To put it simply, exoplanets are planets that are not inside our Solar System. These planets were long theorised in science-fiction and by prominent astronomers, but it took until 1992 for the first confirmation of an exoplanet, a planet now known as HIP 65426b. Nearly 30 years have passed since the first confirmation and in that time, 4,687 exoplanets have been discovered ranging from planetesimals half the size of our moon, to giant planets 30 times larger than Jupiter, from systems orbiting binary stars to planets around the nearest star to Earth. All of this begs the question however, are any of these places habitable?
What is required for habitability?
To help us narrow down the list of potentially habitable exoplanets, we need to determine what makes a planet habitable. Unfortunately for Humans, this is a long list. In order for humans to be able to survive on an Exoplanet, the planet must;
- Be orbiting in a star’s Goldilock’s Zone; a region of space where a planet is far enough away from its parent star to keep liquid water on the surface
- Be orbiting a star that will remain stable for several billion years
- Orbit a star that emits radiation similar to our own
- Have a rocky surface
- Be similar in size, mass and gravity to Earth
Now that we have a more complete understanding of what makes a planet habitable, let’s take a look at some exoplanets to find out if they’re habitable.
Image: Artist’s impression of view from one of the middle planets in the TRAPPIST-1 planetary system.
By ESO/N. Bartmann/spaceengine.org, CC BY-SA 4.0
Proxima Centauri B:
The closest star to us in the Universe, Proxima Centauri, is a red dwarf star orbiting the Alpha Centauri binary star system. In 2016, after analysing the light coming from Proxima Centauri, astronomers made the exciting discovery that an exoplanet was orbiting it. Not only that, but the newly detected exoplanet was orbiting inside its parent stars habitable zone!
Upon further examination, it was discovered that Proxima Centauri B has a year length of just over 11 Earth days, and was similar to Earth in mass, radius and gravity. All of these factors made the exoplanet appear to be habitable. Unfortunately, due to the proximity of the exoplanet to its parent star, being only 5% the distance Earth is to the Sun, Proxima Cenaturi B likely experiences stellar winds over 2000 times stronger than the Earth does. This could result in the exoplanet’s atmosphere having been stripped away. Another possibility is that the exoplanet had become tidally locked with its star. Tidally locked means that the same face of the exoplanet is always facing the star, becoming extremely hot, while the other faces away, becoming extraordinarily cold.
The TRAPPIST-1 system gained massive attention in 2015 after the announcement of 7 exoplanets orbiting the star. One of the exoplanets, TRAPPIST-1e, especially fascinated astronomers. The exoplanet was found to be orbiting within the habitable zone for its parent star, and displayed properties similar to the Earth, having a comparable radius, mass and surface gravity.
TRAPPIST-1e was confirmed to be a rocky planet in 2018, along with confirmation of a compact atmosphere that might be similar to Earth’s own atmosphere. . The exoplanet was also observed to be tidally locked, which means there could still be a small habitable region along the terminator (the line where dark and light meet on the planet).
Teegarden’s Star b
Teegarden’s Star is a low mass red dwarf star located just over 12.5 light years away from Earth. Due to its extremely low brightness, it was overlooked until 2003 during a survey through historical data. The red dwarf was also found to have a two exoplanets. Teegarden’s Star b, the closer of the two exoplanets, was found to have characteristics that are similar to Earth’s. Astronomers found the star has a mass greater than 1.05 times the Earth and likely has an average surface temperature of around 25 °C. The exoplanet’s host star is also noteworthy for the exoplanet’s potential habitability. Most red dwarf stars, like Proxima Centauri, are active and emit strong flares that rip atmospheres off their planets. Teegarden’s Star however, is inactive, and no such flares occur. Due to the inclination of the exoplanet, it is difficult to determine much else about the radius, size or rotational period of the exoplanet.
Kepler-442b was observed over 1200 light years from Earth by the Kepler spacecraft in 2015. This exoplanet orbits within its parent star’s habitable zone and is likely a rocky planet based off it’s radius. As the exoplanet is over 2 times heavier than the Earth and it is assumed that the surface gravity of the Kepler-442b would be 30% than Earth’s. Kepler-442b is distant enough from its parent star to avoid being tidally locked, meaning the exoplanet would have a regular day/night cycle. Unfortunately, for Kepler-442b its parent star could still pose a threat to any future colonists due to the emission of strong solar winds and radiation.
Riley is an experienced astronomy guide who has been working within the astronomy community for several years. He is incredibly passionate about the night sky and is currently completing a Bachelor’s Degree in Science, Math and Education.
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