Astronomy

Gravity of a gaseous planet without a core

Gravity of a gaseous planet without a core


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Both Jupiter and Saturn have rocky cores. Is there such of a thing as a gaseous planet without a core? And would a planet without a core have gravity?


The gravitational force on a small mass on the outside of a planet is always the Newtonian $$F_{G}=-frac{GM}{r^2},$$ so any planet, and particularly, any mass in the universe produces a gravitational field acting on everything else. So if, for example, the mass is $M=2 imes 10^{27} m kg$ (i.e. one Jovian mass), then the gravity field outside the planet will always be the same (apart from Tides, higher order moments), no matter whether the mass is in Hydrogen or solids.

For the gas giants Jupiter and Saturn in our solar system, the mass in heavy refractories (i.e. everything heavier than Helium) is about $M_{ m ref}approx 15-20 m m_{oplus}$, where $ m m_{oplus}$ is an Earth mass. The rest of $M$ is hydrogen/helium. For Jupiter this is 300 $ m m_{oplus}$, Saturn about $75 m m_{oplus}$.
This is a relatively large number of refractories in those gas giants, compared to solar composition, which is why we think that they have been formed via core-accretion, see Pollack (1996).

However there is another idea of how to form gas giants out there, which is that of gravitational disc instability, see Boss (2002). This idea posits that at very massive protostellar discs, which form planets, can become unstable and fragment into large clumps, which form gas giants directly. Those disc instability giant planets would have solar metallicity, i.e. a Jupiter-mass planet would have a refractory mass of only $M_{ m ref} approx 3 m m_{oplus}$.

Those refractories would presumably sink to the planetary center and form a small core. Exoplanets that were found at large semi-major axis distances (hundreds of AU, compared to the Jovian 5 AU) from their stars, such as YSES 2b, are candidates for such disc instability models, and hence would host such small core. But that is as small of a core as it gets, you cannot have a core much much less massive than this.


Anything with mass has gravity, so yes, such a planet would have gravity.

However, gases tend to disperse in their surrounding environment, so you'd need a very massive gas cloud to collapse into such a planet for gases not to disperse. This raises the question of the pressure at the centre of this planet; it would be high enough to turn the gas at least into a liquid, if not a solid. Another possibility is that the gas at the core would turn into a plasma (such as in the centre of the Sun) because of the heat-a plasma is basically a hot gas stripped of some of its electrons.


How planets like Jupiter form

Young giant planets are born from gas and dust. Researchers of ETH Zürich and the Universities of Zürich and Bern simulated different scenarios relying on the computing power of the Swiss National Supercomputing Centre (CSCS) to find out how they exactly form and evolve. They compared their results with observations and were able to show amongst others a big difference between the postulated formation mechanisms.

Astronomers set up two theories explaining how gaseous giant planets like Jupiter or Saturn could be born. A bottom-up formation mechanism states that first, a solid core is aggregated of roughly ten times the size of the Earth. "Then, this core is massive enough to attract a significant amount of gas and keep it," explains Judit Szulágyi, post-doctoral fellow at the ETH Zürich and member of the Swiss NCCR PlanetS. The second theory is a top-down formation scenario: Here the gaseous disk around the young star is so massive, that due to self-gravity of the gas-dust, spiral arms are forming with clumps inside. Then, these clumps collapse via their own gravity directly into a gaseous planet, similar to how stars form. The first mechanism is called "core-accretion," the second one "disk instability." In both cases, a disk forms around the gas-giants, called the circumplanetary disk, which will serve as a birth-nest for satellites to form.

To find out which mechanism actually takes place in the Universe, Judit Szulágyi and Lucio Mayer, Professor at the University of Zürich, simulated the scenarios on Piz Daint supercomputer at the Swiss National Supercomputing Centre (CSCS) in Lugano. "We pushed our simulations to the limits in terms of the complexity of the physics added to the models," explains Judit Szulágyi: "And we achieved higher resolution than anybody before."

