Why “super-Earth” exoplanets are a scientific catastrophe

It’s time to expose a scientific catastrophe: the myth of the super-habitable super-Earth planet.

A comparison of Earth, at right, with a theorized super-habitable planet, at left. In theory, planets orbiting lower-mass stars than our Sun, with slightly larger radii and masses than our planet, and closer to the centers of their so-called habitable zones may be more likely to have life survive and thrive, and be home to greater biodiversity than Earth. Without evidence, this idea is tantamount to little more than guesswork.

Some call super-Earths the most common and most habitable of all exoplanets.

When we take into account all of the nearly 5000 exoplanets known at the start of 2022, we can see that the greatest number of planets can be found in between the sizes of Earth (at -1.0 on the x-axis) and Neptune (at -0.5 on the x-axis). However, that does not mean that those worlds are the most abundant, nor that they’re even, as we’ve long been calling them, legitimate “super-Earth” worlds. However, the gap between Neptune-like and Jupiter-like worlds is real; we do not know why there are so few of them.

It’s true that we’ve found more super-Earth exoplanets than any other type.

The more than 5,000 exoplanets confirmed in our galaxy so far include a variety of types – some that are similar to planets in our Solar System, others vastly different. Among these are a variety we lack in our Solar System that are largely mis-named “super-Earths” because they are larger than our world. However, all but the hottest planets that are more than about ~130% of Earth’s radius will likely be mini-Neptunes, not super-Earths, and their potential habitability remains dubious, despite the contrary assertions of a few vocal exoplanet scientists.

It’s also true that, if rocky, they have more surface area and organic ingredients than Earth-sized worlds.

The most common “sized” world in the galaxy is a super-Earth, between 2 and 10 Earth masses, such as Kepler 452b, illustrated at right. But the illustration of this world as “Earth-like” in any way may be mistaken, as it’s more likely to either have a large, volatile gas envelope, making it a mini-Neptune, or to be a hot, stripped planetary core: like a scaled-up version of Mercury.

But that doesn’t translate into “super-Earths” being more abundant or more habitable.

The mass, period, and discovery/measurement method used to determine the properties of the first 5000+ (technically, 5005) exoplanets ever discovered. Although there are planets of all sizes and periods, we are presently biased toward larger, heavier planets that orbit smaller stars at shorter orbital distances. The outer planets in most stellar systems remain largely undiscovered, but those that have been discovered, largely through direct imaging, are difficult to explain the way we think most exoplanets form: via the core accretion scenario.

We have two primary methods for finding exoplanets.

The idea of the radial velocity method is that if a star has an unseen, massive companion, whether an exoplanet or a black hole, observing its motion and position over time, if possible, should reveal the companion and its properties. This remains true, even if there’s no detectable light emitted from the companion itself.

The radial velocity method more easily reveals massive, closely orbiting systems.

When planets pass in front of their parent star, they block a portion of the star’s light: a transit event. By measuring the magnitude and periodicity of transits, we can infer the orbital parameters and physical sizes of exoplanets. However, from only a single candidate transit, it is difficult to draw any such conclusions with confidence. When transit timing varies and is followed (or preceded) by a smaller-magnitude transit, it may indicate an exomoon as well, such as in the system Kepler-1625.

The transit method has exactly the same bias.

The discovery of the first 5000 exoplanets, as recorded by year and by method. For the first ~15 years or so, the radial velocity method was the dominant method of discovery, later superseded by the transit method beginning with NASA’s now-defunct Kepler mission. In the future, microlensing may surpass them all, as microlensing will be sensitive to low-mass (i.e., Earth-mass and below) exoplanets in a way that the prior two main methods have not been with current instrumentation. These confirmed planets represent only a fraction of the total planetary candidates.

Neither method is optimized for finding Earth-sized or smaller worlds.

When a gravitational microlensing event occurs, the background light from a star gets distorted and magnified as an intervening mass travels across or near the line-of-sight to the star. The effect of the intervening gravity bends the space between the light and our eyes, creating a specific signal that reveals the mass and speed of the object in question.

The dearth of small exoplanets is because of detection sensitivity, not intrinsic populations.

