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Posts Tagged ‘exoplanets’

Slouching to the Right of the Drake Equation

And what rough beast, its hour come round at last,
Slouches towards Bethlehem to be born?

— William Butler Yeats, “Second Coming”

The last few years have been heady for planet hunters. First the hot Jupiters; then the will-o’-the-wisp Glieslings and their cousins; and in the last year, the results from the Kepler mission which detected planetary systems in the low thousands; one of these is Kepler 22.

Kepler 22 is a G5 type star (our sun is G2, about 10% bigger and hotter) 600 light years away with a planetary entourage. For anyone who was in a sequestered jury room or a silently running nuclear submarine, what got splashed across the news media on December 5 was the confirmation that one of the Keplerings is a super-Earth (2.4 times the radius of our planet) that is solidly within the habitable zone of its primary – habitable defined as the region where water can remain liquid. It circles its primary in 289 days and its estimated average temperature is a balmy 22 C/75 F if (big if) it has an atmosphere thick enough for a mild greenhouse effect.

That’s what we know, and it’s important and exciting enough. Here’s what we don’t know, which makes the exclamations of “Twin Earth!” annoying: we don’t know its mass (though the wobble velocity puts an upper limit of 36 Earth masses on it), its composition, the composition of its atmosphere or if it has any moons. Equally annoying are the suggestions to name it The Christmas Planet or the barely less mawkish Hope, right down there with the naming of putative Gliese 581g something like Betty (not even Elizabeth, which at least would celebrate an unforgettable historic figure, plus several literary ones).

The Kepler findings are pinning down the still-loose middle terms of the Drake equation by strongly indicating that most suns have planetary systems, and most planets are of the small rocky variety. Of the approximately 2,000 systems Kepler tentatively identified, about fifty have planets within the habitable zone, of which perhaps ten are “Earth-like” (loosely defined).

Half a percent may not sound like much. But given the quarter trillion suns in our galaxy, the numbers mount up quickly. Plus, of course, the size and location of the newcomer inevitably raises expectations: if Kepler 22b is rocky and has decent amounts of water and a reasonably thick atmosphere, the probability of life moves into the “likely” zone. So it’s not surprising that the Allen Array turned its dishes in the direction of Kepler 22 (no requests for Warren Zevon yet, but the night is still young) – or that the concept art is coming in thick and fast.

It is a great pity that Kepler 22b is so far. Even expeditions with quasi-exotic propulsion systems (or exceptionally nice humans in flawless arkships) would take a long time to reach it. But the lengthening list of not-quite-Earths is a powerful enticement not to abandon the faltering beacon of space exploration. Once again, I will close with what I said about Gliese 581g:

“Whether [Kepler 22b] is so hospitable that we could live there or so hostile that we could only visit it vicariously through robotic orbiters and rovers, if it harbors life — even bacterial life, often mistakenly labeled “simple” — the impact of such a discovery will exceed that of most other discoveries combined. Unless supremely advanced Kardashev III level aliens seeded the galaxy like the Hainish in Ursula Le Guin’s Ekumen, this life will be an independent genesis, enabling biologists to define which requirements for life are universal and which are parochial.

At this point, we cannot determine if [Kepler 22b] has an atmosphere, let alone life signatures. If it has non-technological life, without a doubt it will be so different that we may not recognize it. Nor is it a given, despite our fond dreaming in science fiction, that we will be able to communicate with it if it is sentient. In practical terms, a second life sample may exist much closer to home — on Mars, Europa, Titan or Enceladus. But those who are enthusiastic about this discovery articulate something beyond its potential seismic impact on biology and culture: the desire of humanity for companions among the sea of stars, a potent myth and an equally potent engine for exploration.”

Image: 1st, one of the four Kepler 22b imaginings by space artist Ron Miller. 2nd, comparison of Sol and Kepler 22 (NASA/Ames/JPL).

Postscript: Immediately after my discussion of Kepler 22b, Christopher Jones interviewed me for Trek.fm. He asked me many interesting questions about the 100-Year Starship symposium, long-generation starships and the future of humanity on- and off-earth. You can hear the interview here.

New Worlds: Stranger than Fiction

There’s no better way to fire up my imagination than to introduce me to a cool new place, real or imagined. Something about imagining what it’s like Elsewhere just captivates me. That’s a big part of why I like speculative fiction: fantasy and science fiction are full of exotic new worlds to explore. But it’s also a big part of why I study planets for a living. There are plenty of actual exotic new worlds to explore out there in the universe, and I thought I’d share some of my favorites in the hopes of sparking your imagination too.

