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 ...
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  • 9,893
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 ...
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  • 27.1k
34 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. ...
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  • 1,189
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 ...
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  • 1,903
25 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 ...
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  • 3,536
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 ...
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  • 114k
23 votes
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How did Mars come to have a 24 hour 39 minute day?

"It's believed that the Earth was rotating about once every 5 hours before the theorized collision with a Mars sized coorbiting object referred to as Theia." Almost. Theia did not have to be co-...
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  • 1,060
22 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 ...
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  • 27.1k
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 ...
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  • 87.6k
15 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 &...
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  • 31.9k
14 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, ...
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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 ...
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  • 1,770
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 ...
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  • 114k
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 ...
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  • 27.1k
10 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 ...
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  • 33.7k
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. ...
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  • 114k
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 ...
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  • 7,135
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 ...
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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 ...
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  • 1,189
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....
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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, ...
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  • 327
8 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 ...
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  • 22.9k
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/...
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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 ...
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8 votes
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How do scientists know that Earth used to spin much more rapidly?

The Moon's gravitational force exerted on the Earth's tidal bulge results in a net torque opposite the direction of Earth rotation, pictured very nicely in the below wikipedia graphic. The Earth's ...
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  • 13.6k
7 votes
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Maximum and minimum gas giant & ice giant densities

Here is a plot I generated in 5 minutes at the site exoplanets.org To construct this I took planets discovered by the transit method and which had a $M \sin i$ measured using radial velocities. I ...
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  • 114k
7 votes
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Do most planetary systems have fewer planets than the solar system?

Perhaps the way to answer this is ask - could we detect the planets in our solar system if we were looking at the Sun, using current technology, from distances of many light years? The short answer ...
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  • 114k
7 votes
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How close can planets form to one another?

If you're thinking about how close planets can be, you should probably consider each planet's Hill sphere, the region in which it can retain satellites. Fang & Margot (2013) did an analysis of ...
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  • 33.7k
7 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 ...
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  • 87.6k
7 votes

Are rogue planets ever born in isolation?

Astronomers call the type of object you are describing-- one that condensed from an isolated nebula but was too small to undergo hydrogen fusion-- a sub-brown dwarf. They are quite difficult to ...
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