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!

Video credit: Kepler Exoplanet Candidates by blprnt.

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!

11 thoughts on “The Planet with Nowhere to Land

  1. The figure you show from the paper, Figure 2, is the spectrum of the star, not the planet. The transmission spectrum of the planet is shown in the paper’s figure 10. There are NO obvious signatures of water.

  2. Michael @1,

    Thank you for that. The analysis must be very good to be able to discriminate between the models that figure 10 shows.

    Do you think that the lack of obvious water signatures means that there are alternative explanations to what the authors state? If you have good reasons to doubt their conclusions, I would be excited to hear what you think is going on instead.

  3. Excellent article here. Thankyou.

    What a wonderful watery world. 🙂

    Reminds me of Gliese 436 b and some other similar mass Gas dwarf / “SuperEarth” (a term Ithink is pretty misleading seeinga sthese are very unearthly places btw) exoplanets.

  4. There are many molecules composed of common elements with molecular weights similar to that of water. N2, CO2, NH3, to name a few. Those molecules would produce spectral signatures in the range sampled by this paper which are similar to the flattish model produced by water.

  5. It’s just fascinating to glimpse the variety that the universe exhibits. I’m trying to wrap my head around “hot ice”. Since I was a boy it seemed obvious that exoplanets were commonplace in the universe and it’s rewarding to have lived long enough to see our technology prove it.

  6. As far as I can tell they infer water indirectly, from the low density, not directly from the spectrum (Fig 10 just uses water content as a proxy for molecular weight – a ‘toy model’, as they state). The spectral evidence rules out H2 and He, so the low density must be due to water: the implicit assumption is that planets are made of mixtures of varying proportions of ‘rock’ (silicates and iron), ‘ice’ (water, but also methane, ammonia etc.) and ‘gas’ (H2/He).

    Your comment that “not every solar system is going to have the same chemical composition as ours” may be somewhat ironic: I think they (and the planet-building models they reference) have effectively assumed that the commonest low density planet ingredient after H2/He is water. If GJ1214’s C/O ratio > 1 this might not be true; instead you might have a carbon planet with a methane atmosphere.

  7. @4 Michael Richmond
    Please source your speculation; and tell what the various components in the atmosphere might mean. If you know; please educate me. Thanks.

    Several papers discuss the possibility of water on planet GJ1214b. For example, says, “We present an investigation of the transmission spectrum of the 6.5M⊕ planet GJ 1214b based on new ground-based observations of transits of the planet in the optical and near-infrared, and on previously published data… The planet’s atmosphere must either have at least 70% H2O by mass or optically thick high-altitude clouds or haze to be consistent with the data… The combined data set spans the visible to the infrared 90.6 to 4.5 µm), which makes it one of the most complete exoplanet transmission spectra obtained to date… A new measurement of the system’s trigonometric parallax using modern technology would further our knowledge of this important planet.”

    Another paper says, “GJ1214b is thought to be either a mini-Neptune with a thick, hydrogen-rich atmosphere, or a planet with a composition dominated by water.”

    Another, “The exoplanet GJ1214b presents an interesting example of compositional degeneracy for low-mass planets. Its atmosphere may be composed of water, super-solar or solar metallicity material.”

    Another “We report observations of two consecutive transits of the warm super-Earth exoplanet GJ1214b… cloud-free atmospheric models require more than 30% metals (assumed to be in the form of H2O by volume to be consistent with all the observations.”

    Another, “The composition of the atmosphere of GJ1214b is presently unknown. Possibilities include a composition that is H-dominated, with solar or super-solar composition, CO2-dominated or H2O-dominated”

    OK that’s enough.
    I’m lost in the jargon of exoplanet atmospheres and have no planet deveolpment model context.
    I assume except for this case (not GJ1214b )
    that models imply atmospheric content.

    So what would really be nice (Ethan or someone) is a short education on planet evolution theory and models and what kinds of combinations of data (e.g. this mass, radius and spectral hints) suggests jupiter (gas giant), earth (life habitable), mercury (a cold rock).. Otherwise I can’t put any of this atmosphere jargon in perspective.

    I plead ignorance. I just need to be educated. Thanks.

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