JPL scientists discuss Exoplanet Exploration

A couple months ago we featured NASA scientist Olivier Guyon on PlanetQuest. Guyon won a MacArthur "genius" grant, a major achievement that speaks very highly of the potential that Guyon's colleagues see in him.

Guyon is really interested in bringing the message of exoplanets to the people, as illustrated in this great TEDYouth talk he delivers about how to find exoplanets. His talk begins at about 3:15. Later in the podcast, JPL Mars celebrity Bobak Ferdowsi also talks to the group.

Kepler may be the exoplanet mission you hear most about these days, but for the past few years, it hasn't been the only one.

Sadly, that may be changing soon.

The CoRoT mission, built by our friends over at the French space agency CNES, has suffered a major computer failure that's prevented scientists from downloading data from the telescope. They are currently in the midst of trying to remedy the problem, but if it persists, the mission is doomed.

The CoRoT spacecraft is significant for several reasons -- for starters, it was the first dedicated exoplanet mission ever, predating the launch of Kepler by a little over two years.

CoRoT looks for exoplanets using the same method as Kepler, watching for stars that dim in brightness when planets move directly in between them and the telescope. It took just 3 months for the mission to find its first exoplanet discovery, and so far it's made 25 exoplanet finds. Like the Kepler mission, you can tell that a planet is a CoRoT discovery if it has the word CoRoT in the name, i.e. CoRoT-2 b

Unlike Kepler, which takes a broad look at a huge group of stars in the sky, CoRoT has focused on studying a small number of planets in-depth. The CoRoT planets represent some of the most well-studied exoplanets yet. It's also made breakthroughs in the fields of astroseismology, helping scientists understand how stars work.

Should this be the end for CoRoT, it leaves behind a rich field of about 600 exoplanetary candidates, the result of over five years of observing the sky. Follow-up with other instruments will help scientists determine if these are actual exoplanet discoveries, so the number of CoRoT planets could increase even after the mission hardware has gone dark.

CoRoT's demise is a bummer and also a pretty poignant reminder of how tough it is to keep sensitive instruments running in the harsh environment of space. Kepler itself has had some hardware problems -- one if its reaction wheels, which allows the telescope to accurately point itself at the sky -- went offline earlier this year. Another wheel failure would mean the end of the mission.

Still, when it comes to exoplanet finding, space is most definitely the place. CoRoT and Kepler's many discoveries and huge fields of candidates are proof positive that finding planets is much more successful with instruments that don't have to contend with the effects of Earth's atmosphere. Here's hoping that future exoplanet space missions will build upon the amazing, ground breaking work of the CoRoT mission.

I may work for a website called PlanetQuest, but I myself am not a planet hunter as much as I am a planet-communicator, helping the scientists of NASA get word of their discoveries out to everyone else.

But there's a way the amateurs among us can get our hands dirty with real Kepler data and help with the hunt for new exoplanets. It's called Planet Hunters, and amateur exoplanet enthusiasts like you have already used it to discover real exoplanets in the Kepler data.

The gist of Planet Hunters is pretty simple. The program serves up slices of Kepler data, and you use the sophisticated pattern-recognizing software in your brain to look for signs of light curves, which can be the signatures of transiting exoplanets. First, you're asked a couple questions about the general shape of the light data, then you're given the chance to highlight any events that you think look like exoplanet transits.

It sounds easy, but picking out the light curves can be tricky. First of all, stars don't shine with light that's nearly as constant as what looking out into the night sky might lead you to believe. Many of the stars I looked at had wild shifts in light intensity, yo-yoing up and down over the course of days or climbing in intensity over the course of weeks. In fact, one of the big discoveries of the Kepler mission has been its finding that our sun is actually a pretty docile star compared to most other stars its size.

Because transit events only last a couple of hours, I had to toss these light curves, leaving the data for a more powerful computer than I to process.

My search continued for about 15 minutes - calling up new targets from the computer, squinting and changing the scale and finally determining that what I was seeing wasn't a light curve from an exoplanet.