Although more than 5,000 confirmed exoplanets are known, with more than half of them uncovered by Kepler, there are no true analogues of the planets found in our Solar System. Jupiter-analogues, Earth-analogues, and Mercury-analogues all remain elusive with current technology. The overwhelming majority of planets found via the transit method are close to their parent star, are ~10% the radius (or, equivalently, ~1% the surface area) of their parent star or more, and are orbiting low-mass, small-sized stars.

Moreover, nearly all so-called super-Earths aren’t Earth-like at all.

The eight most Earth-like worlds, as discovered by NASA’s Kepler mission: the most prolific planet-finding mission to date. All of these planets orbit stars smaller and less bright than the Sun, and all of these planets are larger than Earth, with many of them likely possessing volatile gas envelopes. Although some of them are called super-habitable in the literature, we don’t yet know if any of them have, or ever had, life on them at all, but the border between “rocky” and “gas-rich” is still being studied, and most or even all of these selected Kepler planets may yet have volatile gas envelopes around them.

The majority are Neptune-like, possessing large, volatile gas envelopes.

When we classify the known exoplanets by both mass and radius together, the data indicates that there are only three classes of planets: terrestrial/rocky, with a volatile gas envelope but no self-compression, and with a volatile envelope and also with self-compression. Anything above that becomes first a brown dwarf and then a star. Planetary size peaks at a mass between that of Saturn and Jupiter, although there are a few “puffy” super-Jupiters, with a likely unusually light composition.

With crushingly thick atmospheres, the prospects for habitability are dim.

When an exoplanet passes in front of its parent star, a portion of that starlight will filter through the exoplanet’s atmosphere, allowing us to break up that light into its constituent wavelengths and to characterize the atomic and molecular composition of the atmosphere. If the planet is inhabited, we may reveal unique biosignatures, but if the planet has a thick, gas-rich envelope of volatiles around it, the prospects for habitability will be very low. Nearly all so-called “super-Earth” worlds that have had their transit spectrum measured have revealed these characteristic volatile envelopes, suggesting that they’re mini-Neptunes instead of super-Earths. K2-18b is no different.

Moreover, the rocky super-Earths are suspiciously Mercury-like: hot and close to their stars.

An artist’s illustration of a world that would be classified as a rocky super-Earth. If you’re hot enough to boil off the atmosphere of a large planet, you can wind up with a rocky super-Earth: a stripped planetary core. The temperatures will be so high that you’ll roast your planet. If you’re more than about 30% larger in radius than Earth and aren’t too close to your parent star, you’ll collect a large envelope of volatile gases, and be more like Neptune than Earth.

They’re likely bare planetary cores, and, like Mercury, they may undergo mantle-stripping.

This cutaway view of the four terrestrial planets (plus Earth’s moon) shows the relative sizes of the cores, mantles, and crusts of these five worlds. There are compelling similarities between Earth and Mars, as they both have crusts, mantles, and metal-rich cores. However, the much smaller size of Mars means that it both contained less heat overall initially, and that it loses its heat at a greater rate (by percentage) than Earth does.

Being ~twice Earth’s mass and ~1.3 times Earth’s radius is probably an exoplanet’s maximum “Earth-like” size.

The CHEOPS mission discovered three planets around the star Nu2 Lupi. The innermost planet is rocky and contains only a thin atmosphere, while the second and third planets discovered have large, volatile-rich envelopes. Although some are still calling them super-Earths, it’s very clear that not only are they not rocky, but most of the planets we call super-Earths are not like Earth at all in any meaningful way. This extends to all exoplanets with a radius above 1.7 Earth radii, with many of smaller sizes still having hydrogen and helium envelopes.

Super-Earths are inappropriately named. These mini-Neptunes and stripped planetary cores are anything but life-friendly.

Our notion of a habitable zone is defined by the propensity of an Earth-sized planet with an Earth-like atmosphere at that particular distance from its parent star to have the capacity for liquid water, without a cover of ice, on its surface. Although this describes the conditions that Earth possesses, it is unknown whether this is a requirement, or even a preference, of life. None are known to be inhabited, but a few raise tantalizing possibilities: largely among the Earth-sized planets, not the super-Earth-sized ones with large, volatile gaseous envelopes around them.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.