Planet Definition and 820 Threads

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. P

    Pressure at the center of a planet

    Homework Statement Find the pressure at the center of a planet Homework Equations dP/dr = -ρg (Hydrostatic Equilibrium) g = GM/r2 (acceleration due to gravity) The Attempt at a Solution dP/dr = -ρg Assume that density is constant. subsitute GM/r2 for g in the pressure...
  2. A

    Can a planet exist with such small radius and high acceleration rate?

    Homework Statement I was trying to go about deciding whether a planet such as the following could exist: It has a radius of only 2m. It has an acceleration rate of 9.8 m/s^2 (gravity) This way, you can walk on it, and at the same time, you could see whether you were "upside down"...
  3. A

    Existence of a small planet (theoretically)

    I was trying to go about deciding whether a planet such as the following could exist: It has a radius of only 2m. It has an acceleration rate of 9.8 m/s^2 (gravity) This way, you can walk on it, and at the same time, you could see whether you were "upside down", although, physically you...
  4. wolram

    How Would We Start Life on Another Planet?

    Assuming we can ever find another habitable planet and can get there, how the heck would we start to colonise it. If there is water, vegetation and there is~the same climate of Earth.
  5. B

    What is the Angular Momentum of a Planet Rotating and Orbiting a Star?

    Homework Statement Consider a planet that rotates one revolution in one day and orbits a star in one year. The planet has mass = 5.6 1024 kg, radius = 6.7 106 m, and is 1.7 1011 km from the star. (a) Determine the angular momentum for the rotating planet about its rotation axis (assume a...
  6. P

    Finding the mass of a planet using gravitational acceleration and radius

    Homework Statement Planet Z is 5000 km in diameter. The free-fall acceleration on Planet Z is 7.00 m/s^2. What is the mass of Planet Z? Homework Equations Well Newton's gravitational law G(M1M2)/R^2 And I cannot think of much else The Attempt at a Solution On one of the...
  7. T

    Exploring the Physics of a Metal Planet

    Alright, I'm not really a physics major or a scientist, but I do have a basic understanding of physics. In fact, I'm actually a writer and right now I'm trying to research the physics of what this thread is titled as, a metal planet. Problem is since I'm not a physics major or scientist, I sort...
  8. A

    Apparent Weight of Planet Problem

    Homework Statement Planet X rotates in the same manner as the earth, around an axis through its north and south poles, and is perfectly spherical. An astronaut who weighs 950.0 N on the Earth weighs 919.0 N at the north pole of Planet X and only 855.0 N at its equator. The distance from the...
  9. P

    Derive angular momentum of planet with elliptical orbit

    Homework Statement A planet of mass m orbiting the Sun(mass=m_s along an elliptical orbit, with aphelion r1 and perihelion r2. Find the angular momentum of the planet relative to the centre of the Sun. Homework Equations Angular momentum, M=m(R\times v)=mvR sin\theta Distance from Sun...
  10. M

    Cannon shot on airless planet with x gravity

    I've been struggling with this problem for over two hours...PLEASE HELP! Homework Statement A huge cannon is assembled on an airless planet. The planet has a radius of 5.0 \times 10^{6}\;{\rm m} and a mass of 2.6 \times 10^{24}\;{\rm kg}. The cannon fires a projectile straight up at 3800...
  11. M

    Green Planet (My project for energy)

    Hi Everyone, My teacher gave me a project to created anything that need not to be build out physically. I gave the tittle green planet. So the idea is that one could create a VACUUM tunnel from the north to south pole (or any 2 points that is a diameter of the earth) and a "vehicle"...
  12. J

    Calculating the Year Length of Planet Y Around Alpha

    Planet X orbits the star Alpha with a 'year' that is 200 Earth days long. Planet Y circles Alpha at nine times the distance of planet X. How long is a year on planet Y? I know that I have to use the kepler's law : T^2 = (4pi^2/GM)r^3 but I'm not too sure how to do solve this... Please...
  13. A

    Radial acceleration around a planet

    Homework Statement Consider a planet of radius 7.31 x 10^6 m with a rotation period of 25.4 hours. Compute the radial acceleration of a point on the surface of the planet at the equator owing to its rotation about its axis. Homework Equations T = [2(pi)r]/v a_r = -[(v^2)/r] The...
  14. S

    Projectile Motion, finding g on a planet

    Homework Statement An astronaut on a strange planet finds that she can jump a maximum horizontal distance of 10.0 m if her initial speed is 2.40 m/s. Homework Equations What is the free-fall acceleration on the planet? (Ignore air resistance.) The Attempt at a Solution So in order to...
  15. S

    Calculating an Astronaut's Weight on Planet X

    Homework Statement An astronaut weighs 672 N on the Earth. What is her weight on planet X, which has a radius Rx = Rearth / 2.00 and a mass Mx = Mearth/9.00? I know that the radius of Planet X is 3189.05 and that the mass of Planet X is 6.638x10^23 Homework Equations so i know that i...
  16. J

    Acceleration of Gravity inside a planet

    Homework Statement Find the acceleration of gravity within the mantle (at some radius R) of a planet that has a core and a mantle, each with different constant density. Rp= radius of planet Rc= radius of core Pm= density of the mantle Pc= density of the core P= density I know what...
  17. B

    Can the mass of a planet or any object affect the growth of a living thing?

