the colors the pollinators see may not be the colors we see.

Animals have colors to camouflage themselves and attract mates. Some plant and animal coloring is designed to warn off predators. The red eye you see in flash pictures of your friends is a reflection of their eyes' retinas. Photographs of dogs show their retinas reflect greenish light. Is retinal color related to color vision? Most humans have color vision and dogs are color blind.

The colors we see around us are generated by different mechanisms, which can reflect (pun intended) on its use by an organism. The color of a pigment depends on the colors it absorbs and those it reflects. Chlorophyll is a green pigment, and hair and skin colors result from pigments as well.

Polar bear fur only looks white.

Polar bears' black skin pigmentation helps keep them warm. The bears' white fur only looks white in bulk. Individual hair follicles are actually transparent, so that they carry sunlight down from the "top" of the fur coat to the bear's skin, where all the colors of sunlight (you've seen them in a rainbow made by differential refraction, another mechanism!) are absorbed by the black skin, helping to keep the polar bear warm. The fiber optics we use to transfer data over the internet or between components in your home entertainment system carry light in the same way.

The iridescent color of bird feathers is produced by another mechanism, the same one that makes detergent bubbles and thin slicks of oil on water show colors. The structure of feathers and thickness of detergent and oil layers permits waves of light to "interfere" with each other. You've seen wave interference in a quiet pool or pond when you throw two small objects into the water and the circular waves move out from each impact point. When the waves cross over each other, their height is greater where the peaks combine and flat where a peak and a valley combine.

A similar thing happens with light waves in iridescent materials. In the feathers, waves of a particular color are reflected and combined before they are shunted out of the feather, while the other colors are absorbed by a black pigment. The colors come from the spacing of tiny reflectors, called lamellae, in the feathers: change the spacing and the color coming from the feather is different. In detergent bubbles and oil slicks, change the layer's thickness and you change the color seen.

So where might we expect blue-skinned aliens? My answer is on an exoplanet orbiting a cool, red star. Why? Because the alien probably wants to absorb as much stellar energy as it can from its star, and blue pigments absorb red light. It would be well-camouflaged in the blue vegetation trying to absorb as much energy from the red sun as it could.

PlanetQuest home page visitor Pramod M. looked at the website's 3D New World Atlas and wondered "Why do all planets orbit in almost the same plane? Why don't they orbit in all orientations and fill a sphere? The same with a galaxy: Why is it shaped like a disk and not a sphere? With gravity a sphere is the natural shape to form."

Pramod has synthesized his observations of different objects that have similarities and his understanding of the workings of gravity to ask quite reasonable questions. This is one way that scientists "do science."

The answers to his questions come from looking at the processes involved in planetary system and galaxy formation. In both processes gravity is the controlling force, overall. In the chaos of a primordial cloud of gas (for a galaxy) and a cloud of gas and dust (for a planetary system) , somewhere in that volume of space the material will be denser. The denser volume's gravitational attraction will draw other material towards itself. Each individual parcel of material will have an orbit around (or collide with) the density maximum. If gravity were the only force that mattered, it is natural to assume that a spherical cloud with a high density core would grow over time.

But the cloud material doesn't flow cleanly through other cloud material. The parcels interact with "friction" – viscosity – and they slow down. Continue this process long enough and the core grows even faster and material that hasn't fallen in slowly assumes the shape of a disk, minimizing the collisions that would otherwise continue. Density enhancements in a stellar-scale disk turn into planets eventually, or in a forming galaxy they turn into star forming clouds limited to a thin disk in the plane of the galaxy.

Now the descriptions diverge. During the formation of a planetary system all the forming planets suffer collisions and keep growing but they also "kick" debris into random orbits. Any large (Jovian) planets will be very good at this, and most left over debris ends up in a large spherical cloud: we call it the Oort Cloud, composed of comet nuclei. This debris "clean-up" is a form of planetary viscosity and the planets' orbits migrate. If a Jovian planet and a smaller planet migrate so they eventually get close, they will alter each other's orbit, with the smaller planet making a larger change in its orbit.

We now have observations of a planetary system with two planets having very different orbital planes. This is probably the result of one or more encounters with other planets. We don't see multiple planets with large differences between their orbital planes because there may not have been enough migration or enough planets or enough time for the orbits to fill a sphere (as the comet nuclei in an Oort Cloud do). In addition, our exoplanet discovery techniques are not optimal for finding systems like these.

