# Tag Info

83

In addition to Undo's fine answer, I would like to explain a bit about the motivation behind the definition. When Eris was discovered, it turned out to be really, really similar to Pluto. This posed a bit of a quandary: should Eris be accepted as a new planet? Should it not? If not, then why keep Pluto? Most importantly, this pushed to the foreground the ...

73

Since we're talking about terminology, we need to remember that none of this really matters, outside of clarity when communicating. Still, some people tend to have rather strong opinions on it, thus confusion about how many planets are really in the solar system arises. The people The most trusted source in Astronomy would have to be the people that set ...

15

It probably is a dwarf planet. (It almost certainly is a dwarf planet.) The naming procedures at the IAU are that "Objects that have an absolute magnitude (H) less than +1 [...] are overseen by two naming committees, one for minor planets and one for planets. [...] All other bodies are named by the minor-planet naming committee alone." source—wikipedia ...

13

Unfortunately, the paper is not available on ArXiv (oh, what hardships we must overcome!), but I have found it here. In it, where the "900" figure is mentioned (2nd page, I believe), the authors (Trujillo and Sheppard) say that they ran simulations with the data already found and their additional findings, and found that 900$^{+800}_{-500}$ bodies larger ...

13

Yes, easily. 1955 AU is a long way, but it is only 0.03 of a light year. The sun would still be less than 1/100 of the distance to the next star. The brightness of the sun would be much less: about 4000000 times less bright, but that is about 16 magnitudes. The sun from Earth is -27 magnitude, so the sun would appear as a magnitude -10 star. It would be ...

12

I suggest taking a look to https://en.wikipedia.org/wiki/Clearing_the_neighbourhood . Quoting it, In the end stages of planet formation, a planet (as so defined) will have "cleared the neighbourhood" of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its satellites or those ...

12

Yes, Pluto is still a dwarf planet. According to the IAU website, it still fits the criteria for a dwarf planet, fails to meet the criteria for a planet, and still carries the "dwarf planet" label, whatever its future status may be. I'm sorry I can't provide a longer or more detailed answer, but this is really a yes-or-no question.

12

The correct answer is 8 (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune). Pluto is not longer a planet since 2006 when the IAU adopted a formal definition of planet

10

Planets For a body to be classified as a planet it must have a few physical characteristics: Mass It must have enough mass to have a strong enough gravity to overcome electrostatic forces to bring it to a state of hydrostatic equilibrium. Hydrostatic equilibrium is important because early in a planets life it is nearly entirely fluid, crust and all ...

10

Do I have a point here or am I missing one? You are missing several points. Point 1: "Clearing the neighborhood" does not mean getting rid of every last spec of mass in the vicinity of a planet. The concept instead addresses the ability to somehow "clear" the vast majority of the mass in the neighborhood of a planet. The total mass of all of Jupiter's ...

9

A lot of the naming conventions were originally "because they remind us of things we already called this", or simply "tradition". How we name things has slowly but surely adjusted with time as more objects were found, and a more robust classification system was needed. Imagine it like having bins to sort your toys into. If you have a small number of balls,...

9

That actual IAU Resolution B5 adopted at the IAU General Assembly in 2006 states: (1) A planet is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. The ...

8

It was the first thing I thought too, but I would say it won't count in the statistics. The earlier paper by Batygin & Brown was talking about bodies orbiting further out. 2015 RR245 orbits between 33.7 and 129 AU. The "cluster" of bodies considered in the paper have aphelion distances from 150 AU upwards. So I think it would be a mistake to try and ...

8

In 2006 the Internaional Astronomical Union (IAU) (that's a download link, 85KB) came to the resolution that a planet is: a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood ...

8

The mass of the Trojans (both camps as far as I know) is estimated at around 0.0001 Earth masses (reference) (Note that this number is debated). The density of them vary but the highest found is around 2.5 g/cm^3 (reference) Assuming that the composition is mainly rocky/icy with very little gas, the act of combining all of them should make a rocky/icy object ...