I’ll start here on Earth. You might think earth is a bit, well, mundane, but there are some really weird places hidden away even on our familiar home planet that are just begging to be the setting for some fiction. Take, for instance, la Cueva de los Cristales (the Cave of Crystals) in Mexico. The cave is brutally hot and humid, but it is filled with translucent crystals the size of tree trunks, forming a breathtaking natural cathedral.

Another hellish but spectacular place on earth is the Kawah Ijen sulfur mine in Indonesia. I only recently learned about this place from a feature on The Big Picture photo blog, which showed some surreal photos of this volcanic mine where oozing molten sulfur burns with a blue flame and the air is filled with acidic gases. And of course, the ultimate alien locale on our own planet is the deep sea, where entire ecosystems are still being discovered, with creatures more imaginative and terrifying than any fiction.

Burning molten sulfur in the Kawah Ijen sulfur mine.

But really, that stuff is pretty tame compared to the rest of the universe, so let’s take a look at some other awesome places. We’ll start off with Mars. What’s so special about Mars? It’s just a big desert right? Well it has a few claims to fame. You may have heard that it boasts the tallest volcano in the solar system – Olympus Mons – which towers almost three times as high as Mount Everest. Mars is also home to Valles Marineris, the largest canyon in the solar system, which stretches for 4000 km and is 200 km across at its widest point. Ok, so it has a couple planetary tourist traps. To get to the really strange stuff you have to head south. The south polar residual cap on Mars is made of frozen carbon dioxide – a.k.a. dry ice – and it is slowly disappearing. As the cap sublimates, it forms some really bizarre features. Take a look at this picture of the “swiss cheese terrain”:

HiRISE image of the "swiss cheese terrain" of the martian south polar ice cap.

Believe it or not, the smooth, fractured areas in this picture are the high ground (the lighting is from the lower right). The rounded pits are formed as the dry ice turns to gas. Even cooler are the south polar “spiders” – dark splotches that can appear on the ice in a matter of days. One of the leading theories for how these form is that the ice acts like a greenhouse, allowing sunlight to pass through it until it hits a darker layer. As that layer absorbs sunlight, it warms up, vaporizing the ice and creating a high-pressure pocket of gas that erupts, dumping dust onto the surface. How about some fiction set on an unstable landscape of sublimating carbon dioxide, where every step could set off a violent geyser or collapse the roof of an icy greenhouse?

Not impressed by swiss cheese and spiders? Then let’s head out to the icy moons of the outer solar system! We’ll bypass Io’s sulfur-laden volcanoes and Europa’s icy ocean because those are pretty well known in sci-fi. Instead I’d like to focus on two of Saturn’s moons. The first is Iapetus, a familiar name to anyone who has read the book 2001: A Space Odyssey. (In the movie the obelisk was re-located to Europa because they had trouble making convincing-looking rings for Saturn) Arthur C. Clarke picked a great location in which to hide an alien artifact though, because Iapetus is decidedly weird and artificial-looking. First of all, it is two-faced. One hemisphere is black as coal, while the other is as white as snow. And it’s not a smooth transition between the two either: recent photos of the dividing line show splotches of pure black and pure white. It’s like looking at a close-up of a dalmation. The leading theory for this abrupt color change is that Iapetus sweeps up debris blasted off the moon Phoebe. Since Iapetus is tidally locked with the same hemisphere facing Saturn at all times, this material always hits the same hemisphere, causing it to darken. Once it darkens a little bit the sun takes over, heating up the darkened areas and causing surface ice there to sublimate away and condense on the brighter areas. Over time this darkens the dark spots and brightens the bright spots.

The dark hemisphere of Iapetus, showing the strange equatorial ridge.

The other weird thing about Iapetus is that it has a 13 km high ridge of mountains running for 1300 km precisely along its equator, and nobody really knows why it’s there! The leading theory is that Iapetus used to rotate much faster and so was fatter at its equator. It has since cooled (and therefore become more rigid) and also has slowed its spin. The ridge might have been formed as Iapetus tried to change shape in response to its new, slower rotation rate. Still, to anyone with a science-fictional bent, that equatorial ridge brings to mind all sorts of more exotic possibilities. As the saying goes, “That’s no moon…

