Habitable zones refer to the region around a star where the potential temperature of a planet orbiting at a specific distance from a star could allow liquid water to exist on the surface, if the atmosphere where to be strong enough. The bounderies aren’t specifically fixed, but it depends on the system’s star’s characteristics, such as it’s luminosity and temperature. Hotter stars will have their goldilocks zone being further away from the star, whereas colder ones, like red dwarfs, it will be closer in. Though, there are other factors involved, as red dwarf stars put a lot of X-Rays and UV radiation out into their systems.

An exoplanet’s atmosphere should have greenhouse gases, to allow for trapping of the heat on the surface, and also allowing for CO₂ and H₂O vapor being present in the atmosphere.

CO₂ helps us regulate temperatures using the Green house effect. O₂ is then a byproduct of CO₂ being produced by biological processes. On Earth, this is by plant’s photosynthesizing.

Ozone, or O₃, is a byproduct of that oxygen then interacting with a star’s light, which acts as a shield against harmful UV radiation, which is important for life.

Gliese 180, often shortened to GJ 180, is a small, red dwarf star in the Equatorial Constellation of Eridanus, at a distance of about 39 light years away, and is coming closer to us at a speed of -14.6km/s. Based on study of the star, is not considered to be an active star, at an age of about 5 billion years old.

There are 3 planets that have been identified in the system, labelled b, c, and d.

The main planet that is marked as habitable is Gliese 180c. It’s 24 day orbital period puts it at the closer end of the expected habitable zone of it’s parent star, and it has a fairly stable orbit. It’s mass is about 6.4 M_🜨, (earth masses).

HD40307 g is an exoplanet candidate suspected to be orbiting in the habitable zone of HD 40307. HD 40307 is a orange main-sequence star, much like our own, which is 42 light years away, in the constellation of Pictor. It has a mass of 0.75 M_☉, but much younger at just 1.2 Gyr (billion years). It came within 6.4 light-years of the Sun about 413,000 years ago.

HD 40307 g has an orbital period of 197.8 days, and is in a fairly stable orbit, in the star’s habitable zone for much of it’s year. It likely has a mass of 7.09 M_🜨, which does put it close to the 10 M_🜨 limit, and it’s temperature is likely to be roughly 277.6 kelvin, or 4-9^oc

Tau Ceti f, a candidate exoplanet around the recently well known Tau Ceti star, is a planet with the mass of roughly 3.93M_🜨, making it much closer to our own planet’s mass.

Tau Ceti itself is a single star in the constellation Cetus, and is very similar to our sun, at roughly 0.78 M_☉. It is 12 light years away from the Solar system, and is the nearest solitary G-Type star, also being known to appear relatively stable. It is older than our sun though, at 8-10 Gyr, and can be seen with the naked eye.

Tau Ceti F has an orbit at a distance of roughly 1.35 AU, which would be similar to Mars in our solar system, taking 642 _🜨 days to orbit Tau Ceti. However, a further in-depth study suggests it’s orbit could actually be longer, nearing 1000 Earth_🜨 days.

TOI-715 is a red dwarf star located 42 parsecs, or 140 light years, away from the Earth in the constellation Volans, close to our southern celestial pole. It hosts one confirmed exoplanet, TOI-715b, a super-Earth orbiting in it’s habitable zone. Another planet is suspected, but not confirmed. TOI-715 is much smaller and cooler than the Sun, at 0.225M_☉ and 3075K, with an Age of 6.6 Gyr. It was initially spotted by the Transiting Exoplanet Survey Satellite.

TOI-715b was identified in May 24th, 2019, and has a radius of just 1.55 R_🜨, and is orbiting in the star’s habitable zone, with an orbital period of 19 days, and is orbiting it’s host star at a distance of 0.083 AU. It likely has an average temperature of -39^oC. They are hoping that the JWST will be able to confirm it’s atmosphere in the future.

Kepler 452 is a G-type subgiant star, located roughly 1,810 light years away from Earth, in the Cygnus constellation. It has a similar temperature to our own sun, but is brighter and more massive. It has a mass of 1.037M_☉, a temperature of 5757K, and an age of 6 Gyr.

Kepler 452b, sometimes quoted to be Earth 2.0, is a disputed super-Earth exoplanet. It orbits in the inner edge of the habitable zone of it’s parent star, and is the only planet so far located in the system by the Kepler space telescope. It orbits it’s star at a distance of 1.04AU, an orbital period of 385 days, a mass of roughly 5M_🜨, and it’s radius is 1.63R_🜨. It’s gravity is likely to be 1.9g, compared to the 1g of our own. Habitability is debated, as because of it’s proximity to the star, it’s possible it’s subjected to a runaway greenhouse effect, like Venus.