There are spherical and ellipsoidal galaxies, and even many flat spirals have a spherical component. (Visit http://hubblesite.org/gallery/album/galaxy/ for some interesting imagery.) If a large disk galaxy has many companions orbiting it, over time it will cannibalize them as they make many pass-through collisions. The large galaxy's gravity rips apart the smaller galaxies and the smaller galaxies can disrupt the larger galaxy's disk. Stars get scattered into orbits that ultimately give the combined galaxies an ellipsoidal or spherical shape.

It is important to realize that the spherical shape of a moon or planet or star is caused by gravity's pulling every part of the object as close to the center as it can. The situation is different for systems which are not "packed" and have freely orbiting components.

Reader Mark Z. comments: Hello Steve, great blog btw. I was wondering, given how precious telescope-time is, whether there are any plans to image known extrasolar-systems and then to carry-out this new processing technique on them? I appreciate Kepler is teasing out edge-on systems and has already bagged a large number of candidates but that only applies to systems suitable for transits... yet it seems as if the majority of 'imaged exoplanets' are presenting themselves almost face-on (ie we're looking down/up at them). I wonder how many non-transiting systems we will find (and not just in existing data archives).

Has anyone been able to offer a guesstimate, given the ratio of transiting systems to imaged systems, as to what percentage of the Milky Way's stars are likely to have an entourage of planets (covering all stars, irrespective of their metallicity, age, lithium ratio etc? The more refined question would be what percentage of 'sun-like' stars have systems?

Reader Mark has asked some good questions in his comment on my "Steve Stymied?" blog so he wins two blog replies. This is the second, looking at the assumptions behind his questions. The first blog addressed the questions themselves.

Some of Mark's questions illustrate biases from what are called "selection effects," which play a significant role in studies of exoplanets and exoplanet systems. Selection effects can bias both results and interpretations. Biases come about from the way objects are chosen for study or the way they were studied.

Following his question about processing other images the way the imaging of the HR 8799 system was processed (http://hubblesite.org/newscenter/archive/releases/2011/29/full/), he says "...it seems as if the majority of 'imaged exoplanets' are presenting themselves almost face-on." But if you aren't keeping track of all the imaged exoplanet systems, your memory (and mine!) of what's been published could lead you to the wrong generalization about the orientation of exoplanet systems – a selection effect. In fact, the only other telescopic generally-accepted image of an exoplanet was released in 2008: the Hubble Space Telescope's (HST) image of Fomalhaut, which shows its exoplanet on the inside edge of a protoplanetary debris ring (http://hubblesite.org/newscenter/archive/releases/2008/39/full/). Notable is that our point of view is about 45 degrees out of the plane of the ring. There is no majority orientation between two imaged systems.

There is a large majority of systems that are edge on (transiting), if we assume that most of the Kepler candidates are actually exoplanets. But this is a selection effect too. Kepler is only looking for transiting systems and can look for many at once. It is much more efficient than radial velocity searches made star by star so it's finding many of them.

Mark's last question also runs through the selection effects obstacle course. The searches presently being made are all studying Sun-like stars. That means they are fairly similar to the Sun in mass, diameter, and age. The range spans main sequence (meaning not evolved) stars of types F, G, and K, F being warmer and more massive, G being the Sun's classification, and K being cooler and less massive. Even some cooler, light-weight M stars are getting attention. But left out of the search game are the massive, fast burning O and B stars, and even the hot A stars (there are good reasons for this, but I don't have enough space to cover that in this blog) and stars with a wide range of other parameters that are not necessarily Sun-like.

Radial velocity (RV) searches are limited to nearby F, G, and K stars. Breaking down a star's light into its component colors requires brighter stars, even with the world's largest telescopes, and closer means brighter. Also, RV measurements usually cannot tell us the orientation of an exoplanet's orbit: the exoplanet could be an exoJupiter or an exoEarth, and we can't tell. Transit measurements have limitations on the sizes of exoplanets that can be detected (on the small end of the range).

At this point in our searches for exoplanets, we have intentionally de-selected certain classes of stars. That immediately limits extending our conclusions regarding not just the full population of the Milky Way, but even to the Sun's neighborhood.

by guest blogger Joshua Rodriguez

A couple weeks ago, I wrote a story for PlanetQuest looking back on Kepler's many discoveries in 2011. Two years into its mission, the Kepler spacecraft has really hit its stride, with more amazing findings sure to come in the future.

Turns out, Kepler wasn't just the biggest exoplanet story of 2011 – it was one of the biggest stories in the entire field of astronomy! The graphic above shows some of the plaudits Kepler earned in 2011.