In their studies published in the Monthly Notices of the Royal Astronomical Society, the researchers found a big difference between the two formation mechanisms: In the disk instability scenario, the gas in the planet's vicinity remained very cold, around 50 Kelvins, whereas in the core accretion case the circumplanetary disk was heated to several hundreds of Kelvins. "The disk instability simulations are the first that can resolve the circumplanetary disk around multiple protoplanets, using tens of millions of resolution elements in the computational domain. We exploited Piz Daint to accelerate the calculations using graphics processing units (GPUs)," adds Mayer.

This huge temperature difference is easily observable. "When astronomers look into new forming planetary systems, just measuring the temperatures in the planet's vicinity will be enough to tell which formation mechanism built the given planet," explains Judit Szulágyi. A first comparison of the calculated and observed data seems to favour the core accretion theory. Another difference that was expected didn't show up in the computer simulation. Before, astrophysics thought that the circumplanetary disk significantly differs in mass in the two formation scenarios. "We showed that this is not true," says the PlanetS member.

Gravitational instability simulation: Two snapshots in the early and late stage of the simulation at 780 years and 1942 years. The second snapshot shows only 4 clumps remaining among those initially formed. Credit: Lucio Mayer & T. Quinn, ChaNGa code

Luminous shock front detected

Regarding the size of the new born planet, observations can be misleading as the astrophysicist found in a second study together with Christoph Mordasini, Professor at the University of Bern. In the core accretion model the researchers had a closer look at the disk around planets with masses three to ten times bigger than Jupiter's. The computer simulations showed that gas falling on the disk from the outside heats up and creates a very luminous shock front on the disk's upper layer. This significantly alters the observational appearance of young, forming planets.

"When we see a luminous spot inside a circumplanetary disk, we cannot be sure whether we see the planet luminosity, or also the surrounding disk luminosity," says Judit Szulágyi. This may lead to an overestimation of the planet's mass of up to four times. "So maybe an observed planet has only the same mass as Saturn instead of some Jupiter masses," concludes the scientist.

In their simulations, the astrophysicists mimicked the formation processes by using the basic physical laws such as gravity or the hydrodynamical equations of the gas. Because of the complexity of the physical models, the simulations were very time consuming, even on Europe's fastest supercomputer at CSCS: "On the order of nine months running time on hundreds to several thousands of computing cores" estimates Judit Szulágyi: "This means that on one computing core it would have taken longer than my entire lifetime."

Yet there are still challenges ahead. Simulations of disk instability still do not cover a long timescale. It is possible that after the protoplanet has collapsed to the density of Jupiter its disk will heat up more like in core-accretion. Likewise, the hotter gas found in the core-accretion case would be partially ionized, a favourable environment for effects of magnetic fields, completely neglected so far. Running even more expensive simulations with a richer description of the physics will be the next step.

More information: J. Szulágyi et al. Thermodynamics of Giant Planet Formation: Shocking Hot Surfaces on Circumplanetary Disks, Monthly Notices of the Royal Astronomical Society: Letters (2016). DOI: 10.1093/mnrasl/slw212

J. Szulágyi et al. Circumplanetary disks around young giant planets: a comparison between core-accretion and disk instability, Monthly Notices of the Royal Astronomical Society (2016). DOI: 10.1093/mnras/stw2617


If you knew the average density of "rock", you could calculate the mass of a sphere of a specified radius. Then you could calculate the surface gravitation for that radius. You could then calculate the radius for a 1 g gravity.

If you knew the average density of "rock", you could calculate the mass of a sphere of a specified radius. Then you could calculate the surface gravitation for that radius. You could then calculate the radius for a 1 g gravity.

Attachments

If you knew the average density of "rock", you could calculate the mass of a sphere of a specified radius. Then you could calculate the surface gravitation for that radius. You could then calculate the radius for a 1 g gravity.

The graph I sent you is real empirical data, including solar system planets and exoplanets.