And then I saw it.

A light curve with the telltale signs of exoplanet activity, with a steady baseline interrupted by sharp drops. The signature of an exoplanet making its brief passage in front of its star and blocking some of the light.

Granted, it make take some time before my "discovery" is confirmed, and there's a decent chance that Kepler scientists have already spotted this light curve and either logged it as a discovery, or found out that it's a "false positive" like an eclipsing binary star. I'll be watching the discussion page on my discovery to see if others find the same result.

But the feeling of finding my own exoplanet transit was pretty awesome, and gives me a glimpse into how challenging it can be for astronomers to pick planets out of the mountains of Kepler data -- and how cool it can be to realize you're looking at the light signature of a distant exoplanet, orbiting its star.

For generations, the idea of finding exoplanets was considered a dream at best. Now, you can hunt for them in your pajamas. Sure, we might not have lightsabers and hover-boards yet, but I'd say that the future is very much upon us.

For more information and to hunt for your own planets, check out Planet Hunters.

Attention Scrabble players: here's a good word to have in your back pocket: "Exosyzygy."

Sometimes, exoplanets pass directly between their host star and the Earth. That very fortunate coincidence is called a "transit". Literally thousands of potential exoplanet discoveries have been made using this method.

Sometimes, more than one planet will transit its star. That's even cooler because it allows scientists to get info about multiple planets at once. And because multiple planets tug on each other as they orbit, astronomers can use the variations in the timing of the transits to prove, with a very high degree of confidence, that what they're looking at is indeed a bonafide set of exoplanets.

But do you call an event where two planets happen to not only cross in front of their host star as they orbit - but also cross one in front of the other, as a group of astronomers recently discovered?

New Scientist reports that there's some disagreement as to what's the proper nomenclature for such an event. Among the potential names are "double transit," "planet-planet eclipse," and, curiously, "exosyzygy."

Exosyzygy? What?

The term is a play on the word "syzygy," which is a term applied to events where three space objects, like stars and planets, arrange in a straight line. The spring tide is a phenomenon when the sun, moon, and Earth are all arranged in such a fashion.

Having trouble pronouncing the word? Don't worry - astronomers believe that finding planets that not only transit, but line up as they do so is extremely rare, so don't expect to hear it used very often.

Read the rest of the New Scientist article here. See the text of the discovery paper here.

Guest blogger Connie Lu is an undergraduate at the University of Chicago who spent her summer with NASA's Exoplanet Exploration Program at JPL in Pasadena, California. For more information about student internships at JPL, visit http://careerlaunch.jpl.nasa.gov/.

How did you find this internship?

During winter break, all of my friends seemed to know what they were doing. They were all applying for internships in investment banking, the U.N., tech startups, and I was lying in bed, reading poetry. Not that there was anything wrong with reading poetry‚ I love poetry‚but I had no idea how I was going to find a summer job that I enjoyed. So I made a list of subjects that interested me: poetry and literature, physics, music, etc. Then I started trawling Google for relevant organizations with internships that pertained to each one. JPL was the only NASA center that I was really dreaming about interning at, but I'd already dismissed as being a little far-fetched. I submitted my resume to the general pool anyway. Months later, I got a call from JPL asking if I was still available for the summer. How could I have said no? Lessons learned: strange things happen, and you should let them. Don't limit yourself.

What got you interested in space?