Titan, another one of Saturn’s moons, is even more unusual than Iapetus. Titan is strange because it is the only moon with a thick atmosphere. In fact, its atmospheric pressure is greater than the pressure here on Earth despite its lower gravity! Earth is the only other place in the solar system with a significant nitrogen atmosphere. But the similarities with Earth don’t stop there. The Cassini mission has also found river beds, lakes, seas and thunderstorms on Titan! That would be interesting enough, but the really wild thing is that the surface temperature of Titan is -180 degrees C (-292 F)! At that temperature, water is frozen so hard you can basically think of it as a mineral. It turns out that instead of a water cycle like the Earth, Titan has a hydrocarbon cycle. Methane and ethane take the place of water, condensing to form violent thunderstorms which rain down forming rivers, lakes and seas. The surface is obscured by a smog of heavier hydrocarbons that also gradually settle out of the atmosphere forming great deserts of coal-like sand. There is 1000 times more hydrocarbon locked up as sand on Titan than there is in all the coal on Earth, not to mention all the liquid natural gas that fills Titan’s lakes and seas! What I love about Titan is how it looks so familiar but is so bizarrely different at the same time.  Only nature could be creative enough to come up with an icy moon literally drenched in fuel! I’m just waiting for the first interplanetary expedition led by Exxon and BP.

A false-color radar view of the seas and lakes of methane and ethane near Titan's north pole.

But that’s just the tip of the iceberg. We still haven’t even left the solar system, but to date there are 500 known exoplanets orbiting 421 stars, and new planets are being discovered all the time. If the most exotic places in our own solar system don’t seem science fictional enough for you, then just consider some of the awesome exoplanets that have been discovered:

First of all, we have the “hot jupiters” – gas giants that are so close to their stars they orbit in a matter of hours or days rather than decades. The extreme temperatures drive some pretty crazy weather: Astronomers have detected winds blowing  10,000 km per hour on the hot Jupiter planet HD209458b! Even more mind-blowing is that on some of these planets, instead of clouds made of water and ammonia, there are clouds made of silicate minerals or iron!

Rocky planets close enough to their stars might also have some pretty exotic clouds. Planets orbiting close to their stars are “tidally locked” just like our moon, so the same side always faces the star. That means that on a planet like COROT-7b, a possibly rocky planet with a mass about five times that of earth, the sunlit side gets absurdly hot. Estimates put the sub-solar temperature for COROT-7b at 2600 K (4220 F), which is hot enough to vaporize rock and metal, giving the planet clouds of glowing yellow sodium gas and silicate minerals. If you thought methane rain on Titan was weird, stay clear of the olivine sleet on COROT 7b! Obviously these places wouldn’t be very pleasant places for humans to visit, but the great thing about science fiction is that you can bend some rules. Maybe the higher temperatures make silicon-based life more plausible. Or maybe future post-humans scoff in the face of mere high temperatures and visit these planets to mine the clouds.

Artist's rendition of a hot jupiter, complete with incandescent clouds.

I’ve always been fascinated by the idea of these tidally locked worlds because even though the day side can be hellishly hot, the night side temperatures would plummet to nearly absolute zero, and somewhere on the terminator (the transition between day and night) the temperature would be nice and comfortable. But that’s only if the planet has no atmosphere. Things get a lot more complicated and more interesting if there is a way to convect heat from the hot side to the cold side.  A couple months ago I heard a very cool talk by Ray Peirrehumbert about the climate on the possible earth-like planet Gliese 581g. (As a side note, the Gliese 581 system, with at least six planets, is just crying out to be the setting for some sci-fi!) He described a whole range of possible climates for this tidally-locked world, the coolest and most-habitable of which was the “eyeball earth” scenario. In this case, the planet is mostly ocean with an atmosphere not all that different from our own. Under the sub-solar point the dark open water absorbs enough energy to stay liquid at a cozy 37 degrees C, but as you go farther away, temperatures drop and the ocean freezes. The result is an “iris” of warm open water on an otherwise icy world. You can tweak the amount of greenhouse gas in the atmosphere to vary the size of the open water pool. The only downside is that if the eyeball earth does somehow ice over completely, the ice reflects enough sunlight to prevent the water pool from opening up again. I, for one, would love to read the story of a civilization on Gliese 581g struggling to prevent their world from freezing over. Instead of Martians struggling to survive by building canals, you could have Glieseians building giant CO2 factories!

From Pierrehumbert's paper on the possible climate of Gliese581g

You can get some even weirder planets if you change their composition. For example, the planet WASP 12b is thought to be a “carbon planet”, which as the name suggests, is unusually rich in carbon. WASP 12b is a gas giant, but a carbon-rich rocky planet would be a very interesting place. Instead of normal rocks, you would have mountains made of graphite, diamonds and asphalt. Or if a carbon planet doesn’t float your boat, what about an earth-sized planet made mostly of water? (It would, of course, have to be named Sea World) With oceans hundreds of kilometers deep, the pressures at the bottom would grow so high that exotic forms of ice would form even at high temperatures. Considering the alien life in our own oceans, just imagine what might live in the depths of Sea World!

