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75 votes
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Considering our methods of exploration, how likely is it that there are unfound planets (not dwarf planets) in our solar system?

There are no undiscovered planets between the sun and Neptune. Objects closer (to Sun) than Neptune that are large enough to be considered planets (and not dwarf planets) can't remain 'hidden'. If it'...
Ingolifs's user avatar
  • 4,165
52 votes
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How much of the surface of other planets is lit by the sun?

OK, let start with some assumptions: spherical objects and no atmospheric effects. Here's the relevant geometry with the object on the left representing a planet and the object on the right ...
GrapefruitIsAwesome's user avatar
49 votes
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What makes protoplanetary disks start rotating? (Initial energy needed to rotate)

Two rocks placed in space with no relative motion are going to be attracted by gravity, and hit. 3 rocks, placed in space with no carefully rigged symmetry, will likely miss each other, as the ...
Wayfaring Stranger's user avatar
46 votes
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Why wasn't the planet Uranus recognized by ancient cultures?

The source of light that is Uranus was observed as far back as 128 BC. However it was misidentified as star as late as the 1760's. Then it was observed by Herschel in 1781, who also misidentified it,...
StephenG - Help Ukraine's user avatar
44 votes
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Why do the planets orbit in the same direction?

The same reason (almost) all of them rotate in the same direction: because of the conservation of angular momentum. Before a star and its planets exist, there’s just a cloud of disorganized gas and ...
Sir Cumference's user avatar
44 votes
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Does this smartphone photo show Mars just below the Sun?

That is a camera artefact caused by the bright sunlight reflecting within the lens on your phone. It’s more pronounced than on a large camera because of the small lens size. This is a secondary image ...
Tim's user avatar
  • 577
43 votes

What is the likelyhood that planet X is a glass (not a typo for gas) planet? What would it mean if it were for what we know of astronomy?

Zero chance. Even if the planet was made of glass, and was transparent, it would still radiate heat, as all warm objects do. Thus, we could find it with an infrared telescope. In fact, this is likely ...
Rob's user avatar
  • 2,065
42 votes
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Why are most discovered exoplanets heavier than Earth?

There are a number of methods of detecting exoplanets, but all of them favour detection of larger planets over smaller ones, albeit for slightly different definitions of large: Radial velocity ...
Steve Linton's user avatar
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41 votes
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Which planet, viewed from which other planet would appear the largest?

Here is a table with planets, their minimum/maximum distances from the Sun, their minimum distance (assuming the planet's aphelion and its neighbour's perihelion coincide, which they don't, so the ...
Glorfindel's user avatar
  • 4,800
38 votes

Do celestial objects need to be big to have liquid water on their surfaces?

Liquid water can't exist in a vacuum. If there is no pressure, then the boiling point will drop to the freezing point and so there will either be ice or water vapour. And if the world is "small&...
James K's user avatar
  • 126k
38 votes
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If planets are ellipsoids why don't we have 3 diameters?

It is possible for a rotating body in hydrostatic equilibrium to be a triaxial ellipsoid. This solution was found by Jacobi in the mid 1800s, thus it's known as the Jacobi ellipsoid. An example in the ...
PM 2Ring's user avatar
  • 15k
37 votes
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Why do planets and satellites in the Solar system look so wildly different if they came from more or less the same matter?

This questions can be split in two; for planets and satellites. The diversity of planets reflects in part the diversity in terms of chemical composition of the protoplanetary disk. We know that UV ...
Swike's user avatar
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37 votes
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Kepler space telescope undetected planets

That's right. The inclination of the orbital plane around stars is considered to be random throughout the galaxy, thus the planets we can detect by the transit method is just a tiny fraction of the ...
Swike's user avatar
  • 3,926
36 votes
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Why is there so little nitrogen in the Martian and Venusian atmospheres?

Nitrogen, with a molecular mass of 28 atomic mass units, is too light to have remained in Mars's atmosphere. Carbon dioxide, with a molecular mass of 44 amu, could (and does) exist on Mars, but it is ...
David Hammen's user avatar
  • 34.1k
32 votes
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Why do most dwarf planets have mass comparable to moon?