Please don't laugh at this story. Both my parents are computer engineers, so there's been a computer of some sort in the house for as long as I can remember. In elementary school, we had this big desktop that I played Putt-Putt and Oregon Trail on. The screensaver (I promise this is relevant) was the one with the star field, and when I got bored during piano practice (which was often), I'd turn around and look at it and the bookcase. I'm not sure why I thought looking at them would help me escape the bench sooner, but logical reasoning develops with age, and I was young. So one day it finally occurred to me that the screensaver looked like a window progressing further into a star field. Wait, what, stars were in 3-D? So I became obsessed with the local San Jose Tech Museum and the Exploratorium, started reading more, asked my parents irritating questions. When I found out that there existed actual pictures of space, thanks to Hubble, that just really cemented it all. I hadn't realized it was so beautiful. And that was that, and now I'm irritating JPL scientists and engineers with my questions instead. I'm also a volunteer with the telescopes at the Adler in Chicago, so though space and physics didn't really pan out as a course of study or a career path, they're still in my life, and I'm happier for it.
[[IMAGE||252582_2100395831242_4895653_n.jpg||left||250||jpl summer intern connie lu|| ||0||0]]
Did you know anything about exoplanets before this job?

I knew that planets outside of our solar system existed, and I knew about all of the basic components to the science behind them‚for example, spectroscopy, gravitational lensing, accretion, etc. But I had no idea of how planets were discovered, how many had been discovered, what they were like, or that star shades existed. Speaking of which, star shades are unbelievably cool. Probably the favorite thing I've learned about in these past ten weeks. Or maybe it's that the scientists and engineers at JPL are figuring out the logistics of directly imaging exoplanets that are both extraordinarily distant and dim in comparison to their host stars. The idea that we can see photographs of exoplanets is unbelievable to me.

What's the most astounding thing you've learned during your time here? What is something poetic you've learned about the universe?

Ever? In freshman year, my physics teacher went on a brief tangent that amazed me, and still does -- that entropy is irreversible, and everything will die. I know that sounds morbid, but the finality of it all is incredibly poetic‚ and it's already been addressed in poetry. Robert Frost's "Fire and Ice" is one of his best-known: "Some say the world will end in fire, / Some say in ice." As for what I've learned here, researching and writing about exoplanets has made me realize how incomprehensibly large the universe is. I know the scale of the universe and the Earth's relative inconsequence is also a common topic that people like to write about‚ Carl Sagan, to name a big one‚Äîbut it's for good reason. Transience and death are two of the few topics that are hard to exhaust.

At the University of Chicago, you're majoring in English and Economics. How do you stay interested in such disparate majors?

I think of it that way to stay focused: my English readings and papers as fun, or as close as academic work can come to fun, and Economics coursework and problem sets as tasks. Love and work.

How would you describe the atmosphere at JPL?

I ride the bus to work and sometimes laugh at the thought that most of the strangers I'm sitting next to or exchanging "Good morning" with are likely to be both very brilliant and among the best in their fields. The atmosphere on lab is similar. It feels like a college campus filled with adults who happen to work on space‚ complete with cafeterias. Part of the MSL team is working in the basement of the building I'm in, and one day I saw one of the lead flight engineers slide down the stair railing while carrying on a conversation. I wouldn't say that's typical, but JPL's the kind of place where that can happen and not be considered as totally irreverent or out of place.

How did you feel about the people you met?

The scientists and engineers I've talked to are some of the most patient people I've ever met. Everyone is humble to almost a ridiculous extent, and nobody has a superiority complex about their prior accomplishments. I got to meet the principal investigator on Hubble's WFPC-2 camera, one of the major scientists involved in correcting the telescope mirror's spherical aberration, and he would just go on these amazing, completely offhand tangents about that work.

How did it feel to be here during the Curiosity landing?

It was unbelievable. I'm trying not to exaggerate or be too excessively hyperbolic, but it was unbelievable. I already couldn't believe that I was going to be working at JPL for the summer, and then I got here and of course the landing -- not its success, but experiencing it -- was so much better than I'd imagined. I was with a lot of other people from JPL that night, watching the landing at the Civic Center, and when the control room confirmed a safe landing, the auditorium burst into spontaneous cheering and applause and a standing ovation. The woman next to me was sobbing. That was a little unnerving, but it was sweet, and it really illustrated how inspiring the successful landing was. A few of my friends were abroad, and one brought back a newspaper from Tunisia with Curiosity's landing on its front page for me. Everyone on lab was grinning the day after MSL landed. I'm so glad that I got to witness it all. Seeing the control room where everything took place in 230/the Space Flight Operations Facility was definitely a highlight of this internship as well.

