Planets | Can’t Sleep? Explore Our Solar System

Planets come in an impressive variety of sizes, temperatures, and compositions. This episode explores what makes a planet a planet, how planets form, and the different types of worlds found throughout our solar system and beyond. Along the way, you’ll hear about rocky planets, gas giants, planetary orbits, and the strange conditions that make each world a little different from its neighbors.
It’s steady and consistent, with no whispering and no sudden changes, just enough to give your mind something to follow as you wind down. Happy sleeping!
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Transcript

Welcome to the I Can't Sleep Podcast, Where I help you drift off one fact at a time. I'm your host, Benjamin Boster, And tonight's episode is about planets. A planet is a large, Rounded, Astronomical body that is generally required to be in orbit around a star, Stellar remnant, Or brown dwarf, And is not one itself. The solar system has eight planets by the most restrictive definition of the term. The terrestrial planets Mercury, Venus, Earth, And Mars, And the giant planets Jupiter, Saturn, Uranus, And Neptune. The best available theory of planet formation is the nebular hypothesis, Which posits that an interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk. Planets grow in this disk by the gradual accumulation of material driven by gravity, A process called accretion. The word planet comes from the Greek πλανηταί, Wanderers. In antiquity, This word referred to the sun, Moon, And five points of light visible to the naked eye that moved across the background of the stars, Namely Mercury, Venus, Mars, Jupiter, And Saturn. Planets have historically had religious associations. Multiple cultures identified celestial bodies with gods. And these connections with mythology and folklore persist in the schemes for naming newly discovered solar system bodies. Earth itself was recognized as a planet when heliocentrism supplanted geocentrism during the 16th and 17th centuries. With the development of the telescope, The meaning of planet broadened to include objects only visible with assistance. The moons of the planets beyond Earth, The ice giants Uranus and Neptune, Ceres and other bodies later recognized to be part of the asteroid belt, And Pluto, Later found to be the largest member of the collection of icy bodies, Known as the Kuiper belt. The discovery of other large objects in the Kuiper belt, Particularly Eris, Spurred debate about how exactly to define a planet. In 2006, The International Astronomical Union, IAU, Adopted a definition of a planet in the solar system, Placing the four terrestrial planets and the four giant planets in the planet category. Ceres, Pluto, And Eris are in the category of dwarf planet. Many planetary scientists have nonetheless continued to apply the term planet more broadly, Including dwarf planets, As well as rounded satellites like the Moon. Further advances in astronomy have led to the discovery of about 6, 445 planets outside the solar system, Termed exoplanets. These often show unusual features that the solar system planets do not show, Such as hot Jupiters, Giant planets that orbit close to their planet star, Like 51 Pegasi b, And extremely eccentric orbits, Such as HD 20782 b. The discovery of brown dwarfs and planets larger than Jupiter also spurred debate on the definition regarding where exactly to draw the line between a planet and a star. Multiple exoplanets have been found to orbit in the habitable zones of their stars, Where liquid water can potentially exist on a planetary surface. But Earth remains the only planet known to support life. Such a habitable zone is complicated by many factors, Such as the atmospheric composition, If there is an atmosphere, Of the planet. Mars, For example, Has the temperature during the day to form liquid water on its surface, On the equator, If only the pressure were higher. It is not known with certainty how planets are formed. The prevailing theory is that they coalesce during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, Surrounded by a rotating protoplanetary disk. Through accretion, A process of sticky collision, Dust particles in the disk steadily accumulate mass to form ever larger bodies. Local concentrations of mass known as planetesimals form, And these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become increasingly dense until they collapse inward under gravity to form protoplanets. After a planet reaches a mass somewhat larger than Mars' mass, It begins to accumulate in extended atmosphere, Greatly increasing the capture rate of the planetesimals by means of atmospheric drag. Depending on the accretion history of solids and gas, A giant planet, An ice planet, Or terrestrial planet may result. It is thought that the regular satellites of Jupiter, Saturn, And Uranus formed in a similar way. However, Triton was likely captured by Neptune, And Earth's moon and Pluto's Charon might have formed in collisions. When the protostar is grown such that it ignites to form a star, The surviving disk is removed from the inside outward by a photo evaporation, The solar wind, Pointing Robertson