© NASA, ESA, L. Hustak (STScI)

A Super-Earth is a classification of an exoplanet that falls between the mass of Earth and the ice giants like Neptune and Uranus. These planets can consist of gas, rock, or a combination of both, and typically range from about 1 to 10 times the mass of Earth. The term “super-Earth” specifically denotes the mass of the planet and does not provide information about its surface characteristics or potential habitability.

The initial discovery of super-Earths occurred with the identification of two such planets, named Poltergeist and Phobetor, each approximately 4 times the mass of Earth, orbiting the pulsar PSR 1257+12. Subsequent advancements in exoplanet research revealed that super-Earths, previously unrecognized planetary bodies, are quite prevalent. Among the over 5,000 confirmed exoplanets, approximately 30 percent are classified as super-Earths.

Super-Earths are a class of exoplanets that are larger than Earth but smaller than Neptune. While there is no universally accepted definition, these planets are generally characterized by their masses and sizes. The term “super-Earth” is commonly used to describe planets with masses ranging from 1 to 10 times that of Earth. Most astronomers agree that planets exceeding 10 Earth masses should be classified as either massive solid planets, mega-Earths, or gas giants, depending on their composition.

In addition to mass, super-Earths are also defined by their radii. According to the Kepler space telescope personnel, super-Earths are planets larger than Earth-like planets (0.8 to 1.2 Earth-radii) but smaller than mini-Neptunes (2 to 4 Earth-radii). Some authors suggest that the term “super-Earth” should be limited to rocky planets without significant atmospheres or planets with solid surfaces or oceans and a clear boundary between the liquid and atmosphere. This distinction separates super-Earths from the four giant planets in our Solar System, which are primarily composed of gas.

CHARACTERISTICS OF SUPER-EARTH

Super-Earths are a class of exoplanets with masses larger than Earth‘s but significantly less than those of ice giants like Uranus and Neptune. While there is no strict mass range that universally defines a super-Earth, they are generally considered to be between 1 to 10 Earth masses. Here are the main characteristics of super-Earths:

DENSITY AND COMPOSITION

The density and composition of super-Earths can vary significantly:

• TOI-244 b is a super-Earth with an unusually low density of 4.2 g/cm^3, suggesting it is composed of iron and silicates in a proportion similar to Earth but with a significant amount of volatile elements. Its density is below what would be expected for an Earth-like composition.
• In general, super-Earths have higher densities than Earth because the outer layers of the planet crush the inner layers, making them more compact. The exact relationship between a super-Earth’s mass and density depends on the composition and internal structure.
• Kepler-452b is a super-Earth with a radius 50% larger than Earth’s and a mass about 5 times greater. If it is terrestrial, it is likely a super-Earth with many active volcanoes due to its higher mass and density. Its equilibrium temperature is a bit warmer than Earth’s.
• Super-Earths in the 1-10 Earth mass range may have a wide variety of compositions, including water worlds, snowball planets, or planets composed largely of dense gas like Neptune. The upper limit of the super-Earth size range overlaps with mini-Neptunes.

ATMOSPHERE

Several super-Earth exoplanets have been observed to have diverse atmospheres:

• GJ 1252 b, a rocky super-Earth, likely has a very minimal atmosphere or possibly no atmosphere at all due to its extremely high surface temperatures reaching over 1200°C. Its scorching temperatures and low surface pressure suggest the atmosphere, if present, would be very tenuous.
• GJ 9827d, a super-Earth about twice the size of Earth, has water vapor detected in its atmosphere by the Hubble Space Telescope. However, it remains unclear if the atmosphere is predominantly water vapor or if the water is just a minor component in a hydrogen-rich atmosphere. The planet is too hot at 400°C to be habitable.
• WASP-39 b, a “hot Saturn” exoplanet, had its atmosphere studied in detail by the James Webb Space Telescope. Webb detected a wide range of atoms, molecules, and signs of active chemistry and clouds in the planet’s atmosphere. The data suggests the clouds are broken up rather than a uniform blanket.
• Other super-Earths like GJ 1214 b have also shown evidence of clouds in their atmospheres.

In general, super-Earths exhibit a wide diversity of atmospheric compositions, from hydrogen-rich to potentially water-rich, and show evidence of clouds and active chemistry. However, many of the super-Earths discovered so far orbit very close to their stars and have high temperatures, making them unlikely to host life as we know it. Continued observations with powerful telescopes like Webb will further reveal the nature of super-Earth atmospheres.

