Small stars, with their relatively low luminosity outputs, have a habitable zone much closer to them compared with Sun-like stars. Because the climates (and hence, surface temperature) of planets with thin atmospheres are dominated by external energy input from the host star, a star’s habitable zone ( 26, 27) is based on distance from the host star. Surface liquid water, in turn, requires a suitable surface temperature. A habitable planet is generally defined as one that requires surface liquid water, because all life on Earth requires liquid water. The diversity of exoplanets, both observed and theorized, motivates a revised view of exoplanet habitability ( 25) ( Fig. A review of Earth’s atmospheric evolution is in ref. At a later stage, the physical processes operating at the top or bottom of the atmosphere still sculpt the atmosphere, including thermal and nonthermal atmospheric escape of light gases, volcanism, and plate tectonics. For terrestrial planets, the primordial atmosphere may be completely changed by escape of light gases to space, continuous outgassing from an active young interior, and bombardment by asteroids and comets. The amount of gas captured or outgassed is not known and may vary widely. A planet’s atmosphere forms from outgassing during planet formation or is gravitationally captured from the surrounding protoplanetary nebula. It is nonetheless worth summarizing some key factors controlling a planet atmosphere. The atmospheric mass and composition of any specific exoplanet are not predictable ( 23), and in addition, observations are not yet able to measure atmospheric composition or yield estimates of atmospheric mass.
The diversity of exoplanet masses, sizes, and orbits illustrates the stochastic nature of planet formation, and we expect this diversity to extend to exoplanet atmospheres in terms of both atmospheric mass and composition.
16) circumbinary planets ( 17) compact multiple planet systems ( 18), including at least one with five planets orbiting interior to what would be Mercury’s orbit ( 19) and hot rocky worlds that are expected to have surfaces heated by their star to over 2,000 K, which is hot enough to create liquid lava surfaces. Other highlights of exoplanet diversity include a preponderance of sub-Neptune–sized planets ( 14, 15) that are between Earth and Neptune sizes with no solar system counterpart and formation that is not yet understood (e.g., ref. One of the most surprising exoplanet findings is that the most common type of planet is not a Jupiter-sized planet but a planet about two times the size of Earth or smaller ( 3). Instead, astronomers have found that exoplanets and exoplanetary systems are incredibly varied, with planets of nearly all conceivable masses and sizes as well as orbital separations from their host star ( 13). It seems that less than 10–20% of Sun-like stars could host solar system copies. Solar system analogs must be somewhat rare although they are relatively challenging to detect, none are yet known. In the past 20 y of exoplanet discovery, one of the most significant findings is the sheer diversity of exoplanets.
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