Starting from the lower left and looking only at the solar system planets, you can see Mars - Venus - Earth - Uranus - Neptune - Saturn - Jupiter

thanks, though by "large" I was referring to the size.

could there exist an exoplanet somewhere out there with a composition light enough to be larger than the Earth, solid enough to walk on, yet have a gravity not exceeding 1g?

and if they do exist, how large they could get.

they'd be a plus for human colonization. wide, open expanses of land, without a cripplingly strong gravity.

I'm kinda Malthusian, thinking of the future exponential expansion of the population.

Anyway, reasonings aside, I'm just wondering how large 1g planets can get.

Any planet formed the normal way will almost certainly have an iron core.

I think we are looking more at a moon here. If we take the same process that formed our moon except have that process happen to a super-earth, It's concievable that we could have a moon larger than the earth without an iron core and so could achieve that 1g we are looking for. Without an iron core though I'm not sure how it could generate a magnetic field to protect us though so not sure how habitable it would be. Maybe the super-earth's magnetic core could be large enough to encompass the moon also.


Stars [ edit | edit source ]

Star [ edit | edit source ]

Stars are the common center of all Star Systems that emit larges amounts of Solar Radiation that can be absorbed by Starships to increase their antimatter regeneration. However many different stars have different effects and they may generate a Coronal Mass Ejection if random events are on.

- Yellow stars emit solar flares which increase shield regeneration by 20%.
- Blue stars possess solar storms around the star which causes all ships to suffer 50% loss on their Hull and Shield regeneration rates.
- Red stars have Solar Interference which interrupts weapon cooldown rates by 15% and decreases weapons range by 10%.
- Green stars cause ships to suffer from gamma exposure, reducing shield mitigation by 10%.

Neutron Star [ edit | edit source ]

The Neutron Star is a small Stellar Remnant that contains great mass for its small size. The high gravity impairs some ships abilities and destabilizes phase space around the star.

  • Special Effect: Removes 30% of ships hull and 100% of ships antimatter reserves (Without Destabilization protection). Removes 15% of ships hull and 50% of ships antimatter reserves (With Destabilization Protection).

Pulsar [ edit | edit source ]

A pulsar is the rapidly spinning core of a dead star which unleashes crippling waves of radiation. Ships around the star suffer increased damage and jammed weapon sensors.

  • Special Effects: Ships take 50% more damage when in the gravity well of the pulsar. Ships also have their accuracy reduced by 50%.

Black Hole [ edit | edit source ]

Somehow Black Holes have formed in systems without destroying planets. The intense gravity from the black hole makes phase jumps more difficult and the stress from singularity damages nearby ships.

  • Special Effect: Causes 50 damage every few seconds to nearby ships. Slows phase jump speeds by 50% while traveling through phase space. And slows ships Phase Jump Acceleration by 350%.

Disk Stability Parameters

One commonly used parameter when analyzing protoplanetary disk stability is the Toomre Q-parameter. This is given by , where:

  • is the sound speed in the disk (For astrophysical gases, the sound speed where P and are the pressure and density of the gas, respectively)
  • is the epicyclic frequency (this is roughly equal to the Keplerian frequency in protoplanetary disks. The Keplerian frequency is the frequency at which an object at a given radius from the central star would orbit– for example the Keplerian frequency of our solar system at 1 A.U. is 1 orbit per year, since the Earth is at 1 A.U. from the Sun)
  • G is the gravitational constant
  • is the 2-D surface density of the disk

Toomre showed in his seminal 1964 paper that for an infinitesimally thin disk to fragment, this dimensionless Q parameter must be less than about 1. Real disks are obviously not infinitesimally thin, but there is strong evidence that they can be quite thin (the thickness of the disk being sometimes of order 1/100 the radius of the disk), which gives us confidence in the Toomre analysis.

Another important scale is our old friend, the Jeans mass (or equivalently, the Jeans length), which we have talked about in previous astrobites. This is the scale at which gravity overcomes thermal pressure in a gas cloud. There are other parameters which are often used, such as the critical cooling time , which parametrizes how quickly the disk cools (we would expect that a faster-cooling disk would have a higher chance of losing its thermal support and being vulnerable to gravitational collapse).

