Astronomy

Are gas giants supported by thermal pressure?

Are gas giants supported by thermal pressure?

I've heard gas giants are supported because there is an equilibrium between thermal pressure and gravity. That is, if Jupiter were to suddenly begin compressing, temperatures would increase to the point that it would expand to its original size. Since it is in equilibrium, both the compression and thermal pressure counter one another, so its size never fluctuates.

However, Jupiter's average temperature is extremely low, so much so that its gases are electron degenerate. Is this a contradiction, or has the information above been just hearsay?


I am not sure what you mean by "thermal" pressure. Jupiter is supported by pressure, just like all objects that are in (approximate) hydrostatic equilibrium.

That pressure is provided by your everyday, temperature-dependent Maxwell-Boltzmann ideal gas pressure in the outer parts, but the free electrons in the interior become degenerate and so in these regions I suppose you would more accurately describe the pressure as being due to (partial) electron degeneracy pressure.

At the very centre there may be a liquid or even a solid core. In the metallic hydrogen model then this would still be degenerate electrons contributing the pressure. For a solid, rocky core, well solids are rather incompressible.

You can only describe Jupiter as being in approximate hydrostatic equlibrium. It is losing energy from its surface; this energy is supplied from gravitational potential energy and Jupiter is shrinking at a rate calculated to be about 2 cm per year. As it does so, the interior gas becomes denser and more and more degenerate and the pressure will become more independent of temperature. As a result, the rate of contraction will slow down and Jupiter will tend towards the radius of a "cold" body of that composition.


Heating Up the Guts of Gas Giants

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites the original can be viewed at astrobites.org.

Title: The Intrinsic Temperature and Radiative-Convective Boundary Depth in the Atmospheres of Hot Jupiters
Authors: Daniel P. Thorngren, Peter Gao, Jonathan J. Fortney
First Author’s Institution: University of California, Santa Cruz
Status: Submitted to ApJL

Artist’s impression of HAT-P-7b, an inflated hot Jupiter. [NASA, ESA, and G. Bacon (STScI)]

Jupiter-sized gas-giant exoplanets in close orbits around their stars, commonly referred to as hot Jupiters, have been the prime targets for probing planetary atmospheres beyond our solar system. One of the many mystifying features of hot Jupiters — which, ironically, also makes them easier to detect and characterize — is their inflated radii. A good fraction of known hot Jupiters have sizes larger than those predicted by evolutionary models that take into account the properties of the system like temperature, age, and metallicity of the system. What could be causing these hot Jupiters to puff up?

A proposed mechanism to explain hot-Jupiter inflation is deposition of energy from stellar irradiation deep into the interiors of the planet. However, in addition to inflating the planet, energy from stellar flux heating up the planetary interiors can also radically alter the thermal structure (temperature variation with altitude) of its atmosphere which has direct consequences on its inferred atmospheric properties. Today’s paper attempts to draw a connection between the stellar irradiation of hot Jupiters and their intrinsic temperature, and how that ultimately affects the observations and our understanding of the atmospheres of these gas giants.

Structuring the Atmosphere of a Gas Giant

The vertical thermal structure — also referred to as the pressure–temperature profile — of a planetary atmosphere is directly related to change in the mode of heat transport (radiation or convection) within the atmosphere at different heights. You can think of this in the context of the Earth’s atmosphere: closer to the surface heat exchange occurs through convection, with hot parcels of air rising up and adiabatically expanding and cooling. This causes the temperature to steadily decrease as you go up until a certain altitude called the tropopause above this you hit the stratosphere, where the air absorbs most of the heat from ultraviolet radiation from the Sun, causing the temperature to now increase with altitude. Even before this happens convection begins to weaken considerably and radiation takes over as the dominant mode of heat exchange. The altitude or the pressure level at which this happens is called the radiative-convective boundary (RCB see Figure 1 for example). Such stratification of atmospheres is very commonly seen in planetary atmospheres in the solar system and has been studied extensively from measurements by probes like Galileo and Cassini-Huygens.

