The Planet with Nowhere to Land

"Day after day, day after day,

We stuck, nor breath nor motion;

As idle as a painted ship

Upon a painted ocean.

Water, water, every where,

And all the boards did shrink;

Water, water, every where,

Nor any drop to drink." -Samuel Taylor Coleridge, Rime of the Ancient Mariner

Despite the discovery of dozens of worlds -- planets and moons -- in our own Solar System, as well as hundreds (soon to spill into the thousands) of confirmed planets orbiting other stars, our Earth is still unique.

Image credit: The Suomi NPP Blue Marble, NASA / NOAA.

At least, it's unique as far as we know.

A smaller, dense rocky world with not just continents, but also oceans and life, Earth is the only one of its kind. So far, that is. And yet given the huge variety of stars and worlds found to date, it makes you wonder just what sort of unusual places this Universe contains.

Image credit: NASA, ESA and G. Bacon (STScI).

The first exoplanets found were Hot Jupiters: gas giant planets orbiting very close to stars even hotter and brighter than our own. And this is no surprise: it's much easier to detect large, heavy planets that are close to their parent star. Conversely, it's much more difficult to detect planets when they're any combination of smaller, lighter, or farther away from their parent star.

In the last few years, our technology has improved enough that we've found a vast variety of stars, from the smallest, coolest, reddest stars up through big, bright hot blue stars (and even a few red giants and supergiants) that have planets orbiting them. In the image below, all 2,326 exoplanet candidates discovered by the Kepler Mission are shown.

Image credit: NASA's Kepler Science Team / Jason Rowe.

Based on the temperature and luminosity of the star that's central to each of these solar systems, each planetary system has a region of space where -- according to our current knowledge of biology and chemistry -- life may be likely to arise. We call this region the Habitable Zone, and planets found within it are excellent candidates for the one Earthlike condition we prize over any other: liquid water.

Planets too close to their star, like Mercury in our own Solar System, are going to be too hot for this. Rocky worlds that are too far distant, like those out beyond Mars, will be too cold. We have reached the point where we've discovered solar systems other than our own that have planets in all three zones!

Image credit: wikimedia commons user Henrykus.

Like the vast majority of stars in the galaxy, the parent star of the exoplanet system above, Gliese 581, is a red dwarf star with less than 40% the mass of the Sun. While their habitable zones are tiny compared to the one around our Sun, there are already many excellent rocky planetary candidates found interior to and within their habitable zones.

In fact, the Kepler team has put together a tremendously illustrative video of all the candidate planets found. Have a look for yourself!

Now, if we were to rescale these raw distances, we'd find that there are a great number of planetary candidates closer to their star than Mercury is to the Sun that would be too cold to support life, but that -- by far -- the most common type of exoplanet found so far is still the one too close to its star to support Earth-like life.

But one of the most interesting things to consider is that not every solar system is going to have the same chemical composition as ours. How's that?

Image credit: La Silla Observatory / MPG / ESO.

The way you form stars -- as the star forming region NGC 3324 illustrates -- is by collapsing molecular gas clouds down into regions with a central star and a protoplanetary disk. Over time, the atoms in the disk accrete into planets, with typically the densest elements closest to the parent star and the least dense elements the farthest away. It is no coincidence that Mercury, Venus, and the Earth (and Earth's Moon) have significantly higher densities than all other planets, moons, comets and asteroids in our Solar System.

But the reason we have as many heavy elements as we do is because the molecular cloud that formed us had enough enriched elements from deceased, previous generations of stars to form us. Stars that were formed much earlier in the Universe, as well as stars that formed later in less enriched regions, would have solar systems containing planets with much lower densities. Even the innermost planets.

Image credit: David A. Aguilar (CfA).

And by measuring not just the mass and radius of the planet, but the star's light filter through the planet's atmosphere, we can measure the infrared color of "sunset" on the world, and hence determine the elemental composition of the planet's atmosphere!

What did they find? A thick, dense, featureless atmosphere composed primarily of water vapor! As the Harvard-Smithsonian Center for Astrophysics reports:

"The Hubble measurements really tip the balance in favor of a steamy atmosphere," said [Zachory] Berta.

Since the planet's mass and size are known, astronomers can calculate the density, which works out to about 2 grams per cubic centimeter. Water has a density of 1 g/cm3, while Earth's average density is 5.5 g/cm3. This suggests that GJ1214b has much more water than Earth, and much less rock.

As a result, the internal structure of GJ1214b would be very different than our world.

"The high temperatures and high pressures would form exotic materials like 'hot ice' or 'superfluid water' - substances that are completely alien to our everyday experience," said Berta.

That strong signal favoring an atmosphere composed 100% of water vapor is definitely there, as the signatures of models with significant other atmospheric components are clearly disfavored in Figure 10 (thanks, Michael Richmond) from the paper:

Image credit: Z. K. Berta et al.

The conclusion that the planet has this uniform, thick H₂O atmosphere around it indicates a large, watery chemical composition. The sheer amount of water indicated by this data -- combined with the information about the overall density of the planet -- renders a solid, rocky surface is inconceivable!

This world most likely is a waterworld, with a 100% oceanic surface!

Image credit: retrieved from Steven Andrew.

This planet is likely to be at very different temperatures and pressures from Earth, and so will likely have unusual states of water. It should be constantly boiling (at an average temperature of 450 degrees Fahrenheit (230 Celsius), and there should be strange states of matter, like "hot ice" or "superfluid water" composing the surface/atmosphere.

There are stranger worlds out there than most of us have ever imagined, and this one orbiting this star -- GJ 1214 -- just 42 light years away, is the first super-Earth that's ever had its atmosphere detected.

I can't help but look on with awe, and enjoy wondering what else is out there!