Discovering the first alien planet is a dream for most astronomers. With today’s advanced technology, finding new exoplanets is a common thing, but it takes something special to discover planets which bear more Earth-like characteristics and have similar planetary settings.
To discover an Earth-like exoplanet, astronomers look for few essential things. The first and foremost is that if the extraterrestrial plant in question orbits its host star in the right distance or not. If a planet orbits the star in a particular zone, also known as the Goldilocks or simply habitable zone, then there is a greater chance of finding abundant water on its surface.
Before speculating life on exoplanets, researchers look for rocky surface and an atmosphere. Back in 2011, a group of astronomers proposed the Earth Similarity Index or ESI, which classifies all known planetary objects, inside and outside the solar system on how similar they are to the Earth.
Focusing on ESI, we have compiled a list of 15 most Earth-like planets, where life might be possible. All ESI values are taken from Planetary Habitability Laboratory site.
15. Gliese 581c
Gliese 581 system, superimposed on a picture of our own solar system. Image credit: National Science Foundation/Zina Deretsky
ESI Value: 0.70
Gliese 581c is one of the three confirmed planets revolving around a red dwarf designated as Gliese 581, located about 20 light years away from the earth in the constellation Libra. The exoplanet was discovered back in 2007 and it immediately became a subject of interest for astronomers since it was reported as the first potentially earth like planet in the habitable zone of its parent star.
Initial studies found out that the planet’s temperature is adequate to harbor water on its surface and may also support a unique organism that thrives in extreme geological conditions. Based on the data retrieved from the radial velocity method, which is used for detecting far-off exoplanets, researchers have come to a conclusion that its mass is at-least times that of the Earth.
14. Gliese 832 c
Orbit of Gliese 832 c around its parent star
ESI Value: 0.81
Gliese 832 c is one of the closest potentially habitable exoplanets to the earth discovered to date. It’s orbiting Gliese 832, a red dwarf located in the constellation of Grus about 16 light years away. Data derived from satellite observations indicates that the planet has a mass 5.2 times that of the Earth.
Although its average surface temperature is more or less similar compared to Earth’s, extreme fluctuations are common due to the high eccentric orbit around its parent star. Researchers also believe that due to its mass, it may have a dense Venus like atmosphere, making it more of like “Super Venus” than “Super Earth”.
13. K2-72 e
Exoplanet K2-72 e compared to the Earth Image Courtesy: MarioProtIV
ESI Value: 0.82
K2-72 e is one of the four planets revolving around a cool dwarf star K2-72 located around 227 light years away from the Earth. The exoplanet’s mass and radius is close to that of the Earth. Moreover, it appears to be a rocky exoplanet with a strong possibility of the existence of water. There is a greater chance that K2-72 e is tidally locked (one side is face locked with the star).
12. Kepler-452 b
Kepler-452 b, an artist’s concept
ESI Value: 0.83
At the distance of 1,400 light years, Kepler-452b is the farthest Earth like exoplanet in this list. This exoplanet is gravitationally bound by Kepler-452, a Sun-like star located in the constellation Cygnus. It has five times the mass and 50% times the radius of the Earth.
Though its characteristics are still not decisive, the planet appears to be rocky. Due to its many intriguing features, Kepler-452 b has earned a nickname of Coruscant. And it’s also dubbed as Earth’s closest cousin.
However, extensive research shows that the planet can harbor potential life for just another 500–900 million years before it is pushed out of the habitable zone of Kepler-452.
An artist’s impression of the Kepler-62e
ESI Value: 0.83
Kepler-62e lies within the habitable zone of Kepler-62, a K-type main sequence star located 1200 light years away from the Earth. The star hosts another planet designated as Kepler-62f, which is also a potentially habitable exoplanet with an ESI value of 0.67.
Kepler-62e was discovered using the transit method, in which astronomers look for the dimming effect caused by planets when they ‘transit’ or crosses in front of its host star. The planet appears to be rocky with a great possibility of the existence of water.
10. Kepler-442 b
Confirmed small exoplanets in habitable zones (Kepler)
ESI Value: 0.84
Kepler-442b is a distant, super-Earth exoplanet, with both its radius and mass greater than that of the Earth. The exoplanet is revolving around a K-type main sequence star designated as Kepler-442, located 1120 light years away in the direction of the constellation Lyra.
The planet is considered as one of the most Earth like exoplanet found till date, at least by its size and temperature.
Since the K-type main sequence stars are much smaller, they tend to live much more compared to our Sun. But, these stars are known for high stellar activity, especially at the beginning of their lives and pose a serious threat to any kind of life. However a big uncertainty over its age makes exoplanet Kepler-442 b a good contender to support life.
9. Gliese 667 Cc
An artist’s impression of the planet Gliese 667 Cc Image Courtesy: ESO
ESI Value: 0.85
The initial discovery of Gliese 667 Cc was done by ESO’s HARPS satellite in 2011 and was later confirmed by the researchers at the Carnegie Institution of Science in 2012. The exoplanet is orbiting a red dwarf Gliese 667 C within its habitable zone.
Its parent star itself is a part of three star system located about 23.62 light years away from the Earth in the constellation Scorpius. This exoplanet is characterized as a super-Earth, with radius and mass greater than that of the Earth, but smaller than that of Neptune or Uranus.
The planet’s black body temperature calculation suggests that it might be a little warmer than the Earth due to slightly increased electromagnetic radiation it gets. It is most probably tidally locked.