It is interesting to note here that many of the parameters and techniques used to study protoplanetary disks are also used to study accretion disks around compact objects, as well as the stability of disk galaxies!


Do all planets have an iron core?

If not, what can the core be made from? Is an iron core essential for life? How do we find the core composition of planets?

All of the Terrestrial planets in our solar system have a nickle/iron core. We don't know what's in the core of the gas giants, Saturn and Jupiter. Likewise we don't have a good enough understanding of the ice giants, Neptune and Uranus, to know what's at their cores either. And this is just scratching the surface of planets, since we are just looking at the planets in our solar system, a system which is probably somewhat unique (the Sun is somewhat atypical in terms of it's metal content for it's age, and the Earth has alot more superheavy elements --like gold and uranium-- than can be explained by supernova alone) indicating that the molecular cloud the solar system was forged in was probably something of an anomaly and very rich in heavy elements.

We just don't know enough about accretion disks and stellar system formation to accurately answer this question with a high degree of certainty. I doubt it though, I doubt iron is all that common in most stellar nurseries, and there are probably plenty of gas giants out there that formed with relatively little iron around, some of the older gas giants out there in the universe are probably wholly Hydrogen, Helium, and Lithium, with no heavier elements in them, since they formed in ancient molecular clouds that were never seeded by supernova at all.

some of the older gas giants out there in the universe are probably wholly Hydrogen, Helium, and Lithium, with no heavier elements in them

The problem is that it's actually pretty tough to form gas giants without a core, at least within our simulations.

The generally accepted view is that most gas giants form through core accretion. Out beyond the snow-line where the temperature is low enough for water to be in solid phase, you can grow protoplanetary cores pretty quickly with the benefit of both dust and ice (as opposed to inside the snow-line, where cores form only from dust and you generally only get terrestrial planets). It's much easier to reach that 5-10 Earth-mass threshold where the core suddenly has enough gravity to start accreting hydrogen gas in a relatively short time period.

There is the alternate gas giant formation hypothesis that's been kicking around for decades now, disc instability. This suggests there are some initial extra-dense clouds of gas in the young protoplanetary disc that through self-gravity start attracting more and more gas, thereby forming a giant planet without a core. The problem here is that in most realistic simulations, this process takes a long time as the gas has to cool to collapse down to planet-sized volumes - and those timescales are generally significantly longer than the time needed before the proto-star ignites and starts blowing all the gas out of the young solar system.

As it is, though, this is still in the realm of computer simulations. Hopefully we'll know a lot more about the process next year when the Juno spacecraft arrives at Jupiter, making tight orbits around the planet to probe the higher-moments of the gravitational field, thereby revealing the size (and perhaps formation details) of the planet's core.


Ancient Jupiter: Gas Giant Is Solar System's Oldest Planet

The gas giant's core had already grown to be 20 times more massive than Earth just 1 million years after the sun formed, a new study suggests.

"Jupiter is the oldest planet of the solar system, and its solid core formed well before the solar nebula gas dissipated, consistent with the core-accretion model for giant planet formation," lead author Thomas Kruijer, of the University of Munster in Germany and Lawrence Livermore National Laboratory in California, said in a statement. [Photos: Jupiter, the Solar System's Largest Planet]

About 4.6 billion years ago, the solar system coalesced from an enormous cloud of gas and dust. The sun formed first, and the planets then accreted from the leftover material spinning around the newborn star in a vast disc.

Theoretical work strongly suggests that Jupiter took shape quite early in the solar system's history, but the planet's precise age had remained a mystery, Kruijer and his colleagues said.

The researchers dated Jupiter's formation and growth by analyzing the ages of certain iron meteorites &mdash shards of the metallic cores of ancient planetary building blocks &mdash that have fallen to Earth. These ages were determined by measuring the abundances of molybdenum and tungsten isotopes. (Isotopes are versions of elements with different numbers of neutrons in their atomic nuclei.)

This work indicated that the meteorites came from two distinct "reservoirs" that were spatially separate for 2 million to 3 million years, beginning about 1 million years after the solar system formed, the researchers said.

