What is Planet: Definition and 819 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.

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  1. G

    I Range of projectile launched from a non-rotating spherical planet

    I figured this would be a problem in some classical mechanics book but so far I can't find an answer anywhere. Assume there is no drag or lift, and since the planet is not rotating we don't have to worry about Coriolis effects. I'm working on a solution but I want to see if my work is correct...
  2. yh5598

    A planet with yearlong solar eclipse

    I want to write a story about a planet that has yearlong eclipse that how its natives and ecology adapt to this celestial condition. I am just wondering how realistic/feasible it is for some planet to have a moon in stable orbit producing eclipse for 1-3 years, every 70-100 years.
  3. T

    B What is the difference between a rocky planet's core and the Sun's core

    What is the difference between a rocky planet's core and the core of the sun?
  4. G

    What is the power per square meter of a sun on a planet?

    From the previous question I got the star is giving off around 4.01*10^27 W. I am unsure how to find the answer it asks for. Please help.
  5. xpell

    B At what distance would a Venus-like planet have a HZ temperature?

    Well... that: Please, could you please assist me in obtaining a rough estimate of the distance from the Sun at which a planet resembling Venus or a hypothetical Venus-like planet (for the sake of simplicity) would have a habitable-zone surface temperature? A "back-of-the-envelope" educated...
  6. A

    A planet of mass M and an object of mass m

    HI! I tried to solve this exercise, by assuming that it is an inelastic collision, the planet is spherical, and that the rotation axis is parallel to the z-axis, see the figure attached. (1) before the collision, (2) after the collision.I started by assuming angular momentum conservation, which...
  7. Parziter

    Solar System Forces -- Simulating the planetary orbits for my project

    TL;DR Summary: Solar sytem forces on Unity Hello ! For my last year in my school, I've got a project to do, and I wanted to recreate the Solar system with forces on Unity. My forces are Velocity and Acceleration (I'm using the Frenet's formulas). I'm sorry I'm not a physicist and that's why...
  8. H

    Motorized Watercraft on a Hyperdimensional Planet

    Consider a ship with a propeller. A propeller has to be perpendicular to the way you want the ship to move. It has to be perpendicular to the forward direction. In 3D that means that the propeller rotates in the right-up plane. In 4D we have other choices. The ana-right plane is also...
  9. H

    Equator of a Planet with Four Spatial Dimensions is a 4D Torus

    A 4D planet has no axis of rotation. Nothing special about planets, all freely rotating 4D bodies have two perpendicular planes of rigid rotation. (Clifford proved this in the 19th century.) Now there is nothing stopping us from thinking of planes of rotation here in 3D Universe. It's the...
  10. H

    Clocks On a Hyperdimensional Planet

    On a planet with four spatial dimensions and one of time a clock would look like this. The hands are connected with pivots. The height of the jewel corresponds to the height of the Sun in the sky. If it is above white then it is daylight, if above black it is night. Here the sun has height...
  11. S

    Does the gravitational rate of acceleration increase within a planet?

    Example: The radius of the Earth is 6371 km. It has an average density of 5.5 g/cm3. Earth's inner core has the highest density at 12.9 g/cm3 [more than double the average]. Its surface gravity is measured in units of acceleration, which, in the SI system, are meters per second squared. It may...
  12. AotrsCommander

    How do you measure time on a tide locked planet?

    This is both a biomechanical and sociological question. There is a planet. It orbits around a large star whose goldilocks zone is significantly far from the star that the orbit is measured in thousands of Earth-years1. The planet is tide-locked2. The planet is otherwise the same size and...
  13. S

    B Is there a theoretical size limit for a planet?

    Jupiter is huge. TrES-4 is 1.8 times the size. How big can planets actually get? is there a limiting factor? cheers.
  14. J

    B Could a black hole just be a planet?

    So the theory is the center of a "black hole" is nothing more than a planetoid of incredibly dense material, and the black hole is just this planet's atmosphere for lack of a better description. Thoughts?
  15. S Holtom

    B Celestial mechanics: Why is my planet orbiting faster and faster?

    I've tried to make a "naive" implementation of a planet orbiting a gravitational point source, in Processing (basically Java). Gravity is a constant, and adds to the planet's velocity inversely proportional to the square of distance. I start the planet off with a tangential velocity. I get an...
  16. P

    B Can a Planet Made of Glass Generate Free Energy?

