A: Earth is considered to be in area known as the “Goldilocks Zone” around our Sun. As the existence of life beyond Earth is unknown, planetary habitability is largely an extrapolation of conditions on Earth and the characteristics of the Sun and Solar System which appear favorable to life's flourishing. [4] calculate this zone to be a ring 7 to 9 kiloparsecs in radius, including no more than 10% of the stars in the Milky Way,[5] about 20 to 40 billion stars. From the principle of mediocrity (extended from the Copernican principle), they argued that we are typical, and the universe teems with complex life.

[69] The Atacama was the subject of study in 2003 that partly replicated experiments from the Viking landings on Mars in the 1970s; no DNA could be recovered from two soil samples, and incubation experiments were also negative for biosignatures. The two giant planets then drifted out again to their present position. Secondly, smaller planets have smaller diameters and thus higher surface-to-volume ratios than their larger cousins. [21] After an energy source, liquid water is considered the most important ingredient for life, considering how integral it is to all life systems on Earth.

These findings confirm that the Sun is not unique among stars in hosting planets and expands the habitability research horizon beyond the Solar System. However, the majority of stars in barred spiral galaxies populate the spiral arms rather than the halo and tend to move in gravitationally aligned orbits, so there is little that is unusual about the Sun's orbit. Relative abundance in space does not always mirror differentiated abundance within planets; of the four life elements, for instance, only oxygen is present in any abundance in the Earth's crust.

Further research—including a consideration of the amount of photosynthetically active radiation—suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.[79]. The Rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a galactic habitable zone, a central star and planetary system having the requisite character, the circumstellar habitable zone, a right-sized terrestrial planet, the advantage of a gas giant guardian like Jupiter and a large natural satellite, conditions needed to ensure the planet has a magnetosphere and plate tectonics, the chemistry of the lithosphere, atmosphere, and oceans, the role o… [128] There are suggestions that complex life could arise in sub-surface conditions which may be similar to those where life may have arisen on Earth, such as the tidally heated subsurfaces of Europa or Enceladus. [6] would halve these numbers; they estimate that at most 5% of stars in the Milky Way fall in the galactic habitable zone. ⋅

A smaller amount of metal makes the formation of planets much less likely, under the solar nebula theory of planetary system formation. [82], Ward and Brownlee argue that for complex life to evolve (Rare Earth equation factor

[32] Rapid rotation reduces the daily variation in temperature and makes photosynthesis viable. {\displaystyle f_{m}} In its astrobiology roadmap, NASA has defined the principal habitability criteria as "extended regions of liquid water,[1] conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism". N Carbon has an unparalleled ability to bond with itself and to form a massive array of intricate and varied structures, making it an ideal material for the complex mechanisms that form living cells. [45] A proper atmosphere must reduce diurnal temperature variation. [59] This position remains controversial. [citation needed], A terrestrial planet of the right size is needed to retain an atmosphere, like Earth and Venus.

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