"The most plausible mechanism for this efficient separation is the formation of Jupiter, opening a gap in the disk and preventing the exchange of material between the two reservoirs," the researchers wrote in the new study, which was published online today (June 12) in the journal Proceedings of the National Academy of Sciences.

Jupiter's core would have to be about 20 times more massive than Earth to keep the two reservoirs from mixing, Kruijer and his team calculated. So, the results suggest the nascent gas giant was already that big within the first 1 million years of solar system history, the researchers said.

Jupiter's growth rate slowed thereafter, they said. The gas giant didn't reach 50 Earth masses until a minimum of 3 million to 4 million years after the sun's formation, the researchers determined. (Jupiter is currently about 318 times more massive than Earth.)

"Our measurements show that the growth of Jupiter can be dated using the distinct genetic heritage and formation times of meteorites," Kruijer said in the same statement.

The new study could also help explain why the solar system lacks worlds intermediate in mass between Earth and "ice giants" such as Uranus and Neptune. Such "super-Earths" are relatively common in other star systems.

"One important implication of this result is that, because Jupiter acted as a barrier against inward transport of solids across the disk, the inner solar system remained relatively mass deficient, possibly explaining its lack of any 'super-Earths,'" the researchers wrote in the new study.


Why don't gas giants, like Jupiter, condense into solid planets?

Given their immense size and gravity, you would expect that they would be pulled together very quickly. They also have many satellites which have condensed around them, so what keeps them apart?

Astrophysics grad student here.

For the same reason that the sun is hot on the inside.

Gravity has this property that as things come closer together, energy is liberated -- things want to fall together, in a sense. All of this energy has to go somewhere. It goes towards heating up the particles which are falling together.

Stars and planets are in a balance between gravity pulling inward and pressure support pushing outward. When you raise the temperature of some particles without expansion, the pressure goes up. This is thermal pressure. Other pressure support can come from radiation pressure, which is important in sufficiently large stars where the nuclear fusion rate is very high. There is also electron degeneracy pressure which is important in white dwarfs (and some stars).

If you take a class that teaches stellar structure (like Astro 1) you will learn all of this.

If you don't mind, I'm going to use this opportunity to ask a question about astrophysics which I don't quite understand:

So all planets can be described by the Schwarzschild metric (neglecting rotation and all that higher order stuff), and this metric always has a Schwarzschild (S) radius, and I remember hearing something like the planet radius must be at least 12.5% larger than the S-radius for stability--does this mean that all stable planets (the Earth, for example), have an S-radius, and particles inside this S-radius effectively don't interact with particles outside the S-radius? Isn't this backwards from Newtonian gravity where, by symmetry weɽ argue that at the center of the Earth the gravitational pull is zero (since there's equal amounts of mass in all directions)?

EDIT: Shorty after posting this I think I may have realized that I've been assuming the Schwarzschild exterior solution inside of a planet, which I think is a no-no (. ). Then the Schwarzchild interior solution shouldn't have any singularities? Any help would be great.

It is important to note that since Jupiter is a planet and not a star only thermal pressure matters here. More importantly the notion of gas inside Jupiter is not like you think gas in a room it is incredibly dense. The definition of gas is that the particles have little to no interaction between eachother. This means that there are no chemical bonds from one atom to another. The temperatures from the thermal pressure make chemical bonds impossible (basically the atoms are too excited). In order to get a liquid or solid you need those bonds otherwise you have a very dense gas. Note you can say it is a little vague since usually a gas is thought of the particles being far apart but in this case they really aren't so they are bouncing off each other a lot. However, it still doesn't meet the definition of a liquid (or a solid for that matter). In short unless you have chemical bonding going on you have a gas no matter the pressure (this gets an asterix at certain temperatures you can start to form liquids but trust me inside Jupiter this isn't exactly possible).

I would make sure to say its the same reason the Sun first got hot on the inside. Now it's because of a sustained runaway fusion reaction.

