What is Planets: Definition and 514 Discussions

A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and – according to the International Astronomical Union but not all planetary scientists – has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Apart from Earth itself, five planets in the Solar System are often visible to the naked eye. These were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union (IAU) officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the current definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as planets under the current definition of planet.
The planets were thought by Ptolemy to orbit Earth in deferent and epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. About the same time, by careful analysis of pre-telescopic observational data collected by Tycho Brahe, Johannes Kepler found the planets' orbits were elliptical rather than circular. As observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology.
Planets in the Solar System are divided into two main types: large low-density giant planets, and smaller rocky terrestrials. There are eight planets in the Solar System according to the IAU definition. In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites, the two exceptions being Mercury and Venus.
Several thousands of planets around other stars ("extrasolar planets" or "exoplanets") have been discovered in the Milky Way. As of 22 June 2021, 4,768 known extrasolar planets in 3,527 planetary systems (including 783 multiple planetary systems), ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter, have been discovered, out of which more than 100 planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth from the Sun, i.e. in the circumstellar habitable zone. On 20 December 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.
Around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone.

View More On Wikipedia.org
  1. J

    I Plotting the orbits of the planets from Ephemeris data

    I would like to plot the position of planets around the Sun for different dates and describe their orbits. I got the Ephemeris data for each planet from the JPL Horizons System, I got: Date__(UT)__HR:MN R.A.___(ICRF)___DEC Ang-diam ObsSub-LON ObsSub-LAT ObsEcLon ObsEcLat however...
  2. D

    I Question about Special Relativity: Mary from planet B flys at 0.8c to Tom on planet A...

    Quite simple question. There are two planets A and B, they are 4 light years apart measured by tom on planet A. Say both A and B remain stationary. Mary flys 0.8c to tom on planet A. When she passes planet B she should see Tom or planet A 2.4 light years away, am I correct? What about tom now...
  3. DiamondTiara

    I What is the Possibility of Habitable Planets Around Brown Dwarfs?

    I have not seen a whole lot on this topic, so I'm just going to ask a few general questions here that have crossed my mind recently. Habitable meaning, of similar temperature to that of the Earth. 1 - What would we expect habitable planets around Brown Dwarfs to be like? 2 - What types of...
  4. M

    B Sunlight reflection on planets

    If the Sunlight lets say would go off, exactly how long would astronomers on Earth still see sunlight reflected on the surface of Mercury, Venus, Mars, the asteroids, Jupiter Saturn, etc.? I mean Sun goes dark at 0:00 from the point of view of Earth-based observers. At that exact moment, if...
  5. Strato Incendus

    Tidally-locked planets flipping around their axes

    Recently, I’ve heard from Anton Petrov on YouTube that some tidally-locked planets around red-dwarf stars (such as TRAPPIST-1) have been suspected to flip around their own axis every once in a while — so that the former day side becomes the night side, and vice versa. This is presumed to happen...
  6. A

    History Were Rahu and Ketu considered true planets in ancient times?

    TL;DR Summary: Were Rahu and Ketu considered true planets in ancient times before 500 AD and is there any evidence for this in the ancient scriptures? Nowadays when people ask what Rahu and Ketu are, it is always said that those names refer to the lunar nodes. Now I thought that that...
  7. C

    A N-Body Simulations of Chaos in the Orbits of Trappist System Planets

    As a retired physics professor with a long experience in complex simulation software for high energy physics experiments (e.g. the LHC) I revisited last July the n-body planetary simulations which I taught in an undergraduate physics course during the Spring 2017. It was then that the...
  8. jaumzaum

    I Kepler orbits for planets of similar masses

    When we use the third Kepler law to calculate the period, distance and velocity of the Earth, we consider that the Sun is fixed. We know this is not true, because the Sun is also attracted by the Earth. I was wondering, how could we use Kepler laws to calculate the period, distance and velocity...
  9. J

    B Gas planets with solid satellites

    I assume that planets and their satellites are created more or less at the same time. Why gaseous planets like Saturn have solid rings and satellites?
  10. N

    B What would the planets look like in real life?