8. TRAPPIST-1 e
ESI Value: 0.85
The TRAPPIST-1 planetary system is without a doubt a prime focus in our quest to discover habitable exoplanets. It consist of seven terrestrial planets, of which three lies within the habitable zone. One of those three promising planets is TRAPPIST-1 e. The exoplanet is similar to the Earth in almost every manner, whether it is mass, density, radius, temperature or stellar flux.
One important thing to note about this planetary system is that the TRAPPIST-1 is an ultracool red dwarf, a class of stars which have a normal life span of more than 4 trillion years, way more than Earth’s Sun. It is likely that the TRAPPIST-1 will be among the few remaining stars in the universe. Available data on the TRAPPIST-1 b indicates that the planet might be tidally locked.
7. Proxima Centauri b
Artist’s impression of Proxima Centauri b
ESI Value: 0.85
In 2016, Proxima Centauri b became the nearest most Earth-like exoplanet. While the existence of such a planet was initially bought into the attention by professor Mikko Tuomi of the University of Hertfordshire but the actual confirmation was done by an international team of astronomers three years later in 2016.
The planet revolves around a red dwarf known as Proxima Centuari, which itself is a part of a triple star system. Since it is orbiting in the habitable zone of the star, atmospheric properties and conditions are capable of sustaining liquid water.
Extremely strong solar flux, however, is a major problem for establishing habitability there as solar winds are about 2,000 times more powerful than anything experienced on Earth.
6. Kepler-296 e
Image Courtesy: NASA
ESI Value: 0.85
Located about 1,200 light years away from the Earth, Kepler-296 e is one of the most distant earth like exoplanet observed to date. It’s orbiting in the habitable zone of star Kepler-62. The planet orbits its host star in every 122.3 days at a distance of 0.42 AU (slightly more than the Mercury’s distance from the Sun).
Researchers believe that the planet is not likely to be older than 7 billion years. They also believe that the planet may be entirely covered with ocean. On the other hand, multiple studies have shown that exoplanets with radius more than 1.6 times Earth’s have generally volatile composition and are gaseous much like a mini-Neptune, and this might be the same case.
5. Gliese 273 b
ESI Value: 0.86
Gliese 273, popularly known as Luyten’s star is an extremely faint red dwarf located at a distance of 12.36 light years from the Sun in the direction of Canis Minor. In 2017, two planets were discovered orbiting the star. While the innermost planet is completely inhabitable, the outer planet Gliese 273b or Luyten b is showing signs of a potentially habitable planet.
Gliese 273 b is characteristically a super-earth, with 3 times the total mass of the Earth. The planet has an orbital period of 18.69 days, however, it receives only 6% more incident flux than the earth.
In late 2017, Messaging Extraterrestrial Intelligence organization or METI sent an audio message carrying some scientific tutorials and music towards the planet to establish contact with any potential advanced extraterrestrial civilization.
4. Ross 128 b
An artist’s impression of Ross 128 b Image Courtesy: European Southern Observatory (ESO)
ESI Value: 0.86
Ross 128 b is an Earth-sized, rocky exoplanet that orbits within the habitable zone of star Ross 128. It is the second closest exoplanet to the Earth only after Proxima Centauri b and it’s located 11 light years in the constellation of Virgo. The planet was discovered with the help of HARPS after decades of searching and analyzing data.
While the exact measurements of this exoplanet is still unknown as no transit of Ross 128 b is observed till date, researchers have estimated that it is 35% more massive than the Earth and receives only 38% stellar light. The temperature is also suitable for liquid water to prevail, however that depends upon the existence of an atmosphere.
3. Kepler 438 b
An artist’s impression of Kepler 438 b
ESI Value: 0.88
Kepler 438 b holds a special place in the huge catalog of the Kepler objects. In 2015, it became the 1000th exoplanet to be discovered. The planet is gravitationally bounded by the star Kepler-438, a red dwarf in the constellation of Lyra. Currently, it is the only planet revolving around the star and it’s located astonishingly 470 light years away.
It is now accepted that Kepler 438 is orbiting in the habitable zone of its parent star. This region is generally believed to have a perfect condition for liquid water to prevail. But there is one big problem.
A team of researchers from the University of Warwick argues that Kepler 438 b has hostile environment and its non-habitable due to the immense amount of radiation it receives from the parent star. The stellar flares emitted by the Kepler 438 are much more violent than what we see on the Sun, and it would be enough to erase the humanity on Earth.
2. Gliese 3323 b
An artist’s impression of Gliese 3323
ESI Value: 0.89
Gliese 3323 is currently the second most Earth-like planet based on the Earth Similarity Index. It orbits around a red dwarf star known as Gliese 3323 or LHS 1723, located 17.4 light years away from us. Unlike Earth, this planet is extremely close to its parent star, completing one orbit in just 5 days.
However, it only receives about 21% more sunlight than the Earth. Gliese 3323 b was discovered by the HARPS telescope using the radial velocity method in 2017.
1. TRAPPIST-1 d
ESI Value = 0.9
TRAPPIST-1, officially known as 2MASS J23062928-0502285 is a sub stellar, red dwarf star located in the direction of constellation Aquarius 39.6 light years away from the Sun.
One of those three exoplanets that lies within the habitable zone of the star is a small, water rich planet known as TRAPPIST-1 d. Detailed researches prior to 2016 revealed that the TRAPPIST-1 d has the least mass in that entire planetary system and 5% of that mass is likely to be water.
Numerous computer simulation has helped astronomers to calculate the planet’s accurate density, which is around 61% (3.39 g/cm3) of the Earth. Researchers have also confirmed that the planet has a terrestrial atmosphere. Moreover, it receives 4.3% more sunlight than the planet Earth.