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46 votes

Why is the Sun's density less than the inner planets?

The sun isn't the same density all the way through. According to MSFC's solar interior page, the core density at the centre of the sun is a whopping 150,000 kg/m$^3$. Surrounding it the radiative ...
Andy's user avatar
  • 2,467
39 votes
Accepted

Why doesn’t the Sun fill the sky on Mercury?

From where we stand on Earth, Mercury is pretty small about 13 arcseconds across at most. The sun, by comparison is about 1800 arcseconds across, so if you are to see Mercury as a disc, you need to ...
James K's user avatar
  • 119k
33 votes

Why doesn’t the Sun fill the sky on Mercury?

[The real answer is in @James K's answer (it's to do with the field of view of your top image being tiny, but the second one is quite wide). This is to translate the situation into one that might be ...
diwhyyyyy's user avatar
  • 431
31 votes

Why is the Sun's density less than the inner planets?

Fusion inside of a star affects the sun's density (which does not happen with a planet). It produces an outward pressure that balances against the attraction of gravity, thereby reducing the density ...
called2voyage's user avatar
  • 6,312
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
  • 33.7k
23 votes

Why is the Sun's density less than the inner planets?

The density of matter depends not only on its composition, but also on temperature and pressure. It's not meaningful to say that substance A is denser than substance B without specifying the ...
Ilmari Karonen's user avatar
22 votes
Accepted

Does anything orbit the Sun faster than Mercury?

The recently discovered asteroids 2019 LF6 and 2020 AV2, each taking 151 days to orbit the Sun, have the shortest periods currently listed in the JPL Small Body Database. Vulcanoids are difficult to ...
Mike G's user avatar
  • 18.6k
22 votes
Accepted

What rotational speed would Mercury need to have to achieve a temperature comfortable for humans

There is no rotation speed which can achieve that - globally. For a local analysis see below. The global equilibrium surface temperature in the absent of any atmospheric greenhouse effect and with an ...
planetmaker's user avatar
19 votes

Why doesn’t the Sun fill the sky on Mercury?

I can make a similar image using Stellarium, where the Sun seems huge compared to Saturn. Yet when on the surface of Saturn, the Sun seems much smaller in the sky than it does from Earth. Or in other ...
usernumber's user avatar
  • 17.5k
16 votes

Does anything orbit the Sun faster than Mercury?

There is a problem with very high velocity dust particles orbiting the Sun at distances much closer than Mercury with masses of micrograms and below. These particles can do damage to spacecraft that ...
uhoh's user avatar
  • 31.3k
16 votes
Accepted

Why is Mercury's orbit so unusual?

Mercury appears to be on the boundary between being stable and unstable in the Solar System (e.g. Lithwick & Wu 2011). Its orbit can be highly perturbed by interactions, principally with Venus and ...
ProfRob's user avatar
  • 149k
15 votes

Why is the Sun's density less than the inner planets?

I'd say the most important answer is because the volume of stars is counted differently than for (inner) planets.For the former, most of the gas surrounding the dense core is counted. The latter don't ...
kaay's user avatar
  • 259
14 votes

Why doesn’t the Sun fill the sky on Mercury?

For intuitive understanding: if Mercury were a lot bigger, but in the same orbit, the view of the sun from the surface of the planet (2nd photo) would be the same, but from our perspective (1st photo),...
JustJoost's user avatar
  • 141
14 votes

What are the "weather" (and other) implications of long "days" and short "years" on Mercury?

Mercury has no atmosphere, so it experiences direct radiative heating and cooling. The path of the sun in the sky is a slow motion lasting many Earth days and is due as much to the orbit of Mercury (...
James K's user avatar
  • 119k
14 votes

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

I directed this question to Mike Brown, and he answered on Twitter. Mike Brown is about as authoritative as possible. The real answer here is to not get too hung up on definitions, which I admit is ...
Schroeder's user avatar
  • 513
11 votes

Are there ever any simultaneous transits of both Mercury and Venus as seen from the Earth?

As others have calculated, there are no predicted double transits. Since Venus transits for about 12 hours each hundred years (roughly), Venus is in transit for about 1/100000 of the time. Thus ...
James K's user avatar
  • 119k
11 votes

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

One subtle aspect of all this is that a "planet" need not be currently in hydrostatic equilibrium". From https://solarsystem.nasa.gov/planets/in-depth/: The IAU therefore resolves that ...
Oscar Lanzi's user avatar
  • 1,113
10 votes
Accepted

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

Will Mercury ever become locked to the Sun?

Mercury is tidally locked; it has a 3:2 resonance with the Sun, where it rotates three times for every two orbits. It is well understood that Mercury is tidally locked, but modern explanations for how ...
Justin T's user avatar
  • 3,404
9 votes
Accepted

Can we find rocks from Venus or Mercury on Earth?

You can think of it in terms of Hohmann transfer orbits, which define the minimum $\Delta v$ that needs to be applied to bring something from one orbital radius to another orbital radius when orbiting ...
ProfRob's user avatar
  • 149k
9 votes

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

No planet is in perfect hydrostatic equilibrium as no planet can be considered an ideal fluid but always has some finite rigidity. So their rotational flattening is always different from that of an ...
Thomas's user avatar
  • 2,883
8 votes
Accepted

Is it possible that Mercury was originally the moon of Venus after a giant impact?

This was originally going to be a comment, but it ran too long, so I'm making it an answer. Some models argue that the scenario of a satellite of Venus escaping like this is unlikely. Alemi & ...
HDE 226868's user avatar
  • 36.3k
8 votes

Could Venus or Mercury have a moon that we haven't detected?

It's unlikely that either Mercury or Venus could have moons to begin with. Both of these planets are pretty close to the Sun — and in general, this prevents moons from finding stable orbits. If a ...
Sir Cumference's user avatar
8 votes

Path of Mercury and general relativity

And we know that many of astronomers know that mercury orbit does not precess ... Mercury's orbit does precess, by a good amount. The greatest amount is explained by Newtonian mechanics. Venus, ...
David Hammen's user avatar
  • 33.7k
8 votes

Does anything orbit the Sun faster than Mercury?

A hypothetical asteroid that orbited the sun inside the orbit of Mercury would be called a vulcanoid. No vulcanoids have so far been discovered, although there is a population of asteroids that cross ...
gandalf61's user avatar
  • 657
8 votes
Accepted

Could Mercury be a some kind of Chthonian?

Surely not a gas giant... Mercury is a rock of 0.05 $\rm m_{\oplus}$, such a mass could never grow into a gas giant (For detail see Piso & Youdin (2014), fig. 6, where you can see that below a ...
AtmosphericPrisonEscape's user avatar
7 votes
Accepted

Can Mercury's core "boil over" if it gets to close to the Sun?

I was reading about how mercurys core makes up most of the volume of the planet. Im guessing this is both because of the small size and the distance to the sun. There are at least two hypotheses ...
Heopps's user avatar
  • 647

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