SURFACE CONDITIONS

• Super-Earth exoplanets can have a wide range of surface conditions depending on their mass, radius, composition, and orbital distance from their host star.
• Super-Earths are rocky planets larger than Earth but smaller than Neptune, typically between 1-4 Earth radii. Their masses range from 1-10 Earth masses.
• Planets like TOI-2095 b and TOI-2095 c, which are 1.39 and 1.33 times wider than Earth respectively, have deep gravity wells and high mean densities of 11.5 and 17.3 g/cm^3, indicating they are likely rocky super-Earths. Their surface gravities are 2.6 and 4.2 times Earth’s gravity.
• The super-Earth LP 890-9 c, which is 40% larger than Earth, orbits within the habitable zone of its red dwarf star. However, its actual surface temperature depends on its atmosphere, which is unknown. It could potentially have a runaway greenhouse effect like Venus.
• The super-Earth 55 Cancri e, which orbits very close to its star, has extreme temperature differences of over 2300°F (1300°C) between its day and night sides due to tidal locking. The day side reaches nearly 4400°F (2700°C).
• Super-Earths like TOI 270 b, which is 25% larger than Earth, likely have rocky compositions. But larger super-Earths can have gaseous envelopes, like the mini-Neptunes TOI 270 c and d which are 2.4 and 2.1 times Earth’s size.

ORBITAL CHARACTERISTICS

Super-Earth exoplanets have a wide range of orbital characteristics:

• TOI-715 b orbits a red dwarf star 137 light-years away and has a year of only 19 Earth days due to its tight orbit. The second planet in the system, TOI-2095 c, takes 28.2 days to orbit.
• Kepler-452b orbits a Sun-like star 1,800 light-years away at a distance of 1.04 AU, similar to Earth’s orbit, with a period of 385 days.
• In general, super-Earths can have orbital periods ranging from less than a day to hundreds of days, depending on their distance from their host star. Those orbiting red dwarfs tend to have very short periods of just days or weeks due to the small size of their stars’ habitable zones.
• Super-Earths have been found orbiting within the habitable zones of their stars, where liquid water could potentially exist on a planet’s surface. However, the habitability of super-Earths depends on many factors beyond just orbital distance.
• Measuring a super-Earth’s mass and radius via transit and radial velocity methods allows its average density to be calculated, providing clues about its composition and potential to be rocky versus gaseous. Smaller super-Earths under 1.5 Earth radii tend to be rocky, while larger ones are more likely to be mini-Neptunes with thick atmospheres.

POTENTIAL FOR HABITABILITY

Super-Earth exoplanets, larger than Earth but smaller than ice giants like Neptune, have sparked interest in their potential habitability. These planets, such as Gliese 581c and LHS 1140b, are within the habitable zones of their stars, where conditions might allow for liquid water on their surfaces. The habitability of super-Earths depends on various factors like atmospheric composition, magnetic fields, and surface conditions, including the presence of liquid water and suitable temperatures.

Scientists are exploring the potential diversity of life beyond Earth by considering other solvents like methane or ammonia in the search for habitable exoplanets. The discovery of super-Earths, like TOI-715 b and LP 890-9 c, in the habitable zones of their stars has fueled optimism about finding Earth-like planets that could potentially support life.

CLIMATE PATTERNS

Super-Earth exoplanets exhibit diverse climate patterns depending on their orbital and atmospheric properties. Recent studies have provided insights into the weather and climate regimes of these distant worlds.

Exoplanets can be broadly classified into three temperature regimes based on their effective temperature (T_eff, P): cool planets (T_eff, P ≤ 1200 K), transition planets (T_eff, P = 1400–1800 K), and hot planets (T_eff, P ≥ 2000 K). These temperature thresholds determine the types of clouds and atmospheric chemistry present on the planets.

One well-studied example is 55 Cancri e, a super-Earth with a radius nearly twice that of Earth. Despite its extremely close orbit around its host star, with a year lasting only 18 hours on Earth, 55 Cancri e exhibits a strong temperature difference between its dayside and nightside. This suggests that processes like high winds or lava flows, similar to those found in our solar system, are at work on distant exoplanets.