Reader Mark Z. comments: Hello Steve, great blog btw. I was wondering, given how precious telescope-time is, whether there are any plans to image known extrasolar-systems and then to carry-out this new processing technique on them? I appreciate Kepler is teasing out edge-on systems and has already bagged a large number of candidates but that only applies to systems suitable for transits... yet it seems as if the majority of 'imaged exoplanets' are presenting themselves almost face-on (ie we're looking down/up at them). I wonder how many non-transiting systems we will find (and not just in existing data archives).

Has anyone been able to offer a guesstimate, given the ratio of transiting systems to imaged systems, as to what percentage of the Milky Way's stars are likely to have an entourage of planets (covering all stars, irrespective of their metallicity, age, lithium ratio etc? The more refined question would be what percentage of 'sun-like' stars have systems?

Reader Mark has asked some good questions in his comment on my "Steve Stymied" blog so he wins two blog replies. This is the first, and will address the specifics of his questions. The second blog looks into the assumptions behind his questions.

Mark wonders if the techniques used on the recently re-processed images of the exoplanets orbiting HR 8799 will be applied to other known exoplanet systems. The researchers have said they plan to apply it to other known systems with images already in the Hubble archives.

What about finding new systems with the new technique? A member of the research team visited JPL a few days after the release of their HR 8799 results. He addressed this question briefly at his seminar. He pointed out that they knew approximately where to look for HR 8799's exoplanets, making it easy to adjust the process to bring the exoplanets out well. That will not be the case for completely new systems, where they don't know where to look.

The percentage of Milky Way stars that have a system of planets is one of the primary goals of the Kepler Project. To permit a transit, the inclination of an exoplanet's orbit around a star is so limited that we can, with a large enough sample, use the results of a survey like Kepler's to make an estimate of how many non-transiting systems there are in the Milky Way. This is useful, but still incomplete since minor differences in inclination between exoplanets orbiting the same star may prevent visible transits of more than one planet. In other words, Kepler will provide a sample of transiting exoplanets but not a good sample of exoplanet systems.

A distant observer studying the Sun would only be able to see one transit from one of the Sun's eight planets. This is based on the known orientations of the orbits of the planets in our Solar System. An exoKepler spacecraft with the same capabilities as Earth's Kepler would not be able to detect Mercury and would have to wait from about 2/3 of a year (Venus) to almost 165 years (Neptune) for a first transit of the Sun to be observed. Patience is a virtue for exoplanet searchers, from wherever they are looking.

By guest blogger Joshua Rodriguez

The biannual American Astronomical Society meeting is a kind of science summit, a conference where astronomers from across the U.S. gather to talk shop, present their latest research, and connect with other members of the community.

More and more, though, the conference is becoming something of an exoplanet extravaganza. Last year, Kepler used the AAS meeting to announce that the mission had discovered its first rocky planet.

This year, Kepler has upped the ante even more, with the announcement of three new rocky planets, one as small as Mars, and also revealed that the mission had found two more "circumbinary" planets - worlds that orbit two stars, like Tatooine in Star Wars.

Another group of astronomers also revealed evidence that the Milky Way likely contains more than 100 billion exoplanets, a staggering number that could redefine how astronomers understand the galaxy. My colleagues with the Kepler and Spitzer missions have been working non-stop this weekend, preparing for one exciting release after another.

As someone who's been attending AAS meetings for a few years now, it's amazed me to see the rising profile of exoplanets. This year's meeting had three straight days of exoplanet sessions, most of which were packed with hundreds of scientists eager to learn about the latest news. The conference hall was practically buzzing with excitement about missions such as Kepler and Spitzer, which have been outrageously productive planet-finding instruments.

I'm also noticing that young people are increasingly getting into this exciting new field of astronomy. Once considered a niche subject, exoplanets are quickly becoming mainstream, as a new generation of astronomers eagerly jumps into a field once considered the realm of science fiction.

Sometimes I like to imagine how an astronomer of 50 years ago would react to walking around the AAS exhibit hall, seeing the amazing new instruments and discoveries that are a part of the field today. I think that person would probably be pretty blown away by the staggering pace - and very proud of a field that continues to outdo itself and push the boundaries of knowledge.

When I talk to people about exoplanets, I like to tell them how lucky we are to be alive during this time, when we're right on the doorstep of finding Earth-like worlds, and perhaps just a few more steps away from searching for signs of life on them.

Get excited! History is being made at a breakneck pace.

This is an intriguing question, made more so by Kepler findings of Earth-size planets in just the last few weeks. Science fiction authors might be more qualified to reply to this question than I am. But let's consider the details.

