The locked heavens

When you picture an exoplanet, it's easy to fall back into the typical science-fiction concept of an alien world -- almost always like some odd, vaguely hostile version of Earth, with a different-colored sky and lots of big rocks.

Kirk: "Six to beam down.  Be prepared to beam back three of us in about forty-five minutes."

As astronomers have discovered more and more actual exoplanets, though, they've found there's far more variety in their characteristics than the creators of Star Trek ever dreamed.  There are hot Jupiters -- like our own largest planet, made primarily of thick layers of hydrogen and helium, with a solid core, but so close to their parent stars that not only are they extremely hot, they have orbital periods of ten days or under.  There are ice planets, water planets, planets made entirely of molten rock.  One of the most curious features of the exoplanet menagerie, though, is that some are tidally locked -- like our own Moon, they always have the same face pointing toward their orbital center.

And just last week, scientists at the Max Planck Institute for Astronomy have found a tidally-locked planet whose daylight face has temperatures in the habitable range.  If it has an atmosphere as thick as Earth's (something currently not known), it's estimated to have an average temperature of around 13 C.

Called Wolf 1069b, thirty-three light years away in the constellation Cygnus, it has almost exactly the same radius as Earth, but an orbital period of only 15.6 days.  Not that the inhabitants would be able to determine that easily; on a tidally-locked planet, their sun would always be in the same position in the sky, so it wouldn't be obvious that they were orbiting around anything. 

Think of how bizarre it would be to live on Wolf 1069b.  On the always-daylight side, things are reasonably clement, but as you approach the twilit border, conditions go downhill fast.  Because weather is caused by convection -- changes in air pressure driven by uneven heating -- Wolf 1069b would experience a degree of weather never seen on Earth.  Its star would heat the atmosphere on the daylight side, causing the air to expand and rise; this would pull cold air from the nighttime side of the planet, creating a convection cell that dwarfs the strongest trade winds imaginable.  As you got closer to the edge between day and night, you'd walk into an increasingly powerful, freezing cold headwind, moving at a speed that would make a hurricane seem like a gentle breeze.

Then, on the nighttime side -- nothing but frozen wasteland, forever pointing outward into the starlit sky, never seeing the warm light of the parent star.  Not survivable for any life form I can conceive of, certainly not the ones evolved on and adapted to the daylight side.  Imagine what kinds of stories the inhabitants would tell -- of a hospitable region where the sun shines down from high in the heavens, fixed in place as if the sky was a crystalline sphere, eternal and unchangeable.  The known world ringed by an impassible boundary of screaming winds and bitter cold.  On the other side of which is... the unknown.  Eventually, if they developed sophisticated enough technology, surely they'd venture there, as we now dive down in submarines to investigate the deepest oceanic trenches.  

What would they think, the first time they traveled into the region of perpetual night -- and saw stars?

The wild diversity of astronomical objects we're discovering absolutely beggars belief.  There's a planet called TrES-2b that is the darkest exoplanet ever studied -- the same overall hue as a piece of charcoal -- and no one knows why.  55 Cancri-e is hot and carbon-rich, and might be composed chiefly of diamond.  HR 5183b has an extremely elliptical orbit, lasting around 74 Earth years -- starting out farther away from its parent star than Jupiter is from the Sun, but screaming in to slingshot around it closer than the orbit of Mercury -- earning it the nickname of "the whiplash planet."  WASP-76b has a surface hot enough to vaporize iron -- meaning it rains, but it rains molten droplets of iron metal.

Fiction writers like myself would have a hard time coming up with anything odder than what the astronomers are actually discovering in the skies above us.  It recalls the quote from Carl Sagan: "We all have a thirst for wonder.  It's a deeply human quality.  Science and religion are both bound up with it.  What I'm saying is, you don't have to make stories up, you don't have to exaggerate.  There's wonder and awe enough in the real world.  Nature's a lot better at inventing wonders than we are."

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