Observations of 55 Cancri e using the Spitzer Space Telescope revealed that its nightside temperature is around 1,380 K (1,107°C), while the dayside temperature is about 1,300 K (1,027°C) hotter at 2,700 K (2,427°C). The authors also identified a hot spot on the planet, which they suggest could be caused by strong atmospheric winds or low-viscosity lava flows.

Another notable super-Earth is Kepler 452b, which orbits a Sun-like star and resides in the habitable zone. Simulations using a three-dimensional fully coupled atmosphere-ocean climate model suggest that Kepler 452b could be habitable if its atmospheric CO2 concentrations are comparable to or lower than those on present-day Earth. However, if there is a lack of silicate weathering to limit CO2 concentrations, Kepler 452b could become too hot to be habitable.

The diversity of super-Earth exoplanets and their potential for hosting life make them prime targets for future research. Upcoming missions like the James Webb Space Telescope (JWST) and the Transiting Exoplanet Survey Satellite (TESS) are expected to provide more insights into the atmospheres of these distant worlds. By studying the atmospheric composition of super-Earths, researchers aim to detect signs of life, such as the presence of water vapor, oxygen, and methane.

DISCOVERIES

Here is a list of some notable super-Earth exoplanets discovered over the years, highlighting their discovery year and significant characteristics:

EARLY DISCOVERIES

PSR B1257+12 c (1992)

PSR B1257+12 c was discovered in 1992, orbiting a pulsar star. This planet has a mass of 4.3 Earths and is located at a distance of 0.36 astronomical units from its star. It takes approximately 66.5 days to complete one orbit. PSR B1257+12 c is classified as a super-Earth due to its size, being 1.91 times the radius of Earth. The discovery of this exoplanet was a significant milestone in astronomy, marking one of the first confirmed detections of planets outside our solar system, expanding our understanding of planetary systems beyond traditional main-sequence stars.

2000s

55 Cancri e (2004)

55 Cancri e is located 40 light-years from Earth, orbiting its star in just 18 hours at a distance of 0.01544 AU. With a mass 8 times that of Earth, 55 Cancri e has a surface temperature exceeding 2,700°C, hot enough to turn rock into lava. Observations suggest the planet may have a carbon-rich composition, possibly including diamond and graphite. However, its atmosphere appears to be dominated by hydrogen and helium, more like a gas giant than a rocky world.

Gliese 581c (2007)

Gliese 581c was discovered in 2007, residing in the Gliese 581 system about 20 light-years from Earth. Initially thought to be Earth-like and potentially habitable due to its location in the star’s habitable zone, recent research suggests it may have a Venus-like environment with extreme heat. With a mass at least 5.5 times that of Earth, Gliese 581c orbits its M-class dwarf star in about 13 days, likely experiencing tidal locking. Its surface temperature could range from 0°C to 40°C, depending on its composition.

CoRoT-7b (2009)

CoRoT-7b is a remarkable super-Earth exoplanet orbiting the star CoRoT-7 in the constellation of Monoceros, located 489 light-years from Earth. Discovered in 2009, it was initially thought to be rocky like Earth, with a radius 1.68 times that of Earth and a mass initially estimated to be up to 21 times that of Earth. However, further observations revealed its mass to be 4.8 times that of Earth, indicating a density similar to Earth’s. This density classification makes CoRoT-7b a terrestrial planet, not a gas giant like Jupiter. Its scorching surface temperature of about 2,000°C highlights its extreme proximity to its star, orbiting at a distance of 2.6 million km every 0.85 days.

2010s

Kepler-22b (2011)

Kepler-22b is orbiting within the habitable zone of a Sun-like star, Kepler-22, located about 640 light-years from Earth in the constellation of Cygnus. Discovered in 2011, it has a radius of 2.1 times that of Earth and a mass estimated to be less than 9.1 Earth masses. This planet, often referred to as a “water world,” may have a volatile-rich composition with a liquid or gaseous outer shell, potentially making it conducive to life. With an equilibrium temperature of approximately 6°C, Kepler-22b represents a significant discovery in the search for Earth-like planets beyond our solar system.

Gliese 667 Cc (2012)

Gliese 667 Cc is orbiting the red dwarf star Gliese 667 C, located just 22 light-years from Earth. With a mass 3.8 times that of Earth, it takes Gliese 667 Cc only 28.1 days to complete one orbit around its star at a distance of 0.125 AU. This places the planet squarely within the habitable zone, where liquid water could potentially exist on its surface. However, Gliese 667 Cc likely experiences tidal locking, with one side in perpetual daylight and the other in eternal darkness. Its host star is much cooler and dimmer than our Sun, emitting only 1% as much light.