I like to use "Earth-like" to describe an exoplanet that has about the same mass and about the same radius and about the same energy input from its host star as Earth does. I would be comfortable calling such a planet an exoEarth. So by this definition, a planet out of the host star's habitable zone can't be Earth-like.

Can I conceive of ways that an exoplanet that is Earth-radius and Earth-mass but outside the star's habitable zone might be Earth-like because it has liquid water? Yes. The big limitation outside the star's habitable zone is that water freezes. If an exoplanet can have liquid water on its surface in the star's ice zone it will certainly seem a lot like Earth, though daylight illumination will be weaker.

Here are some of my ideas (and I bet you readers will have more to offer). A sufficiently thick atmosphere of greenhouse gases, like carbon dioxide or methane (both are common around planets), could warm up the planet enough for water to be liquid. This is actually one of the considerations in Snowball Earth discussions of our planet's early history.

Volcanism on a cold Earth-size exoplanet could provide islands of warmth, which might, of course, get occasionally too hot when there's an eruption. Alternatively, and I don't know if geologists would buy into this idea, perhaps a magma sea beneath but near the planet's surface might keep wide areas of the surface warm enough for liquid water to exist. Such an idea has some basis in fact. Yellowstone National Park in Wyoming, USA, has very cold, snowy winters but its geyser fields stay free of snow because of the heat coming from below the ground.

What about an exoplanet in a binary star system? If the planet is outside the habitable zone of both stars but orbiting only one of them, it might get enough warming from the pair to have liquid water on its surface. The "days" and "nights" and the seasons would be highly dependent on the relative positions of the stars, which might cause problems (too warm and too cold) on different time scales. Without doing some simulations, it's not clear that it's possible for this hypothetical planet to stay in a relatively stable and circular orbit. It might get ejected from such a system in a geologically short time. Celestial mechanicians might take me to task on this idea.

Solid body tides, akin to those affecting Jupiter's volcanic moon Io, might be "just right" for an Earth-size exoplanet in the ice zone to be warm enough for liquid water. Realistic mechanisms for generating such tides might be difficult to find, or might mean this object is an Earth-size exo-moon.

Do you have any ideas that will keep liquid water on an Earth-size planet outside the host star's habitable zone? And a bigger challenge to you: Could there be habitable zones on an Earth-size planet inside the inner edge of the host star's habitable zone? I have an idea.

After reading "Do they have to be Earthlike and Sun-like?", Ken Stuczynski wondered in a comment about the philosophy of scientists that leads them to making best-guesses that may be rendered obsolete with new data.

I look at "best guessing" about potential habitats for life not as busy work, as he suggests, but as preparation and envelope expansion. It is certainly true that new data can make assumptions, and even accepted explanations, obsolete. That is the nature of science! But science sometimes advances because someone figures out a way to test a best-guess. Whatever the result, the test has expanded our knowledge. And the test procedure itself often has wider application. Without acquiring new data, all we would have are what we observe, which can be very limiting: Humanity "knew" that everything in the night sky circled Earth until Galileo tested Jupiter with his telescope and found its moons apparently circling Jupiter and not Earth.

When I talk about "preparation" above, I mean exploring the possibilities, considering their implications, and then figuring out what to do with that knowledge. A well-developed best-guess can lead to instruments, tests, and/or observations that expand our knowledge. For example, our view of solar systems was vastly expanded when the first individual exoplanets and systems of exoplanets were identified. All of a sudden planets orbiting a star didn't have to range, outward from the star, from small to giant to large, as we see in the Solar System. Giant planets could be closer to their stars than Mercury. Planet orbits could be in much more extended elliptical orbits (egg-shaped) than we see with Mercury and Mars. Exoplanets in a system don't have to lie in the same plane. Speculations about how Mars would be different if it were closer to the Sun or how Titan and Europa will be different when the Sun becomes a red giant can lead to best-guesses that let us both question and test.

"Envelope expansion" comes from aircraft flight testing. Test flights of new aircraft begin conservatively. Over time, the flights slowly expand the aircraft's "flight envelope": the range of altitude, speed, center of gravity, and angle of attack over which the aircraft will fly safely and not, for example, stall or become unstable.

By expanding the envelope in a subject, I mean exploring to the limits of the possibilities as best we can with the knowledge at hand. We can consider the types of stars or the masses of planets that might harbor life. There are many variables when you consider all the potential habitats across a range of planet sizes and types and the range of star types and their states of evolution. Biologists keep finding new and surprising habitats on Earth, expanding the scope of where we might look for life as we know it.

Best-guesses are part of the creative process that expands our understanding of the universe. Scientists, science fiction writers, and everyone else can join in to advance science, entertain, or for their own satisfaction.