Why ‘super-Earth’ exoplanets are a scientific disaster

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

The more than 5,000 exoplanets confirmed so far in our galaxy include a wide variety of types—some similar to the planets in our solar system, others very different. Among these are a variety we lack in our solar system that are largely misnamed “super-Earths” because they are larger than our world. But all but the hottest planets more than about 130% Earth’s radius are likely to be mini-Neptunes, not super-Earths, and their potential habitability remains doubtful, despite the contrary claims of some vocal exoplanet researchers.

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. However, the illustration of this world as “Earth-like” may somehow be incorrect, as it is more likely to either have a large, volatile gaseous envelope, making it a mini-Neptune, or to be a hot, stripped planetary core: like a scaled-up version of Mercury.

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 currently biased towards larger, heavier planets orbiting smaller stars at shorter orbital distances. The outer planets of most star systems remain largely undiscovered, but those that have been discovered, largely through direct imaging, are difficult to explain how we think most exoplanets form: via the core growth scenario.

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

The discovery of the first 5000 exoplanets, recorded by year and method. For the first 15 years or so, the radial velocity method was the dominant method of detection, which was later replaced 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 exoplanets (ie, Earth mass and below) in a way that the previous two main methods have not been with current instrumentation. These confirmed planets represent only a fraction of the total planet candidates.

Although more than 5,000 confirmed exoplanets are known, with more than half of them discovered by Kepler, there are no true analogues to 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% of the radius (or, equivalently ~1% of the area) of their parent star or more, and orbit low-mass, small-sized stars.

The eight most Earth-like worlds discovered by NASA’s Kepler mission: the most prolific planet-finding mission to date. All of these planets orbit stars that are smaller and less luminous than the Sun, and all of these planets are larger than Earth, and many of them likely have volatile gas envelopes. Although some of them are called superbodies in the literature, we do not yet know if any of them have, or ever had, life on them at all, but the boundary 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.

When we classify the known exoplanets by both mass and radius together, the data indicate that there are only three classes of planets: terrestrial/rocky, with a volatile gaseous 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 Saturn and Jupiter, although there are some “bulky” super-Jupiters, with likely unusually light compositions.

When an exoplanet passes in front of its parent star, some of that starlight will filter through the exoplanet’s atmosphere, allowing us to break the light into its component wavelengths and characterize the atmosphere’s atomic and molecular composition. If the planet is inhabited, we can 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. Almost all so-called “super-Earth” worlds that have had their transit spectra measured have revealed these characteristic volatile envelopes, suggesting that they are mini-Neptunes instead of super-Earths.

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 could end up in a rocky super-Earth: a stripped-down planetary core. The temperatures will be so high that you will fry your planet. If you are more than about 30% larger in radius than Earth and not too close to your parent star, you will accumulate a large envelope of volatile gases and be more like Neptune than Earth.

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. Note that Mercury has a core that is 85% of its interior in radius; Venus’ core/mantle boundary is highly uncertain; and that Mercury itself is the only such world we know of without a crust. Yet only the Earth exhibits plate tectonics; the other three rocky planets all have only single plates as far as we know.

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 envelopes. Although some still call them super-Earths, it is very clear that not only are they rocky, but most of the planets we call super-Earths are not at all like Earth in any meaningful way.

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 an ice cover, on its surface. While this describes the conditions Earth has, it is unknown if 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.