drag, And other effects. Thereafter, There still may be many protoplanets orbiting the star or each other. But over time, Many will collide, Either to form a larger, Combined protoplanet, Or release material for other protoplanets to absorb. Those objects that have become massive enough will capture most matter in their orbital neighborhoods to become planets. Protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture or remain in belts of other objects to become either dwarf planets or small bodies. The energetic impacts of the smaller planetesimals, As well as radioactive decay, Will heat up the growing planet, Causing it to at least partially melt. The interior of the planet begins to differentiate by density, With higher density materials sinking toward the core. Smaller terrestrial planets lose most of their atmospheres because of this accretion, But the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets. Smaller planets will lose any atmosphere they gain through various escape mechanisms. With the discovery and observation of planetary systems around stars other than the Sun, It is becoming possible to elaborate, Revise, Or even replace this account. The level of metallicity, An astronomical term describing the abundance of chemical elements with an atomic number greater than two, Appears to determine the likelihood that a star will have planets. Hence, A metal-rich population 1 star is more likely to have a substantial planetary system than a metal-poor population 2 star. According to the IAU definition, There are eight planets in the solar system, Which are an increasing distance from the Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, And Neptune. Jupiter is the largest at 318 Earth masses, Whereas Mercury is the smallest at 0. 055 Earth masses. The planets of the solar system can be divided into categories based on their composition. Terrestrials are similar to Earth, With bodies largely composed of rock and metal. Mercury, Venus, Earth, And Mars. Earth is the largest terrestrial planet. Giant planets are significantly more massive than the terrestrials, Jupiter, Saturn, Uranus, And Neptune. They differ from the terrestrial planets in composition. The gas giants, Jupiter and Saturn, Are primarily composed of hydrogen and helium, And are the most massive planets in the solar system. Saturn is one-third as massive as Jupiter, At 95 Earth masses. The ice giants Uranus and Neptune are primarily composed of low boiling point materials, Such as water, Methane, And ammonia, With thick atmospheres of hydrogen and helium. They have a significantly lower mass than the gas giants, Only 14 and 17 Earth masses. The Moon, Io, And Europa have compositions similar to the terrestrial planets. The others are made of ice and rock like the dwarf planets, With Tethys being made of almost pure ice. Europa is often considered an icy planet though, Because its surface ice layer makes it difficult to study its interior. Ganymede and Titan are larger than Mercury by radius, And Callisto almost equals it. But all three are much less massive. Mimas is the smallest object generally agreed to be a geophysical planet, At about 6 millionths of Earth's mass, Though there are many larger bodies that may not be geophysical planets. An exoplanet is a planet outside the solar system. As of October 8, 2026, There are 6, 534 confirmed exoplanets and 4, 908 planetary systems, With 1, 080 systems having more than one planet. Known exoplanets range in size from gas giants, About twice as large as Jupiter, Down to just over the size of the Moon. Analysis of gravitational microlensing data suggests a minimum average of 1. 6 bound planets for every star in the Milky Way. In early 1992, Radio astronomers Alexander Fulshan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257 plus 12. This discovery was confirmed and is generally considered to be the first definitive detection of exoplanets. Researchers suspect they formed from a disk remnant left over from the supernova that produced the pulsar. The first confirmed discovery of an exoplanet orbiting an ordinary main-sequence star occurred on October 6, 1995, When Michael Mayer and Didier Quellos of the University of Geneva announced the detection of 51 Pegasi b, An exoplanet around 51 Pegasi. The catalogue of Kepler candidate planets consists mostly of planets the size of Neptune and smaller, Down to smaller than Mercury. In 2011, The Kepler Space Telescope team reported the discovery of the first Earth-size exoplanets orbiting a Sun-like star, Kepler-20e and Kepler-20f. Since that time, More than 100 planets have been identified that are approximately the same size as Earth. 