## Super-Earths and Mini-Neptunes

Low-Mass Habitable Zone Planets (artist images)

Our Solar System did not prepare us for what we would discover orbiting other stars. Instead, it told us that planets fall into neat categories: Gas giants made mostly of hydrogen and helium (of which Jupiter and Saturn are the archetypes), ice giants made mostly of water (for which Uranus and Neptune are representatives), and solid terrestrial planets with comparatively thin atmospheres — that would be the planets of the inner solar system and the one right under your feet). Since the discovery of thousands of planets orbiting other stars, and the measurement of their masses and densities, it has become clear that not all planets fit into this paradigm. Significantly, unless rocky worlds have an optimistically high abundance, what may be the most abundant type of planet in the Galaxy is a sort of mix between low-density, volatile-rich Neptune-like planets and rocky terrestrial planets. The Solar System features no such planet — after Earth, the next most massive planet is Uranus at ~14.5 times as massive. A casual look at the entirety of discovered transiting planet candidates discovered by Kepler reveals the magnitude of this problem.

While Kepler is no longer observing its original field, the massive amount of data can still be combed through to reveal new planet candidates. Here, previously discovered planet candidates are blue dots, and newly announced planet candidates are yellow. A few things are noteworthy. Firstly, the overwhelming majority of the newly discovered planet candidates have reasonably long orbital periods. This can be expected as shorter period planets have been detectable in the existing data for longer, and have had time to be spotted already. Secondly, and not really the point of this post… they’re still finding warm Jupiters in the data? Wow! What’s up with that? I would have thought those would have been found long ago.

With the obvious caveat that lower regions of that diagram feature harder to detect planets leading to that part being less populated than would be the case if all planets were detected, it would appear that there is a continuous abundance of planets from Earth-sized to Neptune-sized. While radius and mass may only be loosely related, it may also be that there is a continuous abundance of planets from Earth-mass to Neptune-mass, as well. Not having an example of such an intermediate planet in the Solar System, we really don’t know what to expect for what these planets are composed of. As such we began to call them (sometimes interchangeably) super-Earths or Mini-Neptunes. Are they enormous balls of rock with Earth-like composition extending up toward maybe 10 Me? Are they dominated by mass by a rocky core with a thick but comparatively low-mass hydrogen envelope? Do they have some fraction of rock, water and gas? Are they mostly entirely water with a minimal gas envelope? Answering this question would require some constraints on the masses of these planets, as it would allow one to know their density.

The first data point was CoRoT-7 b, the first transiting super-Earth — discovered before Kepler. The host star is very active, leading to a lot of disagreement in the literature about its mass, but further work seems to have settled on a rocky composition for the planet with ~5 Me. Great! Next data point was the transiting super-Earth orbiting GJ 1214, a ~6.5 Me planet with a much lower density, which is too low to be explained by even a pure water composition. This is decidedly not Earth-like. Additional measurements by highly precise spectrometers (namely HARPS and SOPHIE) of Kepler discovered planets have allowed for more data to be filled in, and an interesting trend can be seen.

Mass-Radius Diagram of Extrasolar Planets with RV-Measured Masses

Interestingly, planets less than ~1.6 Earth-radii seem to have not only solid, but Earth-like compositions. It’s worth noting that only planets where the mass measurement is acquired through Doppler spectroscopy are shown here. Planets like the Kepler-11 family where the masses have been derived by transit timing variations are not shown. If these planets are added, the adherence to the Earth-like composition is much less strict. This may imply that planets which have masses measurable by detectable transit timing variations have had a different formation history and therefore a much lower density. Further data will be very useful in addressing this issue.

On a somewhat unrelated topic, several new habitable planet candidates have been validated by ruling out astrophysical false positives. Among them is Kepler-442 b, which appears to me to be a more promising habitable planet candidate than even Kepler-186 f. Some newly discovered but not yet validated habitable planet candidates have been found as well, including one that appears to be a near Earth-twin.

New Kepler habitable planet candidates

## Bulk Properties of the Planets II

When a planet transits its star (see here), and we’re able to determine it’s mass through some other means, typically Doppler spectroscopy (see here and here), then we can derive its density rather easily.

$\displaystyle \rho_{pl} = \frac{m_{pl}}{V_{pl}} = \frac{3m_{pl}}{4 \pi r_{pl}^3}$

Where $V$ is the volume of the planet, equivalent to $4/3 * \pi r^3$.

Planets of similar densities are not necessarily the same in bulk composition. With the bulk composition being held constant, the radius will increase with mass in a non-linear way such that the increase in the radius slows, while the density increases. After enough mass is built up, the trend reverses, with the gravity compressing the planet more as additional mass is added.

For planets made purely of one substance (half-seriously referred to as “mathematician’s planets”), one can plot their mass and radius on a diagram and see that we actually can use a planet’s observed mass and radius to infer something about its composition, assuming iron, rock, water and gas are the dominant things out of which a planet will be comprised. This seems to be a reasonable assumption based on what we know of how planets form and the observed composition of planet-forming disks.

Plotting the Solar System planets (minus Mercury and Venus) on such a diagram, we see that the radius of Earth and Mars are consistent with being almost entirely rock, with Earth being obviously too dense to be entirely made of rock, requiring an iron component. Uranus and Neptune are slightly too large for their mass to be explained by a composition of 100% water. So a (small, by fraction of mass) gaseous component must exist. Jupiter and Saturn are far too large for their mass to be explained by being mostly water in composition, so it’s clear that they are mostly made of gas.

These things are, for the most part, able to be determined through direct study of the planet themselves, but this is not easy to do for extrasolar planets. For the transiting planets, we’re afforded a radius and a mass and left to interpolate a composition based on it. A complication arises in that there are not unique solutions for the composition of a planet for some values of the radius and mass. A 100% gas planet will stick out as being purely gas, while a 100% iron planet will clearly be an enormous cannonball. But planets can have layers of all three and this can create degeneracies in models of their interior. An small ball of iron with a large layer of water and gas will have a similar radius to a large rocky planet with a small layer of gas. As a real example (and as of the time of this writing), it is not clear if GJ 1214 b is a sort of “mini-Neptune,” with a larger core and a lot of water, or a sort of “micro Jovian,” with a small core and a large gaseous envelope. Even Uranus and Neptune show that there’s some ambiguity here. The two are believed to have solid cores under water mantles, but the mass of the rock and/or iron is hard to disentangle from the mass of the water mantle.

So it is clear that different compositions can be thrown together to blur the classification of planets into neat categories of terrestrial planets, ice giants, and gas giants. Indeed, they do not separate out very well in a mass-radius diagram.