Astrobiologists have long dreamed of finding another planet similar to our Earth in order to explore whether life could exist elsewhere, and new research shows that such planets may be far more widespread than once believed.
Thanks to advanced planet-hunting techniques, astronomers have discovered thousands of planet candidates outside our Solar System – many Earth-sized worlds orbiting Sun-like stars within the habitable zone.
Size and Mass
Astronomers search for planets similar to Earth for two reasons. One, they believe the more similar a world is to ours, the higher its likelihood is of supporting life – prompting astrobiologists to search specifically for rocky, terrestrial planets within habitable zones of their stars and especially recent expansion of this search into extrasolar systems such as Milky Way.
To be considered an Earth analog, a planet would need a silicate mantle, an ocean filled with liquid water below its surface and an atmosphere suitable for life to exist on it. Furthermore, such a planet must possess stable orbits within the Goldilocks region of its host star’s star system as well as Earth-like climate, surface materials, oceans rivers lakes etc – in other words it must share some commonalities with Earth – climate – surface materials – climate change-sensitive ecosystems similar to Earth – for best results in being truly comparable planet to our own home planet Earth analog! Finally a planet once covered in forests or cities is an even better candidate to represent true Earth analog.
Astronomers are using cutting-edge planet hunting technology to identify thousands of exoplanets – planets that orbit other sunlike stars – including many Earth-sized planets in the habitable zone of their stars, though finding an actual Earth-like planet remains challenging.
Kepler-186f is one of the closest matches to Earth among all of the recently discovered planets, yet still doesn’t satisfy as an exact replacement. While its terrain resembles ours closely, its orbit lies too close to its star for habitability; additionally, its rotation axis tilts significantly away from it creating an extreme hot climate on this world.
Recently discovered LP 890-9 b and c are approximately 40 percent larger than Earth, yet still lie within their parent star’s habitable zone. Unfortunately, however, they’re more than twice as hot than our planet at 257 degrees Fahrenheit; therefore it seems likely that similar Earth-sized planets might share more similarities than differences and could potentially host complex extraterrestrial life forms.
Atmosphere
Atmospheres of planets consist of layers of gases and aerosols that extend from its ocean, land and ice-covered surfaces outward to space, where their density decreases with altitude. At the surface of Earth is called the troposphere; here you’ll find gases such as Nitrogen and Oxygen along with Carbon Dioxide, Water vapour and other substances; solar radiation can ionize this layer, allowing radio signals to penetrate deep within planets; it is then surrounded by thermosphere and exosphere that reaches out into outer space.
All planets in our Solar System possess atmospheres, although their composition varies significantly from each other. Mercury’s atmosphere consists of virtually indetectable sodium and potassium clouds blown off the planet’s surface by solar wind; Venus has an abundant, hot atmosphere comprised of 78% nitrogen, 21% oxygen and other gases with a surface pressure of 9.2kPa; finally Earth has predominantly nitrogen atmosphere with trace elements and chemicals such as trace hydrogen present, with surface pressure estimated at 100kPa.
Mars and Jupiter contain predominantly hydrogen and helium with some small amounts of other gases; Saturn and Uranus on the other hand feature atmospheres consisting of almost 100% Hydrogen and Helium gas with only trace amounts of methane present.
Most of the Sun’s radiation consists of visible light, but it also emits invisible infrared and ultraviolet rays that reach Earth. Planet atmospheres absorb some infrared rays while reflecting others back into space to maintain an energy balance sufficient for life on planet surfaces to exist. Without this energy balance, life could no longer survive on Earth’s surfaces.
Astrobiologists theorize that Earth analogs could exist elsewhere in the Universe. A number of criteria are employed to identify potential candidate planets for consideration: planet size and surface gravity, orbital distance from parent star, stability, temperature, atmospheric composition composition axial tilt rotation oceans etc. Additionally important characteristics may include potential extraterrestrial life forms as well as magnetic fields or environments suitable for their existence.
Surface Temperature
Since the discovery of planets in our Solar System, astronomers have been keen on discovering Earth-like worlds – or astrobiologists refer to them – which might hold potential for extraterrestrial life to exist and evolve. Astrobiologists suggest that similar planets with higher similarities to ours could provide more chances for complex extraterrestrial organisms to exist on them and potentially develop life.
Scientists have discovered a few planets which astrobiologists consider Earth-like. This includes planets with characteristics such as their size and mass, orbital period stability, axial tilt rotation rate, whether or not tidally locked to its star and surface temperature/atmosphere considerations.
Planets similar in size to Earth tend to be warmer than larger or smaller planets due to sunlight being absorbed more directly by their surfaces and by having thick atmospheres which trap heat, providing additional energy sources and affecting surface temperatures by trapping it there.
Temperature levels on planets depend on their distance from and size of their host star, with those closer in orbiting within its habitable zone being hotter than those further away.
55 Cancri e is located 41 light years from Earth and rotates once every 18 hours, locked tidally into its host star so that one side (termed the dayside) gets exposed to intense radiation from it while its opposite (called nightside) remains cooler.
Due to being so close to its star, this planet may lack an insulating atmosphere and experience extreme surface temperatures – sometimes reaching up to 430deg Fahrenheit during daytime! At night however, with no atmosphere present the surface can cool to around 40 degrees minus one.
Astronomers have identified a group of exoplanets that might host an atmosphere and temperatures suitable for liquid water to exist on their surfaces, including TRAPPIST-1 e, f, g and h. Led by astronomer Travis Quick’s team they believe these could be ocean worlds similar to Europa or Enceladus and believe there could be as many as six billion potentially habitable Earth-like planets around sun-like stars in our Milky Way Galaxy that might meet this criteria.
Water
Astrobiologists and astronomers seek planets similar to Earth to test for extraterrestrial life. Criteria used for such planets include their size and mass, surface gravity, orbital distance, star size/type (including luminosity), axial tilt/rotation angle, climate/oceans/air and weather conditions; possible complex lifeforms present, strong magnetospheres as well as other processes present on such a world.
Planets that could potentially support life have many desirable traits that make them suitable for water, including proximity to their “triple point”, which is the temperature at which water molecules change states from solid ice into gaseous form and back again to solid state. Water must exist on its surface so it can either fall as rain or snow or be stored in oceans.
Scientists recently used modeling to ascertain the likelihood that rocky planets within the habitable zones of their host stars contain surface or subsurface water, specifically with reference to TRAPPIST-1e – one of seven known exoplanets orbiting TRAPPIST-1. Analysis indicated this particular planet as being relatively small with comparable densities as Earth, as well as being within its host star’s habitable zone.
The team also conducted extensive analysis on other potential candidates, including planets of Kepler-138 planetary system located 218 light years away. Researchers observed evidence for liquid ocean beneath its icy outer layers – something Europa Clipper will soon be able to investigate for signs of life.
Astronomers recently discovered another exoplanet named GJ 667Cc may also support life. It has similar diameter and density to Earth, is within its host star’s habitable region, but further observations showed its surface was extremely hot – disqualifying this planet as suitable habitat.