Kepler-186f (2014)

Kepler-186f is an Earth-sized exoplanet orbiting within the habitable zone of the red dwarf star Kepler-186, located about 580 light-years from Earth in the constellation of Cygnus. Discovered in 2014 by NASA’s Kepler spacecraft, it is the outermost of five planets orbiting the star. With a radius 1.11 times that of Earth, Kepler-186f potentially has a mass 1.44 times that of Earth. This exoplanet receives only 32% of the light Earth receives from the Sun, but its location within the habitable zone suggests the possibility of liquid water if its atmosphere contains sufficient carbon dioxide. Kepler-186f, along with other exoplanets like Kepler-442b and Kepler-62f, is considered a strong candidate for potential habitability.

Kepler-452b (2015)

Kepler-452b is orbiting within the habitable zone of a G2-type star, Kepler-452, located approximately 1,400 light-years from Earth. Discovered in 2015 by NASA’s Kepler spacecraft, this planet has a radius 1.63 times that of Earth and a mass of 3.29 Earths. With an orbital period of 384.8 days and an orbital radius of 1.046 AU, Kepler-452b is often referred to as an “Earth-cousin” due to its similarities to our planet. While it orbits at a distance similar to Earth from the Sun, receiving slightly more solar energy, its potential habitability remains uncertain, with the possibility of a runaway greenhouse effect due to its star’s increasing luminosity.

Proxima Centauri b (2016)

Proxima Centauri b is orbiting within the habitable zone of the red dwarf star Proxima Centauri, the closest star to the Sun at a distance of about 4.2 light-years. Discovered in 2016, it has a mass of at least 1.07 times that of Earth and orbits its star every 11.2 Earth days at a distance of about 0.05 AU. Despite being in the habitable zone, the planet faces challenges such as extreme ultraviolet radiation that could strip away its atmosphere over time.

LHS 1140 b (2017)

LHS 1140 b is orbiting within the habitable zone of the red dwarf star LHS 1140, located approximately 39 light-years away from Earth in the constellation of Cetus. Discovered in 2017, this planet is about 1.43 times larger than Earth, with a mass around 6.6 times that of Earth. It is likely composed of rock with a dense iron core, lacking a substantial gas envelope. Positioned 10 times closer to its star than Earth is to the Sun, LHS 1140 b receives about half as much sunlight, making it a promising candidate for further studies on potentially habitable exoplanets.

K2-18b(2018)

K2-18b is orbiting a red dwarf star about 110 light-years from Earth. Discovered in 2015, it is eight times the mass of Earth and is the only known exoplanet outside our solar system with both water and temperatures potentially supporting life. Recent atmospheric studies using the Hubble Space Telescope detected water vapor, hydrogen, and helium in its atmosphere, marking a significant milestone in exoplanet research.

2020s

TOI-700d (2020)

TOI-700 d is located in the habitable zone of the red dwarf star TOI-700, about 100 light-years away from Earth. Discovered by NASA’s TESS mission, this planet has a radius 1.05 times that of Earth and orbits its star every 37 days, receiving 86% of the solar insolation Earth receives. TOI-700 d is the first Earth-sized planet found in the habitable zone of a stellar system by TESS.

Gliese 486 b (2021)

Gliese 486 b is orbiting a red dwarf star, Gliese 486, just 26 light-years from Earth. Discovered in 2021, it has a mass of 2.82 Earths and orbits its star at a distance of 0.01734 AU, completing one orbit in 1.5 days. This planet has become a pivotal subject for exoplanetary studies, offering insights into planetary atmosphere models and composition. With its precise characterization, Gliese 486 b presents opportunities for investigating magnetic fields, atmospheric composition, and potential habitability.

LP 890-9 c (2022)

LP 890-9c is orbiting a red dwarf star about 100 light-years away in the constellation Eridanus. Discovered accidentally, it is the second-most favorable target for potential habitability among known terrestrial planets. With a mass around 6.6 times that of Earth, LP 890-9c orbits its star every 8.5 days, receiving low stellar irradiation due to its star’s cooler nature. This exoplanet’s proximity to the inner boundary of the habitable zone suggests the potential for liquid water on its surface, pending further observations, particularly by the James Webb Space Telescope, to study its atmosphere and composition in more detail.