Figure 1: Pressure-temperature profiles for hot Jupiters at different distances from a Sun-like star, and hence different equilibrium temperatures (Teq). Note that on y-axis, the pressure decreases as you go up, corresponding to going higher up in the atmosphere. The thick parts of the profiles mark the regions of the atmosphere that are convective, and you can see how the radiative–convective equilibrium boundary moves to lower pressures for hotter planets. [Thorngren et al. 2019]

100 K has an RCB around the height corresponding to the pressure of 0.2 bars (1 bar = pressure at sea level on Earth). In the case of hot Jupiters, on the other hand — which, given their proximity to the star, receive radiation of thousands of times that received by Jupiter — the atmosphere remains radiative to a much greater depth. Here the RCB can be expected to lie much deeper, at pressures of around 1 kilobar (remember pressure increases with depth). However, this is a good estimate only if you assume Tint

100 K for hot Jupiters as well. As mentioned before, observed radii inflation of hot Jupiters points toward possible heating of their interiors by stellar irradiation (the strength of which is reflected by the equilibrium temperature of the planet Teq). This implies that hot Jupiters can have much higher Tint, which would push the region of convection and hence the RCB to larger altitudes (lower pressures). Since Tint and Teq both affect the height of the RCB, and Tint also depends on Teq, at what height should we expect the RCB for a hot Jupiter with a given Teq?

To answer this question, the authors calculate temperature–pressure profiles from thermal equilibrium atmospheric models of archetypal hot Jupiters with a range of Teq, and they then investigate how the height of the RCB changes with respect to different levels of stellar irradiation (see Figure 1 and 2).

Marking the Boundary

As is evident from Figure 1, the RCB moves to lower pressures (larger altitudes) with higher Teq, similar to how Tint increases with Teq. The surface gravity and metallicity of the planet also affect the RCB height, as seen in Figure 2.

Figure 2: The RCB pressure level with respect to the Teq of the planet, as calculated for different surface gravities and metallicities of the planet. Note that the RCB ends up at higher pressures for higher surface gravity and lower pressures for higher metallicity. [Thorngren et al. 2019]

With more exoplanet discoveries from TESS and exoplanet characterization opportunities from JWST on the horizon, we can hope to obtain a stronger constraint on atmospheric boundary conditions such as these, which would be important for accurate interpretations of exoplanet atmosphere observations.

About the author, Vatsal Panwar:

I am a PhD student at the Anton Pannekoek Institute for Astronomy, University of Amsterdam. I work on characterization of exoplanet atmospheres to understand the diversity and origins of planetary systems. I also enjoy yoga, Lindyhop, and pushing my culinary boundaries every weekend.


Feeling Gassy?

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites the original can be viewed at astrobites.org.

Title: The Boundary Between Gas-rich and Gas-poor Planets
Author: Eve J. Lee
First Author’s Institution: California Institute of Technology
Status: Accepted to ApJ

Astronomers often compare exoplanets to the planets in our own Solar System — Jupiters, Neptunes, super-Earths, etc. — because they are familiar. But the distinction can be made even simpler: planets that are gas-rich, and those that are not. Where does the boundary between the two fall, and how does it arise? Today’s paper addresses that very question.

An Excess of Sub-Saturn Planets

Figure 1. In the core accretion model of planetary formation, rocky cores form within the gas disk around the star, accrete gas as they cool, and, if they formed massive and early enough, experience runaway accretion to become gas giants. [jupiter.plymouth.edu]

The core-accretion story of planet formation results in a binary picture of planets: those with large gaseous envelopes relative to their cores, and those with small envelopes. But what about the planets in the middle? The core-accretion model suggests that we should expect to find a lot of Jupiters (planets sized 8󈞄 R, where R is Earth’s radius) and a lot of Neptunes or rocky planets (<1𔃂 R), but not much in between. Defying theory, such in-between “sub-Saturns,” which are on the verge of runaway accretion with gas-to-core mass ratios (GCRs) of

0.1𔂿.0, are observed at the same rate as gas giants!

Gassy … or Not?

The fact that sub-Saturns are observed as often as gas giants suggests that the story is a bit more complicated. The cooling of the core is not the only process that must be considered when simulating the formation of planets in a gas disk. Complex interactions between the gas in the planet’s atmosphere and the gas remaining in the disk can play a large role in a planet’s ultimate fate.

