Hot answers tagged

45

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 zone is around 20,000 - 200 kg/m$^3$ (already less dense than water). Eventually at the edge is the convective zone - the density at the part that we see is much ...


28

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 as long as the star is burning. Once a star the mass of the sun is no longer able to sustain fusion, what is left is a white dwarf which is in fact much denser ...


21

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 conditions under which the comparison is being made. For a simple everyday example, at room temperature (and pressure) water is significantly denser than air. But ...


11

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 have significant enough amounts of it. This is even more pronounced with larger stars. VY Canis Majoris: "With an average density of 0.000005 to 0.000010 kg/...


9

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 there is a (roughly) 1 in 100000 chance that a randomly chosen transit of mercury will coincide with a transit of Venus. Since Transits of Mercury occur every 10 ...


9

EDIT: As it turns out, I'm not the first or even the second person to run calculations like this: https://www.fourmilab.ch/documents/canon_transits/ http://www.solexorb.it/SolexOld/Simtrans.pdf Meeus' work (second link) mentions the 13425 CE event in "Table 1. Simultaneous and near-simultaneous transits of Mercury and Venus, years 1 to 300,000" Within the ...


8

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 moon were too close to the planets, it would fall within the Roche limit and be torn apart by tidal forces. If a moon were too far from the planets, it would fall ...


8

The most-widely accepted hypothesis at the moment is that Mercury was struck by a large impactor that removed a significant fraction of its mantle (I believe this theory was originally proposed by Cameron & Benz in 1987, and the qualitative theory hasn't changed very much). For planets that are close to their parent stars (such as Mercury), the collision ...


8

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 a massive body. This calculation takes into account that the two objects have Keplerian orbits where the objects begins with at least the orbital speed of the ...


7

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 & Stevenson (2006) have explored the possibility of a prior Venusian moon, starting from the assumption that Venus would not have been able to avoid a giant impact. ...


7

Both effects combined. Day being longer than year would just have retrograde motion of the Sun on sky, but no direction change. Variation of the distance alone happens on Earth, and we have no such effect. But the combination of both factors, in the precise amount they have on Mercury, makes this effect happen.


7

The implication of the question is that this extra 1000 miles should be added to Pluto's radius. The answer is no. For all of the solid planets, it's that solid surface (or solid+liquid surface in the case of the Earth) that counts, not the outer reaches of the atmosphere. The surface is a clear-cut, non-arbitrary boundary. The atmosphere? They can extend a ...


6

It's liquid. As detailed here, To figure out whether Mercury's core was liquid or solid, a team of scientists led by Jean-Luc Margot at Cornell University measured small twists in the planet's rotation. They used a new technique that involved bouncing a radio signal sent from a ground telescope in California off the planet and then catching it again in ...


6

Some things we know about Mercury's orbit: Semi-major axis: 0.387 AU, about 57.9 million km Eccentricity: 0.205 We can calculate the semi-minor axis, $b$ from the equation $$e=\sqrt{1-\frac{b^2}{a^2}}\to b=0.379\text{ AU}$$ We can also calculate the distance to the focus from the center of the ellipse, $f$, as $$f=ae=0.078\text{ AU}$$ I used Mathematica's ...


5

I've just rewritten this answer - @MikeG caught a glaring error by pointing out a really basic handy relationship called the Rayleigh criterion. \begin{align} {\theta}_R \approx1.22 \frac{\lambda}{D}. \end{align} It's better to read the (or any) article, but very briefly, the angular resolution is roughly the ratio of the wavelength to the diameter of a ...


5

Mercury's angular diameter on transit day will be 12 arcseconds. A camera obscura using a 12 mm aperture could resolve it; one lens from +0.75 diopter reading glasses, if you can get them, will project a bright 12 mm image of the Sun at a distance of 1.33 m. Note that a larger aperture or a shorter focal length will make the Sun image hotter than direct ...


5

The logic that In-The-Sky.org uses to decide whether events are observable or not doesn't currently work very well for Mercury and Venus. I have some plans to fix this in the long term, but always have far too many projects on the go. As you correctly say, it's blazingly obvious that Mercury is highest in the sky at lunchtime, and the code I wrote to ...


