Earth analog
The possibility is of particular interest to astrobiologists and astronomers under reasoning that the more similar a planet is to Earth, the more likely it is to be capable of sustaining complex extraterrestrial life.
[3] Scientific findings since the 1990s have greatly influenced the scope of the fields of astrobiology, models of planetary habitability and the search for extraterrestrial intelligence (SETI).
Likewise, until the 1960s, Venus was believed by many, including some scientists, to be a warmer version of Earth with a thick atmosphere and either hot and dusty or humid with water clouds and oceans.
Though having a dramatically different chemical makeup, discoveries such as the confirmation of Titanian lakes, rivers and fluvial processes in 2007, advanced comparisons to Earth.
Solar analog), orbital distance and stability, axial tilt and rotation, similar geography, oceans, air and weather conditions, strong magnetosphere and even the presence of Earth-like complex life.
The formation, presence, influence on these characteristics of the Moon (such as tidal forces) may also pose a problem in finding an Earth analog.
As anthropologist Vincent Ialenti's work on the epistemology of analogical reasoning has shown,[22] some planetary scientists are "more comfortable making the leap of faith to bridge time and space and pull together two disparate objects" than others are.
The closest known examples are Mars and Titan and while there are similarities in their types of landforms and surface compositions, there are also significant differences such as the temperature and quantities of ice.
There are several factors that can determine planetary temperatures and therefore several measures that can draw comparisons to that of the Earth in planets where atmospheric conditions are unknown.
[11] Thus extrasolar planets (or moons) in the Goldilocks position with substantial atmospheres may possess oceans and water clouds like those on Earth.
In contrast, the Rare Earth hypothesis asserts that if the strictest criteria are applied, such a planet, if it exists, may be so far away that humans may never locate it.
Astrobiologists assert that Earth analogs would most likely be found in a stellar habitable zone, in which liquid water could exist, providing the conditions for supporting life.
Some astrobiologists, such as Dirk Schulze-Makuch, estimated that a sufficiently massive natural satellite may form a habitable moon similar to Earth.
Several current scientific studies, including the Kepler mission, are aimed at refining estimates using real data from transiting planets.
A 2008 study by astronomer Michael Meyer from the University of Arizona of cosmic dust near recently formed Sun-like stars suggests that between 20% and 60% of solar analogs have evidence for the formation of rocky planets, not unlike the processes that led to those of Earth.
[41] Meyer's team found discs of cosmic dust around stars and sees this as a byproduct of the formation of rocky planets.
[44] In 2013, a Harvard–Smithsonian Center for Astrophysics using statistical analysis of additional Kepler data suggested that there are at least 17 billion Earth-sized planets in the Milky Way.
[46] Terraforming (literally, "Earth-shaping") of a planet, moon, or other body is the hypothetical process of deliberately modifying its atmosphere, temperature, surface topography or ecosystems to be similar to those of Earth to make it habitable to humans.
