planet, broadly, any relatively large natural body that revolves in an orbit around the Sun or around some other star and that is not radiating energy from internal nuclear fusion reactions.
The discovery of large organic molecules in Titan’s atmosphere
In addition to the above description, some scientists impose additional constraints regarding characteristics such as size (e.g. the object should be more than about 1,000 km [600 miles] across, or a little larger than the largest known asteroid, Ceres), shape (it should be large enough to have been squeezed by its own gravity into a sphere i.e. roughly 700 km [435 miles] across, depending on its density), or mass (it must have a mass insufficient for its core to have experienced even temporary nuclear fusion).
As the term is applied to bodies in Earth’s solar system, the International Astronomical Union (IAU), which is charged by the scientific community with classifying astronomical objects, lists eight planets orbiting the Sun; in order of increasing distance, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Titan is Saturn’s largest moon and the second largest moon in the Solar System. MSSL Planetary scientists used measurements made by the Cassini spacecraft, as it flew through Titan’s upper atmosphere, to discover very large electrically charged organic molecules in Titan’s atmosphere. By measuring their mass and considering the chemistry involve to create them we found out that they could be created by complex chemistry in the atmosphere. The energy to drive this process comes from sunlight and from particles in Saturn’s magnetosphere raining down on the atmosphere. These particles slowly rain down onto Titan’s surface, becoming neutral as they do so, and become a type of heavy molecule known as a Tholin. We think these are important for the origins of life and may have been important early in the history of the Earth. So in a sense Titan is like a window into the past history of the Earth.
Planetary geology
The best known research topics of planetary geology deal with the planetary bodies in the near vicinity of the Earth: the Moon, and the two neighbouring planets: Venus and Mars. Of these, the Moon was studied first, using methods developed earlier on the Earth. Planetary geology focuses on the celestial objects that exhibit a solid surface or have significant solid physical states as part of their structure
Cosmochemistry, geochemistry and petrology
One of the main problems when generating hypotheses on the formation and evolution of objects in the Solar System is the lack of samples that can be analysed in the laboratory, where a large suite of tools are available and the full body of knowledge derived from terrestrial geology can be brought to bear. Direct samples from the Moon, asteroids and Mars are present on Earth, removed from their parent bodies and delivered as meteorites. Some of these have suffered contamination from the oxidising effect of Earth’s atmosphere and the infiltration of the biosphere, but those meteorites collected in the last few decades from Antarctica are almost entirely pristine.
Atmospheric science
The atmosphere is an important transitional zone between the solid planetary surface and the higher rarefied ionizing and radiation belts. Not all planets have atmospheres: their existence depends on the mass of the planet, and the planet’s distance from the Sun – too distant and frozen atmospheres occur. Besides the four gas giant planets, almost all of the terrestrial planets (Earth, Venus, and Mars) have significant atmospheres. Two moons have significant atmospheres: Saturn’s moon Titan and Neptune’s moon Triton. A tenuous atmosphere exists around Mercury.
History
The ordered worlds are boundless and differ in size, and that in some there is neither sun nor moon, but that in others, both are greater than with us, and yet with others more in number. And that the intervals between the ordered worlds are unequal, here more and there less, and that some increase, others flourish and others decay, and here they come into being and there they are eclipsed. But that they are destroyed by colliding with one another. And that some ordered worlds are bare of animals and plants and all water.
In more modern times, planetary science began in astronomy, from studies of the unresolved planets. In this sense, the original planetary astronomer would be Galileo, who discovered the four largest moons of Jupiter, the mountains on the Moon, and first observed the rings of Saturn, all objects of intense later study. Galileo’s study of the lunar mountains in 1609 also began the study of extraterrestrial landscapes: his observation “that the Moon certainly does not possess a smooth and polished surface” suggested that it and other worlds might appear “just like the face of the Earth itself”
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