    I am of the total understanding that most astrophotography pictures are not a real representation of what objects actually look like... I guess I mean, it is not an accurate representation of what things would look like if you were actually the same distance closer as the picture's magnification...
  11. C

    How might plant life look different on other theoretical planets?

    Got interested on how plants (and other things such as fungi) could evolve on other planets along with how plant life may flourish depending on the position of the planet to the sun, moon etc. I'm looking for any article talking about this or something similar.
  12. C

    B Which Way Do Planets in Our Galaxy Spin?

    Do all the planets in all the solar systems in our Galaxy spin in the same direction ? Clockwise or anti clockwise depending on the viewers POV.
  13. D

    I Does gravity compress matter in planets?

    Will there be a compression of matter that the planet is made out of due to gravity when we are talking about planets? If so I would like to know what the effect would be in the example given in the summary.
  14. brochesspro

    I The centripetal acceleration of the planets in our solar system

    Relevant formulae:- Angular velocity in uniform circular motion ##=## ##\omega## ##=## ##\frac {2\pi} t##, where ##t## is the time taken to complete one revolution. Centripetal acceleration in uniform circular motion ##=## ##a## ##=## ##\omega^2r##, where ##r## is the radius of the circular...
  15. momoneedsphysicshelp

    How do mass and radius affect gravitational pull on planets?

    Can someone please verify if my reasoning is accurate? I chose E) Planets B and D because they both have the same ratio of mass to radius which is the lowest of all the other planet options. Due to the fact that they have mass and radius evened out the gravitational pull will pull weight down...
  16. J

    Gravity and Energy -- Two masses above two different planets

    So far, I have found g of the foreign planet, Vf of the ball, and realize that GPEiA = KEfA (Am I right here?). Thus, since GPEiA = GPEiE, GPEiE also equals KEfE. I also understand that, since the same impulse is applied to catch the objects, both objects have the same momentum at the moment...
  17. Cerenkov

    B Tidal locking for planets of K type (orange) dwarf stars

    Hello. If its possible I'd like to find out more about tidal locking for planets orbiting K type dwarf stars. Specifically, at what distances from their host stars would exoplanets have to be to avoid becoming tidally locked. I'm specifying K dwarves because, from what I've read (see...
  18. DaveC426913

    B Plotting the Solar System's planets' distances logarithmically

    I'm designing a growth chart for my granddaughter with the planets on it. I wanted them to be proportional* as far as distance goes - with the sun at the floor and Pluto at, say, six feet, but obviously that's impossible since all the inner planets would be in the first inch or two from the...
  19. V

    Orbital Periods of Planets vs Comets

    Could someone please tell me if I'm on the right track with understanding this. The periods of the comet and Uranus is different because of Kepler's second law? Is it because planets orbit the sun in a circular path whereas for comets it's a highly elliptical orbit? Thank you.
  20. M

    What kind of telescope can see planets in other galaxies?

    I was recently looking at a comic reprinting of Retro Sci Fi Tales # 9, and the synopsis on the site spoke about a story of the "Exposition Universelle", where at a fictitious worlds fair in Paris in 1878, they unveil a "grand inter-galactic telescope so powerful that it can view the surfaces of...
  21. R

    B What force keeps the planets orbiting normally?

    Let G be a graph where vertices are heavy planets and edges are forces between the incident vertices. A complete graph K6 of six planets (of different masses) has 15 edges, why don't the planets collapse to the center?
  22. Haorong Wu

    I How to communicate between planets?

    Hello, I am recently studying communication in curved spacetime. For example, if i use a laser in 1000nm and the initial beam waist is 1m. Then the rayleigh range is ##3 \times 10^6m##. The nearest distance between Earth and Mars is about 5500km. So the beam waist at the Mars will be around ##2...
  23. S

    I What is the rate of ejecta deposited between planets?