5

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 about the causes of Mercury's composition: Early in the Solar System's history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass ...


5

usrLTK's answer provides a lot of good details and in particular explains why Mercury wouldn't have much of an atmosphere. Let me complicate the picture a little by pointed out that some recent research indicates that magnetic fields may not be the guaranteed, automatic atmosphere-protection devices that conventional wisdom suggests. In particular, Gunell ...


5

Mercury's orbit is highly eccentric: 0.21 according to Wikipedia. Therefore, the actual time between repeating occurrences will vary depending on the year. If you were to perform your calculations for many periods, the average should approach the value given by Stellarium. The theoretical synodic period, using the sidereal period of Earth and Mercury, is ...


4

Yes, Vulcanoids larger than 5.7 km diameter are considered to be ruled out: A Search for Vulcanoids with the STEREO Heliospheric Imager. An object sufficiently large to be called "planet Vulcan" would change Mercury's orbit in a detectable way by gravity. So, there are two lines of evidence, that such a planet doesn't exist. More to read in Wikipedia: ...


4

GJ 581d and GJ 667c are candidates for showing spin-orbit resonance. This paper mentions GJ 3634b and 55 Cnc b as two further candidates. Spin-orbit resonances different from 1:1 are expected to be rather common among rocky planets orbiting close to a star. Direct observational evidence is difficult to obtain; results are based on model calculations.


4

If we look at the planet's cores and I'm going to ignore liquid vs solid and focus on size overall. Mars: Core estimated 1,794 +/- 65 km radius. The planet is 3,390 km radius. About 53% of the planet's radius is its core. Mars also has more sulfur in it's core and more Iron in it's mantle than Earth, suggesting that it probably didn't mix as well as ...


4

Addressing @Allure's comment below @ JohnHoltz's excellent answer, the synodic period is simply a function of the two periods. It will return something like the average value between two successive events where the planets would line up if they orbited in the same plane, but it does not predict the exact times as pointed out in that answer. In addition to ...


3

All the other answers address the density of the sun, but I feel that none of them actually addresses the OP's misconception. OP seems to think denser material should sink, but this is not the case. Thus Pluto is denser than Uranus, but orbits further out. There is nothing strange about this. The reason is that orbital energy is conserved indefinitely ...


3

Yes, after a close encounter with other planets, almost anything can happen, including a split of Mercury into smaller bodies by tidal forces, or capturing as one or more moons of one or more planets, or a sequence of captures by, and escapes from planets, conversion to a ring of all or some of the fragments, collisions with other moons, fragments falling ...


3

In very a general post-Newtonian metric for a two-body system with the first body oblate, where $M\equiv m_1+m_2$ is the total mass, $\mu\equiv m_1m_2/M$ is the reduced mass, and $p\equiv a(1-e^2)$ is the semi-latus rectum of the orbit, the perihelion advance per orbit is $$\small\delta\varpi = 6\pi\frac{GM}{pc^2}\left[\underbrace{\frac{2-\beta+2\gamma}{3}}_{...


3

The MESSENGER probe was able to take many true-color pictures of Mercury. A full list can be found on JPL's Photojournal. It is clear that Mercury is light grey in color. (source: nasa.gov) (source: nasa.gov) (source: nasa.gov) In terms of the actual surface, Mercury is very similar to the Moon. It's surface is speckled with craters, with some ...


3

If I'm reading your question correctly, your suggestion is that Mercury, having a small magnetic field, should have some atmosphere, perhaps more atmosphere than Mars, with no magnetic field, though Mars does have some surface generated magnetic field regions, it just lacks a field generated by its core. Solar stripping due to no magnetic field isn't the ...


2

Assuming circular, coplanar orbits, Trans Venus Insertion (TVI) is 8 km/s from Mercury's surface. That's with horizontal take off and no gravity loss. That's for the minimum energy Hohmann transfer. Exiting the Hohmann transfer and soft landing on Venus would take 11.9 km/s if Venus had no atmosphere. But much, maybe all of that 11.9 km/s could be done by ...


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