To quantify the effects of these additional processes, Lee ran a series of planetary formation simulations. She first determined the best-fit core mass distribution through comparison with observations. Notably, this paper is the first time a single core mass distribution reproduced both the observed plethora of sub-Neptunes and the similar numbers of gas giants and sub-Saturns (see Equation 5 in the paper). Considering planets with orbital periods between 10� days, Lee generated a range of planetary cores with masses from 0.1� M (where M is Earth’s mass) from the best-fit core mass model. These cores were placed in a gas disk at uniform times between 0 to 12 million years and evolved until the end of the 12 million years. The bottom line is perhaps unsurprising: the planet’s fate depended both on the initial core mass and when during the disk’s lifetime the planet formed.

More interestingly, by taking into account processes beyond cooling, Lee’s simulations resolved the discrepancy between the expected and observed number of sub-Saturns. The simulations also revealed four distinct core mass ranges that ultimately result in different planet types (see Figure 2):

  1. Core masses <0.4 M can only accrete a small amount of gas through cooling and remain sub-Neptunes and super-Earths.
  2. Core masses between 0.4󈝶 M accrete gas through cooling until the gas disk dissipates, while interactions between the atmosphere and gas disk decrease the amount of gas that falls onto the core. These planets do not reach runaway accretion and so remain sub-Saturns.
  3. Core masses between 10󈞔 M experience runaway accretion but growth is ultimately stymied by fluid interactions between the planet’s atmosphere and the gas disk. These planets become Jupiters.
  4. Core masses >40 M accrete gas so quickly that they carve deep gaps in the disk and ultimately deprive themselves of further accretion. These planets are massive Jupiters.

Figure 2. The resulting GCR given an initial core mass and time available for accretion. Each point is one planet formation simulation, and darker colors indicate that the core formed later in the disk’s lifetime. The regions A,B,C,D are described in the text. [Lee et al. 2019]

The Gassy Conclusion

Today’s paper is the first study that is consistent with observations across all core mass ranges. Furthermore, Lee shows the importance of including the fluid interactions between the planet’s atmosphere and the gas disk, resolving the discrepancy between the expected and observed number of sub-Saturns. As both observational and computational techniques improve, we will move closer to a comprehensive and complete description of planet formation.

About the author, Stephanie Hamilton:

Stephanie is a physics graduate student and NSF graduate fellow at the University of Michigan. For her research, she studies the orbits of the small bodies beyond Neptune in order learn more about our solar system’s formation and evolution. As an additional perk, she gets to discover many more of these small bodies using a fancy new camera developed by the Dark Energy Survey Collaboration. When she gets a spare minute in the midst of hectic grad school life, she likes to read sci-fi books, binge TV shows, write about her travels or new science results, or force her cat to cuddle with her.


Are gas giants supported by thermal pressure? - Astronomy

The atmosphere is the gaseous outer portion of a planet. Atmospheres have been detected around all planets and many satellites. Some are very dense, and blend gradually into fluid envelopes which contain the bulk of the planet's mass, while others are extremely tenuous. The compositions vary from the hydrogen and helium dominated atmospheres of the gas giants to those found around the terrestrial planets and satellites, with substantial fractions of nitrogen, carbon dioxide, or sulfur dioxide. Trace elements, such as methane or ammonia, can be a negligible mass fraction of an atmosphere and yet play a substantial role in our observations of the atmosphere. All atmospheres are controlled by the same physical processes, and utilize a similar photochemistry. Most feature condensation and clouds, the upper layers are typically strongly modified by solar radiation, and variations in temperature and pressure lead to winds. We state below the basic layers of an atmosphere.

    Thermosphere: a thermal classification of the atmosphere. In the thermosphere, temperature increases with altitude due to the strong incident UV solar flux. It includes the exosphere and part of the ionosphere. On Earth, the temperature rises to 1000K at 500 km, and is isothermal above this level.


Feeling gassy?

Astronomers often compare exoplanets to the planets in our own Solar System — Jupiters, Neptunes, super-Earths, etc. — because they are familiar. But the distinction can be made even simpler: planets that are gas-rich, and those that are not. Where does the boundary between the two fall, and how does it arise? Today’s paper addresses that very question.