Guest blogger Joshua Rodriguez is the editor of NASA's PlanetQuest website.

Every few months, the NASA Kepler mission science team releases an updated database chock-full of light curves and planet candidate information. It's become something of an event for the many astronomers who comb the data to follow up on possible planet discoveries.

Two separate teams in the past couple of weeks have announced 41 new confirmed exoplanets, sifted out of the massive Kepler data archives. Jiwei Xie at the University of Toronto released a paper with details about 24 new exoplanets in 12 separate solar systems, while a team led by Jason Steffen at the Fermilab Center for Particle Astrophysics announced 27 planets orbiting 13 stars.

Most Kepler confirmations are made using ground-based telescopes, which use the radial velocity technique to confirm that possible Kepler discoveries are indeed real planets. But these newest discoveries were confirmed using what are called Transit Timing Variations.

A lone exoplanet orbiting a star will transit, or pass between the Earth and its star, on a very regular schedule. But the presence of other planets in a solar system can change the timings of those transits, as the planets pull on one other, speeding each other up and slowing each other down.

From those variations in timing, astronomers can confirm that a particular Kepler light curve is an actual exoplanet observation and not something else, like one star moving in front of another. This level of confirmation is what separates a "confirmed" Kepler discovery from a "candidate" one.

It's a cool find because it means that many Kepler planets may be able to be confirmed without additional ground-based observation, which can be a time-consuming process. We might start to see the many Kepler candidates become confirmed at a quicker pace, as scientist refine techniques like TTV and dig deeper into Kepler's treasure trove of data.

Guest blogger Joshua Rodriguez is the editor of NASA's PlanetQuest website.

One of the coolest things about Curiosity's first images of the Martian surface is, pinkish tint aside, just how familiar everything looks. If you've spent time in the deserts of Earth, you've seen scenery like this before — craggy mountains thrusting out above a dusty landscape strewn with rocks and boulders. The few pictures we have of the surface of Venus show similar features. Despite the massive differences between these three planets, certain ties seem to bind us with the rest of the rocky planets and moons in our solar system.

The key difference between Earth and these other worlds, of course, is that Mars, Venus, Ganymede, Triton, and the rest of the rocky bodies in the solar system are completely devoid of beachfront property — severely lacking the lakes and rivers and oceans of liquid water that are Earth's signature. Instead, the rest of the solar system appears mainly to be a bleak tableau of hellish deserts and frigid expanses of ice, interrupted by the occasional volcano or hydrocarbon lake.

So when you consider that humans spent years imagining that Mars was full of canals and the Moon a hunk of cosmic Havarti, the findings of those first planetary missions must have seemed like kind of a bummer. Five missions of Apollo astronauts found nothing particularly friendly on the lunar surface, the few probes that managed to land on Venus barely survived more than an hour apiece, and even Carl Sagan was said to have been disappointed when the first images of Mars from Viking 1 showed an unfriendly expanse of red rocks stretching into the distance. With success came disappointment — we've reached other worlds, only to find that there's nobody there.

[[IMAGE||venusPano.jpg||center||494||Shot of Venus' surface||The few spacecraft that managed to land on the surface of Venus found that the planet's surface wasn't exactly people-friendly. This photo was taken by the Russian Venera-13 mission in 1982. The spacecraft lasted for about two hours on the Venusian surface.||0||0]]

Consider this: barely 20 years ago, scientists wondered if other stars even had planets at all — and many had resigned themselves to thinking that if they did, we'd never find them. Ten years later, dozens of exoplanets had been discovered, but they were all massive, gas giant planets like Jupiter, many arranged in punishingly hot orbits that practically grazed the surfaces of their stars. We'd found planets, but they were so alien and unlike Earth that scientists wondered if planets were common, but friendly ones like ours were not.

