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The exoplanets most likely to harbour life

The Habitable Exoplanets Catalog maintained by the Planetary Habitability Laboratory at the University of Puerto Rico in Arecibo has been listing potential homes of aliens for six years. There are two lists of potentially habitable exoplanets, a conservative list and an optimistic list. The conservative list has more stringent requirements, and thus more chances of harbouring life. The planets here are from the conservative list. The naming convention is the name of the star followed by an alphabet, which denotes the number of the planet, with the list starting from b. For example, Earth would be named Sun d. Proxima b is on the conservative list, but has been excluded as recent evidence suggests that the exoplanet is too old and bombarded with too much radiation to support life.

Many of the planets in this list are in orbit around a red dwarf star, the most common type of star in the Milky Way. An orange dwarf star, which is stable for much longer a G dwarf star such as the Sun, allows for more time for life to emerge. Three planets in the list are in orbit around orange dwarf stars. None of the planets are in orbit around a G dwarf star. The planets here are all rocky, just the right distance from their host stars for liquid water to exist on the surface, and consequently most likely to have the conditions necessary to support life as we know it on Earth.

Gliese 677 Cc

A triple sunset on GJ 677 Cc. Image: ESO/L. Calçada

From the surface of GJ 677 C c, also known as Gliese 667 C c, an observer can behold three stars in the sky at once. The stars are all smaller than the sun. Gliese 667 C c is in orbit around the smallest star, a red dwarf. The planet may be tidally locked to the star. In this case, one side is forever in cold darkness, while the other side enjoys an hot endless day. The twilight zone between the two sides, known as the terminator line, is where life is most likely to exist. The host star is about 23 light years away and appears in the constellation of Scorpius.

LHS 1140 b

LHS 1140 b. Image: Harvard-Smithsonian Center for Astrophysics.

The planet passes in front of the star every 25 days as seen from the Earth, which makes it a great candidate for future observations by more powerful telescopes. Astronomical instruments such as the James Webb Space Telescope can in the future, scan the light that passes through the atmosphere of the planet, for signs of life. The planet is a super Earth, about 40 percent larger than the Earth, and about 6.6 times denser. The planet could have once been inhospitable because of the harsh ultraviolet radiation from an energetic young star, but now the host star has cooled down. LHS 1140 is 40 light years away in the constellation of Cetus.


TRAPPIST-1 f. Image: Star Engine.

The TRAPPIST-1 system is incredibly interesting, considering that there are seven rocky planets in orbit around the ultracool red dwarf star. Three of the planets, e, f and g are all in the habitable zone. All seven planets are closer to the host star than Mercury is to the sun. The planets are tidally locked. Between the two sides is a zone of perpetual twilight, where anyone on the surface can enjoy a sunset frozen forever in time. An observer on the surface can see all the other planets in the sky, and discern surface details as well. The star is about 40 light years away and appears in the constellation of Aquarius.

Kepler-186 f

Kepler-186 f by NASA Ames/JPL-Caltech/T. Pyle

Kepler-186 f was the first Earth sized exoplanet to be found in the habitable zone of its host star. The world is only about ten percent larger than the Earth, but its composition and mass are not yet known. The planet receives only about a third of the energy as the Earth receives from the Sun. For an observer on the surface at noon, the star would appear about as bright as the Sun on Earth just before sunset. There are four other planets in the system, which are too close to the host star to support life. Kepler-186 is about 550 light years away and appears in the constellation of Cygnus.

Kepler-442 b

Kepler-442 b in Space Engine

This planet is very similar to the Earth, and right in the middle of the habitable zone of the host star. The host star is an orange dwarf, which means that there is plenty of of time for life to emerge and thrive on this system. However, early in their life, orange dwarf stars bombard the planets with an excessive amount of ultraviolet radiation. Currently the planet basks in moderate warmth, receiving about 70 percent of the light that Earth receives from the Sun. The planet takes a trip around the host star about once every 112 days. This star is 1,120 light years away and appears in the constellation of Lyra.

Kepler-62 f

Kepler 62 f by NASA Ames/JPL-Caltech/T. Pyle.

Kepler-62f is a super Earth about 40 percent larger than the Earth. The composition and mass of the planet is however, not yet known. The planet is the outermost one in a system containing at least five other planets. Kepler-62 is an orange dwarf star, cooler and smaller than the Sun. The planet receives about 41 percent of the light that Earth receives from the Sun. The system is pretty ancient as the star is more than seven billion years old. The planet is right in the heart of the habitable zone around the star. The star is pretty distant, at 1,200 light years away in the constellation of Lyra.

Aditya Madanapalle

Aditya Madanapalle

An avid reader of the magazine, who ended up working at Digit after studying journalism, game design and ancient runes. When not egging on arguments in the Digit forum, can be found playing with LEGO sets meant for 9 to 14-year-olds.