“Most stars have planetary systems, probably like our own.” ~ American astronomer Debra Fischer
A planet is a major celestial body, massive enough to be rounded by its gravity, but not so sizable as to catch fire in thermonuclear fusion.
The term planet is ancient, tied to mythology as firmly as to astronomy. Early cultures often considered planets themselves divine, or at least the emissaries of deities.
Even after astronomy supplanted astrology, solar planets were named after mythological beings, as were many moons. Then, with the 1851 discovery of 2 moons orbiting Uranus, English astronomer William Lassell started a tradition of naming Uranus’ satellites for characters in the works of William Shakespeare and Alexander Pope.
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Planets are a byproduct of star formation. They emerge not long after a star gets its start. As a young star gathers sustenance from surrounding clouds of gas and dust, the incoming material forms a flat, spinning disk around the aspiring star.
Planets start as small clumps within a stellar disk, managing through gravitational attraction to put on weight. As a planet pulls in more material, it leaves a wake in its trail, creating a gap in the stellar disk of dust.
Despite planets creating gaps, a star still gets fed. Streamers of gas from the outer portion of a disk are pulled in by the planets, adding to their girth. Alas for the nascent planets, much of the pulled plumes pass the planet that enticed them. In overshooting, the filaments head inward, further feeding the star.
As the universe aged, an unfathomable number of star systems formed. While stars now number 100–300 billion trillion, the number of planets within and without star systems are many times that.
The growing embryos of large planets suck in hydrogen and helium from the disk encircling an infant star. These become gas giants, such as Jupiter in our solar system.
Planets that form too close to a star risk being eaten. As a star grows, it may gobble a gas giant in close orbit.
Smaller planets accumulate debris and form a rocky core. Cosmologists do not know how large a rocky planet may be before its girth lets it become a gas giant. A rocky planet has been found that is 17 times Earth’s mass; far outside the hoary 10-times rule of rocky cores turning into gas Goliaths. This illustrates how little is known of planet formation dynamics.
Solar systems with rocky planets first formed after some unknown threshold was crossed in accumulating heavy elements in natal interstellar clouds. From this horizon arose a vast variety of planetary metallicity: elements exclusive of hydrogen and helium.
“Unlike the gas giants, the occurrence of smaller planets is not strongly dependent on stars with a high content of heavy elements. Planets that are up to 4 times the size of Earth can form around very different stars; also stars that are poorer in heavy elements.” ~ Dutch astrophysicist Lars Buchhave
22–40% of the planets around low-mass stars like the Sun are in a habitable zone. Even those that are not, such as Mercury, may have regions livable to extremophilic microbes. Further, as Europa and Titan evidence, planetary moons may offer a home to organisms. In the face of enormous cosmic diversity, the universe seems to favor life.
The Milky Way is home to 200 billion free-floating planets that do not orbit a star. Many were tossed from a star system: flung for having the wrong trajectory to fit in.
Planets can come together on their own, rather than synthesizing from the leftovers of a star on the make. Small round clouds shear from the dusty pillars of gas sculpted by young stars. These balls of debris are pushed from the center of a nebula by radiative pressure from the hot stars there. So sped on their way, such clouds develop the spin to coalesce into planetary free floaters that never catch fire, nor become attached to a star system. Their hardscrabble upbringing means that freeborn planets invariably are compact, with dense cores.
Brown dwarfs, sometimes called failed stars, are bodies with a mass between that of planets and stars. Akin in outcast status, they are big brothers to the rogue rocks that roam the cosmos without a star to call their own.