    As far as I know, if vacuum antigravitates, it makes the light that travels through it redshift, e.g. in a very large empty box with mirror walls, light will just lose energy. I am interested in the reverse effect, if we had a very large box or planet with mirror walls, that is filled with...
  17. Melbourne Guy

    Hiding an orbital colony 'behind' a planet from a flare star

    So, I think it's possible to place an object in an orbit such that it remains perpetually in the planet's shadow, like in my awesome graphical representation. But is it?
  18. A

    Astronauts landing on the planet Mars

    Summary:: Astronauts landing on Mars When will the Crewed space mission for Mars exploration event taking place in future? Do you feel that in case life exists for survival on Mars planet, the future generations may actually visit and stay there for many years? Do you feel there will be a...
  19. L

    A satellite in orbit in a system with one planet and two moons

    1) Considering the forces on one of the moons, I have: ##\frac{GMm}{(10R)^2}+\frac{Gm^2}{(20R)^2}=m\frac{v^2}{10R}\Leftrightarrow v=\sqrt{\frac{G}{10R}(M+\frac{m}{4})}.## 2) Considering the initial situation in which the satellite is at rest on the surface of the planet...
  20. M

    I How to find Heliocentric latitude data for each planet?

    Does anyone know where to find information (or software or calculator) able to show on which days / time (2021) where Mercury (and other planets) are as low/high as possible on the heliocentric latitude For example: like the animation seen in the link below. The problem with this site is that...
  21. anven

    B Listen to Record from Planet w/ Extreme Gravity

    Hi everyone. I am new here and have a question that has been bothering me for a few days. Maybe you guys can help me out. Suppose a band gives a 60 minute concert on a planet with extremely strong gravity. This concert is recorded on site on a cassette, which has space for 60 minutes. Then a...
  22. A

    I Weightless or Heaviness Descent into Planet spaceship

    I was working on something where I was trying to imagine an astronaut flying into Jupiter. I convince myself that as the spaceship descends, as it slows down, whether or not the astronaut feels heavier or feels more weightless. I believe the answer is that the astronaut will feel heavier and I...
  23. Lren Zvsm

    Sapient Life Indigenous to a Planet Orbiting a Red Dwarf

    I am considering writing a story that features aliens indigenous to a planet orbiting a red dwarf star. Even so, because of its proximity to the red dwarf, the planet's surface receives much more radiation than Earth's surface does. What is more, the planet has no moon to stabilize its...
  24. Zeeshan Ahmad

    I Evidence for existence of a 9th Planet in the solar system

    Could the existence of the 9th planet of the solar system is possible under these arguments of existence of it some where in the out scurt of solar system? The evidence for Planet 9 comes from its gravitational pull on other bodies. If the planet exists, its gravity will affect the orbits of...
  25. E

    Find the gravitational field strength of planet X

    weight/mass = gravitational field strength. my working is -> weight = 150kgx10m/s² = 1500N mass = 150kg gravitational field strength= 10N/kg. is this correct?
  26. S

    Waterworld Possibility: Can Planet Be Covered in Water?

    Hello All Is it possible to have a planet completely covered in water, like in the film Waterworld? Is it the ultimate fate of the Earth for all the land to be eroded into the oceans, and all the ice to melt, giving a planet showing no land? best regards ... Stef <minor edit - mentor>
  27. S

    I Why is Mars not considered a "full-fledged" planet?

  28. J

    Fundamental Forces Problem: Acceleration from Planet

    I know its something to do with my unit conversions because my answer is the right numbers but too small by a factor of 10000. I convert the radius to meters by multiplying it by 1000. I convert the density to kg/m^3 by dividing it by 10. I find the volume using the equation (4/3)*pi*radius^3...
  29. Mark Everglade

    Gravity, heat, flooding, and centrifugal force on a tidal-locked planet

    Hi. I'm a science fiction author whose first novel Hemispheres, published through RockHill Publishing, explored a tidal-locked planet (Gliese 581g) where a group of activists increased its rotation to bring daylight cycles to both hemispheres. Now I'm writing the sequel, and there's ecological...
  30. fresh_42

    Thank you to all the nurses on this planet

    I am not sure whether this thread will get many responses, nor am I sure that any nurse will ever see it. Maybe @Greg Bernhardt can promote it on facebook. Anyway, I strongly feel the urgent need to acknowledge what they all do for us. I had the chance to closely observe the work nurses have to...
  31. mgkii

    I Gravity transition directly at the underside of a "shell planet"

    I'm watching the Stanford University Lecture series: Einsten's General Theory of Relativity presented by Leonard Susskind (who incidentally has to be one of the greatest educators I've ever watched). Whilst deriving the basic divergence equations relating acceleration, mass density, and...
  32. L

    Causing periodic storm events on a fictional planet

    Hi, I’m looking for ideas and guidance (maybe even formulas?) on making an on-planet situation match a hypothetical solar system. It’s for a fantasy role-playing, but the sentient species must necessarily care (and believably calculate) aspects about their solar system. (However, because it’s...
  33. S

    B Why is the momentum of a star equal to the momentum of a planet?

    I was wathcing a video about radial velocity method for seeking exoplanet(video) and on 3:05 author writes that momentum of a star equal momentum of a planet. Why?
  34. R

    B Detecting Gravitational Waves: Earth-Like Planet Possibility

    Is it theoretically possible that one day a gravitational wave detector will be developed, that is sensitive enough to detect gravitation at the order of magnitude that an Earth like planet has?
  35. Bob Walance

    What if everybody on the planet received stimulus checks?