Flash forward to 2012 — 17 years after the first exoplanet discovery. Recent findings have estimated that there are more planets in the galaxy than stars. A cosmos that many astronomers once thought was barren has revealed itself to be practically chock-full of planets.

Not only does the galaxy appear to be crammed with planets and solar systems, but the Kepler mission's broad planet-finding net has found that small, rocky planets are likely to be much more common than big ones like Jupiter. Which means that the pictures we're seeing from Curiosity could be representative of millions, if not billions of similar viewpoints on planets strewn across the Milky Way.

And if even a small percentage of those rocky worlds happens to be the right distance from their stars and have the right mix of volcanic activity, stellar radiation, and liquid water to support life as we know it?

Let's just say that beachfront property, lakeside retreats, and lush, Earth-quality real estate might not be as rare in the universe as we might have once thought.

Finding those planets will take time, and it will be even longer before realtors are selling prime parcels on alien planets.

But if the greatest astronomical disappointment of the 20th century was finally reaching other worlds, only to discover them barren and lifeless, then perhaps the greatest accomplishment of the 21st is finding that our own solar system is just the beginning — one outpost in a galaxy teeming with possibility.

Guest blogger Joshua Rodriguez is the editor of NASA's PlanetQuest website.

If you follow the search for other Earth-like planets, you've probably heard of the so-called "Goldilocks zone," the area around a star where life as we know it could exist.

The current definition of the habitable zone around a star is pretty simple – it's the range of temperatures where liquid water – an essential factor for life as we know it – can exist.

"The problem is that an exoplanet likely needs more than just liquid water to harbor complex life," says Paul Mason, a scientist with New Mexico State University and the University of Texas at El Paso. "For example, we know that UV rays from the sun can destroy DNA. We are looking at habitable niches that exist around some single and binary stars."

Mason and Joni Clark have found that Earth-like habitability, one that takes into account other factors besides water, such as UV radiation and planetary synchronization times. "We find Earth-like conditions may be maintained on a planet orbiting a close binary; twin K-stars, for much longer than is possible in the solar system" he explains.

By examining habitable niches where conditions are most Earth-like, Mason's new description is more restrictive than previous models, but likely more realistic. "We get a better perspective on whether or not an exoplanet might have complex life when we combine as many Earth-like factors as possible."

Mason's research has also found that some binary star systems may be able to harbor habitable exoplanets. "A pair of stars that are cooler than the sun wouldn't emit as much UV radiation and have much longer life-times," he says. "A planet at around 90% of the Earth's distance from the sun might be able to harbor life orbiting such a pair of twin stars."

Mason has also researched how tidal factors – the pull of a star's gravity on the planets in its orbit – can influence potential habitability. "A planet that experiences too much tidal force could be a bad place for life," he explains. "Jupiter's moon Io, for example, experiences severe tidal forces that make it a very volcanic world with no water."

Some tidal forces may be important for life or at least beneficial to life. Mason hopes that future research will be able to more clearly target follow-up studies on potential planets with complex life.

Guest blogger Joshua Rodriguez is the editor of NASA's PlanetQuest website.

One of the most revolutionary space missions ever launched, Kepler has astounded exoplanet scientists with its massive amount of data on thousands of stars, many of which could harbor exoplanets.

Most of the work of the Kepler science team has been processing this data in order to make it useful for scientists across multiple disciplines, from exoplanet hunters to astronomers who study stars.

Erik Petigura, a second-year graduate student at the University of California, Berkeley, has taken a new approach to dealing with the "firehose" of Kepler data.

"The Kepler science team has a challenge because they have to produce data for a lot of different people," Petigura explains. "You have astronomers looking for small planets, for long-period planets, for stellar oscillations ...it's a broad audience, because different signals have different timescales. A hot Jupiter transit happens once a day, whereas an Earthlike transit happens once a year."

Petigura's approach to Kepler data processing is to focus specifically on the narrow range of timescales he's looking for – in this case, small, Earth-size planets orbiting at distances from their star similar to Earth's. "By tossing out everything else, I make the data I'm working with simpler," Petigura says.