An Excess of Sub-Saturn Planets

The most successful theory of planet formation to date is that of core accretion (Figure 1). In this theory, planets first form as rocky cores embedded within the star’s gas disk. As the core cools, the decreased thermal pressure allows more and more gas to accrete onto the core. The outward thermal pressure of the atmosphere supports additional accreted gas in hydrostatic equilibrium until the mass of the gas envelope approaches the core mass. After this critical point, the system experiences runaway accretion and the planet becomes a gas-rich giant planet. Critically, runaway accretion occurs only if the core and atmosphere become massive enough before the end of the typical 10-million-year lifespan of the gas disk. More massive cores will accrete gas faster and therefore be more likely to trigger runaway accretion before the dissipation of the gas disk.

The core accretion story of planet formation results in a binary picture of planets: those with large gaseous envelopes relative to their cores, and those with small envelopes. But what about the planets in the middle? The core accretion model suggests that we should expect to find a lot of Jupiters (, where is Earth’s radius) and a lot of Neptunes or rocky planets (), but not much in between. However, these “sub-Saturns,” which are on the verge of runaway accretion with gas-to-core mass ratios (GCRs) of

0.1-1.0, are observed at the same rate as gas giants!

Gassy…or not?

The fact that sub-Saturns are observed as often as gas giants suggests that the story is a bit more complicated. The cooling of the core is not the only process that must be considered when simulating the formation of planets in a gas disk. Complex interactions between the gas in the planet’s atmosphere and the gas remaining in the disk can play a large role in a planet’s ultimate fate.

To quantify the effects of these additional processes, Lee ran a series of planetary formation simulations. She first determined the best-fit core mass distribution through comparison with observations. Notably, this paper is the first time a single core mass distribution reproduced both the observed plethora of sub-Neptunes and the similar numbers of gas giants and sub-Saturns (see Equation 5 in the paper). Considering planets with orbital periods between 10-300 days, Lee generated a range of planetary cores with masses from (where is Earth’s mass) from the best-fit core mass model. These cores were placed in a gas disk at uniform times between 0 to 12 million years and evolved until the end of the 12 million years. The bottom line is perhaps unsurprising: the planet’s fate depended both on the initial core mass and when during the disk’s lifetime the planet formed.

More interestingly, by taking into account processes beyond cooling, Lee’s simulations resolved the discrepancy between the expected and observed number of sub-Saturns. The simulations also revealed four distinct core mass ranges that ultimately result in different planet types (see Figure 2):

A. Core masses can only accrete a small amount of gas through cooling and remain sub-Neptunes and super-Earths.

B. Core masses between accrete gas through cooling until the gas disk dissipates, while interactions between the atmosphere and gas disk decrease the amount of gas that falls onto the core. These planets do not reach runaway accretion and so remain sub-Saturns.

C. Core masses between experience runaway accretion but growth is ultimately stymied by fluid interactions between the planet’s atmosphere and the gas disk. These planets become Jupiters.

D. Core masses accrete gas so quickly that they carve deep gaps in the disk and ultimately deprive themselves of further accretion. These planets are massive Jupiters.

Figure 2 shows the wide variety of planets that can be formed given an initial core mass and time available for gas accretion. In particular, more massive cores can span the full GCR range depending on when they formed, becoming gas-rich or gas-poor planets. Conversely, low mass cores will only ever become gas-poor planets. This provides a potential explanation for why metal-rich solar systems with more massive elements appear to host a wider variety of planets.

The Gassy Conclusion

Today’s paper is the first study that is consistent with observations across all core mass ranges. Furthermore, Lee shows the importance of including the fluid interactions between the planet’s atmosphere and the gas disk, resolving the discrepancy between the expected and observed number of sub-Saturns. As both observational and computational techniques improve, we will move closer to a comprehensive and complete description of planet formation.


What Makes Gas Giants Unique?

Initially, planets were just clumps of debris. By gravitational force, the debris clumped together into larger chunks. This repeated more and more as the planets grew bigger, because as the total mass increased, so did its gravitational pull.

This is how we expect a planet’s core is formed.