    It seems (to me) that if everybody received the same amount of money then the health of the global economy would not be affected. Any other ideas?
  36. T

    Orbital speed variation as a planet orbits the Sun

    Summary:: At what distance from the Sun will the speed of the planet be equal to the average orbital speed? I'm not sure where to place this question, please move it in the right thread. [Mentor Note -- thread moved from the technical forums, so no Homework Template is shown] At what...
  37. RemotePhysics

    Require help with angular velocity and people flying off the planet

    My solutions (attempts) : a> w=v/r | r=6.35x10^6m | therefore V=7.04x10^-5 m/s b> speed of rotation is faster at the equator than the pole as w=v/r. As w remains constant, as r increases towards the pole V has to decrease. c> F = W - R d> Stuck here. I presume that I have to use the equation...
  38. mollwollfumble

    B Would lightning be possible on brown dwarfs?

    Lightning is well known from planets, and to some extent the bigger the planet, the more powerful the lightning. Would it also be possible on brown dwarfs (Y, T, L, M)? What about red dwarfs (M)? Other stars?
  39. S

    Do We Sense the Rotation on a Rapidly Spinning Planet?

    It's often said that you don't feel Earth rotation because the gravity acts against the centrifugal force. Of course this is true but also your body is turned around once each 24 hours. So I wonder on a planet which is rotating once each 3 seconds and has same g=9,81: Would you feel the rotation?
  40. D

    How the mass of a planet affects the evolution of life on that planet

    Summary:: Specifically, how Earth and life on it would have evolved if our planet was twice as massive but with the same composition of building elements? Could humans or sentient, upright walking creatures appear in the evolutionary scale in a twice as strong gravitational field? How that...
  41. TaurusSteve

    Stargazing Wasp-76b is a planet where it rains iron

    https://www.bbc.co.uk/news/science-environment-51828871 "Wasp-76b, as it's known, orbits so close into its host star, its dayside temperatures exceed 2,400C - hot enough to vaporise metals. The planet's nightside, on the other hand, is 1,000 degrees cooler, allowing those metals to condense and...
  42. Y

    Radioactivity-Warmed "Rogue" Planet

    Main Question or Discussion Point How much radioactive warming would occur in the astronomical body specified below? I want to write a story about a very advanced civilization, not yet of Dyson Sphere capability, able to artificially create large planets up to Saturn's mass, of any...
  43. Buzz Bloom

    I Tidal forces acting on planet NGTS-10b

    I am hoping a PF participant can help me understand why the tidal forces cause the NGTS-10b planet to fall towards it's star. I more-or-less understand why our moon's tidal forces on the Earth cause the Earth's rotation period to decrease, and the conservation of angular momentum of the...
  44. K

    I Sending Signal from Planet Near Gargantua: A Thought Experiment

    The other day my friend asked me a really interesting question regarding the scene from interstellar where they go down to Miller's planet, where every hour on this planet is 7 years of Earth time. He asked me if they were to send a signal to the spaceship where Romilly was, what would happen...
  45. tanaygupta2000

    What is Kepler's Second Law and How Does it Relate to a Planet's Time Period?

    By using the above formula I am getting value of time-period T as 6.757*10^6 hrs. But the answer given is 1.7 - 1.8 hrs.
  46. jim mcnamara

    How will public react if we discover life on another planet?

    There are lots of decent articles about this, examples: https://www.space.com/40435-finding-aliens-humanity-reaction.html https://en.wikipedia.org/wiki/Potential_cultural_impact_of_extraterrestrial_contact - in part, discusses how speculative this subject can be Well, since Jenner first...
  47. Petronius

    Determine the mass of the planet using Newton’s Law of Universal Gravitation

    Hello, and thank you again to anyone who can confirm if I have the right answer or who can give me some suggestions. This question felt like a bit of a surprise because we have not yet covered one where the mass of a planet was missing. Thus, my confidence in my work is low. Part b felt like a...
  48. A

    I Inside The 24/7 Search For Another Habitable Planet Within 100 Ly

    Hi, Forbes just published an article about the Habitable Exoplanet Hunting Project: http://www.forbes.com/sites/jamiecartereurope/2019/11/25/inside-the-247-search-for-another-habitable-planet-within-100-light-years-of-earth/#149fbf103442 I hope you find it interesting! We welcome any type of...
  49. S

    Reverse gravity-assist, using the moons of a planet

    Hello, We hear a lot about gravity-assist, which usually means using a planet like Jupiter to speed a spacecraft on its way to another destination, like Pluto. But what about using a moon of a planet as a reverse gravity-assist? For example, if you wanted to place a spacecraft in orbit...