    Would there be subtle differences or large differences on how a human or any other living creature grows up from childhood to adulthood into old age if that person were raised on a planet like Earth vs. a place like Jupiter or the moon? In other words , would the transitions from childhood to...
  18. H

    How to solve planet orbital question

    Ok I remeber a question on my physics test. where there's a planet orbiting a star in some other solar system. heres what was given to me. the radius,and the time it took for it to finish 1 revolution. I was asked to find the mass of the sun, that the planet was orbiting around so here's what...
  19. H

    What is the Orbital Period of Mars in Earth Years?

    Mars travels in a nearly circular orbit of radius 2.28 x 10^11 m around the Sun. The mass o Mars is 6.27 x 10^23 kg. The gravitational force of attraction between Mars and the sun has a magnitude of 1.63 x 10^21 N. a) what is the speed of Mars? i got 2.4 x 10^4 m/s b)Determine the period...
  20. C

    Mass and Radius of a Planet given v

    Homework Statement While visiting Planet Physics, you toss a rock straight up at 13m/s and catch it 2.7s later. While you visit the surface, your cruise ship orbits at an altitude equal to the planet's radius every 210min . What is the mass and radius of the planet Physics? Homework...
  21. T

    Uncovering the Mass of an Object on Planet X

    Homework Statement When venturing forth on Planet X, you throw a 5.24kg rock upward at 13.0 m/s and find that it returns to the same level 1.51 s later. What does the rock weigh on Planet X? Homework Equations W=m*a The Attempt at a Solution To find the acceleration i good the...
  22. U

    How many more dwarf planets can be named after deities?

    Hi, A dwarf planet discovered in 2005 has now got a name, http://www.sciencedaily.com/releases/2008/09/080918234427.htm how many more do you think are out there? I am betting a lot. Utwig
  23. U

    Does Geological Evidence Support the Existence of Planet X?

    Hi, I’m new to the forum, having joined after 6 pints on Saturday night while looking for historical evidence of the mythical 10th planet as described by the ancient Sumerians. The purpose of this post is to spark debate, hopefully backed up with facts that can be verified (and I don’t...
  24. B

    Astronomy: Calculating Neptune's Mass from Triton's Orbit

    This question has been stumping me on my Astronomy homework for the past couple of days. I've tried to input the question into any of the other formulae that we've studied, but I keep drawing a blank on it. Homework Statement Given the size of Triton's orbit(a = 355,000,000 meters) and...
  25. F

    Video: How Tiny is Planet Earth?

    Ever wondered about just how small Earth is? Well here is a quick documentary i made showing just how tiny Earth is. Planet Earth Is Tiny - Video Feedback is most welcome. This sort of puts things in perspective. http://www.future-films.org"
  26. J

    Finding distance of a planet whose orbital period is 2x Earth

    A small planet has an orbital period that is twice that of Earth. What is the planet's orbital distance? From Kepler's 3rd law of planetary motion, I can assume that: rp3 / rE3 = Tp2 / TE2 where rp is the orbital radius of the planet and Tp is the orbital period of the planet...
  27. D

    How does the planet earth exist?

    How does the planet Earth exist? can you please give me a lead on where should I start on this topic? what are the aspects why Earth exists? thank you.
  28. L

    Calculate Density of Spherical Planet - 2.58hr Orbit

    Homework Statement A satellite is in orbit very close to the surface of a spherical planet. The orbital period is 2.58 hours. What is the density of the planet? Assume the planet has a uniform density. Homework Equations T^2=(4pi^2/GM)*r^3 Density=m*v The Attempt at a Solution...
  29. R

    Planet Velocity at Aphelion: Find the Answer

    Homework Statement At perihelion a planet in another solary system is 175 x 10 ^ 6km from its Sun and is traveling at 40km/s. At aphelion it 250 x 10 ^ 6 km distant and is traveling at? Homework Equations IiWi = IfWf The Attempt at a Solution What do i do?
  30. R

    How Much Would You Weigh on a Smaller Planet?

    Homework Statement The mass of a hypothetical planet is 1/100 that of Earth and its radius is 1/4 that of Earth. If a person weighs 150 lb on earth, what would he weigh on this planet Homework Equations I think F = G*m1*m2 / r^2 The Attempt at a Solution I believe F = 150lbs (im...
  31. Evo

    Walking ‘damages planet more than going by car’

    Well, I thought I would start walking to the nearby stores instead of using the car to be more "environmentally friendly" and get some excersize at the same time. Turns out that walking is bad for the environment because the extra food consumed to offset the lost calories is worse for the...
  32. C

    Planet gravitation represented as a single point

    Homework Statement planet with mass m and uniform density and radius r, show by integration that the force this body generates on say a satelite is equivalent to a point mass m at the center of the planet Homework Equations i know that gravitational pull is inversley to r*r The...
  33. T

