Super-Earths are planets that have a mass higher than Earth’s but substantially below those of the Solar System’s ice giants, Uranus and Neptune, which are 14 and 17 times Earth’s, respectively. Super-Earths are expected to have rocky compositions similar to Earth’s, in contrast to the large low-density planets that we refer as “mini-Neptunes”.
What is a super-Earth?
A super-Earth is defined as an exoplanet (a planet outside our Solar System) with a mass higher than Earth’s but significantly below the mass of the Solar System’s ice giants Uranus and Neptune. Typically, super-Earths are defined as having a mass between 2 and 10 times that of Earth.
While there is no precise upper limit for the mass of a super-Earth, 10 Earth masses is commonly used as a cutoff. Planets above 10 Earth masses begin to accumulate thick hydrogen/helium atmospheres and are more appropriately classified as mini-Neptunes or gas dwarfs.
In terms of size, super-Earths have radii about 1.3 – 1.7 times larger than Earth. Their sizes place them in the ambiguous region between terrestrial planets like Earth and the ice giants of the outer Solar System.
Despite the “super” designation, these worlds are still significantly smaller and less massive than any of the Solar System’s giant planets.
What makes super-Earths so interesting to astronomers?
Super-Earths are among the most common types of planets in our galaxy, at least among those we have been able to detect so far. Statistical studies indicate that about 30-50% of stars host a super-Earth sized planet orbiting within their habitable zones, where liquid water could exist on a planet’s surface.
The proximity of many super-Earths to the habitable zone makes them compelling targets in the search for potentially habitable worlds and alien life. Their sizes and masses also make their bulk properties, like density and atmospheric composition, easier to constrain compared to smaller terrestrial worlds.
In many ways, super-Earths are excellent analogs to help understand exoplanets that are slightly larger than our own planet. Studying super-Earths can provide key insights into processes governing planet formation, evolution, and potential habitability.
What are the defining characteristics of super-Earths?
Here are some of the main physical characteristics used to identify super-Earths:
- Mass: Between 2-10 times Earth’s mass, typically measured via radial velocity or transit timing variation methods.
- Size: Around 1.3 – 1.7 times Earth’s radius, typically measured during planetary transits.
- Density: Since they likely contain significant fractions of rocks and ices, super-Earths have densities between 1.5 – 5 g/cm3.
- Orbit: Super-Earths span a broad range of orbital distances, from being very hot worlds orbiting extremely close to their stars out to cold planets at distances beyond the traditional habitable zone.
- Composition: Super-Earths are expected to have large rocky interiors surrounded by thick atmospheres composed of lighter elements like hydrogen, helium, and water vapor.
A key characteristic of super-Earths is that they appear to represent a transitional zone between small, rocky planets like Earth and the ice giants of our Solar System. Their physical and orbital properties overlap between these two very different classes of worlds.
How do super-Earths form and evolve?
There are two main theories for how super-Earths are able to form:
- In situ formation: Super-Earths form in their current orbital locations from the traditional core accretion process, as solid cores gradually attract thick atmospheres of gas.
- Migration: Super-Earths form farther out in the protoplanetary disk and migrate inwards over time via interactions with the disk material or with other planets.
The diverse orbital distances of known super-Earths likely means that both formation scenarios contribute to the population. Additional mechanisms like giant impacts between protoplanets may also play an important role.
After formation, many super-Earths may evolve substantially through processes like photoevaporation, where stellar radiation strips away the planet’s atmosphere over billions of years. The extent of atmospheric loss can transform small Neptune-like worlds into super-Earths over time.
How do super-Earths compare to Earth?
