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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36 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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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
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36 votes
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Intuitive explanation of the source of energy that cause Jupiter and Saturn to migrate outward in the Grand Tack Hypothesis?

First let's try to understand why planets migrate inwards. Planets are formed in a protoplanetary disk; a huge disk of gas and dust that accretes on to a newly forming star at the centre. ...
lucas's user avatar
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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
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26 votes

How did the Earth come to be in orbit around the sun?

The picture was so much cleaner 20 to 25 years ago. I'll present that nice clean picture first. Stars form from the gravitational collapse of huge clouds of interstellar gas. Those gas clouds ...
David Hammen's user avatar
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24 votes
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How did the lighter elements end up in the center of the solar system? Solar System Formation

The solar system contains very little of elements heavier than Helium - less than 2% by mass. This is reflected in the chemical abundances measured in the photosphere of the Sun. i.e. The Sun does ...
ProfRob's user avatar
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17 votes

Why do most dwarf planets have mass comparable to moon?

Consider tomatoes.... There are cherry tomatoes, salad tomatoes and beefsteak tomatoes. Why do all salad tomatoes have about the same size (to within a couple of orders of magnitude)? Well if I ...
James K's user avatar
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16 votes
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Which came first: Galaxies <=> Stars <=> Planets?

The structure we see in the Universe has formed from the gravitational collapse of the matter that was once an almost smooth density field of gas ("baryons") and dark matter$^1$. The word &...
pela's user avatar
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16 votes

Why do planets tend to rotate in the same direction although they have formed from tumbling asteroids?

You are right that the tilt of the asteroids are distributed in very random way, and that the rotation of the Solar nebula is a minor contributor to that tilt, and only skews it a little. However, ...
SE - stop firing the good guys's user avatar
15 votes

At what point in history was the idea of planets being spit out by the sun abandoned?

That specific idea doesn't seem familiar to me, and I work in that field. Particularly this idea would only be called 'theory' when making testable predictions. The only thing that comes to mind here ...
AtmosphericPrisonEscape's user avatar
13 votes

Why is there so little nitrogen in the Martian and Venusian atmospheres?

3.5% of all atmosphere in Venus still accounts for more partial pressure of nitrogen than on Earth. Venus has ~90bar pressure at the surface, 3.5% of them are ~3.2 bar nitrogen. Earth has only 0.8 bar ...
fraxinus's user avatar
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12 votes
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At what point in history was the idea of planets being spit out by the sun abandoned?

Between 1965 and 1980 I recall a book from the "Ladybird" series of short non-fiction books for children which included two possibilities for the formation of the planets. One was "...
James K's user avatar
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12 votes

Why is it so difficult to believe that the Sun once had a companion star

No, it would not. Because the companion star would in nearly all cases leave a remnant - a white dwarf, or for heavier stars, a neutron star or black hole. We do not see such a remnant. The reason ...
Anders Sandberg's user avatar
11 votes
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Why don't we have in-between planets?

Super-Earths and Mini-Neptunes are the "in-between" types of exoplanets you're looking for. A sweeping generalization would put most in the range of $\sim1$-$10M_{\oplus}$ (Earth masses), with some ...
HDE 226868's user avatar
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11 votes
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Is there any theoretical or empirical research about planetary systems with a black hole in their center?

Since planets are known to exist around neutron stars (e.g. see Wolcznan & Frail (1992); and see the list of "pulsar planets" - technically, these were the first exoplanets ever discovered), then ...
ProfRob's user avatar
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11 votes
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How is asteroidal rock formed?

Another major theory regarding the early solar system is that there was a relative abundance of short-lived radioactive isotopes at the time the solar system formed. These short-lived isotopes would ...
David Hammen's user avatar
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10 votes
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How large (that is, radius) could a planet be?

Puffy planets tend to be Jupiter or Saturn like, probably lower mass than Jupiter, perhaps lower metalicity but the most important factor is heat. Either close to the sun or recently formed. Heat ...
userLTK's user avatar
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10 votes
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Are rogue planets ever born in isolation?

The minimum mass of a "planet" forming from a gas cloud (definitions of what a planet is are rather slippery, and some would say this is not a planet at all) is not determined by the time available. ...
ProfRob's user avatar
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10 votes
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Are there any simulations of the Grand tack?

Kevin Walsh, lead author of the original Grand Tack paper has a page on his website discussing the Grand Tack model and subsequent work. At the bottom of this page there is a movie of the evolution ...
astrosnapper's user avatar
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10 votes
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Why is Mercury's Density So Low?

The actual density depends on the mineralogy, we don't have a crystalline iron core and silicon crust. You do have a lot of oxygen available, too when you look at the overall elementary abundance. So ...
planetmaker's user avatar
10 votes
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How do rocky objects between 1cm and 1m accrete to form planetesimals?

This is indeed a tricky problem, and the accretion of pebbles to form planetesimals is a big question in planetary science. You are right to say that small particles can stick together through ...
lucas's user avatar
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10 votes

How is asteroidal rock formed?

One major theory for the evolution of the asteroid belt is that planetesimals (early precursors to planets) formed early in the development of the Solar System in Solar orbits around the asteroid belt....
Connor Garcia's user avatar
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9 votes
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Why Only Saturn Has Visible Rings

By "visible to the naked eye", I take it you mean "visible from Earth with a small telescope". Saturn's rings are largely water ice, and so they reflect more sunlight back to us. Jupiter's rings, ...
hartacus's user avatar
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9 votes
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Existence of planets larger than their host star?

The answer to the question depends on the exact definition of planet that is used. A possible example is the L dwarf 2M 0746+20 (2MASS J07464256+2000321) and its planet 2M 0746+20 b. The radius of ...
aventurin's user avatar
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9 votes

At what point in history was the idea of planets being spit out by the sun abandoned?

Spit out? In 1955 my parents gave me an illustrated book on Astronomy, which gave two hypotheses: the Kant-Laplace (nebular) hypothesis, and Sir James Jeans' idea that a passing star pulled some ...
Simon Crase's user avatar
8 votes
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What is the origin of the dust near the sun?

There are two primary dust populations near 1 AU, interplanetary dust (IPD) and interstellar dust (ISD) [Mann, 2010]. I also discussed dust observations in detail at https://physics.stackexchange.com/...
honeste_vivere's user avatar
8 votes
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Why do rocky planets form?

The nebula from which the sun and the solar system formed contained mostly hydrogen and helium, but also small amounts of heavier elements, including "dust". Any dust that ended up in the centre of ...
James K's user avatar
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8 votes
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Can a donut-shape planet or star be formed?

In principle, yes. In practice, no. The question has been studied for a long time. The classic treatment is Dyson's papers on "anchor rings" in 1893 (paper I, paper II) but it goes back to ...
Anders Sandberg's user avatar
8 votes
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How can it be known that Venus does not have plate tectonics?

There's much less data available from Venus. Some data exists. As mentioned in HDE 226868's answer, maps of Venus's surface exist. Like Earth's atmosphere, Venus's atmosphere is transparent to some ...
David Hammen's user avatar
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