Locked in place

The Moon orbits the Earth in such a way that the same side always faces us.  Put another way, its periods of revolution and rotation are the same; it takes the same amount of time for the Moon to turn once on its axis as it does to circle the Earth.

This seems like a hell of a coincidence, but there is (of course) a physical explanation for it.  Close orbits -- either of a planet around its host star, or a satellite around a planet -- generate a high tidal force, which is the gradient in the gravitational force experienced by the near side of the orbiting body as compared to the far side.  There's always going to be a tidal force; even tiny Pluto has a greater pull from the Sun on the near side than it does on the far side, but with a small body at that great a distance, the difference is minuscule.  (You're experiencing a tidal force right now; the Earth is pulling harder on your feet than on your head, assuming you're not upside down as you're reading this.)  But the Moon's proximity to the Earth means that the tidal force it experiences is comparatively huge.  So even if it once rotated faster than it revolved, the higher pull on the near side slowed its rotation down -- a sort of gravitational drag -- until the two matched exactly.

The result is called 1:1 tidal locking, and is why (apologies to Pink Floyd) there is no permanently dark side of the Moon.  There's a near-Earth and a far-Earth side, but no matter where you are on the Moon, you'll have a 28-day light/dark cycle.  However, the apparent position of the Earth in the sky doesn't change.  If where you stand on the Moon's surface, the Earth appears to be hovering thirty degrees above the western horizon, that's where it will always be from that perspective.

It's been known for some time that planets can also be tidally locked.  Once again, it's more likely to happen when they orbit close to their host star, which means a lot of tidally-locked planets are probably so hot they're uninhabitable.  But the situation changes if the host star is a red dwarf -- small, low-luminosity stars that are incredibly common, making up almost three-quarters of the stars in the Milky Way.  These stars have such a low heat output that the "Goldilocks zone" -- the distance from the star in which the conditions are "just right" for liquid water to form -- is very close in.

So a star in a red dwarf's habitable zone might well also be tidally locked.

Think of how bizarre a situation that would be.  If the planet is at the right distance for the lit side to be comfortable, there'd be a region of perpetual twilight bounding it, and on the other side of that, permanent, freezing-cold night.  Not only that; this would create the convection cell from hell.  Weather down here on Earth is largely caused by uneven heating of the planet's surface; air warms and rises near the Equator, cools, eventually becoming cool enough to sink and completing the circle.  The Earth's rotation and topography complicate the situation, but basically, that convective rise-and-fall is what generates wind, clouds, rain, snow, and the rest of the meteorological picture.

On a tidally-locked planet, these processes would be almost certainly be amplified beyond anything we ever see on Earth.  Especially the twilit boundary zone -- the constant heating of the bright side, and loss of heat to radiation on the dark side, would cause the atmosphere on the bright side to rise, drawing in cold air from the dark side fast.  The result would be a screaming hurricane across the boundary.

At least, so we think.  We don't have any tidally-locked planets to study, only airless moons.

Until now.

A study out of McGill University has confirmed the first tidally-locked exoplanet, LHS 3844b, a "super-Earth" that was identified by measuring the light coming off the planet at different places in its orbit -- something that allowed the researchers to estimate its temperature.

Artist's impression of the dark side of LHS 3844b [Image credit: NASA/JPL-Caltech/R. Hurt (IPAC)]

Chances are, LHS 3844b doesn't have much of an atmosphere, so the convective hellscape I described above might not apply to it.  Still, the idea that astronomers have identified that an exoplanet is tidally locked is kind of astonishing.  The first exoplanet was only discovered in 1992; in the intervening thirty-odd years not only have we found thousands of them, we're now getting so good at analyzing them we can figure out the size of their orbits, how fast they rotate, and the probable composition of their atmospheres.

Our understanding of the universe has accelerated so much, it's hard even to imagine where it might be headed.  The idea that we could not only find an exoplanet around a distant star, but determine that the same side of the planet always faces the star, boggles the mind.

The future of astronomy is looking pretty stellar, isn't it?

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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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