Pedagoguery

Last issue, I discussed Earth's history and how it could help us identify Earth-like planets. We have not yet found any, but various surveys have discovered super-Earths. These are planets that are smaller than the gas giants in our solar system, but larger than the rocky planets. Depending on how they are discovered, we can conceivably tell quite a lot about their composition and atmospheres.

Typical super-Earths would orbit small, red M-class stars, since those are the most numerous stars out there. If they were in the habitable zone, they would have to be much closer to their star than the Earth is to the Sun – around 2 million miles compared to 93 million. At such a close distance, the star would appear huge. A star with 20% of the mass of the sun would have about 30% of its radius. At a distance of 2 million miles, that star would cover an angle more than seven and a half time larger than the sun does from Earth. In addition, the close distance would also mean that the planet would be tidally locked to its star, with one side in perpetual daylight and the other in perpetual night. A year would be short – about 10 days or so.

Super-Earths, being larger than Earth, would also have a higher gravity. Most of the planets we have found are estimated to have masses between 2 and 10 times Earth's mass. Naturally, they would also have larger radii than Earth, but it would still translate to surface gravities much higher than ours.

Most of these worlds have been discovered through radial velocity measurements. Those measurements detect planets by detecting the miniscule wobble back and forth that the planet causes in the star. The disadvantage of this technique is that it provides only a minimum mass for the planet, and tell us nothing about the planet's radius. Another method is transiting, which is what the Kepler satellite does. It watches for the tiny dip in light from a star that occurs when a planet passes in front of the star. The advantage of this technique, especially when later paired with the radial velocity method, is that it provides relatively precise values for mass and radius, and thus density. If we know the density, then we can get a decent idea of the composition of the planet. This technique also provides the opportunity to learn more about the planet's atmosphere through spectroscopy of the star's light as it passes through the planet's atmosphere.

Two planets discovered through the transiting technique are Corot-7b and Gliese 1214b. Corot-7b orbits a K0 star at a mere 1.6 million miles. It has 5 times the mass of Earth and 1.7 times Earth's radius. This gives it a density of 5.6 grams/cubic cm – just a shade higher than Earth's. It probably means that Corot-7b has a large nickel-iron core, and is made up primarily of silicate materials, just like Earth is. It would have almost twice Earth's surface gravity, however.

Corot-7b would be blisteringly hot. There is additional evidence that there are more planets in the same system, and the gravitational tug-of-war would mean that the planet is highly volcanic – due both to the tidal forces of the star and the other planets as well as the fact that a larger planet would both retain more of the initial heat of its formation plus it would have more of the radioactive elements that provide most of the heating within Earth's core.

Gliese 1214b is, if anything, more potentially interesting. It orbits a red dwarf planet at 1.3 million miles. It has 6.6 Earth masses and 2.7 times Earth's radius, yielding a density of only 1.9 grams/cubic cm. Such a world can be of several different types. One possibility is a very small gas giant – a gas dwarf, if you would. This would have a nickel-iron core, a silicate mantle, surrounded by an envelope of hydrogen. Another possibility would be a mini-Neptune. It would also have a nickel-iron core and a silicate mantle, but around that would be a layer of water ice, then a hydrogen/helium envelope.

The most intriguing possibility is a water world. Once again, we start with a nickel-iron core and silicate mantle, both of which would be somewhat smaller than in the previous examples. Surrounding that would be a layer of water. Near the mantle, the water would actually be a form of high-temperature ice -- it would be ice due more to pressure than temperature. This ice layer would gradually give way to a water vapor atmosphere. But there would be a thin layer between them of superfluid water, which is not quite liquid and not quite gas.

All of this demonstrates that there are some very strange places in the uinverse.

Next time, different ways that time itself could end.