In order for a gas to turn into a solid, it doesn't need to just "condense", it needs to cool down quite a bit. Jupiter is mostly made up of hydrogen, which would have to be incredibly cold to even become a liquid. Though wikipedia does say there is a "metallic hydrogen" core.

The more it condenses, the more it will actually heat up, and the molecules will become more active and push each outward more, which is what maintains its size in very basic terms (also I'm far from an expert).

It's not just temperature, but pressure. They're gas on the outside, but they get liquid/solid on the inside. Think of it like looking at our cloud layer.

Random awesome side-note: Jupiter is mostly hydrogen which is an non-metal gas. Because Jupiter is so massive, the hydrogen around the supposed rocky core is compressed and heated so much that the chemical composition breaks down (or something--I'm not a chem person) and it behaves like a metal. It sloshes around and generates a magnetic field GREATER than that of the Sun. This always blows my mind.

What was the effect of those comets slamming into Jupiter and causing explosions bigger than our plant? (in the ➐'s) If the entire planet didn't combust, what kept or is keeping the hydrogen of Jupiter intact when a comet explodes in its atmosphere?

Technically, gas giants have huge solid interiors. It's just that they have huge atmospheres. Another question to ask might be, "why don't the rocky inner planets have such huge envelopes of gas?"

Basically, there's a relationship between the temperature of the outer atmosphere (which determines the velocity of the molecules at the right end of the Maxwell-Boltzmann distribution - where they can escape) and the strength of gravity at the planet's outer atmosphere. If the molecules in the planet's outer atmosphere can easily escape (because gravity isn't strong enough to prevent almost all the molecules from escaping). Light molecules, like hydrogen, escape more easily than heavier molecules (simply because less massive particles occupy higher velocities according to the Maxwell-Boltzmann distribution => http://en.wikipedia.org/wiki/File:MaxwellBoltzmann-en.svg). Gas giants have gravities strong enough to hold in all atoms, including hydrogen, so well that no gas can effectively escape (through all sorts of mechanisms - Jeans escape being the most relevant for hydrogen/helium). Lighter planets, however, have gravities that are so weak that they can't prevent lighter molecules from escaping. That's why on Earth, once hydrogen or helium get into the upper atmosphere, they're effectively gone forever into space.

Keep in mind that hydrogen and helium are by far the most abundant elements in the universe (hydrogen constitutes 90% of the molecules and helium consists almost all of the remaining 10%). There's simply more of them that can form. And this can pretty much explain why the gas giants have so much gas compared to planets like Earth - almost all of it is hydrogen and helium anyways. Meanwhile, planets like Earth might have had atmospheres with more hydrogen/helium in the past, but the gases escaped very quickly (because their gravities were too weak to hold the gases in).

Of course, the outer gas giants might have started out with rocky interiors. But once they captured hydrogen/helium from the solar nebula, the hydrogen/helium stayed in (since they were massive enough to capture them - it's also a positive feedback process since all that captured gas made each of the gas giants even more massive). The inner planets, meanwhile, had too little mass to capture them from the solar nebula.