Unlike the terrestrials, these two gas giants had the advantage of having an abundance of hydrogen and helium to pull in. As they did, the mass and gravity of the planets increased rapidly, allowing them to pull in more and more gas. Eventually the hydrogen became the dominant substance within the planet, followed by helium, giving Jupiter and Saturn the title gas giant.

As the planets finished forming, what we end up with are giant planets with high amounts of hydrogen and helium gas.

This also explains why Jupiter and Saturn rotate faster than the other planets

Jupiter and Saturn’s days are small since their rotational velocities are larger than the terrestrial planets and ice giants, but why?

The explanation can be found within conservation of angular momentum. You may know linear momentum and the equation p = mv, or the fact that momentum is always conserved. Angular momentum also always obeys this rule.

Angular momentum can be defined by this equation (don’t worry, no need to know it in detail):

Momentum = moment of inertia x angular velocity

Let’s apply this equation to a forming planet. The moment of inertia (I) would be the distance some gas that’s being pulled in is from the planets center. The angular velocity (ω) is how fast the planet’s spinning.

As the gas gets closer, the moment of inertia decreases. But, if the angular momentum (L) needs to stay the same, then the velocity must increase to balance everything out (i.e the planet needs to spin faster).

The same concept can be applied ballet dancers pulling their arms and leg in when spinning to rotate faster, or playground spinning wheels.


References

  • (1) T. Guillot, Science 286 , 72 (1999).
  • (2) J. M. McMahon, M. A. Morales, C. Pierleoni and D. M. Ceperley, Rev. Mod. Phys. 84 , 1607 (2012).
  • (3) R. Redmer et al, in Shock Compression of Condensed Matter, AIP Conference Proceedings 1195 , 905 (2009).
  • (4) N. Nettelmann et al, Astrophysical Journal 683 , 1217 (2008).
  • (5) B, Holst, R. Redmer and M. P. Desjarlais, Phys. Rev. B 77 , 184201 (2008).
  • (6) T. Guillot, Annual Review of Earth and Planetary Sciences 33 , 493 (2005).
  • (7) E. García-Melendo et al, Nature Geoscience 6 , 525 (2013).
  • (8) J. Leconte and G. Chabrier, Nature Geoscience 6 , 347 (2013).
  • (9) A. Morbidelli, A. Crida, F. Masset and R. P. Nelson, Astronomy and Astrophysics 78 , 929 (2008).
  • (10) J. D. Anderson and G. Schubert, Science 317 , 1384 (2007).
  • (11) D. J. Stevenson, Nature 441 , 34 (2006).
  • (12) L. Mayer, T. Quinn, J. Wadsley and J. Stadel, Science 298 , 1756 (2002).
  • (13) T. Guillot and A. P. Showman, Astronomy and Astrophysics 385 , 156 (2002).
  • (14) G. Chabrier and I. Baraffe, Anual review of astronomy and astrophysics 38 , 337 (2000).
  • (15) E. Gregoryanz et al, Phys. Rev. Lett. 90 , 175701 (2003).
  • (16) E. Kiran, P. G. Debenedetti and C. J. Peters, Supercritical Fluids: Fundamentals and Applications (NATO Science Series E: Applied Sciences 366, Kluwer Academic Publishers, 2000).
  • (17) T. Guillot, Planetary and Space Science 47 , 1183 (1999).
  • (18) M. French et al, Astr. J. Suppl. Ser. 202:5 (2012).
  • (19) V. V. Brazhkin and K. Trachenko, Physics Today 65(11) , 68 (2012).
  • (20) V. V. Brazhkin et al, Phys. Rev. E 85 , 031203 (2012).
  • (21) D. Bolmatov, V. V. Brazhkin and K. Trachenko, Nature Communications 4 , 2331 (2013).
  • (22) V. V. Brazhkin et al, Phys. Rev. Lett. 111 , 145901 (2013).
  • (23) D. Bolmatov, V. V. Brazhkin, Yu. D. Fomin, V. N. Ryzhov and K. Trachenko, J. Chem. Phys. 139 , 234501 (2013).
  • (24) J. Frenkel, Kinetic Theory of Liquids (ed. R. H. Fowler, P. Kapitza, N. F. Mott, Oxford University Press, 1947).
  • (25) The National Institute of Standards and Technology database, http://webbook.nist.gov/chemistry/fluid.
  • (26) A. B. Belonoshko, O. LeBacq, R. Ahuja and B. Johansson, J. Chem. Phys. 117 , 7233 (2002).
  • (27) Yu. D. Fomin, V. N. Ryzhov and V. V. Brazhkin, in press.

