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The stratosphere is the second-lowest layer of Earth's atmosphere. It lies above the troposphere and is separated from it by the tropopause. This layer extends from the top of the troposphere at roughly 12 km (7.5 mi; 39,000 ft) above Earth's surface to the stratopause at an altitude of about 50 to 55 km (31 to 34 mi; 164,000 to 180,000 ft).
The troposphere is the lowest layer of the atmosphere. This extends from the planetary surface to the bottom of the stratosphere. The troposphere contains 75–80% of the mass of the atmosphere, [9] and is the atmospheric layer wherein the weather occurs; the height of the troposphere varies between 17 km at the equator and 7.0 km at the poles.
For the giant planets, the "radius" is defined as the distance from the center at which the atmosphere reaches 1 bar of atmospheric pressure. [11] Because Sedna and 2002 MS 4 have no known moons, directly determining their mass is impossible without sending a probe (estimated to be from 1.7x10 21 to 6.1×10 21 kg for Sedna [12]).
The layers are to scale. From the Earth's surface to the top of the stratosphere (50km) is just under 1% of Earth's radius. The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that the molecules are essentially collision ...
The smallest known extrasolar planet that is likely a "gas planet" is Kepler-138d, which has the same mass as Earth but is 60% larger and therefore has a density that indicates a thick gas envelope. [16] A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature. [17]
Terrestrial planets have a compact, rocky surfaces, and Venus, Earth, and Mars each also has an atmosphere. Their size, radius, and density are all similar. Terrestrial planets have numerous similarities to dwarf planets (objects like Pluto), which also have a solid surface
A theoretical planet that may form via mass loss from a low-mass white dwarf. Helium planets are predicted to have roughly the same diameter as hydrogen–helium planets of the same mass. Hycean planet: A hypothetical type of habitable planet described as a hot, water-covered planet with a hydrogen-rich atmosphere. TOI-270 d, K2-18b (candidates)
A 5 km (3.1 mi; 16,000 ft) deep sodium layer is located between 80–105 km (50–65 mi; 262,000–344,000 ft). Made of unbound, non-ionized atoms of sodium, the sodium layer radiates weakly to contribute to the airglow. The sodium has an average concentration of 400,000 atoms per cubic centimetre.