Mini-Neptunes

A mini-Neptune is an exoplanet, 2 to 10 Earth masses with a density less than 1. Mini-Neptunes are dwarf gas that have a liquid ocean surrounded by a thick atmosphere of hydrogen and helium and a small rocky core. Detection methods exoplanets, are becoming more sophisticated and more precise.
The radial velocity method or the oscillation method is an indirect method for finding exoplanets by observing the Doppler shifts in the spectrum of the star. By measuring these variations, we can calculate the movement described by the star and deduce the presence and characteristics of any planets that accompany it. In our solar system we observe a slight oscillation of the Sun on a 12-year cycle, which corresponds to the cycle of gravity of Jupiter.
The transit method allows telescopes to measurement of radiance to confirm the presence of planets around a star because to each transit of the planet in front of the star darkening takes place. Cyclical variations in brightness reveal the passage of a planet between the Earth and the star.
The astrometric method consists in measuring a star's absolute position in the sky and its movement. When the star describes a regular ellipse in the sky, is that it is influenced by one or more of its planets.
Direct detection of exoplanets is based on a high-resolution imaging and high contrast using adaptive optics.
Detection of gravitational microlensing effect occurs when the gravitational field of a star warps space-time, which deflects the light from a distant star behind. This effect is only visible if the two stars are aligned with respect to the Earth. If the star that acts as a lens has a planet, the field of the planet can have a small but detectable effect.
Since March 6, 2009, Kepler Space Telescope is specialized in the search for extrasolar planets or exoplanets and more specifically exoterres small size, 2 to 20 times the size of Earth. Kepler mission must determine whether there are habitable planets outside our solar system. Kepler will observes more than 100,000 stars in the Milky Way, rather in the regions of Cygnus and Lyra. Kepler observes continuously, two areas of the Milky Way, rich in stars and monitors tens of thousands of stars simultaneously.
Kepler discovered a wide variety of planets that scientists call Jupiter hot, super-Jupiter, planet of helium, super-earth exoterre, subterranean planet, dwarf planet gaseous, planet of transition, gaseous dwarf, planet ocean, planet of metal, iron planet, dwarf of gas, hot Neptune, Neptune cold, giant of ice, carbon planet, silicates planet, metallic planet or mini-Neptune.

A mini-Neptune is a gaseous planet or dwarf of transition. This type of planet is smaller than Uranus (14.5 Earth masses) and Neptune (17.1 Earth masses), about 2 to 10 Earth masses. Scientists believe that these mini-Neptunes have a thick atmosphere of hydrogen and helium, deep layers of ice and rock, liquid water oceans or ammonia or a mixture of both with a small core of matter volatile low density. Theoretical studies of these planets are usually based on the knowledge that one has planets Uranus and Neptune. Without a thick atmosphere, these planets would be kind of planet ocean. These mini-Neptunes do not turn on an orbit close to their stars, if not their thick atmospheres would be blown away by stellar winds.
Properties that differentiate the rocky planets to gaseous planets, are the diameter and the mass. As regards the diameter, the transition is made from two terrestrial diameters, and for the mass this can vary greatly depending on the composition of the planet, it is 2 to 20 Earth masses. Based on the above indicators, several intermediate planets, or mini-Neptunes, were discovered. Currently with the Kepler Space Telescope, 70% of exoplanets discovered by the transit method looks like mini-Neptunes whose size is comprised between that of our planet and that of Neptune. Neptune has a mass equivalent to 17.1 Earth masses and a density of 1638 kg/m3 is density (relative to water) of 1.638. Its atmosphere is composed of 80% of hydrogen, 19% helium, 1% methane.
Example of mini-Neptunes:
Kepler-11f has a mass of 2.3 Earth masses and a density of 0.69, the same as that of Saturn whose mass is 95 Earths. These properties class, this exoplanet in the category of mini-Neptunes or gaseous dwarf which have a liquid ocean surrounded by a thick atmosphere of hydrogen and helium and a small rocky core.
Kepler-11c has a mass of 2.9 Earth masses and a density of 0.66. Its period of revolution around its star (Kepler-11) 191.231 days.
Kepler-11e has a mass of 8 Earth masses and a density of 0.58. Its period of revolution around its star (Kepler-11) 31.9996 days.
Kepler-16b has a mass of 8.45 Earth masses and a density of 0.964. Its period of revolution around its star (Kepler-16) 13.0241 days.
Kepler-87c has a mass of 6.4 Earth masses and a density of 0.15. Its period of revolution around its star (Kepler-87) in 191.231 days.
Kepler-109c has a mass of 2.22 Earth masses and a density of 0.65. Its period of revolution around its star (Kepler-109) 21.2227 days.


Safe Havens for Planetary Formation

A new theory of how planets form finds havens of stability amid violent turbulence in the swirling gas that surrounds a young star. These protected areas are where planets can begin to form without being destroyed. The theory will be published in the February issue of the journal Icarus.

“This is another way to get a planet started. It marries the two main theories of planet formation,” said Richard Durisen, professor of astronomy and chair of that department at Indiana University Bloomington. Durisen is a leader in the use of computers to model planet formation.

