The Life of Super-Earths (9 page)

There is a third possible family of super-Earths and terrestrial planets—carbon planets. These would be extremely rare, as they require more carbon than oxygen to be present in the planet-forming mixture.
¹⁹
Normally carbon is half as abundant as oxygen, as we saw in
Figure 2.1
. But astronomers have observed rare stars in which carbon is more plentiful than oxygen. A planet that forms from such a mixture will be different—it will have a mantle rich in silicon carbide and graphite in its interior.
²⁰
It will still have a precipitated iron core, but its overall size and the chemistry on its surface and crust will be very different. Silicon carbide is a very hardy substance—we use it to make durable ceramics, the disk brakes of sports cars, and tools for other high-stress environments. So volcanism, tectonics, and weathering are
going to be minimal on carbon planets. Also, carbon planets are likely deficient in water.

Figure 5.2
shows some imagined family portraits of super-Earths and compares them to Earth and Neptune. The relative sizes of the super-Earths are accurate to the best of our theoretical models. Carbon planets are not shown in
Figure 5.2
. They will have intermediate sizes between rocky and ocean planets. The Neptune-like giant planet orbiting the star Gliese 436 is shown for comparison too.
²¹

Finally, in the approximate mass range for super-Earths we discover small planets with relatively large amounts of hydrogen and helium—perhaps up to 10 percent by mass—just as we see on Neptune. Over all, of course, the planets would be smaller than Neptune: call them mini-Neptunes. It is still unknown where the transition occurs from planets rich in solids to planets with increasingly more massive hydrogen-helium gas envelopes. Theory gives us multiple possible solutions and no clear choices, so we'll need the observations to show us nature's preferences, and NASA's Kepler is well on its way to provide them. Fortunately, it appears that studying the colors of such planets (a.k.a. spectroscopy) will allow separating the mini-Neptunes from the super-Earths.
²²

Even this doesn't exhaust all the possibilities of terrestrial planets in the Universe. Another possible planet would simply be a bare iron core! Under most conditions, we would not expect to see such a thing because a planet body has to be assembled first (at which point it would mostly be silicate, as we've seen) and then differentiate, and only then have the
iron precipitate in a core. The silicate mantle, without which there would be no iron core, would have to be stripped away to leave behind a bare iron core. That's easy to do to a small body—a twelve-mile-wide asteroid, for example. Super-Earths are a different matter; they are so massive that even two super-Earths colliding head-on would not shatter to bits and expose blobs of pure iron.
²³
Even under such stress, their gravitational reach stretches far beyond their surface and brings rocks back in. In fact, our own planet Mercury, as small as it is compared to Earth (just 1/18 of Earth's mass), is reported to have survived such a head-on collision early in its history and still retains most of its mantle. Nevertheless, iron super-Earths probably do exist, and we may have already discovered them—the pulsar planets.

 

FIGURE 5.2
.
Comparison of planet sizes among different super-Earths, as well as giants.

Pulsar planets were discovered by Alex Wolszczan and Dale Frail in 1992, marking the dawn of the hunt for exoplanets. As the name suggests, these planets orbit a pulsar—an ultradense, very massive (more than our Sun), fast spinning neutron star; a remnant from the explosion of a spent massive star, known as a supernova. We do not know their sizes (and there is no obvious way to measure them in the near future), nor do we have a good idea how they could have formed. Pulsar planets appear to have accumulated from the iron-rich debris left after the supernova explosion. Heavy-metal planets!

There are limits, however. We wouldn't expect, for example, to see a pure-water planet. Even if we allow for the water to be mixed with ammonia and a small percentage of other impurities, a pure super-Snowball is as unlikely as a snowball
on Venus. The problem, as with the pure iron super-Earths, lies again in planet formation. There is no conceivable way to “purify” the snowflakes from the dust in the debris disk or to shield the water-rich planet from accreting rocky bodies. Collisions don't seem to help either, and for the same reason—super-Earths are just too massive. Even in our Solar System, the most water-rich bodies are comets and distant cousins of Pluto with water-rock ratios of about 1:1. If Kepler discovers something with a mean density of water, I would bet on it being an extraterrestrial civilization's container for water storage ... or perhaps just a mixture of hydrogen gas, water, and rocks—a mini-Neptune planet.

Now that we've considered this range of planets, you may be wondering if it's fair to use the name of our beautiful planet Earth for such a plethora of exotic worlds, even if the models of these other planets are fairly based on knowledge of our own. To me, the answer is clearly yes. Our Earth is a super-Earth, part of the big family, and something general and deep unites our planet with those others. It's so deep, in fact, that you'll find it at the bottom of Earth's mantle. Let's see what it is.

 

 

 

 

R
ock hunting is fun: get yourself a hammer, a chisel, and a guidebook and head to the hills. If you're lucky (and persistent), you'll discover a pretty specimen to join your collection on the mantle or the bookshelf at home. There are so many—a seemingly endless variety. And the average rock hunter is just scratching the surface of the planet. If only we could look inside it, what wonders might we find? Actually, if you could go deeper inside the Earth, the variety of minerals would decrease dramatically to just a handful, with one of them—perovskite—dominating the bulk of the body of our planet (40 percent of its mass).

Perovskite, despite its planetary abundance, was not noticed and classified by geologists until 1837. The discovery
was set in motion in the 1820s, when Czar Nicolas I of Russia, eager to map the uncharted riches of Siberia, invited the accomplished German naturalist Alexander von Humboldt to lead an expedition into the region. Humboldt set out east in the spring of 1829, taking two associates with him. They explored the Ural Mountains and the Caspian Sea, and reached the Altai Mountains of central Asia, where China, Russia, and Mongolia converge.