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Jupiter is comprised of 90% hydrogen and 10% helium (a 75/25% mass ratio) and contains small amounts of methane, water, oxygen and ammonia. According to scientists, this chemical composition matches closely to the solar nebula that formed our solar system in the early stages of development. Saturn, Uranus, and Neptune also have a similar chemical makeup, but Uranus and Neptune have less hydrogen and helium. Since Jupiter is a gas planet, there is no solid surface. This means that if one stepped onto the planet, he would sink into it and eventually get crushed by the massive increase in pressure (the gaseous compounds get denser with depth) or be vaporized by the hot temperatures near the center. Thus, what we see when we observe the surface of Jupiter is the atmosphere that extends deep into the planet.

Jupiter orbits the sun in the counterclockwise direction as seen from the north.Its orbit is similar to the shape of a circle (The focus points which make up Jupiter's orbits are very close to each other). The radius of Jupiter's orbit is approximately 5 AU. The period of Jupiter's orbit is about 12 years. Jupiter rotates in about 10 hours.


Giant Storms on Giant Planets

Superimposed on the regular atmospheric circulation patterns we have just described are many local disturbances&mdashweather systems or storms, to borrow the term we use on Earth. The most prominent of these are large, oval-shaped, high-pressure regions on both Jupiter (Figure (PageIndex<11>)) and Neptune.

Figure (PageIndex<11>) Storms on Jupiter. Two examples of storms on Jupiter illustrate the use of enhanced color and contrast to bring out faint features. (a) The three oval-shaped white storms below and to the left of Jupiter&rsquos Great Red Spot are highly active, and moved closer together over the course of seven months between 1994 and 1995. (b) The clouds of Jupiter are turbulent and ever-changing, as shown in this Hubble Space Telescope image from 2007.

The largest and most famous of Jupiter&rsquos storms is the Great Red Spot, a reddish oval in the southern hemisphere that changes slowly it was 25,000 kilometers long when Voyager arrived in 1979, but it had shrunk to 20,000 kilometers by the end of the Galileo mission in 2000 (Figure (PageIndex<12>)). The giant storm has persisted in Jupiter&rsquos atmosphere ever since astronomers were first able to observe it after the invention of the telescope, more than 300 years ago. However, it has continued to shrink, raising speculation that we may see its end within a few decades.

Figure (PageIndex<12>) Jupiter&rsquos Great Red Spot. This is the largest storm system on Jupiter, as seen during the Voyager spacecraft flyby. Below and to the right of the Red Spot is one of the white ovals, which are similar but smaller high-pressure features. The white oval is roughly the size of planet Earth, to give you a sense of the huge scale of the weather patterns we are seeing. The colors on the Jupiter image have been somewhat exaggerated here so astronomers (and astronomy students) can study their differences more effectively. See Figure (11.1.1) to get a better sense of the colors your eye would actually see near Jupiter.

In addition to its longevity, the Red Spot differs from terrestrial storms in being a high-pressure region on our planet, such storms are regions of lower pressure. The Red Spot&rsquos counterclockwise rotation has a period of six days. Three similar but smaller disturbances (about as big as Earth) formed on Jupiter in the 1930s. They look like white ovals, and one can be seen clearly below and to the right of the Great Red Spot in Figure (PageIndex<12>). In 1998, the Galileo spacecraft watched as two of these ovals collided and merged into one.

We don&rsquot know what causes the Great Red Spot or the white ovals, but we do have an idea how they can last so long once they form. On Earth, the lifetime of a large oceanic hurricane or typhoon is typically a few weeks, or even less when it moves over the continents and encounters friction with the land. Jupiter has no solid surface to slow down an atmospheric disturbance furthermore, the sheer size of the disturbances lends them stability. We can calculate that on a planet with no solid surface, the lifetime of anything as large as the Red Spot should be measured in centuries, while lifetimes for the white ovals should be measured in decades, which is pretty much what we have observed.