Artist's rendition of planets around a pulsar.

Finally, if diamond mountains, sodium clouds and eyeball earths still aren’t good enough for you, then take a trip to the first exoplanets ever discovered, orbiting the pulsar PSR 1257+12. These planets are either the charred cinders left over after their sun went supernova, or they formed from the shrapnel generated by that explosion. They are bathed in deadly radiation from their star, and also are in the interesting position of being much larger than the star they orbit (a neutron star is so dense that it can pack the mass of the sun into a sphere the size of a small city). If these planets did indeed form from the gas produced by the supernova, they are almost surely full of exotic radioactive elements generated in the dying gasps of the star. So not only are they blasted by radiation from the pulsar, but they probably produce plenty of radiation themselves. Probably not a nice place to live, but also a fantastic source of resources for anyone who can survive the harsh environment long enough to mine them.

As you can see, the universe is full of fascinating places that are just begging to be the setting for some speculative fiction. I hope these places have sparked your imagination as much as they do mine. It’s fun to imagine other planets, but if there’s anything I’ve learned from studying them, it’s that our imagination pales compared to what is really out there waiting for us.

Once Again with Feeling: The Planets of Gliese 581

Gliese 581 may be small as stars go, but it looms huge in the vision field of planetfinders.  As of late last week, measurements indicate the system has six planets of which three are Earth-size and -type, within the star’s habitable zone, with stable, near-circular orbits.

The Gliese 581 system has a persistent will-o-the-wisp quality.  Almost each of its planets (c, d, e and now g) has been pronounced in turn to pass the Goldilocks test, only to have expectations shrink when the data get analyzed further.  The first frisson of excitement arose when 581c was determined to be Earth-type, which quickened the usual speculations: atmosphere? water? life?  We don’t know yet and our current instruments cannot detect biosignatures at that distance (short of an unencrypted request for more Chuck Berry).  But there are some things we do know.

Gliese 581 is a red dwarf, a BY Draconis variable.  This makes it long-lived; on the minus side, it may produce flares and is known to emit X-rays.  Planets in its habitable zone are so close to it that they are tidally locked, always presenting the same face to their star.  The temperature differentials resulting from the lock imply hurricane-force winds and tsunami-like tides.  Gliese 581g, like 581c, is large enough to retain an atmosphere; the hope is that, unlike 581c or Venus, its specific circumstances have not resulted in a runaway greenhouse effect.

The real paradigm shift is the discovery that this solar system has many earth-size rocky planets, in contrast to the hot-Jupiter/hot-Neptune preponderance in most others.  The second enticing attribute of Gliese 581 is its relative closeness — a distance of merely 20 light years.  It is still millennia away by our present propulsion systems.  But I nurse the dream that if we see anything remotely resembling a biosignature, we will strive to reach it.  In the meantime, I suggest we give it a name that fires the imagination.  Perhaps Yemanjá, the Yoruba great orisha of the waters, in the hope that the sympathetic magic of the name will work.  Perhaps Kokopelli, the trickster piper of the American Southwest cultures, who may entice us thither.  I will conclude with the final words of my first article on Gliese 581:

“Whether Gliese 581c [g] is so hospitable that we could live there or so hostile that we could only visit it vicariously through robotic orbiters and rovers, if it harbors life — even bacterial life, often mistakenly labeled “simple” — the impact of such a discovery will exceed that of most other discoveries combined. Unless supremely advanced Kardashev III level aliens seeded the galaxy like the Hainish in Ursula Le Guin’s Ekumen, this life will be an independent genesis, enabling biologists to define which requirements for life are universal and which are parochial.

At this point, we cannot determine if Gliese 581c [g] has an atmosphere, let alone life. If it has non-technological life, without a doubt it will be so different that we may not recognize it. Nor is it a given, despite our fond dreaming in science fiction, that we will be able to communicate with it if it is sentient. In practical terms, a second life sample may exist much closer to home — on Mars, Europa, Titan or Enceladus. But those who are enthusiastic about this discovery articulate something beyond its potential seismic impact on biology and culture: the desire of humanity for companions among the sea of stars, a potent myth and an equally potent engine for exploration.”

Images: Top, comparison of the Sun and Gliese 581 habitable zones (the diagram is by Franck Selsis, Univ. of Bordeaux; the image of 581g was originally created for 581c by Ginny Keller); bottom, Kokopelli playing his flute.