Petigura says that this narrower approach can help all astronomers using Kepler's data – not just those looking for other Earths. "Every astronomer has a timescale of interest, whether it's very short or much longer," he says. "By throwing out all the data that doesn't fit to that timescale, scientists can 'scratch their own itch' and process the data in a way that's more efficient."

This approach has already helped Petigura, with the help of the Kepler science team, discover a flaw in the Kepler data that could obstruct the detection of the smallest exoplanets. "I noticed a pattern of noise that we eventually traced to the cycling of heaters in the spacecraft," Petigura explains.

Petigura hopes that his approach to Kepler data processing will help others who are looking for specific kinds of targets in the mission's data. He's also hopeful that he'll be able to use this method to better pick out the small planets that he's really looking for. "I'm trying to move beyond the low-hanging fruit in the Kepler dataset and find the small planets that may be buried in the data."

Reader Q wonders "Could there be stars with earth-like planets outside our galaxy? That is, not in another galaxy, just outside of our own, in the space between galaxies."

Probably not, unless the star+exoplanet system was formerly part of a dwarf galaxy that is being cannibalized by our galaxy, the Milky Way. We do observe streams of stars torn from dwarf galaxies by tides as the dwarves pass through the Milky Way and its gravitational field. (Visit http://sim.jpl.nasa.gov/files/Chapter-4-LR.pdf and scroll down to p. 10 of the PDF, which is the document's actual p. 48. The "film strip" on the right illustrates, from bottom to top, how a dwarf galaxy orbiting a galaxy like ours would be disrupted. The gravitational interaction generates long star streams as the dwarf makes revolutions around the larger galaxy.)

Freeing stars from a host dwarf galaxy is not very "violent," in the sense that during the encounter the Milky Way's gravity generally makes small, but significant, changes in the motions of some stars. Perturbations to the motion of a host star at this level would probably allow exoplanets orbiting the star to continue to do so as the star began its own orbit around the Milky Way. The stars in streams are no longer bound to their dwarf galaxy but they are still bound to the Milky Way. They will spend a lot of time outside the Milky Way but on occasion pass through our galaxy.

The situation is different for stars that might be ejected from our galaxy, never to return. To eject any star from our galaxy requires that it acquire significant velocity to escape the Milky Way's gravity. This, in turn, can only come from a close encounter with a large mass, and the star was most likely binary before things got exciting. If the binary gets close to the Milky Way's central black hole, one component can get swallowed while the other is ejected with enough energy to never return to the Milky Way. This is a "violent" interaction.

While we now know that exoplanets can orbit binary stars, it is doubtful that any would remain attached to the surviving, ejected component. Such a close pass places strong tides across a star's system of planets, just like the Milky Way places tides across the dwarf galaxy. The result is likely to be similar: the planets get freed from their host star. The planets may fall into the black hole, orbit the Milky Way on their own, or be ejected from the Milky Way. The specific details of what happens are going to be dependent on the starting conditions so I can only discuss this in generalities.

With either encounter, the exoplanets will end up pretty lonely, as a family orbiting a host star orbiting our galaxy or as singletons on their own.

Reader Mark K. offers an intriguing scenario: "I was wondering what the effect on double planets' climate would be if the planets were both Earth-size or slightly greater and were orbiting each other, while in orbit about a red dwarf? How would this affect them becoming tidally locked? And how would this affect the climate on them? Also, to what degree would their axial tilt vary?"

Let's consider the system as Mark describes it. A double planet pair, Earth-size or slightly greater, is orbiting a red dwarf. What would inhabitants there see? I'll give you some general thoughts, but realize that the scenario begs for specifics.

The planets would create tides on each other. The inherent flexibility of planetary materials will lock the pair into mutual synchronous rotation over time, with a fixed hemisphere on each component facing the other's fixed hemisphere. Over time, their rotation axes and the axis of their mutual orbit will all be parallel. It would take some careful computer modeling of specific scenarios to confirm that the double planet pair could actually "hold together" as close to the red dwarf as they need to be to be in its habitable zone.