20 of which orbit in the habitable zone of their star, The range of orbits where a terrestrial planet could sustain liquid water on its surface, Given enough atmospheric pressure. There are types of planets that do not exist in the solar system, Super-Earths and mini-Neptunes, Which have masses between that of Earth and Neptune. Planets more massive than Jupiter are also known, Extending seamlessly into the realm of brown dwarfs. Exoplanets have been found that are much closer to their parent star than any planet in the solar system is to the Sun. There are hot Jupiters, Such as 51 Pegasi b, That orbit very close to their star and may evaporate to become Sonia planets, Which are the leftover cores. There are also exoplanets that are much farther from their star. Although each planet has unique physical characteristics, A number of broad commonalities do exist among them. In the solar system, All planets orbit the Sun in the same direction as the Sun rotates, Counterclockwise as seen from above the Sun's north pole. The period of one revolution of a planet's orbit is known as its sidereal period, Or year. A planet's year depends on its distance from its star. The farther a planet is from its star, The longer the distance it must travel, And the slower its speed, Since it is less affected by its star's gravity. No planet's orbit is perfectly circular, And hence the distance of each from the host star varies over the course of its year. As a planet approaches periastron, Its speed increases as it trades gravitational potential energy for kinetic energy, Just as a falling object on Earth accelerates as it falls. As the planet nears apastron, Its speed decreases, Just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory. The eccentricity of an orbit describes the elongation of a planet's elliptical orbit. Planets with low eccentricities have more circular orbits, Whereas planets with high eccentricities have more elliptical orbits. The semi-major axis gives the size of the orbit. The inclination of a planet tells how far above or below an established reference plane its orbit is tilted. Planets have varying degrees of axial tilt. They spin at an angle to the plane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year. When the Northern Hemisphere points away from its star, The Southern Hemisphere points toward it, And vice versa. Each planet therefore has seasons, Resulting in change to the climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice. Each planet has two in the course of its orbit. When one hemisphere has its summer solstice with its day being the longest, The other has its winter solstice when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. Jupiter's axial tilt is very small, So its seasonal variation is minimal. Uranus, On the other hand, Has an axial tilt so extreme it is virtually on its side. Which means that its hemispheres are either continually in sunlight or continually in darkness around the time of its solstices. The planets rotate around invisible axes through their centers. A planet's rotation period is known as a stellar day. The rotation of a planet can be induced by several factors during formation. There is great variation in the length of day between the planets, With Venus taking 243 days to rotate, And the giant planets only a few hours. The defining dynamic characteristic of a planet, According to the IAU definition, Is that it has cleared its neighborhood. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. In effect, It orbits its star in isolation, As opposed to sharing its orbit with a multitude of similar sized objects. Gravity causes planets to be pulled into a roughly spherical shape. So a planet's size can be expressed roughly by an average radius. For example, Earth's radius or Jupiter radius. A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, Leading to a state of hydrostatic equilibrium. This effectively means that all planets are spherical or spheroidal. Every planet began its existence in an entirely fluid state. In early formation, The denser, Heavier materials sank to the center, Leaving the lighter materials near the surface. Each therefore has a differentiated interior, Consisting of a dense planetary core, Surrounded by a mantle that either is or was a fluid. The terrestrial planet's mantles are sealed within hard crusts. But in the giant planets, The mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such as iron and nickel, And mantles of silicates. Jupiter and Saturn are believed to have cores of rock and metal, Surrounded by mantles of metallic hydrogen. Uranus and Neptune, Which are smaller, Have rocky cores surrounded by mantles of water, Ammonia, Methane, And other ices. The fluid action within these planets' cores creates a geodynamo that generates a magnetic field. All of the solar system planets except Mercury have substantial atmospheres because their gravity is strong enough to keep gases close to the surface. Planetary atmospheres are affected by the varying isolation or internal energy, Leading to the formation of dynamic weather systems, Such as hurricanes on Earth, Planet-wide dust storms on Mars, A greater-than-Earth-sized anticyclone on Jupiter called the Great Red Spot, And holes in the atmosphere on Neptune. Weather patterns detected on exoplanets include a hot region on HD 189733 b, Twice the size of the Great Red Spot, As well as clouds on the hot Jupiter Kepler-7 b, The super-Earth Gliese 1214 b, And others. One important characteristic of the planets is their intrinsic magnetic moments, Which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, Magnetized planets have flows of electrically conducted material in their