# Tag Info

41

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 measurement — this detects the small movement of the star towards and away from us as the planet and the star orbit their mutual barycenter. This movement is fastest ...

36

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 rather sparse. Venus's atmosphere appears to contain a small amount of nitrogen when viewed on a percentage basis. "Only" 3.5% of all of the gases in ...

34

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. Gravitational interactions between the planets and the gas in the disk play a very important role in planetary formation and evolution. As planets orbits within the disk, ...

24

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 contain heavier elements. Your question is the wrong way around; it is not that the heavier elements have not sunk into the middle, it is that the vast majority ...

23

"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-orbiting, just an intersecting orbit. We have no idea what the Earth's spin was before the collision, but it is theorized that the Earth rotation had a 5 hour ...

22

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 inevitable have some net non-zero angular momentum. This forces the gas cloud to change shape from being more or less spherical to being disk-shaped. (Why? That's a ...

15

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 "almost" is important here, for if it had been completely — or even non-completely but much more — smooth, then the collapse would not have had the time ...

15

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 radiation from the sun can dissociate complex molecules or even very simple ones; for example, when UV rays split water molecules the result is free hydrogen and ...

14

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, you are not right that randomness simply adds up. The randomness does in fact cancel out more and more when you combine a large amount of asteroids, until the ...

13

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 nitrogen. Even accounting for the Venus surface temperature (~700K vs ~300K on Earth), one still gets more nitrogen mass per volume. Mars: Most of the ...

11

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 it is difficult to see any fundamental reason why there should not be examples orbiting black holes. In fact, since it is probably feasible to form black ...

10

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 outliers a bit above that. They may have significant quantities of hydrogen and helium in their atmospheres, as well as water, in liquid or vapor form. The latter ...

10

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. The collapse process is rapid - less than a million years. There is a minimum mass though, and what you are referring to is something known as the fragmentation ...

10

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 taken from the numerical simulation that was the basis of the paper and the Grand Tack model. Sean Raymond (one of the other authors on the Walsh et al. 2011 ...

10

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 the abundant materials are Fayalite (${\rm Fe_2SiO_4}$), Olivine (${\rm (Fe,Mg)_2SiO_4}$), Fosterite (${\rm Mg_2SiO_4}$) etc. which make up most of the crust. ...

9

Yes, unless you want to get really particular with the "protoplanetary" part. For example, there are stars forming in the circumstellar disks around Wolf-Rayet stars [reference]. If we were picky, we might not call the circumstellar disk around this Wolf-Rayet star a protoplanetary disk (and instead refer to the planet-forming circumstellar disks of the ...

9

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, have lower proportions of ice, and lots of smaller dust particles that tend to scatter light forward rather than back to us. The ring systems of Uranus and ...

8

No, not really. Stars can form in circumstellar disks, that are, in general, disks surrounding forming stars, but not in protoplanetary disks. Protoplanetary disks are, by definition, flat, rotationing disks composed of gas and dust, found around newly born low-mass stars (see the review by Williams & Cieza (2011)). There are two important point in this ...

8

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 expands gas planets. You're correct that as you add more mass the planet of Jupiter mass tends not to grow larger, but if there's enough internal heat, gas ...

7

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 divided the $M \sin i$ by the sine of the measured inclination angle (this is required to avoid using masses that have been estimated using an assumed mass-radius ...

7

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 is that we could detect Jupiter using the Doppler radial velocity technique, if we observed for more than 10 years (at least one orbit is required). If we were ...

7

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 Kepler data and found that planets had mean values of $\Delta = 21.7$, where $\Delta$ is a parameter given for two adjacent planets by \Delta=\frac{a_2-a_1}{R_{...

7

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 detect as you might imagine, since they "shine" only with the heat of their formation, very dimly in the infrared (the Wikipedia page refers to some that orbit other ...

7

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 the study of "figures of equilibrium" starting with Newton's considerations of the oblateness of the Earth and then continuing with Maclaurin and ...

7

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 low frequency electromagnetic radiation such as those used by radar. These observations are consistent with a planet that has stagnant lid tectonics and ...

7

In most planet formation theories, the boundary is around 10 Earth masses - the build up of the core mass before that is relatively slow, but once it crosses that threshold, the planet gains mass quickly via attracting gas from the surrounding nebula via the core’s gravitational pull, a process called “runaway accretion.” As this summary shows, you can ...

7

Prelude It is now generally accepted in the planet formation community that planets form as a side-product of the star formation process in so-called protoplanetary discs. Protoplanetary discs have initial masses of few to tens of percent of their stellar host masses, are relatively cold (T<150K in about 95% or more of their mass, which is outside the ...

6

Thommes et al. (2001) ran simulations and found that, at optimal conditions (namely, a planet of ~ 10 Earth masses), migration can be complete with ~ 100,000 years. Note that this was done before in-depth research was done on the Nice model, which is very similar. However, the mechanisms are different, as are the planet masses. The difference in timescales ...

6

We can identify two distinct realms of planetesimal formation - the inner Solar System and the outer Solar System. The initial group of small bodies1 in the inner section of the protoplanetary disk was rather quickly accreted into planets via various interactions; some bodies grew bigger and thus dominated the disk around them. In the outer parts of the disk,...

Only top voted, non community-wiki answers of a minimum length are eligible