    I was reading this (which is notable for its non-science content, but I am only referring to this part): https://www.nationalreview.com/2020/11/wokeists-assault-space-exploration Is there a matrix that shows the amount that goes between the planets? (And here, I consider a moon to be a "planet".)
  24. jaumzaum

    Why we jump with the same initial speed on 2 different planets?

    In every single question asking about the maximum height you get from jumping in 2 different planets they consider that the speed you leave the ground is constant. Why is that? Speed would be constant if impulse were constant. Impulse would be constant if both force and time were constants...
  25. J

    I Rotation rates of planets seem odd?

    Ok, I know there are a lot of strange things in our solar system. Can anyone explain why the small planets spin so slowly? and why does Jupiter spin so quickly? It seems like a ball of debris, getting smaller and smaller, would increase its speed like an ice-skater pulling their arms in...
  26. D

    I Remains of collapsed stars as touching binary planets?

    Would it be in any way physically possible for a binary star system to evaporate / explode, leaving behind two spheres of a solid metal such as gold (or an alloy), which were then rigid enough to drift together and maintain their shapes, rather than just collapsing into a larger sphere? And...
  27. Electrical

    How scientists calculated the weight of Mars

    How scientists calculated the weight of Mars to be 6.39 × 10^23 kg?
  28. Epsilon Eridani

    Hypothetical discussion on the effect of magnetic fields on planets

    Ok, so I have long been fascinated with magnetic fields and their mysterious nature. I've been wondering what would happen when the magnetic field of Earth gets bent/disturbed/rippled by some extraneous force. I am aware the extent of effects that magnetic fields have on a planet is great, but I...
  29. J

    Stargazing Planetary orbits -- Why do planets orbit at same “level”?

    Why is it that all of the planets in our solar system (to our knowledge) orbit the sun in such a way that they all go around the sun in roughly similar orbital planes? Why don’t we have planets with orbital planes at significantly different angles?
  30. Uberhulk

    Force Required To Destroy Three Planets

    Astrophysicist Scott Manley determined the energy required to destroy a planet (2.25 x 1032 joules). Is the energy required to destroy 3 planets simply a case of multiplying his result by three? If not, how much energy would it take to destroy 3 planets? This scenario relates to Incredible Hulks...
  31. LCSphysicist

    Doubt about the elliptic orbits of the planets

    I was thinking to myself, if it were ask to me why is the orbit of planets elliptic, how to justify?: The bould phrases i just know to justify with collected dates. IS this a reasonable justification?
  32. S

    I Is it possible for the Moon & a pair of planets to conjunct as smiley face?

    I was reading this: http://www.physics-astronomy.com/2020/04/set-your-alarm-jupiter-venus-and-moon.html It seems that for this to happen the Moon would need to be in-between the pair of planets and the Sun - but the Sun's path defines the Ecliptic, and the bright planets are all very close to...
  33. D

    B How much in the past do we see these planets?

    The HR 8799 star is 129 light-years away from us, in this image we can see thermal emissions coming from these planets. Do these emissions move at the speed of light? If so does it mean that what we are seeing in this image is how the planets looked like in infrared 129 years in the past?
  34. G

    Stargazing Satellite grids around other planets

    We send a lot of rovers to Mars, as interesting as the planet could possibly be. But why are we not putting satellite grids around other planets that could transmit superficial probe data at the very least? That along with whatever other prospects satellite grids could possibly bear us. That...
  35. Anand Sivaram

    I Viewing Planets or stars from a high altitude

    We can not see planets or stars during the day time because of the scattering of the sunlight. But, in space, all planets and stars would be visible. Consider the situation that a rocket is going to space during the day time. At what altitude these stars or planets would become visible? On...
  36. DavidCummings

    I Could S5-HVS1keep its planets?