interiors, Which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called the magnetosphere, Which the wind cannot penetrate. Several planets or dwarf planets in the solar system, Such as Neptune and Pluto, Have orbital periods that are in resonance with each other or with smaller bodies. All except Mercury and Venus have natural satellites, Often called moons. Earth has one, Mars has two, And the giant planets have numerous moons in complex planetary-type systems. The four giant planets are orbited by planetary rings of varying size and complexity. The rings are composed primarily of dust or particulate matter, But can host tiny moonlets, Whose gravity shapes and maintains their structure. Although the origins of planetary rings are not precisely known, They are believed to be the result of natural satellites that fell below their parent planet's Roche limits and were torn apart by tidal forces. The dwarf planets Haumea and Quawar also have rings. The idea of planets has evolved over the history of astronomy. From the divine lights of antiquity to the earthly objects of the scientific age. The concept is expanded to include worlds not only in the solar system, But in multitudes of other extrasolar systems. The consensus as to what counts as a planet, As opposed to other objects, Has changed several times. It previously encompassed asteroids, Moons, And dwarf planets like Pluto, And there continues to be some disagreement today. The five classical planets of the solar system being visible to the naked eye have been known since ancient times and have had a significant impact on mythology, Religious cosmology, And ancient astronomy. In ancient times, Astronomers noted how certain lights moved across the sky, As opposed to the fixed stars, Which maintained a constant relative position in the sky. Ancient Greeks called these lights Planetes Asteres, Wandering Stars, Or simply Planetes, Wanderers, From which today's word planet was derived. In ancient Greece, China, Babylon, And indeed all pre-modern civilizations, It was almost universally believed that Earth was the center of the universe, And that all the planets circled Earth. The first civilization known to have a functional theory of the planets were the Babylonians, Who lived in Mesopotamia in the 1st and 2nd millennia BC. By the first century BC, During the Hellenistic period, The Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. After the fall of the Western Roman Empire, Astronomy developed further in India and the medieval Islamic world. The astronomy of the Islamic Golden Age mostly took place in the Middle East, Central Asia, Island Dallas, And North Africa, And later in the Far East and India. These astronomers, Like the polymath Ibn al-Haytham, Generally accepted geocentrism, Although they did dispute Ptolemy's system of epicycles and sought alternatives. With the advent of the scientific revolution and the heliocentric model of Copernicus, Galileo, And Kepler, Use of the term planet changed from something that moved around the sky relative to the fixed stars to a body that orbited the sun, Directly a primary planet or indirectly a secondary or satellite planet. In the 1950s, Gerard Kuiper published papers on the origin of the asteroids. He recognized that asteroids were typically not spherical, As had previously been thought, And that the asteroid families were remnants of collisions. From the 1960s onwards the term minor planet was mostly displaced by the term asteroid, And references to the asteroids as planets in the literature became scarce, Except for the geologically evolved largest three, Ceres and less often Pallas and Vesta. A growing number of astronomers argued for Pluto to be declassified as a planet because many similar objects approaching its size had been found in the same region of the solar system, The Kuiper belt, During the 1990s and early 2000s. The announcement of Eris in 2005, An object 27% more massive than Pluto, Created the impetus for an official definition of a planet, As considering Pluto a planet would logically have demanded that Eris be considered a planet as well. To acknowledge the problem, The International Astronomical Union set about creating a definition of planet, And in August 2006 settled on the definition proposed by Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi. Under this definition, The solar system is considered to have eight planets. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, And Neptune. The IAU definition has not been universally used or accepted. In 1992, Astronomers Alexander Volshan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257-12. The discovery of exoplanets led to another ambiguity in defining a planet, The point at which a planet becomes a star. Many known exoplanets are many times the mass of Jupiter, Approaching that of stellar objects known as brown dwarfs. The minimum mass size required for an extrasolar object to be considered a planet should be the same as that used in our solar system. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are brown dwarfs, No matter how they formed nor where they are located.

Reviews

A spacey talk to drift off too... Nice one Ben

Always love the outer space ones!