Despite Neptune&rsquos smaller size and different cloud composition, Voyager showed that it had an atmospheric feature surprisingly similar to Jupiter&rsquos Great Red Spot. Neptune&rsquos Great Dark Spot was nearly 10,000 kilometers long (Figure (PageIndex<8>)). On both planets, the giant storms formed at latitude 20° S, had the same shape, and took up about the same fraction of the planet&rsquos diameter. The Great Dark Spot rotated with a period of 17 days, versus about 6 days for the Great Red Spot. When the Hubble Space Telescope examined Neptune in the mid-1990s, however, astronomers could find no trace of the Great Dark Spot on their images.

Although many of the details of the weather on the jovian planets are not yet understood, it is clear that if you are a fan of dramatic weather, these worlds are the place to look. We study the features in these atmospheres not only for what they have to teach us about conditions in the jovian planets, but also because we hope they can help us understand the weather on Earth just a bit better.

The wind speeds in circular storm systems can be formidable on both Earth and the giant planets. Think about our big terrestrial hurricanes. If you watch their behavior in satellite images shown on weather outlets, you will see that they require about one day to rotate. If a storm has a diameter of 400 km and rotates once in 24 h, what is the wind speed?

Speed equals distance divided by time. The distance in this case is the circumference ((2 pi R) or (pi d)), or approximately 1250 km, and the time is 24 h, so the speed at the edge of the storm would be about 52 km/h. Toward the center of the storm, the wind speeds can be much higher.

Jupiter&rsquos Great Red Spot rotates in 6 d and has a circumference equivalent to a circle with radius 10,000 km. Calculate the wind speed at the outer edge of the spot.

For the Great Red Spot of Jupiter, the circumference ((2 pi R)) is about 63,000 km. Six d equals 144 h, suggesting a speed of about 436 km/h. This is much faster than wind speeds on Earth.


A common mystery

Although K2-18b has the right conditions for Earth-like clouds to form, that doesn't make the planet itself Earth-like. Instead, it is classified as a sub-Neptune, a gas giant without a surface. NASA's Kepler spacecraft determined that sub-Neptunes are likely the most common type of exoplanets in the Milky Way, making up more than three-fourths of the planetary population.

Nonetheless, astronomers are having a difficult time understanding the relatively small gas giants, and that's one reason the K2-18b findings are so exciting. "We don't quite know what's going on with these planets," Benneke said. "It's probing this regime of planets that we have a very poor understanding of right now."

Sub-Neptunes have masses that fall somewhere between Earth and Neptune, and there's no analogue in the solar system. Understanding worlds like K2-18b can help improve scientists' knowledge of how planets grow and evolve. "If you want to understand planets as a whole, the diversity of planets, it's very critical that you understand the most common ones," Benneke said.

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Three teams of astronomers have been fascinated by an alien world known as K2-18b. But what's all the fuss about?

This gas-giant exoplanet has water-rich clouds. Here's why it thrills astronomers. : Read more

The report stated "Although scientists have studied exoplanet atmospheres before, those worlds have been larger than K2-18b, which is only about 2.6 times the size of Earth and 8.5 times its mass. The planet's small size made the observations especially challenging, requiring multiple detailed measurements that, combined, could provide a more in-depth probe of the world."

Using the properties for mass and size, the mean density is some 2.66 g cm^3. The eccentricity is 0.2, more like Mercury's orbit compared to the Earth :) The semi-major axis is 0.143 a.u. compared to Mercury's 0.387 a.u. This report is objective in stating, "Although K2-18b has the right conditions for Earth-like clouds to form, that doesn't make the planet itself Earth-like. Instead, it is classified as a sub-Neptune, a gas giant without a surface."

I do not think astronomers know the spin rate either compared to Earth's length of day. More details on K2-18 b properties can be found at Planet K2-18 b At present in my exoplanet studies, I find some exoplanets listed near 39 Mjup masses orbiting pulsars and terms like *planetors*. I am aware of *motaurs*, motorcycle with half-man on top. The motaurs are born to ride :) I am not sure about the *planetors*.

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Watch the video: Brown Dwarfs Vs Gas Giants? Whats The Difference? (September 2021).