    Planet formation and star formation

    When our star the Sun finally accumulated enough matter for fusion to start what would that have done to the planets around it? Afterward what would have happened to the Earth for it to get from when the sun started fusion and blasted the Earth and now, with water and atmosphere?
  34. T

    Can Planets Form Without Stars?

    has anyone ever done research on the accretion of planets into a system without a star forming? Just a dark system. How likely is this to happen as opposed to a system forming with a star?
  35. J

    Fastest Planet Rotation: Calculating Period using Volume & G

    The fastest rate of rotation of a planet is that for which the gravitational force on matter at the equator just provides the centripetal force necessary for that matter to move with circular motion. Show that the period of rotation in this case is given by T= √3π/Gp where the planet is...
  36. G

    Can Lingerie Contribute to Environmental Sustainability?

    http://www.reuters.com/article/lifestyleMolt/idUST29336420080514 What will they think of next?
  37. S

    What causes Venus to be hotter than Mercury despite being farther from the Sun?

    The dense atmosphere on the planet Venus makes Venus the hottest planet in the solar system, even hotter than Mercury the closest planet to the Sun. suggest an explanation for this anomaly that involves the electromagnetic spectrum. - Electromagnetic waves travel slower in denser mediums, and...
  38. C

    Planet is 2.00 a.u. when closest to sun. eccentricty is .500. find period

    Homework Statement a planet is 2.00 a.u. when it is closest to the sun. its eccentricity is .5000. Find its period in years. Homework Equations The Attempt at a Solution i got this so far... don't know if it is right PD + AD = A -AD - PD = F <SUBTRACTING THESE TWO>...
  39. X

    Density of a planet using orbit of a satellite

    [SOLVED] Density of a planet using orbit of a satellite Homework Statement A satellite is in a circular orbit about a planet of radius R. If the altitude of the satellite is h and its period is T, show that the density of the planet is \rho=\frac{3\pi}{GT^2}(1+\frac{h}{R})^3 The...
  40. wolram

    Secure a Seat on the New Planet: What You Need to Know

    The planet is doomed, how do you make sure you get one of the limited seats to the new planet. A person who knows how to smelt iron may be chosen over one that knows how to split atoms, and a person who can build a wind mill may be chosen over one that can build a computer, but then who knows.
  41. Y

    Long planet and Galileo thought experiment

    A long time ago, Galileo stated the vertical motion of a falling body is not influenced by its horizontal motion. Is that still true in moderns terms, taking relativity into account? For example, if a particle is fired horizontally at 0.8c, will it hit the ground at the same time as a...
  42. E

    Count out the distance from origo to planet z

    Hi, I want to confirm this: a=8, b=5, c=7 Count out the distance from origo to planet z = ax + by + c in three different ways. 1) With the aid of linear algebra and geometry (no derivates!). Normalvector is: (a,b,-1) , the length * (8,5,1) = p ,(p=point) p=(x,y,z) gives 8s=x ...
  43. J

    Why don’t planet hunters find black holes?

    Over 200 planets have been discovered by the radial velocity technique. Why no black holes? For similar periods, the signal amplitude should be much greater. TIA. Jim Graber
  44. N

    Elliptical orbit around a massive spherical planet

    Homework Statement The path of a very small satellite S in an elliptical orbit around a massive spherical planet P is shown in the figure. The distance from 3 to 4 is the same as from 4 to 5, 9 to 10, and 10 to 11. (Select T-True, F-False, G-Greater than, L-Less than, E-Equal to, If the first...
  45. S

    What is the surface gravity on this distant planet?

    Homework Statement You have been visiting a distant planet. Your measurements have determined that the planet's mass is twice that of Earth but the free-fall acceleration at the surface is only one-fourth as large. Radius = 1.8*10^7meters Mass = 1.19484 × 10^25 kilograms Homework...
  46. V

    What is the Scientifically Accepted Age of Planet Earth?

    :rolleyes: I've been wondering about the Earth's age. Some says...1000000000000 or what..? other says..almost 7000 years or 6000+..Well, what do you think?
  47. B

    The spectrum of reflected light from a planet question.

    Is the spectrum of reflected light of a planet the same as the spectrum of the star near it but just less in magnitude? Like when you do B(Lambda;T) for it you just multiply that answer by the albedo?
  48. N

    Earth's Average Speed: Calculating its Orbit around the Sun

    The Earth moves around the sun in a nearly circular orbit of 1.50 x 10m ^ 11 During the three summer months (an elasped time of 7.89 x 10s^6) the Earth moves 1/4 the distance around the sun. What is the average speed of the Earth?
  49. N

    Creating a Planet of Sky and Clouds: Is it Possible?

    Sorry if this is in the wrong part of the forum but I am a budding SciFi authro with many ideas. And I just wanted to check if one of my ideas is possible, and if not, how this effect could be best recreated. I'm sure everyone has seen the Star Wars movies. Well if you remeber Star Wars...
  50. V

    Angular Momentum: Constant Velocity, Elliptical Orbit

    if the angular momentum of planet is constant when its orbiting around the sun then velocity X radius is a constant vr=constant but if we consider the velocity and distance from the sun of the planet in an elipticall orbit to be v and r the centrepital force is provided by gravitational...
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