Classifying Exoplanets
How are Exoplanets Classified?
All exoplanets are categorized into a type that consists of 3 variables: the temperature of their star, the temperature of the planet, and the terrain of the planet. ​
Additionally, all planets have an ESI value.
​
Stellar Classification (The Temperature of the Star):
The stellar classification consists of 7 classes of stars based on their temperatures.
​
The temperature of the Planet:
Cold planets: Typically below 200-250 K (-73°C to -17°C)
Medium planets: Around 250-350 K (-23°C to 77°C)
Hot planets: Above 350 K (77°C and above)
The terrain of the planet:
Planets have different terrains, such as rocky planets and gas planets. All habitable planets are rocky since we need to be able to stay on the planet and not just fall through. These rocky planets are called “terran”, and there are 3 types of terrans; Sub-terran, terran, and super-terran. These types represent the mass of the planet compared to Earth.
Sub-Terran = 0.1 - 0.5 * Earth’s Mass
Terran = 0.5 - 2 * Earth’s Mass
Super-Terran = 2 - 10 * Earth’s Mass
​
ESI:
The Earth Similarity Index (ESI) is a measure used to assess how similar an exoplanet is to Earth. It considers key factors such as a planet’s size, density, and surface temperature. The ESI ranges from 0 to 1; a value closer to 1 indicates a higher similarity to Earth. Planets with higher ESI scores, generally above 0.8, are considered more likely to have Earth-like conditions that could support life. This index helps prioritize which exoplanets to study in the search for a potentially habitable world.​
Habitable Exoplanets
What are habitable exoplanets?
Many factors are present in determining a habitable exoplanet.
Firstly, the planet has to be either a super-Earth or terrestrial planet. In our solar system, only Mercury, Venus, Earth, and Mars are terrain planets.
The second factor is the temperature. A planet close to a star will have burning temperatures, while a planet too far from the sun will experience subzero temperatures. The space between these two zones is called the Goldilocks zone. This term derives from the tale of Goldilocks and the Three Bears, where Goldilocks tries to find a soup that's not too hot nor too cold. This is key to a planet being habitable because, with these conditions, a planet can possess liquid water, the essential element of life. In our solar system, the only planets that are in the Goldilocks zone are Earth and Mars.
​
The third parameter for a stable planet is having a stable star system. A star system is stable if it has no transfer of mass from one object to another, no tidal effects, and no perturbation from other forces. Additionally, the stars must have enough energy to sustain continuity.
Lastly, the planet should have an atmosphere. This allows for there to be a chance of having breathable air and water. Furthermore, the atmosphere will protect the planet from radiation, and monitor the temperature.
These are not the only parameters needed for a planet to be habitable, but they are some of the most important ones. Despite there being over 5,700 exoplanets, these four parameters narrow them down to around 70. That means that less than 1.5% of every single exoplanet planet in the universe is habitable when considering these parameters.
Habitable Exoplanets
1. Teegarden’s Star b:
Teegarden's Star is the closest Earth-like exoplanet discovered, and it orbits a red dwarf star only 12 light-years away. It’s located in the star's habitable zone and may have the potential to support liquid water. Teegarden's Star B has the highest ESI value ever recorded, meaning it’s remarkably similar to Earth regarding potential habitability!
​​
​
-
Type: M Warm Terran
-
Discovery method: Radial Velocity
-
Distance from Earth: 12.5 light-years
-
Period: 4.91 days
-
ESI: 0.97
​​​​
​
​3. Kepler-1649 c:
Kepler-1649 c is an Earth-sized exoplanet located about 300 light-years away, orbiting a red dwarf star in the habitable zone. It has a rocky composition and a temperature that might allow for liquid water, making it a strong candidate for potential habitability.
​
-
Type: M Warm Terran
-
Discovery method: Transit
-
Distance from Earth: 300
-
Period: 19.5 days
-
ESI: 0.93
​
2. TOI-700 d:
TOI-700 d is located in the habitable zone of its star and is one of the most promising exoplanets for potentially hosting life. It’s a rocky planet that could have surface water or an atmosphere similar to Earth. TOI-700 d was discovered by NASA’s TESS mission, and its location in the habitable zone makes it a top candidate for further exploration of alien life.
​
-
Type: M Warm Terran
-
Discovery method: Transit
-
Distance from Earth: 101.4 light-years
-
Period: 37.43 days
-
ESI: 0.94
​
​
4. TOI-700 e:
TOI-700 e is a recently discovered planet, located just inside the inner edge of its star's habitable zone. It’s capable of supporting life if its conditions allow liquid water on the surface.
​
​​
​
-
Type: M Warm Terran
Discovery method: Transit -
Distance from Earth: 101.4 light-years
-
Period: 27.8 days
-
ESI: 0.91