Watching his simulations run on a computer monitor, it’s easy to imagine looking down from a vantage point in interstellar space and watching the process actually happen.

A green disk of gas swirls around a central star. Eventually, spiral arms of yellow begin to appear within the disk, indicating regions where the gas is becoming denser. Then a few blobs of red appear, at first just hints but then gradually more stable. These red regions are even denser, showing where masses of gas are accumulating that might later become planets.

The turbulent gases and swirling disks are mathematical constructions using hydrodynamics and computer graphics. The computer monitor displays the results of the scientists’ calculations as colorful animations.

“These are the disks of gas and dust that astronomers see around most young stars, from which planets form,” Durisen explained. “They’re like a giant whirlpool swirling around the star in orbit. Our own solar system formed out of such a disk.”

Scientists now know of more than 130 planets around other stars, and almost all of them are at least as massive as Jupiter. “Gas giant planets are more common than we could have guessed even 10 years ago,” he said. “Nature is pretty good at making these planets.”

The key to understanding how planets are made is a phenomenon called gravitational instabilities, according to Durisen. Scientists have long thought that if gas disks around stars are massive enough and cold enough, these instabilities happen, allowing the disk’s gravity to overwhelm gas pressure and cause parts of the disk to pull together and form dense clumps, which could become planets.

However, a gravitationally unstable disk is a violent environment. Interactions with other disk material and other clumps can throw a potential planet into the central star or tear it apart completely. If planets are to form in an unstable disk, they need a more protected environment, and Durisen thinks he has found one.

As his simulations run, rings of gas form in the disk at an edge of an unstable region and grow more dense. If solid particles accumulating in a ring quickly migrate to the middle of the ring, the core of a planet could form much faster.

The time factor is important. A major challenge that Durisen and other theorists face is a recent discovery by astronomers that giant gas planets such as Jupiter form fairly quickly by astronomical standards. They have to — otherwise the gas they need will be gone.

“Astronomers now know that massive disks of gas around young stars tend to go away over a period of a few million years,” Durisen said. “So that’s the chance to make gas-rich planets. Jupiter and Saturn and the planets that are common around other stars are all gas giants, and those planets have to be made during this few-million-year window when there is still a substantial amount of gas disk around.”

This need for speed causes problems for any theory with a leisurely approach to forming planets, such as the core accretion theory that was the standard model until recently.

“In the core accretion theory, the formation of gas giant planets gets started by a process similar to the way planets such as Earth accumulate,” Durisen explained. “Solid objects hit each other and stick together and grow in size. If a solid object grows to be about 10 times the mass of Earth, and there’s also gas around, it becomes massive enough to grab onto a lot of the gas by gravity. Once that happens, you get rapid growth of a gas giant planet.”

The trouble is, it takes a long time to form a solid core that way — anywhere from about 10 million to 100 million years. The theory may work for Jupiter and Saturn, but not for dozens of planets around other stars. Many of these other planets have several times the mass of Jupiter, and it’s very hard to make such enormous planets by core accretion.

The theory that gravitational instabilities by themselves can form gas giant planets was first proposed more than 50 years ago. It’s recently been revived because of problems with the core accretion theory. The idea that vast masses of gas suddenly collapse by gravity to form a dense object, perhaps in just a few orbits, certainly fits the available time frame, but it has some problems of its own.

According to the gravitational instability theory, spiral arms form in a gas disk and then break up into clumps that are in different orbits. These clumps survive and grow larger until planets form around them. Durisen sees these clumps in his simulations — but they don’t last long.

“The clumps fly around and shear out and re-form and are destroyed over and over again,” he said. “If the gravitational instabilities are strong enough, a spiral arm will break into clumps. The question is, what happens to them?”

Co-authors of the paper are IU doctoral student Kai Cai and two of Durisen’s former students: Annie C. Mejia, postdoctoral fellow in the Department of Astronomy, University of Washington and Megan K. Pickett, associate professor of physics and astronomy, Purdue University Calumet.