6

As of March 6, 2015, Dawn has entered orbit around Ceres. But it's really so far only "captured by Ceres' gravitational pull". Then, it was still 61,000 km from Ceres. It's slowly spiraling down into an orbit that JPL's Dawn Journal calls "RC3", which will be 13,500 km above Ceres. This orbit will last 15 days per revolution. It will take about 15 ...

6

I answered this same question at physics.SE. I specifically joined this part of the SE network to address this duplicate question at this site. The astronomy community faced two crises with regard to what constitutes a "planet", first in the mid 19th century, and more recently at the start of the 21st century. The first crisis involved the asteroids. The ...

6

The reports that Goblin is a planet are incorrect. It is a dwarf planet, at best. Dwarf planet and planet are mutually exclusive terms. Planets are objects whose masses are so large that they have collided with, ejected, or captured the vast majority of the smaller stuff that share the same orbit as the planet. Moreover, Goblin might well not even be a ...

6

This has to do with how minor planets obtain provisional names (see also Wikipedia). The year is divided into 24 half months, with a particular letter associated with each one. Each of these half-months has a number of cycles of length 25 depending on how many minor planets are discovered; a minor planet is assigned a letter corresponding to its order in the ...

5

Geothermal activity can originate from a combination of: Residual heat from planet formation: During the formation of a planet, half of the potential energy of the parent region of dust and gas can be theoretically converted to kinetic energy---this is known as the Virial Theorem. The kinetic energy of the particles translate to a temperature. Internal ...

5

Dawn was not exploring L3. But instead it explored both Vesta and Ceres. In order to do so in a cost-saving manner, Dawn was also a mission demonstrator for an interplanetary mission equipped with ion propulsion. Because of the low specific impulse of ion propulsion, Dawn cannot fly in a straight line, as this would imply a high initial velocity and the ...

4

Even rocky planets would explode. I think there are two ways to see this. From the perspective of forces, the earth is in equilibrium between the compressive force of gravity and the elastic resistance to compression of the materials that make it up. By Newton's third law, the mantle is pressing upwards on the crust with a force equal to the weight of the ...

3

As the comments already say, an object being a dwarf planet is a matter of convention. If the IAU says it's a dwarf planet, it's a dwarf planet. Otherwise, it's not. The requirements you are listing from Wikipedia are the IAU criteria for pronouncing objects as dwarf planets, but that does not mean that all objects fulfilling these criteria are dwarf planets....

3

Sadly, a well referenced answer to this question likely lies behind academic publisher paywalls. However, Wikipedia claims: Collisions between main-belt bodies with a mean radius of 10 km are expected to occur about once every 10 million years.[63] -Source for citation 63 has disappeared from the internet. However, the Veritas asteroid cluster, and the ...

3

As I noted in a previous answer, the official body for naming and classification of astronomical objects is the International Astronomical Union, or IAU. They are the ones responsible for designating bodies as "planets," "dwarf planets," etc. The scientific community recognizes them and their decisions as "official." The IAU meets periodically for General ...

3

There is actually disagreement on this matter (within the IAU?). Dr. Alan Stern (lead of the New Horizons mission) for instance points out that "this rule is inconsistent" (e.g. see Pluto vote 'hijacked' in revolt). Not only has Neptune not cleared its path, but the same holds for Earth, Mars and even Jupiter. Jupiter has a set of asteroids (the Trojans) ...

3

History, and recent history at that: At one time (1980) things seemed clear enough: There were planets (Mercury-Pluto), Asteroids (mostly between Mars and Jupiter) and Satellites. No one care much about whether a body was spherical or not, since it was pretty clear that this was all there was. . . . Then the Kuiper belt happened, and in particular it ...

3

Celestia can run on MacOS. With is, you can travel in the solar system and in time. It can give you a good idea of how far things are from each other.

2

Well, it depends on how you define a planet. According to IAU (InternationalAstronomical Union), an object should satisfy three conditions for it to be called a planet It should revolve around the sun. It should have enough gravity to form itself into a sphere. It should have cleared space around itself (it should be a dominant object in that area) Pluto ...

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