Two orders of magnitude is a very large range. The Moon has a mass of $7.342 \times 10^{22}$ kg, so your question is, why do most dwarf planets have a mass between $10^{20}$ and $10^{24}$ kg? By ...
gerrit's user avatar
  • 2,233
32 votes
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Why is it impossible to infer the surface temperature of Venus by spectroscopy observation from earth?

The important difference is that the word "surface" has a different meaning depending on whether we're talking about the Sun or Venus. When we say "the surface of the Sun," we ...
Tanner Swett's user avatar
31 votes

Is it at all possible for the sun to revolve around as many barycenters as we have planets in our solar system?

The short answer is no; there is only one barycenter. Yes, you can count the Sun/Jupiter barycenter or the Sun/Saturn barycenter, or whichever barycenter you want, but the net effect of all Solar ...
Pierre Paquette's user avatar
29 votes
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Starting a fire in a cold planet that was full of flammable gas

The outer parts of Neptune are mostly hydrogen and helium. There are small amounts of other gases such as methane, ammonia and water vapour. However, there is no oxygen at all. If you took some of ...
James K's user avatar
  • 126k
29 votes
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Why did the dust between the planets disappear during the birth of the solar system?

Dust happens in two ways. "Primordial dust" just condenses out of the protostellar material in the disc providing it gets cool enough and dense enough. "Second generation" dust is ...
ProfRob's user avatar
  • 156k
28 votes
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Daylight hours on Titan?

Titan orbits Saturn in the same plane as the rings, and the planet's equator, but much further out. Saturn is tilted with respect to the plane of it orbit by 27 degrees This tilt means that usually ...
James K's user avatar
  • 126k
27 votes
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How many undiscovered planets might be in our solar system?

Let's take a look at the 2006 IAU planet definition 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 ...
SE - stop firing the good guys's user avatar
27 votes

Why wasn't the planet Uranus recognized by ancient cultures?

I like ScienceSnake's comment - a combination of faintness and its comparatively small apparent motion. It was observed in 1690 by John Flamsteed on several occasions, but because it moved so little ...
Bumptious Q Bangwhistle's user avatar
25 votes

Why doesn't the IAU definition of "Planet" disqualify Mercury and Venus as planets?

You are citing a paper that has been cited only six times in the peer reviewed scientific literature since it was published in 1984, which was almost 40 years ago. One of those six citations was a ...
David Hammen's user avatar
  • 34.1k
23 votes
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Could it be possible to detect planets from stars that went supernova through the resulting nebula shape?

The escape speed of a big planet like Jupiter is about 60 km/s. The speed of the ejecta from a supernova explosion is something like $10^4$ km/s. i.e. Jupiter would have little discernable ...
ProfRob's user avatar
  • 156k
21 votes
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Could Neptune be viewed with the naked eye from Uranus?

According to https://arxiv.org/pdf/1808.01973.pdf, the magnitude of Neptune follows the relationship (formula 17, page 25): $ V = 5 \log_{10} (rd) - 7.00 + 7.944 \times 10^{-3} α + 9.617 \times 10^{-5}...
Pierre Paquette's user avatar
20 votes
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Can the paper narrowing Solar System's barycentre to within 100m help find Planet Nine?

Short answer Short answer is no, it can't prove or disprove the existence of Planet 9. Even if there is a significant difference between the barycenter of the solar system with and without Planet 9 ...
William Miller's user avatar
19 votes

Was Theia a planet or an asteroid?

The IAU definition of "planet" is at best irrelevant here. The IAU defines a planet as a body that has cleared its neighbourhood. But in the early solar system the 8 bodies that are now ...
James K's user avatar
  • 126k
18 votes

Why is it impossible to infer the surface temperature of Venus by spectroscopy observation from earth?

I think the problem is that spectroscopy at visible and infrared wavelengths simply can't penetrate the thick atmosphere of Venus. Thus the information comes from much higher in the atmosphere and not ...
ProfRob's user avatar
  • 156k
17 votes
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Could the 9th planet be virtual?

The ninth planet can absolutely be a "virtual", in the way you describe it, meaning observed data indicating the gravitational influence by an object is not actually caused by such an object. A ...
SE - stop firing the good guys's user avatar
17 votes
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Orbital velocity of a planet - why is my calculation off by about 10%?

Well done you. I double checked the calculations and couldn't fault what you had done. So I contacted the lead author of the paper about it and here is the response: "After checking the numbers in ...
ProfRob's user avatar
  • 156k

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