Reportedly Humboldt did not enjoy the trip much, though it appears to have been very successful.
¹
One of his associates, Gustav Rose, was a geologist. Among the minerals he brought back was a heavy dark piece found in the Ural region near the town of Slatoust. It had not been catalogued, so Rose had naming rights as its discoverer. He named it perovskite (originally, peroffskite), in honor of the Russian geologist Count Lev Alexeevich von Perovski, who was also the expedition's host in Moscow.
²
Gustav Rose had no idea that he had discovered the most abundant mineral on Earth!

Rocks are aggregates of one or more minerals. Minerals are inorganic solids with a definite chemical composition. They range from a single element (e.g., gold) to very complex silicates. On planets the predominant minerals are oxides—one or more oxygen atoms bound to silicon, iron, magnesium, aluminum, and so on. For example, silica, also known as quartz, consists of one silicon atom and two oxygen atoms. In silicate minerals that are most common in rocks, one silicon atom is usually surrounded by several oxygen atoms. Depending on how the atoms arrange themselves in regular repeating patterns, different crystal structures are possible.

The perovskite mineral Gustav Rose discovered in Russia was an oxide of titanium and calcium. There is a rich group (known as the perovskite group) of minerals that all share the same unique crystal structure of connected octahedrons of silicon or titanium bound to six oxygen atoms.
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Despite the fact that Rose discovered a titanium perovskite first, Earth's lower mantle is, by volume, almost entirely taken up by silicon perovskite minerals.

Perovskite minerals contain rare earth metals (such as lanthanum, neodymium, and niobium) as trace elements. Perovskite is famous to physicists as the mineral in which high-temperature superconductivity was first discovered in 1986 by Alex Muller and Georg Bednorz.
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Nevertheless, most people have never heard of it, and it rarely appears in the popular field guides to rocks and minerals.
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(More common is a mineral called enstatite, similar to perovskite, which can be found in a variety of rocks.) Importance need not bring fame.

It turns out that perovskite—specifically as it behaves under high pressure—is very important to the study of super-Earths, and indeed forms the link between our planet and other planets. The breakthrough occurred in 2004, when a long-standing mystery about the Earth's interior was finally explained. Seismologists and other scientists had noticed a thin layer at the bottom of the mantle, just above the core-mantle boundary, that had an unpredicted effect on the behavior of earthquake waves propagating through it. This layer, known as D”, is only 150 kilometers thick, and no one knew how to model it or why it affected seismic waves as it did. A combination of high-pressure experiments by Murukami and team
and theoretical calculations by Oganov and Ono uncovered the culprit—perovskite under very high pressure.
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At that depth—where the pressure reaches 1.3 million atmospheres—the closely packed structure of perovskite deforms into a layered structure of nanoscale sheets—a new phase the discoverers called
post-perovskite
.
⁷

As already noted, this layer is very small on our planet—just 150 kilometers—but turns out to be the main component of a super-Earth! For any super-Earth of about 2 M
_E
and above, the pressure inside most of the mantle exceeds 1.3 million atmospheres; therefore, super-Earths, whether rocky or oceanic, are post-perovskite planets.

You might have noticed the unremarkable naming of the new mineral. No naming rights here. This is because post-perovskite cannot be present in a rock on the surface of the Earth, so no one has held it in their hands yet, or ever will. (This is apparently what it takes to name a mineral according to the medieval rules of the geological societies.) This is because the structure of post-perovskite makes it act like a loaded spring; if it were ever brought nearer to the surface by convection, it would revert back to perovskite.

Discovering post-perovskite was timely for the upcoming study of super-Earths, but we might not be finished with the project yet. The problem is that pressure throughout the mantle of the most massive super-Earths reaches 10 million atmospheres, much higher than what is needed to turn perovskite into a post-perovskite.
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It is possible that other mineral phases might be inside them—they could even be a
post
-post-perovskite?

Theory and experiment have already given some hints to the fate of post-perovskite under higher pressure. The main player in this high-pressure drama is the oxygen atom. After submitting to two dense crystalline arrangements, called packing, the further belt tightening leads the oxygen atoms to rearrange themselves again. Their efficient packing seems to have one more possible crystalline structure—a 12-point phase.

The hints come from the study of a little-known crystal called gadolinium gallium garnet—GGG (Gd
₃
Ga
₅
O
₁₂
). GGG is transparent and very hard under pressure—useful properties to the scientists who try to compress hydrogen to a million atmospheres in the hope of making thermonuclear energy commercially viable.
⁹
The compression of hydrogen (e.g., very small pellets of it) is done by powerful lasers, happens quickly, lasts briefly, and has to be observed through a tiny window that does not break under high pressure. In 2005 scientists in Japan and the United States used strong laser impulses to squeeze GGG to 1.2 million atmospheres and found, somewhat to their surprise, that the crystal deformed into a state that was harder (less compressible) than diamond.
¹⁰
This was a world record. So, GGG emerged as a better material for the high-pressure experiments with hydrogen, replacing the sapphire and diamond that had provided the tiny windows until then.

The GGG experiment shows that an even denser packing of the oxygen atoms is possible in a post-perovskite material. It may be the last such deformation we need to consider inside a super-Earth planet. Fortunately we can do the experiment in a lab, although it is expensive, so the wait for the
answer should not be long. In the meantime, it is already clear that super-Earths are made of the hardest materials known in the Universe: post-perovskite in the rocky and water ones, and diamond in the rare carbon planets. Indeed, this captures part of the beauty of studying super-Earths: the richness of new exotic materials they bring to the table, or lab bench as it may be. Hot water ices VII, X, and XI are as unusual to our world and the rest of the Universe, as are post-perovskite and GGG. Under pressure the solid form of water attains the more compact cubic crystal structure. Ice VII is analogous to crystal rock salt in that way and happens to have similar mean density. What a strange world!