Assuming the pair is stable, the gravitational influence of the pair on each other will be much stronger than the influence of the host M dwarf on the individuals. The star's tidal effect on the pair may keep the pair from settling into a circular orbit around each other. Then their mutual orbit would be forced into a slightly elliptical shape that is always changing. This depends on how close the planets are to each other and how close they are to the star.

Going to extremes

There are two extreme scenarios to consider: the orbital plane of the planets is perpendicular to their orbital plane with the star, or that the orbital planes are the same. First pretend the planes are perpendicular to each other and start with the star in the planets' orbital plane. A visitor on the equator of one planet would see the star passing directly overhead. On the planet-facing hemisphere, the M star would rise, be occulted ("eclipsed") by the co-planet for a time dependent on its diameter and their separation, and then reappear and set. On the planet-far-hemisphere, the M star would simply rise, pass over head, and set. The duration of the period when the star is above the horizon would be half of the planets' mutual orbital period - their orbital motion provides the day-night cycle which a single planet in the habitable zone of a red dwarf would lose due to its synchronous rotation with the star.

When the planets have moved through one-quarter (90°) of their orbit around the M star, the situation has changed drastically. Now the same hemisphere on both is receiving continuous daylight during many continuous double-planet orbits. From a planet's equator, the M star is observed to be cut in half by the horizon and moving continually around it, while on the star-illuminated hemispheres of the planets the M star is at or circling the zenith. On the "night hemispheres," except very near the equator, the M star has been below the horizon for almost one-quarter of the planets' orbit around the M star.

In the course of the next quarter revolution around the star, the M star rises above the horizon again in the other (long-night) hemisphere of each planet. At the halfway point (180°) of their orbit around the M star, the planets are back again experiencing a day-night cycle as I described already. By the third quarter point (270°) of their orbit, the planets' former-night hemispheres are bathed in continuous daylight and the M star is bisected by the horizon at their equators; the former-day hemispheres have continuous night. One more quarter of their orbit around the M star and the starting description is repeated. So if the planets' orbital plane is perpendicular to their orbital plane around the M star, the planets will have extended periods of continuous daylight on one hemisphere and continuous night on the other, kind of like what happens with Uranus and also at Earth's geographic poles (with six months of daylight and six months of night), switching each half-revolution around the M star. Depending somewhat on the atmospheres of the double planets, temperature extremes are quite likely.

A more comfortable scenario

The situation is much simpler and comfortable if the planes of the double-planets' orbit and their mutual orbit around the star coincide. In this case the planets have regular day-night cycles (governed by their mutual orbit) at all positions around their M dwarf host star. Temperatures on the planets would be much more comfortable assuming they orbited each other relatively rapidly. The only significant difference would be that a zone centered on the equator on their mutually facing hemispheres would be slightly cooler since the M star would be occulted "daily" on both planets. Higher latitude zones would have a familiar (to Earthlings) day-night cycle if the planets were separated enough from each other. The cooler equatorial zone would probably lead to different major atmospheric and ocean circulation patterns compared to Earth's so there would be some affect on each planet's weather. The extremes experienced would be nowhere near the extremes experienced in the perpendicular- planes scenario.

Thanks for the mind-stretching question, Mark.

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.

Science fiction is fun to read, and sci-fi movies are fun to watch. Both share the imaginations of their authors or directors as the stories and characters are presented to us. When they are well made they entertain us and encourage us to reflect on what we've read or seen. Both also often require us to suspend disbelief, to a greater or lesser extent depending on our individual backgrounds and our views of science.

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!

Joel asked "If you were to find aliens next to the sun, why would they be blue?"

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.

[[IMAGE||polar_bear.jpg||right||250||Polar bear fur only looks white.||Polar bear fur only looks white.||0||0]]

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 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.

Today's question, from Reader Q: "Could an exoplanet be Earth-like even when it is outside of its star's habitable zone?"

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.