Undreamed Shores - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 259

would be so deep and dense that even at high temperatures the pres-
sure would turn the water into ice.
A candidate for such a water world is Gliese 1214 b, lying some
40 light years from Earth in the direction of the constellation Ophiu-
chus. The planet was initially detected by the dimming effect of its
transit across the parent star, a dimming of 1.5 per cent occurring
every 1.58 days—and indicating that this planet also has a short year
and rapid orbit about its star. Later its wobble was measured, giving
the planet's orbit, radius, and mass: 173 Gliese 1214 b is 2.68 times the
radius of Earth, and 6.55 times more massive, giving the planet a
much lower density overall (1870 ± 400 kg/m 3 ) than Earth (5520 kg/m 3 ).
That lower density is intriguing for it suggests a world rich in water, a
water world of perhaps 25 per cent silicate rock and iron, and 75 per
cent water—although there are alternatives, of which one is a planet
with a silicate core and a massive envelope of hydrogen and helium.
If it is a water world, though, then its diversity of forms of water
would rival that of the Earth. A steam atmosphere would give way to
a hot liquid ocean that, as it became further compressed, would turn
into a form of 'hot ice'. There are various forms of such solid, high-
density water, such as the crystalline 'ice VII', with normal ice being
'ice 1h'. There is also an 'ice IX', a genuine low-temperature, high-
pressure form which is not to be confused with the terrifying 'ice-
nine' that Kurt Vonnegut invented for his novel Cat's Cradle . Vonnegut's
thankfully imaginary ice-nine was evoked as a stable form, crystalline
at room temperature that, on contact with normal water, could make
that water become solid and crystalline too, to threaten satisfyingly
dramatic eco-disaster.
Not all the water worlds need be weird and exotic by Earth's stand-
ards. One of the goals of the exoplanet community is to find 'Earth
twins'—similar-sized planets at a similar distance from their stars,
which might possess both oceans and land. The companion planets