    "Astronomers have spotted an ultrafast star [S5-HVS1], traveling at a blistering 6 million km/h, that was ejected by the supermassive black hole at the heart at the Milky Way five million years ago." -- https://www.sciencedaily.com/releases/2019/11/191112110238.htm If this star had planets 5...
  37. A

    I Why is there a difference between the composition of planets and stars?

    I've been thinking about this from time to time, I'll try to make this short and understandable. So we go back to the early universe within our current best model of extrapolation called the Big bang theory. In the early universe as it was expanding we have a very dense and energetic matter...
  38. T

    I Projectile angle (straight line) between 2 planets

    I have a planetary system with planets orbitting a central star (circular orbits). I want to shoot a projectile P in a straight line from planet A to B and need to calculate the angle or vector to shoot the projectile P. I know the straight line of the projectile is physically not correct, but...
  39. T

    I Why are planets visible in a telescope in the daylight?

    Why are planets visible in telescope in daylight? As far as I understand both 1) total light flux from the planet 2) and that from the atmosphere receved by scope are proportional to the aperture area. So why then we can see the planets? And I have realted question. I am my city humidity and...
  40. T

    B The Mysterious Shape of Jupiter: Why Gas Planets Stay Spherical

    If Jupiter is made of gas, how can it maintain its spherical shape without being contained in a spherical shaped container?
  41. S

    A Gravity on Planets: Does Mass Affect Free Fall?

    My current understanding is that a person falling toward the surface of a planet with no atmosphere, would feel no force acting upon themselves even though they are accelerating relative to the planet. If the mass of the planet suddenly tripled while the person is in free fall, would the person...
  42. G

    Ideas about centrifugal space colony habitats on high-gravity planets

    Centrifuges have long been viewed as the means to provide artificial gravity in the zero-g of deep space. Space colony structures like the O'Neill cylinder and the Stanford torus. Centrifuges could also work their magic on the surfaces of low-gravity planets. Conical or paraboloidal shaped...
  43. MathematicalPhysicist

    I Planets or stars that are composed entirely out from Dark Matter.

    Is there a possibility that there are stars and planets that are composed entirely of DM? How would we detect such stars or planets? Well, I am not sure about stars, as the definition of a star depends on its fuel that it uses, I guess.
  44. M

    Calculation of the distance between planets on a given date

    I'm working on a novel and the action has moved off Earth and into the asteroid belt. I've been using a few websites and apps - notably AstroGrav, Stellarium, and Atlas InfoGraphics - to figure out where objects are in the solar system at the future date of the novel. But I can't find a way to...
  45. K

    I Showing that planets follow elliptical orbits

    Is it difficult to show that planets follow elliptic orbits around the sun, using Newton's theory? I have seen the equations showing it, but from General Relativity, considering the Newtonian limit. How to arrive at them using only Newton?
  46. L

    Question About the Escape Velocity from Different Mass Planets

    Hi! I have a question about escape velocity. If a planet is bigger and have a greater escape velocity than another planet. Do this effect the density of the bigger planet in any way? Or do we have to know the mass of the bigger planet to know if the density is larger or lower for this planet?
  47. H

    I Do trojan satellites follow migrating planets?

    Some or all of the planets are thought to have migrated long ago under the gravitational influence of Jupiter. Would the trojan matter at their L4 and L5 points have followed during their migration to new orbits? In other words, while the L4 and L5 points are approaching or receding from the...
  48. O

    What is the period of the second planet in this exercise?

    Homework Statement In a distant galaxy, a planet orbits its sun at a distance of m with a period of 108 s. A second planet orbits the same sun at a distance of m. What is the period of the second planet? Select one: a. s b. s c. s d. s e. Homework Equations T^2=constant * r^3 The...
  49. diegzumillo

    Stargazing Possibility of small bodies of liquid etc

    I'm a physicist but my field is at the other side of the spectrum, but my curiosity has no bounds :P Based on observations we know of several configurations 'clumps of matter' can take, like planets, gas giants, comets, or basic rocks etc. But I always wondered about more unusual possibilities...