Astrobiologists and advocates of human space colonization have long searched for planets similar to Earth in terms of conditions – with water bodies and an atmosphere that offers protection from cosmic radiation.
Exoplanets circling stars other than our Sun are now estimated in the thousands, yet finding an Earth-like planet within its host star’s habitable zone remains much rarer.
1. Kepler 442-b
Kepler 442-b is the planet most similar to earth, according to a new study. It orbits its host star every 112.3 days and lies within its Goldilocks zone – an area where liquid water could form on its surface.
Researchers led by Guillermo Torres from Harvard-Smithsonian Center for Astrophysics say this makes the planet an excellent candidate for supporting life, marking the first time scientists have confirmed a planet similar to our own in an outer star’s “habitable zone”.
Torres used three years of data from Kepler to compile his list of planet candidates; using computer programs he calculated their likelihood as being real world; only two made the cut: Kepler-438b which orbits a red dwarf star about 35 times smaller and 30 percent cooler than our sun and Kepler-442b, orbiting 50 times smaller but 15 percent cooler.
These worlds, both approximately the size of Neptune, lie within habitable zones of their stars where there would be enough sunlight for water to exist on the planet’s surface – a key indicator that they might be suitable hosts.
Kepler-442b’s discovery does not guarantee its habitability; current telescopes cannot adequately survey it, while even next-generation planet-hunting spacecraft such as TESS or CHEOPS cannot assess its composition or atmosphere with any certainty.
As part of any accurate assessment of Kepler-442b’s habitability index in 2015, more information would need to be gathered on its energy source in order to ascertain if its conditions could support life. Although Kepler-442b scored higher on that scale compared with Earth in terms of habitability index rankings. We need more details before being certain if life could exist there.
2. Gliese 667 Cc
Scientists have recently made the groundbreaking discovery that Gliese 667 Cc is the closest exoplanet known to Earth in terms of similarities. Located just 22 light-years from us in Scorpius constellation, this super-Earth takes 28 days to orbit its star; making it suitable for hosting liquid water and possibly life as we know it.
Gliese 667 Cc stands out for being both its size and composition; moreover, it is also notable for being tidally locked, always facing one way towards its host star. This would require it to have both dayside and nightside sides; also it requires an atmosphere thick enough to redistribute heat evenly without creating deserts.
Atmospheres play an essential role in shaping climate and surface conditions on planets, helping retain heat while deflecting harmful radiation. Gliese 667 Cc has an apparently dense atmosphere but its exact composition remains unknown; it may consist of mixtures of rocks and gaseous substances or have more carbon-rich conditions.
Gliese 667 Cc’s distance from its host star is another key factor influencing conditions on the planet. To maintain a temperate climate, exoplanets must exist within their star’s “habitable zone”, or region with conditions suitable for liquid water presence. Gliese 667 Cc lies comfortably within this zone; consequently it should have an enjoyable temperature level.
Researchers have also discovered a few potential habitable planets within this system, such as Gliese 667 Cd and Cf, although any others in this galaxy likely lie too far from their host stars to support liquid water or life.
3. Enceladus
Enceladus, one of Saturn’s 62 moons, immediately captured astrobiologist attention when it was discovered to contain an ocean beneath its icy shell, spewing plumes of water vapor and particles into space – with organic molecules detected there pointing toward life on Enceladus as evidence for life itself.
Scientists have recently discovered that Enceladus’ ocean contains hydrogen cyanide, a toxic but essential chemical component for life’s origins on Earth’s seafloor. This discovery supports claims that Enceladus might support simple microbial life similar to what could be found at hydrothermal vents on its ocean floor.
Cassini’s mission flew past Enceladus’ erupting plumes and collected samples of its icy material as it shot into space from its south pole, discovering methane, carbon dioxide, ammonia and molecular hydrogen which suggests Enceladus might contain an internal ocean with thick rocky core.
Scientists theorize that frictional heating of hot ice particles within Enceladus’ overlying ice shell drives its geological activity, with Tidal deformation contributing further. When heated, this causes it to expand and crack with deep fractures referred to by scientists as “Tiger Stripes”. Heat can then reach its way beneath to warm its ocean depths while simultaneously emitting water vapor and particles into space.
Cassini also detected signs of carbon monoxide gas in its plumes during its flybys, suggesting the possibility that life could potentially thrive on an icyball’s surface.
Recently, researchers performed an experiment in which they cultivated three strains of bacteria under similar conditions to what exists on Enceladus and in its icy plumes. One strain ate up the hydrogen and carbon dioxide present in these plumes while producing methane as byproduct.
Enceladus’ liquid water contains high concentrations of sodium carbonate (or soda salt), making its oceans extremely salty; yet still has an alkaline pH level similar to Earth’s, providing vitality and life support systems for survival.
4. Callisto
Callisto is one of Jupiter’s moons and, like Europa, may contain subsurface oceans. With low levels of radiation and a thin atmosphere made up primarily of carbon dioxide, Callisto provides humans with an ideal platform to explore Jupiter’s system. Its surface features many craters believed to be due to asteroid impacts over a long period of time; and due to a lack of plate tectonics or volcanic activity its surface may remain relatively unchanged since its creation.
Remarkably, unlike our moon, Mars doesn’t contain mountains. This may be due to its icy surface rather than rock surfaces, which has allowed time for erosion of impact craters by melting snowfall over time. There are, however, two large concentric-ring impact basins known as Valhalla that span approximately 600 kilometers each in diameter.
Callisto was initially observed with telescopes only as having a very dark surface and low reflectivity (albedo), however over time we have come to learn of its water ice surface which, unlike Ganymede, does not appear to have any deposits of other materials covering its surface. Furthermore, its low albedo allows it to absorb much sunlight resulting in very cold temperatures on its surface.
There is evidence of water vapor in Jupiter’s thin atmosphere and magnetic resonance imaging demonstrates it responds to its changing magnetic fields like a conducting sphere – suggesting the possibility of salty liquid water underneath its surface.
Scientists think Callisto’s icy surface is covered by two layers: thick ice and thin rock layers, as well as an ocean 150-200 kilometers deep, possibly filled with salty brine that contains antifreeze chemicals such as ammonia to keep its water from freezing over. This theory has been supported by other moons of Saturn like Dione and Enceladus that also boast salty seas.