Super-Earths share a number of similarities but also key differences compared to Earth:
| Property | Earth | Super-Earth |
|---|---|---|
| Mass | 1 Earth mass | 2-10 Earth masses |
| Radius | 1 Earth radius | 1.3-1.7 Earth radii |
| Density | 5.5 g/cm3 | 1.5-5 g/cm3 |
| Composition | Iron/rock core, silicate mantle, thin atmosphere | Iron/rock core, icy/rocky mantle, thick atmosphere |
| Habitability | Earth-like temperatures, liquid water | Depends on distance from host star and atmospheric properties |
The most significant differences are super-Earths’ larger sizes, lower densities, and thicker atmospheres compared to Earth. These factors can influence the planet’s climate, geology, and potential habitability.
What super-Earths have been discovered so far?
As of early 2023, astronomers have confirmed nearly 200 super-Earth exoplanets. Here are some of the most notable examples:
- Kepler-10c: One of the earliest known super-Earths, at about 17 Earth masses and 2.3 Earth radii. Orbits a Sun-like star every 45 days.
- Gliese 1214b: A low-density super-Earth with a thick, steamy atmosphere. Orbits an M dwarf star every 1.6 days.
- LHS 1140b: A rocky super-Earth in the habitable zone of a small red dwarf star. At 6.6 Earth masses, it may have magma oceans on its surface.
- TOI 700d: A super-Earth located in the habitable zone of a tiny M dwarf just 100 light-years away. It receives 86% of the stellar radiation that Earth does from the Sun.
- K2-18b: A super-Earth with water vapor detected in its hydrogen-rich atmosphere. Orbits within the habitable zone of a cool red dwarf.
These planets showcase the diversity of super-Earths found throughout the galaxy, in terms of their masses, sizes, orbits, and parent stars. Many more such worlds will likely be discovered in the coming decade by new exoplanet surveys.
How many potentially habitable super-Earths are there?
Based on current exoplanet statistics, astronomers estimate there are likely billions of potentially habitable super-Earths in our galaxy. About 30-50% of Sun-like and lower-mass stars are predicted to host a 1-2 Earth mass planet in their habitable zones.
However, some researchers caution that many of these worlds may be too massive to support liquid surface water or Earth-like climates. Planets above 2 or 3 Earth masses may possess dense atmospheres that create extremely hot surface conditions, even in habitable zone orbits.
Additionally, some studies suggest that plate tectonics, which help regulate Earth’s climate over billions of years, may not operate on planets much more massive than Earth. Nonetheless, certain super-Earths could still be habitable if they have the right atmospheric and geologic conditions.
Can we study the atmospheres of super-Earths?
Yes, astronomers are able to study the atmospheres of some super-Earths located close to their host stars. When a super-Earth transits in front of its parent star, starlight filters through the planet’s atmosphere. By analyzing the spectral fingerprints in this filtered light, scientists can infer the chemical composition of the atmosphere.
So far, large amounts of water vapor have been detected in the atmospheres of a few super-Earths. Meanwhile, other super-Earths seem to possess clouds or hazes high in their atmospheres that obscure molecular absorption features. Future next-generation telescopes will enable more detailed studies of super-Earth atmospheres.
Could we live on a super-Earth planet?
In theory, some super-Earths appear capable of supporting life as we know it. However, living on one would likely feel quite different than Earth in many respects:
- Stronger gravity: Super-Earths can have 1.5-3 times Earth’s gravity, making it more difficult to move around and launch rockets.
- Thinner air: Less atmosphere per unit surface area may lower oxygen availability for breathing.
- Extreme climates: Higher mass planets tend to transport heat more efficiently, reducing temperature contrasts across the globe.
- Frequent quakes: Higher mass planets experience more frequent and extreme seismic activity from internal pressures.
While super-Earths may offer exotic destinations for exploration, they pose substantial challenges for establishing long-term human civilizations. But with the right conditions, we can’t rule out the possibility that they could be habitable.
Conclusion
Super-Earths represent one of the most common potentially habitable planet types in our galaxy. Their proximity to the habitable zone of many stars while still being relatively easy to study make them extremely attractive targets in the search for life in the universe. As detection methods improve, astronomers will uncover more super-Earths around nearby stars, giving us an ever better chance at identifying truly Earth-like worlds in our celestial neighborhood.