Astrophile is our weekly column covering curious cosmic objects, from within the solar system to the furthest reaches of the multiverse

Supercritical scenario (Image: University of Texas, National Science Foundation, NASA)

Object type: Extrasolar planet

Composition by mass: 70 per cent rock, 30 per cent supercritical water

Orbital angle: Constantly changing

Imagine that you are floating thousands of kilometres below the surface of a vast ocean that is neither liquid nor gas, but somewhere in between.  Above you, the constellations very slowly shift and change as your watery world and its host star turn somersaults in space. 

That’s what you would see if you could swim on the planet 55 Cancri e, the most watery world discovered to date.  New observations suggest that the planet is probably covered in so-called supercritical water, a kind of water that blurs the line between liquid and gas. 


Not only that, but as the planet and its four planetary siblings orbit their host star, the whole system rotates in space due to tugs from a partner star, as if Saturn and its rings were turning on a spit. 

Planet in hot water

55 Cancri, a sun-like star 41 light years from Earth in the constellation Cancer, is one of only a handful of stars known to host five planets or more.  Its innermost planet, 55 Cancri e, was detected in 2004, given away by the wobbles it induced in its host star. 

Those first observations suggested the planet orbited the star once every 2.8 days.  But last year, a pair of astronomers realised that gaps in the observations had skewed the statistics.  The planet’s year was actually only 17 hours, 41 minutes long, meaning the planet’s distance from its host star is 1/20th that of Mercury from the sun. 

At that distance, the temperature at the planet’s surface is a scorching 2700 °C.  But to astronomers’ delight, the short year also means they can watch the planet cross in front of its star, or transit, in less than one Earth day.  That makes 55 Cancri e the first known planet to transit in front of a naked-eye star. 

Wettest world

More importantly, watching a transit lets astronomers determine the planet’s size, giving a clue to its density and composition. 

This spring, two independent groups using the space telescopes MOST (Microvariability and Oscillations of Stars) and Spitzer did just that. 

Diana Valencia of the Massachusetts Institute of Technology and colleagues have now combined the results, finding that the planet is 2.17 times as wide as Earth.  Combined with its mass of 8.57 Earths, that size suggests the planet has a dense rocky core, surrounded by a 3000-kilometre-thick envelope of nearly pure water. 

The team calculates that it is probably 30 per cent water by mass, making 55 Cancri e the most watery world yet discovered. “ It’s really the most water-dominated planet, by mass,” Valencia says. 

It also places 55 Cancri e in a totally new class of planets, right in the middle of two previously known types.  Other planets with similar masses, known as super-Earths, are thought to be either smaller and rockier, or bigger and puffier, like miniature Neptunes. 

“We have two families, and 55 Cancri e is kind of in between,” Valencia says. “ It is the first one that is right at the edge.” 

Slick fluid

Because of the extreme temperatures and pressures, the water is probably in the supercritical phase, where gas and liquid are indistinguishable.  As easily as the water we are familiar with flows, it is still 10 times as viscous as supercritical water.  Other materials can dissolve in supercritical water, so in theory 55 Cancri e’s oceans could be salty. 

The density of the supercritical water would vary from the rocky core to the edge of space, with no clear boundary between sea and sky.  At a certain distance from the centre there would be a level where humans would be buoyant, Valencia says.  Assuming we could breathe and withstand the temperatures, we would float. 

The star also has a small companion, a red dwarf star that lies about 1000 times as far away as Earth’s distance from the sun.  This red dwarf pulls on the 55 Cancri system, and because all five planets in the system – and their host star – are such a tight-knit family, they behave like ice skaters holding hands, so that the companion star’s tugs cause them all to do somersaults in space. 

“The crazy thing is, over the course of hundreds of millions of years, your orientation relative to the other stars in the galaxy would totally change,” says Nathan Kaib of Queen’s University in Kingston, Ontario, who led the research team that found the head-over-heels motion. “ You’d get flipped upside-down, but it’s a very subtle effect.  The only thing you’re going to notice is that your view of the night sky gets flipped on its head.” 

That might be bad news for any seafaring creatures living in 55 Cancri e’s supercritical oceans and using the stars to navigate. “ You couldn’t use them reliably for any trips that took millions of years,” Kaib jokes. 

Reference: arxiv.org/abs/1110.4783; arxiv.org/abs/1110.5911, to be published in a forthcoming issue of The Astrophysical Journal Letters

Read previous Astrophile columns: Attack of the mystery green blobs, Undead stars rise again as supernovae, The sticky star cluster that’s mostly black hole, The rebel star that broke the medieval sky, Star exploded?  Just another day in Arp 220, Giant star comes with ancient tree rings, Frying pan forms map of dead star’s past, The most surreal sunset in the universe, Saturn-lookalike galaxy has a murky past, The impossibly modern star, The diamond as big as a planet.